Glossary of Terms 

A glossary of terms covering lighting, audio, vision and staging in the event and theatrical industry.

There can be a number of meanings for some terms which can vary from country to country and manufacturer to manufacturer. Information within this guide is indicative and every situation can require a different approach and solution. All care has been taken, however, Image Group NZ accepts no responsibility for inaccuracies. If you have any suggestions, comments or queries regarding this list please contact us.

 A

A-B TEST
Comparing the performance of two or more models of equipment, such as amplifiers or speaker systems, by listening and switching quickly from one to the other.

ABSORPTION
The ability of a room to take up or absorb the acoustic energy radiated within it. There are many types of absorption since it can be frequency dependent. There are certain materials such as acoustical ceilings that may absorb more high frequencies than lows. Diaphragmatic absorptions (caused by loose wall panels or cavities behind the panels) cause certain low frequencies to be absorbed.

AC
Short for ALTERNATING CURRENT.

ACOUSTIC
Relating to the production, effects and transmission of sound waves; the transmission of sound waves through various mediums. Pertaining to the act or sense of hearing, the science of sound, or the sound heard.

ACOUSTICAL
Sound or properties of sound; the acoustical response of a room has to do with the way that room responds to sound.

ACOUSTIC TREATMENT
The application of acoustic or sound absorbing material to a room or enclosure to obtain the desired acoustic characteristics.

ACTIVE SIGNAL
A type of electronic circuitry that can increase the gain or amplitude of a signal. Active gain controls. Active Equalization. Active Direct Boxes. Active Crossover.

ACTIVE MATRIX TFT
The most common type of LCD used in most laptops and projectors. A typical active matrix TFT display is a single panel of LCD glass that controls all three primary colours. TFT displays are noted for their quick response time and ability to display full motion video and animations without image ghosting.

ADAPTOR
A connector which allows two or more electrical devices to be connected to a single power outlet. The connection is normally parallel, that is, each device is fed the same voltage, but the current is divided between them.

ADDITIVE COLOUR MIXING
See COLOUR MIXING.

ALS
Assistive Listening System. This term refers to systems used to augment the regular sound system to allow hearing impaired persons to more clearly hear. These can be either hardwired (earphones) or wireless (inductive loop, FM, infrared). The FM and infrared wireless systems are most common and allow the user to have individual control of the listening volume.

AMPERE
(amp) Named after Andre Ampere (1775-1836), French scientist. A unit of measurement of electrical current (I). One amp of current represents 6.2818 x 10 electrons flowing past a given point in one second, and is equal to one coulomb.

AMPLIFICATION
An increase in signal magnitude from one point to another, or the process causing this increase.

AMPLIFIER
A device capable of increasing the gain (magnitude) or power level of a voltage or current that is varying with time (frequency), without distorting the wave form of the signal. An amplifier is used to increase weak signals (such as those from a program source) to a level sufficient to drive loudspeakers.

ANALOG
A physical variable which remains similar to another variable insofar as the proportional relationships are the same over some specified range. The electrical signal produced by a microphone is an electrical analog of the acoustic sound that the microphone is reproducing. The continuous electrical signal that the microphone produces varies in voltage and frequency as a direct correlation to the nonelectrical acoustic information impressed on the transducer. The electrical signal is analogous to the acoustical sound that the microphone reproduces, i.e., the voltage that the microphone produces is the electrical analog of the acoustic sound source.

ANALOGUE SIGNAL
The way information is transmitted over a continuously changing electrical wave that is similar to, or analogous with, the original signal. A continuously variable signal that can have any value over a given range.

1. In lighting: an analogue voltage within the range 0 to 10 Volts can have values of 0, 2, 8.785 or any value between. Most dimmers require an analogue voltage in order to operate (from 0 to -10V or 0 to +10V depending on the manufacturer). Most lighting control desks produce a digital multiplexed output, which is converted by a demux box to an analogue signal for the dimmer. See also DIGITAL DIMMER.
2. Sound: An analogue recording will record the exact waveform of the original sound, simply converting it to an electrical signal at the microphone, and back into air movement at the speaker. See DIGITAL.

ANECHOIC
Refers to a room in which all surfaces are lined with acoustic absorption material to such an extent that the room absorbs sound energy instead of reflecting it around the room (no echo). A room that offers nearly total absorption is called an Anechoic Chamber and must be quite large in order to accommodate low frequencies.

ANIMATION DISC
See EFFECTS.

ANSI LUMENS
A standard for measuring light output that is used to compare projectors. Unfortunately, there are enough variables that the eye will often disagree radically with the ANSI rating. At best, ANSI lumens do fairly well comparing "apples" to "apples". If however one projector uses Halogen lamps and another metal-halide, the halogen projector will seem noticeably dimmer even if the two units rate the same. Other variables, including type of LCD technology (active matrix TFT, Poly-Si, passive), type of overall technology (LCD vs. DLP vs. CRT), contrast ratios, etc. all effect the end result.

ANTI PROSCENIUM
Originally Ante Proscenium, meaning in front of the proscenium.

ARC LIGHT
See DISCHARGE LAMP.

ARCHITECTURAL LAMP
A type of linear filament lamp with contacts at 90 degrees to the filament which can gives the appearance of a continuous line of light (similar to neon, but dimmable).

ASPECT RATIO
The most popular aspect ratio is 4:3 (4 by 3). Early television and computer video formats are in a 4:3 aspect ratio, which means that the width of the image is 4/3 times the height. Other formats are 5:4 used by the 1280x1024 SXGA resolution, 16:9 used by HDTV, and 3:2 for 35mm slides.
Aspect ratio of 4 units wide by 3 units high can also be described as 1.33 aspect ratio (i.e. width is 1.33 times height).

ATTENUATION
A decrease in signal magnitude from one point to another, or the process causing this decrease.

ATTENUATOR
An adjustable passive network which reduces the power level of a signal without introducing any appreciable distortion.

AUDIO CHAIN
The order of sequence for connecting audio components, i.e. microphone, preamplifier (mixer), effects device, graphic equalizer, crossover, amplifier, and speaker.

AUDIO FREQUENCY
Any frequency corresponding to a normally audible sound wave.

AUDIO RANGE
20 Hz to 20,000 Hz. (Twenty cycles per second to twenty thousand cycles per second). The frequency response spectrum of human auditory perception.

AUTO BALANCE
A system for detecting errors in colour balance in white and black areas of the picture and automatically adjusting the white and black levels of both the red and blue signals as needed for correction.

AUTOMATIC GAIN CONTROL (AGC)
A process by which gain is automatically adjusted as a function of the input level or other parameter. A device used to automatically control the level of a sound system based on the input level.

AUTOMATED FIXTURE
See MOVING LIGHT.

AUX INPUT / AUX OUTPUT
An auxiliary input or output that is generally a direct connection to the device's internal signal bus. These are used to interconnect devices which have the same signal level. That is, where gain is not required such as when connecting a microphone to a mixer. Typical devices that operate at "aux level" are tape decks, CD players, equalizers, signal processors, etc.

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 B

BACKLIT PANEL
Refers to a remote control, or on projector control panel, that has buttons and controls that are illuminated. This is a major asset when using the projector in a darkened or semi-darkened room. Many projectors have backlit remote controls, while the number of projectors with backlit control panels is much smaller. As projectors have gotten brighter, room lights tend to stay on. So while a nice added feature, having backlit controls is no longer important to many users.

BACK ROOM PROJECTOR
A projector with a "long-throw" lens designed to be used from the far back of the room, often in a projection booth, balcony, or the back of an auditorium. Many typical projectors have third party lenses available for "long-throw" applications.

BACK PROJECTION
See PROJECTION.

BACK-UP
A section of a lighting control board (sometimes a separate unit) which provides an alternative method of control should the main board fail.

BACKLIGHT
Light coming from upstage, behind scenery or actors, to sculpt and separate them from the background.

BAFFLE

1. A sheet of material used to prevent a spill of light in a lantern or in part of a set.
2. A panel in a loudspeaker cabinet designed to reduce back interference noise by isolating the front and rear of the loudspeaker diaphragm.
3. The term derives from its original use in preventing or baffling the speaker's rear sound waves from interfering with its front waves.
4. A panel in an auditorium positioned so as to reduce sound reflections and improve the acoustics of the space.

BALANCED CABLE
A cable consisting of a pair of wires surrounded by either a braided shield or foil wrap with a drain wire.

BALANCED LINE

1. A transmission line consisting of two conductors plus a shield, capable of being operated so that the voltages of the two conductors are equal in magnitude (voltage) and opposite in polarity with respect to ground. A balanced line offers common mode rejection, or cancellation by attenuation, of signals electromagnetically induced into the signal lines.
2. See also COMMON MODE REJECTION.

BALLAST
A unit used in conjunction with discharge lamps containing capacitors, inductors and other start-up circuitry. The inductor is initially used to develop a high potential (voltage) to strike the discharge and is then used to limit the current flow while the lamp is lit.

BALLYHOO
Swinging a followspot beam around in a figure of eight pattern.

BANDPASS
Refers to a type of filter that passes a certain band of frequencies uniformly and attenuates or reduces the level of frequencies below and above the specified bandpass.

BANDWIDTH

1. The number of cycles per second (Hertz) expressing the difference between the lower and upper limiting frequencies of a frequency band; also the width of a band of frequencies.
2. The effective carrying capacity of a telecommunications transmission medium. Bandwidth describes how much information can be pushed through an electronic "pipe" at any given time.

BAR
The horizontal metal (steel or alloy) tube (usually 48mm in external diameter, but can be up to 60mm) hung from flying lines (or forming part of a grid) from which lighting equipment and scenery etc. may be suspended. Also known as a BARREL. When vertical, known as a BOOM.

BARE ENDS
Term to describe an electrical cable which has no connector at one end (for example, a SPEAKON to BARE ENDS cable is used to connect the terminals of a speaker cabinet to a speakon socket, and a 63A socket to bare ends might be used to wire in a temporary supply from a power distribution board before connecting equipment. Any installation work of this sort should only be carried out by a qualified electrician, and should never be done "live").

BARNDOORS
A rotatable attachment consisting of two or four metal flaps (hinged) which is fixed to the front of a Fresnel or PC type lantern to cut off the beam in a particular direction(s). Profile lanterns use SHUTTERS to achieve a greater degree of control and accuracy.

BASE PLATE
A metal plate which prevents damage to floors when using scaffolding or trussing.

BASS REFLEX
A type of speaker enclosure in which the speaker's rear sound wave emerges from a critically dimensioned auxiliary opening, or port, to reinforce the bass tones.

BATTENS

1. Timber at the top and bottom of a cloth. A Sandwich batten is used to carry a hanging cloth. It comprises two flat pieces of timber screwed together with the edge of the cloth between them.
2. Timber used for joining flats together for flying.
3. Compartmentalised floodlights set up so as to allow colour mixing. See also GROUNDROW. Low voltage battens are commonly used as light curtains & for colour washes. Known in the US as a STRIPLIGHT.

BARREL
An outward bowing of the picture. A condition in which colours appear to flow into unintended areas of the image.

BEAM ANGLE
The angle of the cone of light produced by a lantern. Defined as the angle within which the lowest intensity in a beam of light from a lantern is not less than one-tenth of the maximum.

BEAMLIGHT
Flood lantern which uses a parabolic reflector and a low voltage high intensity lamp to produce an intense near-parallel beam. Also known as a Beam Projector.

BEGINNERS
A call given by the DSM (Deputy Stage Manager) to bring those actors who appear in the first part of a play to the stage. e.g. "Act One Beginners to the stage, please". The actors/actresses are then called by name.

BELTPACK
Part of the communication ("cans") system in a theatre; the beltpack contains the controls and circuitry to drive the HEADSET worn by crew members. Each beltpack connects into the headset ring and back to a PSU (Power Supply Unit) which is powered from the mains.

BIAMP
Separating the audio spectrum into two bands, i.e., high frequencies (high pass) and low frequencies (low pass) by means of an electronic crossover and using two separate amplifiers or channels of an amplifier. One amplifier or channel passes the high pass signals (high frequencies) to drive the high frequency component or horn of the speaker system. The other amplifier or channel passes the low pass signals (low frequencies) and drives the woofer or low frequency component of the speaker system. The advantage to biamp operation of a sound system is increased headroom and dynamic range.

BIFOCAL SPOT
Profile lantern with two sets of shutters, one of which produces a hard edge, and one a soft edge. Not necessary in zoom profiles, because this requirement is fulfilled by two lenses.

BINARY
BIT - Binary Digit.

BIRDIE
A compact low voltage display luminaire containing a Par 16 or MR16 lamp. So called because it is similar to, but much smaller than, the Parcan, and is hence "one under Par" (It's a golfing reference). Requires a 12 Volt external transformer, but is ideal for concealing in pieces of set or as downstage uplights. See also MR16.

BLACK HOLE
Accidentally unlit portion of the stage.

BLACK LIGHT/BLACKLIGHT
See ULTRA-VIOLET.

BLACKOUT / BO / B.O.
Complete absence of stage lighting. Blue working lights backstage should remain on and are not usually under the control of the board, except during a Dead Blackout (DBO), when there is no onstage light. Exit signs and other emergency lighting must remain on at all times.

BLACKWRAP
(Trade Name) Thin black aluminium product which is used to temporarily mask light coming out of a luminaire's body, or to control spill. A self-adhesive tape version is also available.

BLEED THROUGH
Transformation from a scene downstage of a gauze to another scene upstage, by slowly crossfading lighting from downstage to upstage. If a gauze is lit steeply, or from the sides, it will appear solid. If this light is turned off and light added to the set upstage of it, it will disappear.

BLEEDING
Dimmers which are incorrectly trimmed are said to bleed. That is, the dimmer still gives a small output, causing the lantern to glow, when the control signal is at a minimum.

BLIND PLOTTING
The facility on some lighting control desks for the operator to make changes to the plot, without affecting the state on stage.

BLINDERS
Lamps arranged around the stage directed into the auditorium, originally to prevent spectators seeing the stage during scene changes when the house tabs were not lowered. Now used for effect in rock concerts etc.

BLUES

1. Blue lights used backstage in a performance situation.
2. Also see WORKING LIGHTS.

BNC
(Bayonet Neill Consulman - after original inventor) Coaxial connector used for carrying a composite video signal or radio frequency signal.

BOARD

1. The main control for the stage lighting. Originally known as the switchboard or dimmerboard, it is now usually remote from the dimmers. The lighting operator for a show is said to be "on the board", and is sometimes known as the "board op".
2. Colloquially, any control desk (e.g. Sound Board, Light Board).

BOOM

1. Vertical scaffolding pole (usually 48mm diameter) on which horizontal boom arms can be mounted, carrying lanterns. Often used behind wings for side-lighting etc. Booms have a base plate or stand at the bottom and are tied off to the grid or fly floor at the top (not always necessary for short booms). Booms can also be fixed to the rear of the proscenium arch (Pros. Boom) or hanging from the ends of lighting bars. Sometimes known in the US as a light tree. A light tree mounted upstage of a Tormentor is known as a Torm Tree.
2. An arm mounted on a microphone stand.

BOOST
A term used to indicate an increase in gain of a frequency or band of frequencies when equalizing an audio signal. Opposite of cut.

BOOTH
Control Room.

BORDER
A narrow horizontal masking piece (flattage or cloth), normally of neutral colour (black) to mask the lighting rig and flown scenery from the audience, and to provide an upper limit to the scene. Often used in conjunction with LEGS.

BOUNCE

1. Diffuse light that has been reflected from the stage, walls, cyclorama etc.
2. "Bounce" is sometimes used for a flat (non-curved) cyclorama. Strictly, a bounce is a white or light blue cloth onto which light is bounced to backlight another cloth. A bounce doesn't need to be seamless, whereas a cyclorama should be.
3. Describes the fast in/out movement of "bouncing" flown house tabs, used during curtain calls. This can also apply to the fast blackout/lights up cues that happen at curtain calls.

BOX TRUSS
See TRUSS.

BRAIL
A horizontal rope, wire or chain attached at either end of a piece of scenery or lighting bar pulling it upstage or downstage of its naturally hanging position to allow another flying item to pass, or to improve its position.

BREAKOUT
A connection at the end of a multicore cable which allows the connection of many items to it. (e.g. there is a breakout box at the end of a sound multicore cable which allows you to plug microphone cables into it).

BREAKUP
A commonly used abstract GOBO which gives a textured effect to the light, without throwing a specific pattern onto the stage. Used to add interest to light beams. A leafy breakup is used for outdoor scenes / forests / spooky wood etc. to break up the light on the actors faces.

BRI (BASIC RATE INTERFACE)
The ISDN standard that governs how phones and other electronic devices are connected to the ISDN switch.

BRIDGE

1. A walkway, giving access to technical and service areas above the stage or auditorium, or linking fly-floors.
2. A videoconference bridge is usually made up of back-to-back codecs from different manufacturers to convert signals from one proprietary system to another.

BRIDGE MODE (MONO)
Operating a stereo amplifier in mono via the bridge mode switch, which typically makes Channel A output the positive power rail and Channel B output the negative power rail. Since the signal swings between A and B Channels, the output of the amplifier is twice that of single channel operation.

BRIDGING
Connecting one electrical circuit in parallel with another. Typically used to describe the process of connecting the output of a single device to the inputs of multiple other devices.

BRIGHTNESS
The attribute of visual perception in accordance with which an area appears to emit more or less light. (Luminance is the recommended name for the photo-electric quantity which has also been called brightness.)

BROADBAND
Digital services at rates greater than 1.536 million bits (mbps), capable of supporting voice, video, and data, possibly using multiple channels.

BUBBLE
(Especially TV and Film) Jargon for a replacement lamp.

BUILD

1. During lighting plotting, to construct a state from blackout, or to add to an existing state.
2. An increase in light or sound level. SEE CHECK.
3. A period of set construction ("The Build").

BUMP
A flash or sudden jump in light level (a snap cue). See also FLASH BUTTON.

BURNT OUT
A coloured gel that has lost its colour or melted through due to excessive heat in front of a lantern. Dark blues and greens are most susceptible, and may need replacing during a long run.

BUS
A conductor that serves as a common connector to several signal sources, most often associated with a mixer. A separate signal routing to a specified output.

BUSBAR
Metal bar carrying incoming electrical supply into which portable dimmer racks or other large power requirements can be wired directly. An enclosure containing busbars is a Busbar Chamber.

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 C

C CLAMP
US equivalent of the UK Hook Clamp. Requires a spanner/wrench to tighten. Known as a PARROT CLAMP in the US film business.

CABLE
Wiring, temporarily rigged, to carry electrical current. Depending on the size of the cable (current carrying capacity), cables are used to supply individual lanterns, whole dimmer racks, or carry signals from a microphone etc.

CABLE GRIP
A U-shaped clip and saddle used for terminating wire rope. Also known as a Bulldog, Dog Grip or Wire Rope Clip.

CABLE TIE
Lockable (and sometimes releasable) plastic strap used to tie a bundle of cables together, amongst many other things.

CAD

1. Computer-Aided Design. Using a computer to help with 2D plans and drawings, or increasingly for 3D visualisation of how a set will look, and how lighting will affect it.
2. See also WYSIWYG.

CAN
ACN (Advanced Control Network). A new (2003) ethernet-based control protocol between control desk, dimmers & moving lights. Developed by ESTA and Strand Lighting. DMX nodes are used to communicate with non-ethernet devices.

CANS

1. Headset earpiece, microphone and beltpack used for communication and co-ordination of technical departments during a performance. (e.g. "Electrics on cans", "Going off cans", "Quiet on cans!").
2. Any headphones
3. Short for PARCANs.

CAPACITOR
A device which consists essentially of two conductors (such as parallel metal plates) insulated from each other by a dielectric (a material in which an electric field can be sustained with a minimum dissipation in power) and which introduces capacitance into a circuit, stores electrical energy, blocks the flow of direct current (DC), and permits the flow of alternating current (AC), to a degree dependent on the capacitor's capacitance and the current frequency.

CARDIOID

1. A type of microphone having a heart shape pickup pattern that picks up sound better from the front (on axis) than back (off axis).
2. See also UNIDIRECTIONAL

CAROUSEL
Circular slide magazine; also refers to a 35mm slide projector using this type of magazine (Kodak trade name). See PROJECTION.

CHAIN HOIST
Manually operated or electrically driven hoist for lifting scenery and lighting equipment. The chain hoists are rigged to fixed points in the venue. Commonly used to lift lighting truss into position for touring shows or concerts.

CHANNEL

1. A complete control path for signals in lighting or sound equipment.
2. A path for electrical transmission between two or more points. Also called a link, line, circuit, or facility.

CHASE
A repeated sequence of changing lighting states. A chase can be produced easily by the effects functions of a computerised lighting desk. There are standalone units designed to chase lighting circuits electronically in time to music (sound to light) or mechanically as a repeated sequence (as used in early neon signs).

CHEAT SHEET
A smaller version of the lighting plan, used by the lighting designer during the lighting plot. Also known as a Dimmer Layout or Magic Sheet.

CHECK

1. Opposite of Build; a smooth diminishment of light or sound level.
2. See PREFADE LISTEN.

CHIEF ELECTRICIAN
The senior member of the theatre's stage lighting team, although not necessarily the lighting designer. In common with many theatre jobs, the actual duties of the Chief Electrician vary from theatre to theatre. Some chiefs are responsible for electrical maintenance of the building, some design the lighting for nearly every in-house production and some design no lighting at all, some have a team of eight staff under them, some have two. Many theatres employ casual staff to assist on lighting rigging sessions. Some theatres have a separate sound department, smaller venues have the lighting team also running sound for shows (and doing sound design for some).

CHINAGRAPH PENCIL
Usually white, wax-based pencil used for marking magnetic tape prior to splicing. Also used for marking identifying numbers on lighting gels.

CHROMATIC ABERRATION
An optical defect of a lens which causes different colours or wave lengths of light to be focused at different distances from the lens. It is seen as colour fringes or halos along edges and around every point in the image.

CHROMATICITY
The colour quality of light that is defined by the wavelength (hue) and saturation. Chromaticity defines all the qualities of colour except its brightness.

CHROMINANCE
A colour term defining the hue and saturation of a colour. Often confused with brightness, the two terms are not interchangeable.

CID
(Compact Iodide Daylight) A high intensity discharge lamp that produces a light similar in colour temperature to daylight approx. 5500K). A 1000W CID lamp produces 2.5 times more light than a 2000W tungsten halogen source.

CIRCLE FRONTS
A permanent front of house lighting position in older proscenium theatres. A number of spotlights, sometimes fitted with colour changers, are recessed into the front of the circle balcony above the stalls. Sometimes known as the Balcony Rail position.

CIRCUIT

1. The means by which a lantern is connected to a dimmer or patch panel. Numbered for reference.
2. A complete electrical "loop" around which current can flow.

CIRCUIT BREAKER
An electro-mechanical "fuse" that can be reset, rather than having to be replaced. Available in the same ratings as fuses. See MCB, RCD.

CLEANERS
Auditorium working lights. Used for cleaning and setting up the auditorium before the house lights (usually more atmospheric) are switched on.

CLIPPING
Amplifier overload causing a squaring off or undesirable change in the wave form resulting in distortion or perceptible mutilation of audio signals.

CLOVE HITCH
Invaluable knot that every technician should know.

CLUSTER
An array of loudspeakers or horns designed to act as a single or point source of sound.

COATED OPTICS
A variety of materials are put on to high quality lenses for several reasons. One of the key reasons to coat optics is to minimize the amount of light reflected back to the lamp and the amount of ambient light that mingles with the focused light leaving the lens. Generally good coatings can add 15% or more to the lenses brightness. Other coatings are used for filtering colours.

CODEC
An acronym for Coder/Decoder. This device compresses (for transmission) and decompresses (once received) digital video and analog audio signals so that they occupy less bandwidth during transmission.

COINCIDENT
Two signals are said to be coincident when they correspond exactly, fall upon or meet at the same point. Coinciding or occurring in space or time in exact agreement.

COLOUR

1. We often hear references to 8-bit, 16-bit, or 24-bit colour. These measurements refer to how many unique colours are used to display the image. The more colours used, the smoother the colour gradations will appear.
2. See COLOUR FILTER.

COLOUR CALL
A list compiled from the lighting plan of all the colours needed for the rig, and their size. This term also applies to the act of preparing colour filters and frames from such a listing.

COLOUR CHANGER

1. Scroller, where a long string of up to 16 colours is passed horizontally in front of a lantern. Remotely controlled by the lighting desk. Some scrollers have cooling fans to prolong the life of the gel string. Stronger colours will burn out faster without cooling, or if the focus of the beam is concentrated on the gel. If colours aren't lasting very long in scrollers, try changing the focus of the lantern. HEAT SHIELD clear gel should be used between the lens and the colour scroller to absorb some of the heat.
2. Wheel: Electrically or manually operated disc which is fitted to the front of a lantern with several apertures holding different colour filters which can be selected to enable colour changes. Can also be selected to run continuously.
3. Semaphore, where framed colours are electrically lowered into place in front of the lantern. Remotely controllable, can perform additive colour mixing by lowering two colours into position at the same time.
4. Magazine: Manual semaphore-type device used on the front of a followspot.

COLOUR CORRECTION
The use of colour filters to compensate for the different colour temperatures of different light sources. Important in lighting for TV and film.

COLOUR FILTER
A sheet of plastic usually composed of a coloured resin sandwiched between two clear pieces. The coloured filter absorbs all the colours of light except the colour of the filter itself, which it allows through. For this reason, denser colours get very hot, and can burn out very quickly. There are a number of manufacturers of Colour Filters - Lee (UK), Rosco (US) and Gam (Great American Market - US) are the most popular. Each manufacturer's range has a numbering system for the different colours. It's important to specify which range you're talking about when quoting numbers. A colour filter is sometimes known as a Gel, after the material Gelatine, from which filters were originally made.

COLOUR FRAME
(or Gel Frame) A frame which holds the colour filter in the guides at the front of a lantern. Many different sizes of frames are needed for the different lanterns.

COLOUR MIXING
Combining the effects of two or more lighting gels:

1. Additive: Focusing two differently coloured beams of light onto the same area (eg. Cyc Floods). Combining colours in this way adds the colours together, eventually arriving at white. The three primary colours additively mix to form white, as do the complementary colours.
2. Subtractive: Placing two different gels in front of the same lantern. Subtractive mixing is used to obtain a colour effect that is not available from stock or from manufacturers. Because the ranges of colour are so wide, the need for subtractive mixing is reducing. Combining colours in this way reduces the light towards blackness. The three primary colours mix subtractively to form black (or to block all the light).

COLOUR TEMPERATURE

1. A measure of the "warmth" or "coolness" of light sources and colours. Measured in degrees Kelvin. A higher colour temperature light source will appear whiter (colder). The human brain automatically compensates for different colour temperatures - a film or video camera cannot, and thus what we see as white may appear to have a blue or green tint when no colour correction is used for video. Daylight is approximately 5600K and Tungsten Halogen is approx. 3200K. Many discharge light sources are in use in modern theatrical productions using discharge followspots or moving lights - colour correction filters are used to balance the colour temperatures. Metal halide lamps have very high colour temperatures compared to halogen or incandescent lights which tend to look red in comparison.
2. See also COLOUR CORRECTION.

COLOUR WHEEL
See COLOUR CHANGER.

COMPLEMENTARY COLOURS
Pairs of colours which, when additively mixed, combine to produce white light. Examples are red + cyan, green + magenta, and yellow + blue.

COMPOSITE GOBO
See GOBO.

COMPONENT VIDEO CONNECTION
The output of a video device (such as a DTV set-top box or DVD player), or the input of a DTV receiver or monitor consisting of the three component signals: luminance or brightness (Y) and two-colour difference signals (PB and PR), each on a separate wire.

COMPOSITE VIDEO SIGNAL
The most common video output from VCR's & DVD players. The combined picture signal, including vertical and horizontal blanking and synchronizing signals.

COMPRESSED RESOLUTION
Anytime a projector can accept a higher resolution signal and compress the data down to fit its true resolution. Quality of compressed images varies tremendously. Most but not all projectors offer compressed resolution for handling higher

COMPRESSION
Any of several techniques that reduce the number of bits required to represent information in data transmission or storage, thereby conserving bandwidth and/or memory.

COMB FILTER
When two combining sound waves have different amplitudes, phases, and frequencies, the resultant soundwave develops many nulls or spaces where the energy has cancelled. When viewed on a graphic recorder the resultant frequency response resembles a comb due to the nulls or notches of information that have cancelled.

COMBINING UNIT
Typically used with microphones to combine two or more mics into a single input of a mixer. These devices are usually passive transformer or resistive circuits, but active versions are available which provide a higher degree of isolation between the multiple inputs.

COMMON MODE REJECTION
The ability of an amplifier to cancel a common mode signal (such as interference) that is applied equally to both input terminals of a balanced amplifier, while responding to a signal from the source that is constantly changing direction (alternating current) so it is out of phase with respect to the two balanced signal lines; therefore it is not common mode and will be passed and not rejected.

COMPANDER
A combination of a compressor at one point of a signal path for reducing the level of the signal, followed by an expander at another point for a complimentary increase in signal level.

COMPRESSION
Reduction of the effective gain of an amplifier at one level of signal with respect to the gain at a lower signal level.

CONDUCTOR
A wire, cable or other material (metal, liquids, gases, or plasma) that is suitable for carrying electric current.

CONTINUOUS POWER
This power rating represents the most conservative statement of the capability of an amplifier. It is also called "RMS" power. It denotes the amount of power an amplifier can deliver when amplifying a constant steady tone. It is usually measured at a signal frequency of 1kHz for a specific distortion. Continuous power in watts is expressed as: W = V2/R Power in watts equals the voltage squared divided by the resistance of the load.

CONTINUOUS PRESENCE
The transmission of two or more simultaneous images.

CONTINUOUS PROGRAM MATERIAL
A signal, such as speech or music, that contains voltages continuously changing in both frequency and voltage (time and amplitude).

CONTOUR
A circuit which adds a bass boost to attain equal loudness at lower volumes. Also known as Loudness.

CONTRAST CONTROL
An accrual control to increase the difference between black and white in an image.

CONTRAST RATIO
The ratio between the whitest and blackest portions of an image. The larger the contrast ratio the greater the ability of a projector to show subtle colour details and tolerate extraneous room light.

CONVERGENCE
The ability of a three-panel projector to project uniform images without colour variance.

CONCAVE
Lens shape. Edges are wider than the centre of the lens. Useful to remember that "caves" go inward.

CONDENSER LENS
Loosely applied to any spotlight lens which condenses diverging rays into a beam, but more correctly to the short focus combination of two or more lenses in a jacket used for illuminating a slide or effect disc. Also used in some profile lanterns and followspots to produce a smoother light (especially for gobo work).

CONDUIT
Metal or plastic pipe used to carry electrical conductors as part of a permanent electrical installation. See also TRUNKING.
Also used to add weight to the bottom of a flown cloth.

CONTROL ROOM
Room at the rear of the auditorium (in a proscenium theatre) where lighting and sometimes sound is operated from. Known in the US as the BOOTH. The stage manager calling the cues is very often at the side of the stage (traditionally stage left) but in some venues he/she may be in the control room also. The control room is usually soundproofed from the auditorium so that communications between operators cannot be heard by the audience. A large viewing window is obviously essential, as is a "show relay" system so that the performance can be heard by the operators. Obviously if sound is being mixed, the operator should be able to hear the same as the audience, so some control rooms have sliding or removable windows, or a completely separate room for sound mixing. Where possible, the sound desk is moved into the auditorium so that the operator can hear the same as the audience.

CONVEX
Lens shape. Edges are thinner than the centre of the lens.

CPS
Abbreviation for "Cycles per second", the units for expressing frequency. The term "CPS" has been obsoleted and replaced by "Hertz". Hertz = Cycles per second. 1 kHz = 1 Kc.

CRACKED OIL
A smoke effect which creates a haze in the air to make light beams visible. This effect is rarely used now, because it has been found to be carcinogenic. See WATER CRACKER.

CRESCENT
An open ended adjustable hand wrench originally produced by the Crescent Tool Co.

CRITICAL DISTANCE
The point within a room where the sound level of the direct field radiating from the loudspeaker and the reverberant field within the room become equal in intensity or level.

CROSS FADE
Bringing another lighting state up to completely replace the current lighting state. Also applies to sound effects / music. Sometimes abbreviated to Xfade or XF.

CROSSOVER (X-OVER)
An electronic device that is used to separate an audio signal into two or more bands of frequencies or component signals above and below a certain frequency, said to be the crossover frequency or crossover point. Crossovers can be active or passive.

CROSSOVER, ACTIVE
Electronic or active crossovers are used in biamplified sound systems where two amplifier channels are used to individually operate the woofer(s) and horn(s) of the speaker system. The active crossover is placed in the audio chain just ahead of the amplifiers, and separates the audio signal into low and high frequency groups. Some active crossovers separate the signal into low, mid and high frequency bands for 3-way or triamplified speaker systems. The advantages of active crossovers and biamplification are low distortion, increased headroom and better control over the relative levels of the low and high frequency sections of the speaker system. Disadvantages include the need for additional amplifier channels, higher cost of the crossover compared to passive units and multiple wire runs from the amplifiers to the speakers.

CROSSOVER, PASSIVE
A passive crossover is built into most speaker cabinets in order to separate the full range signal from the output of a power amplifier into low frequency and high frequency bands to operate the woofer and horn (or tweeter) respectively. Some speakers utilize a 3-way crossover which separates the signal into low, mid and high frequency bands. The advantages of passive crossovers are that a single amplifier channel can be used and they can be relatively inexpensive. Disadvantages include higher potential for low frequency distortion caused by saturation of the inductors used in the low frequency section and excess amplifier power consumption due to heat losses within the passive components.

CROSSTALK
Interaction of adjoining channels or circuits. Crosstalk can occur by being induced electromagnetically or electrostatically. Crosstalk is a common specification for mixing consoles.

CSI
(Compact Source Iodide) A high intensity discharge lamp. Most often used in followspots, because it has a colour temperature (approx. 4000K) close to that of the tungsten halogen lamps.

CUE
The command given to technical departments to carry out a particular operation. E.g. Fly Cue or Sound Cue. Normally given by stage management, but may be taken directly from the action (i.e. a Visual Cue).

CUE LIGHT
System for giving technical staff and actors silent cues by light. Cue lights ensure greater precision when visibility or audibility of actors is limited. Sometimes used for cueing actors onto the set. For technical cues, lights are normally now used just as a backup to cues given over the headset system. In the UK, a flashing Red light means stand-by or warn, green light means go. The actor / technician can acknowledge the standby by pressing a button which makes the light go steady. In the US, a red light means warn, and when the light goes off, it means GO. The UK system seems to be more secure, but it depends what you're used to.

CUE STACK
Section of a lighting desk which allows a list of pre-plotted lighting states to be "played back" on the push of a button. These lighting states normally have fade times allocated to them. Lighting desks designed for theatrical use will have this as the primary control, but a rock desk will have more "hands on" control as a priority, only providing a cue stack for occasional use.

CUE TO CUE
("Topping and Tailing") Cutting out action and dialogue between cues during a technical rehearsal, to save time.

CUEING
There is a standard sequence for giving verbal cues: 

•  "Stand-by Sound Cue 19" (Stand-by first)
•  "Sound Cue 19 Go" (Go last).

CURRENT
The rate of flow (measured in amperes) of electricity in a conductor or circuit. The amount of current that flows is determined by the voltage or electrical pressure applied and the conductivity of the substance or material (which also determines the resistance or opposition to current flow).

CUT
A term used to indicate the reduction in gain or attenuation of a frequency or band of frequencies when equalizing an audio signal.

CYCLE OR HERTZ
A unit of motion referenced to a time period of one second. The frequency of a vibration or oscillation in units per second. 100 Hertz or 100 c.p.s. (cycles per second) refers to the number of times a second (100) a string is vibrated or an amplifier is swinging between its positive and negative supply voltage.

CYC FLOOD
A floodlight, usually with an asymmetrical reflector, designed to light a cyc or backcloth from the top or bottom. The asymmetric reflector helps to throw light further down the cloth, producing a more even cover. In the US, a flood at the top of the cyc is a CYC OVER, and a flood at the bottom is a CYC UNDER.

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 D

DAISY-CHAINING
Connecting items of equipment together by linking from one to the next in a chain. Used for connecting demux boxes to dimmers etc.

DAMPING FACTOR
The ratio of the speaker impedance to the amplifier's internal output impedance. Damping factor is a measure of how well an amplifier can actually control the movement of a speaker cone or diaphragm by preventing it from moving farther than it is supposed to. Damping factor is arrived at by dividing the speaker impedance by the amplifier's internal output impedance. The internal output impedance of any amplifier is determined by the transconductivity (internal resistance) of the output devices. Everything connected in the speaker line (including the speaker cable itself or a crossover) looks to the speaker like an increase in the output impedance of the amplifier, thus lowering the effective damping factor. Because any speaker is a mechanical device, it will have its own resonant frequencies, which will cause the cone to continue in motion after the electrical signal has stopped. [see transient distortion]. An amplifier with a high damping factor will damp out the unwanted speaker cone excursion.

DANCE LIGHTING
Lighting design for Dance is reliant on a great deal of sidelight from BOOMS at the side of the stage. There are normally at least three lanterns on each boom, and three heights - SHINS (to light feet and lower legs), MIDS and HEADS.

DATA
Information represented in digital form, including voice, text, facsimile, and video.

DB (DECIBEL)-
A unit for describing the ratio of two voltages, currents, or powers. The decibel is based on a logarithmic scale; when measuring differences in sound pressure level (SPL), the amount of change in sound pressure level perceivable is directly proportional to the amount of stimulus (the more sound present, the greater the change must be, to be perceived).

DBO (DEAD BLACKOUT)
See BLACKOUT.

D-CHANNEL
The Data Channel on the ISDN circuit is used to carry control signals and customer call data at 16 kbps (BRI) and 64 kbps (PRI).

DC
Short for DIRECT CURRENT.

DECAY
The gradual reduction in sound energy once the sound source is turned off.

DE-RIG
The process of removing lanterns & cabling from flying bars or grid - returning the venue to its normal state, or as preparation for the next production.

DEDICATED ACCESS
A private connection between a customer's equipment and a company providing transmission services. The connection bypasses the local switched telephone network.

DEDICATED NETWORK
Sometimes referred to as a private or leased line. This transmission circuit is used exclusively by a single customer.

DEAD

1. A pre-plotted height for a piece of scenery or lighting bar - "that bar's on its dead". The positional indicators on the rope (either PVC tape, or more traditionally cotton tape passed through the strands of the rope) are called DEADS. Sometimes flying pieces are given a number of extra deads, that may be colour coded, in addition to the "in dead" (lower) and "out dead" (higher - out of view). In the US, TRIM has the same meaning.
2. Scenery or equipment not needed for current production - "that table's dead".
3. An electric circuit that has been switched off or has failed - "the circuit's dead, you can change the lamp now".

DIAGONAL SCREEN
A method of measuring the size of a screen or a projected image. It measures from one corner to the opposite corner.

DIAPHRAGM

1. See IRIS.
2. A thin flexible sheet that can be moved by sound waves as in a microphone, or can produce sound waves when moved as in a loudspeaker or compression driver.

DICHROIC FILTER
Glass colour filters which reflect all light except that which is the colour of the filter, which passes through. Normal gels absorb the unwanted colours, turning the light into heat. Dichroic filters run cooler, and produce a much cooler beam of light. Longer lasting, but a lot more expensive, they are predominantly used in moving lights or architectural applications.

DICHROIC LAMP
A low voltage display lamp with a reflector that lets heat pass through it, rather than reflecting it. Results in a much "cooler" light.

DICHROIC
A mirror or lens that reflects or refracts selective wavelengths of light. Typically used in projector light engines to separate the lamp's "white" light into red, green and blue light. Also used to remove the Infrared and Ultra-violet.

DIFFERENTIAL AMPLIFIER
An amplifier whose output is proportional to the difference between the voltages applied to its two inputs. Used to balance or offer common mode rejection of interference signals.

DIFFRACTION
The bending or redistribution of acoustic sound waves in a room caused by some obstacle, such as a column or divider. Only low frequency wave forms can be diffracted.

DIFFUSION

1. See FROST.
2. The scattering of sound waves by a solid object.

DIGITAL
Many electronic devices use digital logic. Information is handled in separate bits (either ON or OFF) rather than continuously variable analogue signals. Most computer lighting boards give a digital multiplexed output, and more and more sound equipment is going digital. In digital audio the continuous analog signal is converted to an encoded discrete value or digital word.

DIGITAL DIMMER
The new generation of dimmers that can respond directly to the digital multiplexed output of the lighting desk. The technology also permits the dimmer to report faults and other data back to the control board.

DIGITAL LIGHT CURTAIN / DLC / D.L.C.

1. A remotely controllable motorised batten fitted with an integral colour changer. The DLC can now be controlled via DMX (via an interface) although it originally used software called Light Moves running on a Mac. The effect produced by this lantern is a wall of light (when used with a HAZE MACHINE).
2. See also LIGHT CURTAIN.

DIGITAL LIGHT PROCESSING (DLP)
The commercial name for this technology from Texas Instruments (TI). The technology inside is often referred to as either "micro-mirrors", or DMD. It works this way: build a few hundred thousand tiny mirrors, and line them up in 800 rows of 600 mirrors each. Now attach a hinge to each of those 480,000 mirrors. Attach each of those 480,000 hinges to its own very tiny motor! Power each motor with electrostatic energy. The motors tilt their mirrors up to 20 degrees at incredible speeds. This allows the mirrors to modulate light from a lamp, and send the "modulated signal" out through a lens on to a screen. The most amazing part of DLP micro mirrors, is the scale of size. The 480,000 mirrors (actually 580,000 are used), hinges and motors are packed onto a "wafer" a bit larger than your thumbnail.

DIGITAL TRANSMISSION
A way of sending coded information via a series of electric or light pulses through the air, over wires, or through glass fibres.

DIM OUT
Reduction of lighting level for a scene change, that isn't quite a BLACKOUT.

DIMMER
Electrical or electronic device which controls the amount of electricity passed to a lantern, and therefore the intensity of the lamp.

DIMMER LAYOUT
See CHEAT SHEET.

DIMMER PER CIRCUIT
A lighting installation where there is no patching system. Each lighting circuit / socket has a dimmer always connected to it. This has advantages in that you never run out of dimmers, but allows no flexibility and can have cost disadvantages in a large space.

DIMMER RACK
A number of individual dimmer circuits mounted in a cabinet.

DIN
Deutscher Industrie Normen. European standard covering audio connectors and tape equalisation characteristics.

DIP

1. Small covered trap at stage level containing electrical outlets. (US equivalent is FLOORPOCKET)
2. Lighting equipment on stands at stage level. (e.g. "We're just focussing the dips now")
3. Low lighting intensity when cross fading between two higher states - "there's a dip between these two states".
4. Transparent lacquer for colouring lamp bulbs - known as "Lamp Dip".
5. An audible reduction (attenuation or cut) in gain at a certain frequency also called a notch.

DIRECTIVITY
Area of coverage of a speaker or microphone.

DIRECT CURRENT

1. Electric current that flows in one direction only (e.g. from a battery). Abbreviated to DC.
2. Also see ALTERNATING CURRENT.

DISCHARGE LAMP

1. A high-powered source of light produced by means of an electrical discharge between two electrodes. An arc light, for example uses a discharge between two carbon rods which are manually or automatically fed together as they are burnt up. The use of this type of lighting is restricted to non-dimming applications such as followspots and projection, where dimming is achieved by mechanical means. Many of the new generation of moving lights use discharge lamps, dichroic filters and mechanical dimming shutters.
2. Also see BALLAST, CSI, CID, MSR, HMI, HTI, Xenon, MBI.

DISPERSION
The spread or distribution or coverage of sound generated from a horn or loudspeaker. For any given frequency, the area of dispersion is defined as that area between the -6 dB down points of that frequency plotted against amplitude. It is measured in degrees related to an imaginary line descending from the center of the speaker cone. As you move away from the imaginary line, up or down, right or left, the loudness level of the sound decreases. When the sound level decreases rapidly on either side of the imaginary line, the dispersion in degrees is relatively small and the speaker is said to be highly directional.

DISSOLVE UNIT
Interface connected between two or more slide projectors and a tape player. Synchronisation signals recorded onto the tape are detected by the dissolve unit and fade up the lamp in one slide projector while changing the slide in the other, and then vice versa, producing a dipless crossfade between the two images.

DISTORTION
Any undesired change in the wave form of an electrical signal passing through a circuit or transducer. Any distortion can be defined as deviation from the original sound, the discrepancy between what the amplifier should do and what it actually does. All distortion is undesirable. Distortion occurs when the amplifier alters the original sound in the process of amplification so that what comes out of an amplifier is no longer a true replica of what went in. Performers, however, will sometimes desire the application of electronically induced distortion for extra-musical effect in the production of their "sound". The undesirability of inherent distortion is associated with high fidelity and should not be confused with the desirability of distortion as it is expected to be produced through circuitry. When reproducing sound, distortion is unwanted.

DISTRIBUTION AMPLIFIER
An amplifier used to maintain a clean noise free signal to the projector over significant distances. Even with good heavily shielded cables, range of video and computer signals is limited to a few metres before noticeable degradation. In ceiling mount situations, where the wiring may pass along side or across electrical conduits, etc. a distribution amp may be needed with shorter distances. Many distribution amps can also split the signal into 2 or more amplified signals for driving multiple projectors and monitors.

DISTRIBUTION BOARD
System of interconnected fuse carriers and cabling that routes an incoming power supply to a number of different outputs. Known colloquially as DISTRO.

DISTRO / POWER DISTRO
See DISTRIBUTION BOARD.

DLP TECHNOLOGY
Digital Light Processing

DMX512
See MULTIPLEXED (MUX) SIGNAL.

DMX512-A
See MULTIPLEXED (MUX) SIGNAL.

DOME
Follow spot location usually at rear of the upper gallery. Also referred to as BIOBOX (shortened version of BIOGRAPH BOX, after its original function as a cinema projection box).

DONUT
A metal plate with a hole in the middle inserted in the colour runners of a lantern to sharpen focus (in the case of a profile) or reduce spill.

DOT PITCH
The distance between pixels in an image.

DOWNLIGHT
A light from directly above the acting area.

DOWNSTAGE

1. The part of the stage nearest to the audience (the lowest part of a raked stage).
2. A movement towards the audience (in a proscenium theatre).

DOWSER
A metal flag used in larger followspots and projection equipment to cut off the light beam without cutting off the electrical supply. Discharge lamps need a period of cooling down when they are turned off before they can be turned on again, so they should not be switched off if needed again within about two hours.

DPI
Dots Per Inch.

DRESS LIGHTING
Providing a low level of light to an open stage while the audience enter the house up until the performance starts. Also known as PRESET.

DRESS REHEARSAL
A full rehearsal, with all technical elements brought together. The performance as it will be "on the night".

DRIFT WIRE
A length of suspension wire of standard length with eyelets at each end between the counterweight bar and the top of the scenic piece flown from it.

DRIVER
The motor structure portion of a horn loaded loudspeaker system that converts electrical energy into acoustical energy and feeds that acoustical energy into the entry of a horn throat or the narrow end of the horn. Most often used when referring to a high frequency compression driver, called a driver for short. The definition also includes the loudspeaker in a horn loaded woofer or mid bass horn.

DRY ICE

1. Frozen solid carbon dioxide (CO2) at a temperature of -87.5° centigrade which produces clouds of steam-loaded CO2 gas forming a low-lying mist or fog when dropped into boiling water. Although non-toxic, caution is required in the storage and handling of dry ice because of its extreme cold. Water is boiled in a large tank offstage, into which the dry ice is lowered in a basket. Fans and ducts then direct the gas onto the stage. Dry ice does not support life, so care should be taken that small animals, actors etc.are not below the level of the dry ice for more than a few seconds.
2. See also LOW SMOKE.

DS-1
The Level 1 standard for digital systems operating at 1.536 mbps (24 DS-0 channels). Also known as T1.

DS-3
Digital Signal Level 3. This term is used to refer to the 45 mbps digital signal carried on a T3 facility.

DVI
Digital Video Interface is a connection standard for linking a video card and a display that requires a digital signal, such as a LCD panel. Analogue video signals can also be used with the DVI interface. A normal DVI cable can support 1600 x 1200 resolution at 60 Hz. A special dual link version can support 2048 x 1536 or even higher resolutions.

DVI-D

DVI-A

DVI-I

DYNAMIC RANGE
In a musical instrument, the dynamic range is the difference in decibels between the loudest and softest level of notes that can be played on that instrument. In electronic equipment, dynamic range is the difference in decibels between the highest (overload level) and lowest (minimum acceptable) level compatible with that piece of equipment.

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 E

E.R.S.
(ERS) Ellipsoidal Reflector Spotlight (US)

EARTH SPIKE
Copper rod inserted into the ground to maintain earth continuity (especially when using generators etc.)

EARTHING
Electrical safety requirement that metal parts of electrical equipment are connected to a common earth or ground point so that in the event of a fault, excess current can be carried away, causing the fuse to blow. Known in the USA as Ground.

ECHO
A wave which has been reflected or otherwise returned with sufficient magnitude and delay to be perceived as a distinct wave, separate from the original wave. A delay in sound of more than 50 milliseconds resulting in a distinct repeat or number of repeats of the original sound.

ECHO CANCELLATION
A technique used in high-speed modems and voice circuits to isolate and filter out unwanted signal energy caused by echoes from the main transmitted signal.

EFFECTS PROJECTOR
Lantern used to project the image from a rotating glass effects disc. Used with an objective lens to produce the desired size of image. Commonly used discs are clouds, flames and rain.

EFFECTS, LIGHTING

1. Animation Disc: A slotted or perforated metal disc which rotates in front of a lantern to provide "movement" in the light. Most effective when used in front of a profile carrying a gobo.
2. Effect Disc: A painted glass disc rotating in front of an effects projector with an objective lens to focus the image (eg lames, Rain, Snow).
3. Flicker Flame: Irregularly slotted rotating metal disc through which light is shone onto a prism-type piece of glass which scatters the beam of light and adds the "dancing" effect of firelight to a scene.
4. Gobo Rotator: Motorised device inserted into the gate of a profile lantern that can be remotely controlled to rotate a gobo, usually with variable speed and direction.
5. KK Wheel: Slotted metal disc which rotates in front of a lantern to break up the light and provide movement. (Flicker Wheel)
6. Lightning: Created through the use of either strobe sources or photoflood lamps.
7. Tubular Wave Ripple: Horizontal linear lamp around which a slotted cylinder is rotated providing a rising light (as reflected from water onto the side of a ship).
8. See also EFFECTS,SOUND, PYROTECHNICS, SMOKE.

EFFICIENCY
The ratio, usually expressed as a percentage, of the useful power output to the power input of a device. The efficiency of a speaker system is the SPL the unit produces at a 1 W RMS input power level measured 1 meter from the unit. Doubling the input power raises the SPL 3 dB. Doubling the number of enclosures raises the SPL 3 dB. Doubling the input power and the number of enclosures raises the SPL 6 dB. Doubling the distance (near field) lowers the SPL 6 dB.

EGGS
Slang term used for Strand Pattern 123's, due to their shape.

ELECTRIC
LX bars are ELECTRICs bars. LX1 in the UK (first bar upstage of the proscenium arch) is FIRST ELECTRIC.

ELECTRICS
See LX.

ELECTRONICALLY BALANCED INPUT
A differentially balanced amplifier; an amplifier whose output is proportional to the difference between the voltages applied to its two inputs. Offers common mode rejection or attenuation of interference signal that was introduced electromagnetically in the signal carrying conductors.

ELEVATION

1. A working drawing usually drawn to scale, showing the side view of a set or lighting rig.
2. See also PLAN.

ELLIPSOIDAL
A profile lantern with an elliptical reflector. Referred to as a LEKO.

EMF
Abbreviation for Electromotive Force, or VOLTAGE.

ENCLOSURE
An acoustically designed housing or structure for a speaker.

EQUALIZATION
The act of obtaining a desired overall frequency response through the implementation of graphic equalizers or tone controls.

EQUALIZER
A device designed to compensate for an undesired amplitude-frequency characteristic of a system or speaker.

EQUITY LIGHT
See GHOST LIGHT.

ER / E.R.
Short for Ellipsoidal Reflector - type of fixed beam profile lantern common in the US. Also known as ERS / E.R.S.

ETHERNET
Computer networking protocol which is installed on many new lighting desks, to allow networking between the main desk, dimmers, and remote desks around the theatre.

EXCITER

1. See ENHANCER.
2. Exciter Lamp - the lamp in a film projector that shines through the optical soundtrack and enables it to be read by a light sensor.

EXCURSION
Movement of the cone of a loudspeaker or the diaphragm of a compression driver. The higher the voltage or amplitude of the signal applied, the greater the movement or excursion of the loudspeaker or diaphragm.

EXIT SIGN
Usually illuminated sign, of standard size, which should always be visible, showing an audience member and the company the nearest exit.New legislation in Europe means that the word "EXIT" has been removed from these signs to be replaced by "Running Man", known more politically correctly as "Person moving purposefully".

EXPONENTIAL HORN
A speaker designed to reproduce the high frequencies. An exponential horn has a flare rate that increases with the square of the distance from the entry to the horn throat.

EXPRESSIONISM
Theatre design and performance style which places greater value on emotion than realism. The trademark Expressionist effects were often achieved through distortion.

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 F

F.B.O.
Abbreviation for Fade to Blackout.

FADE
A fade is an increase, diminishment or change in lighting or sound level.

FADE TIMES
On computerised memory lighting control desks, a lighting fade can have two times - an up fade and a down fade. The Up fade time refers to the time it takes dimmer levels to rise to their new positions. The Down fade time refers to the time for dimmer levels falling to their new positions.

FADER
A vertical slider which is used to remotely set the level of a lighting or sound channel.

FAR FIELD
That portion of the direct field that is at least twice the distance of a frequency's wavelength.

FEED

1. A power supply to a piece of equipment or installation is termed a "feed". Sound equipment and sensitive computer equipment should have a clean feed - that is, a supply that is free from interference from other equipment.
2. A signal from one system to another is also known as a feed (for example, an audio signal from the FOH desk to a TV company videoing a concert is known as a feed.)

FEEDBACK
(Electronic) The return of a portion of the output of a circuit to its input. (Acoustic) A squeal of a sound system caused by the regeneration of a signal from the output of a sound system into a microphone input.

FERGO
A custom made plastic gobo that is suitable in certain lights.

FESTOON

1. Describes tabs which adopt a sculpted shape.
2. A length of cable incorporating a number of lamp holders used for outdoor party lighting etc. Available in multi-circuit form so that the lamps can be "chased".

FIBRE FRAME
A gel frame made from heat resistant fibres, which doesn't get as hot to the touch as a standard metal frame.

FIBRE OPTICS
A method of directing light down a very thin glass fibre. Fibre Optics are used mostly in communication, but find theatre applications in star cloths which are black backcloths with the ends of optical fibres poked through, to create a mass of pin pricks of light. A large bundle or harness of fibres may be fed from one light source, sometimes with a motorised colour or flicker wheel. New technology enables digital sound signals to be sent down optical fibres now, replacing heavy and expensive multicore cables.

FIELD
Refers to the spread of light intensity across a beam. Most profile lanterns have an adjustable field. A Flat field has an even distribution, a peak field has a "hot spot" in the centre of the beam. A flat field is essential when using gobos. See PROFILE.

FILL LIGHT
(especially TV and Film lighting) Light which fills the shadows that key light creates.

FILTER
An electrical or electronic device that permits certain frequencies to pass while obstructing others. Examples include loudspeaker crossovers, equalizers, feedback reducers and even simple bass and treble controls.

FIRE EXTINGUISHER
Essential tools of the pyrotechnician's trade ! In the UK, they used to be colour-coded according to content (Carbon Dioxide (Black), Water (Red), Foam (Cream), Halon Gas (Green) Powder (Blue)) but now, they're all red with a small label saying what they are. Another great leap forward !.

FIRST ELECTRIC
The first LX bar upstage of the proscenium arch. (Also known as LX1).

FIT-UP
Initial assembly on stage of a production's hardware, including hanging scenery, building trucks etc.

FIXTURE
Used to describe a type of moving light.

FLAGGING
When focussing lighting, flagging means waving your hand in and out of the beam of a lantern/instrument in order to see where the beam is hitting on stage. Flagging is particularly useful in high ambient light levels. (e.g. "Can you flag that please ?") Term probably originates from a FRENCH FLAG.

FLASH BOX
A small box containing the socket into which a pyro cartridge is plugged. Also known as a flash pod.

FLASH BUTTON
A push switch on a lighting desk which flashes selected channels / memories / submasters to full (100%) while pressed. Some Flash buttons on submasters can be set to latch (ie they stay on when pushed, until they are pushed again).

FLASH OUT / THROUGH
Method of checking whether lanterns are functioning properly by flashing them on one at a time. It is good practice to flash lanterns to 70%, rather than Full to preserve lamp life.

FLIGHTCASE / FLIGHT CASE
Metal framed wooden box on wheels with a removable lid used for transporting equipment between venues. Flightcases are very strong, and have reinforced corners and edges. Care should be taken when lifting flightcases as they can be very heavy.

FLOOD

1. A lensless lantern that produces a broad non-variable spread of light. Floods are used in battens, or singly to light cycloramas or large areas of the stage.
2. To increase the beam angle of a focus spot by moving the lamp and reflector towards the lens. "Flood that a bit, please !"
3. A flood light is also refered to as a Q.I. Flood.
4. See also CYC FLOOD.

FLOORCANS
A PARCAN with an extra trunnion arm / yoke, and often a short nose, which is designed to sit on the floor.

FLOORPOCKET
A electrical socket mounted under a flap in the stage floor.

FLUORESCENCE

1. The property of some materials to glow when subjected to light. This normally refers to ultraviolet light, although blue visible light (along with many other colours) can cause fluorescence. The materials degrade the UV wavelengths into longer and therefore visible reflected rays.
2. See also PHOSPHORESCENCE.

FLUTTER ECHO
A multiple echo in which the reflections occur in rapid succession caused by large surfaces being acoustically parallel to each other.

FLY BARS
The metal bars to which scenery and lanterns are attached for flying above the stage.

FM BASED REMOTE
A remote control that broadcasts its instructions with an FM transmitter, normally required in large rooms, thanks to long range, and no line of site requirement.

FOCAL LENGTH
The distance between the focal point of a lens or mirror in a projection device and the corresponding principle plane.

FOCUS

1. The session when all the lanterns in the rig are angled in the correct direction, with the correct beam size.
2. Description of how sharply defined a light beam is ("give that profile a sharp focus")
3. Control on projection equipment used to change the focus.
4. Sharpness of a pixel or series of pixels on the projection screen faceplate. Also measured as the spot size.

FOCUS SPOT
Term for both Fresnel and PC type lanterns with adjustable beam size.

FOCUSING
The process of adjusting the direction and beam size of luminaires and projectors. Does not necessarily result in a "sharply focused" image.

FOH
See FRONT OF HOUSE.

FOLLOW-ON CUE / FOLLOW CUE
A cue that happens so soon after a previous cue, that it doesn't need to be cued separately. The follow-on can be taken by the operator once a previous cue is complete, or a lighting or sound cue can be programmed to happen a specific time after a previous cue. Fly follow-on cues are often taken as soon as the operator has completed a previous cue. Often abbreviated to F/O.

FOLLOWSPOT
Usually, a powerful profile lantern usually fitted with its own dimmer, iris, colour magazine and shutters mounted in or above the auditorium, used with an operator so that the light beam can be moved around the stage to follow an actor. Sometimes a beam light or other lantern may be used in the same way. See PICK-UP.

FOOTLIGHTS
A compartmentalised batten sometimes recessed into the front edge of the stage, used to neutralise shadows cast by overhead lighting. Modern lighting equipment renders footlights virtually obsolete except for period/special effects.

FPS
Frames Per Second.

FRACTIONAL T1
A service provided by carriers, where a full T1 link is leased to a customer, but the service charge is based only on the number of time slots used.

FRAME
A single refresh of the entire screen as perceived by the viewer. This is two fields in an interlaced system.

FRESNEL
(pronounced "Fre-nell") A type of lantern which produces an even, soft-edged beam of light through a Fresnel lens. The lens is a series of stepped concentric circles on the front and pebbled on the back and is named after its French inventor, Augustin Jean Fresnel (1788-1827). He developed the lens for French lighthouses so that they could be seen further out to sea and could achieve a longer focal length with a lot less glass than a standard plano-convex lens.

FREQUENCY
The number of vibrations or oscillations in units per second. Measured in cycles or Hertz per second. The rate of repetition in cycles per second (Hertz) of musical pitch as well as of electrical signals. For example, the number of waves per second a vibrating device such as a piano or violin string moves back and forth each second of time to produce a musical tone.

FREQUENCY RESPONSE
A measure of the effectiveness with which a circuit, device or system transmits the different frequencies applied to it. The way in which an electronic device (mic, amp or speaker) responds to signals having a varying frequency. This is a measurement of how well an amplifier reproduces and amplifies a specified audible range with equal amplitude or intensity, for example, 30 to 16,000 Hz.

FRONT OF HOUSE (FOH)

1. Every part of the theatre in front of the proscenium arch. Includes foyer areas open to the general public.
2. All lanterns which are on the audience side of the proscenium and are focussed towards the stage.
3. The backstage areas of the theatre are known as Rear of House (ROH).

FROST
A diffusing filter used to soften the edges of a light beam. Frosts are commonly used in profiles in front of house positions to achieve the same beam edge quality in all lanterns. Different strengths of diffusion frost are available from many colour filter manufacturers. See also SILK.

FULL DUPLEX
A system capable of transmitting and receiving signals simultaneously.

FULL RANGE
The entire audio spectrum, 20 Hz - 20 kHz.

FULL UP

1. A bright lighting state with general cover lanterns at "full" (100%) intensity.
2. Also see FULL UP FINISH.

FULL UP FINISH (FUF)
A shorthand note for manual desk lighting operators to bring all relevant dimmers to full for the end of a song / finale of a show to "draw the applause".

FUSE
Protective device for electrical equipment (E.g. dimmers). The fuse link will melt when excess current flows, preventing damage to people or equipment. Every piece of electrical equipment has at least one fuse in its associated circuit.

FUZZ LIGHT
A lamp with a revolving mirror and a coloured plastic dome. Gives a "police light" effect. Usually 12 Volt or 240 Volt operation.

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 G

GAFFA TAPE
Ubiquitous sticky cloth tape. Most common widths are .5" for marking out areas and 2" (usually black) for everything else. Used for temporarily securing almost anything. Should not be used on coiled cables or equipment. Originally known as Gaffer's Tape, from the Gaffer (Master Electrician) on a film set.

GAIN
An increase in strength or amplitude (voltage) in a signal. The increase in signal power that is produced by an amplifier; usually given as the ratio of output to input voltage, current, or power expressed in decibels.

GAMMA CORRECTION
A variation in the input signal to provide a linear relationship between signal source and display output.

GATE

1. The point of focus in a profile spot where the shutters are positioned and where an iris or gobo can be inserted.
2. A single base section of a folding rostrum system.

GAUZE
Cloth with a relatively coarse weave. Used unpainted to diffuse a scene played behind it. When painted, a gauze is opaque when lit obliquely from the front and becomes transparent when the scene behind it is lit . Many different types of gauze are available ; Sharkstooth gauze is the most effective for transformations, because it is the most opaque. Vision gauze is used for diffusing a scene and for supporting cut cloths. Also known as a Scrim.

GBPS
Gigabits per second. 1 Billion bits per second.

GEL OR GELATINE
See COLOUR FILTER.

GENERAL COVER
Those lanterns in a rig which are set aside purely to light the acting areas. The stage is normally split into a number of areas for this purpose, which can then be isolated or blended together as required by the director. See SPECIAL.

GENERIC
Standard stage lighting instruments, rather than moving lights or other effects. (e.g. "There are 40 instruments in the rig - 20 moving lights and 20 generics").

GERB
See PYROTECHNICS.

GET-OUT
Moving an entire production out of the venue, and into either a large waste-disposal skip, or into transport. Usually preceded by the strike. (aka Load out or Bump out.)

GFI / G.F.I.
(US) Ground Fault Interruptor. See RCD.

GHOST LOAD
A lamp or group of lamps used to smooth out the waveform from electronic dimmers when using an inductive load(ballasts, transformers) rather than a resistive load (lamps).

GHOSTING
A method of determining the exact position of a followspots beam by faintly exposing it on a darker area of the stage or upon the drapes. Often done just before a "pick up" so the operator can have the lantern aimed and ready. A more professional practice is to use sights to line up a followspot.

GHOSTLIGHT / GHOST LIGHT

1. To stop people tripping over bits of scenery when they come into the theatre in the morning.
2. Also refers to the light emitted by a lantern when a dimmer has not been "trimmed" correctly, and is leaking.
3. Also known as the "Equity Light". See link below for more information.

GLOVES
Used when lifting heavier lanterns or other equipment.

GLS
General Lighting Service. Lamps designed for general everyday use.

GOBO
A thin metal plate etched to produce a design which can then be projected by a profile spotlight. There are hundreds of gobo designs available - common examples are breakup (foliage), windows and scenic (neon signs, cityscapes etc.). The image can be used soft focus to add texture, rather than a defined image. A number of composite gobos in different coloured lanterns can, with careful focusing, produce a coloured image (e.g. a stained glass window). Greater detail can be achieved using a glass gobo, which consists of a thin layer of aluminium etched onto glass.

There are a few possible origins for the word GOBO: 

1. It came from the early days of Hollywood. When the Director of Photography wanted daylight excluded from some area of the set, he'd say "GO BlackOut". Loads of people would run around putting black material between the sun and the set. 
2. It stands for Graphical Optical BlackOut.
3. It is short for Go-Between, as the gobo goes between the lamp and the lens.
4. It is short of GO Before Objective (i.e. it goes before the Objective lens).

Material from 1967 uses the word "MASK", and no mention is made of "GOBO", so we can assume the word wasn't in widespread use then. In the US TV/Film industry, a Gobo is a piece of material used to mask or block light, placed in front of a lantern (also known as a SHADOW MASK) and a Cookie (short for Cucaloris(from the Greek kukaloris: the breaking up of light)) is the same as a UK Gobo. PATTERN and TEMPLATE can also refer to a gobo in some areas. In the film industry, the word gobo can be used as a verb (e.g. "We need to gobo off that light so the camera doesn't see it").

GOBO HOLDER
A metal plate designed to hold a gobo of a particular size in a lantern of a particular type.

GOBO ROTATOR
See EFFECTS, LIGHTING

GOING DARK
Warning to people on stage that the lights are about to be switched off. Normally said during lighting plotting sessions or
technical rehearsals.

GRAND MASTER
See MASTER.

GROUND
A heavy cable connected to earth via a metal copper stake for the purpose of grounding electrical equipment. In the U.S. a third wire in our electrical system is connected to this earth ground to provide a means of connecting the chassis of electrical equipment to the earth ground and thus provide protection against hazardous electrical shock.

GROUND LOOP
Hum caused by return currents or magnetic fields from relatively high-powered circuits or components which generate unwanted, noisy signals in the common return of relatively low-level signal circuits. A potentially detrimental loop formed when two or more points in an audio system that are nominally at ground potential are connected by a conducting path.

GROUND PLAN
Scaled plan showing the exact position (seen from above) of all items standing on the stage floor and indicating the position of items suspended above. Typical scales are 1:24 (.5" to 1 foot) or, metrically 1:25 (1cm to .25m). Venues have a base plan showing proscenium, walls, seating etc on which individual set and lighting plans can be drawn.

GROUNDROW

1. A long piece of scenery positioned at the base of a backcloth usually to mask the very bottom of a cloth or lanterns lighting a cloth.
2. Compartmentalised floodlight battens at floor level used to light the bottom of skycloths etc.

GROUP
A subdivision, permanent or optional, of a lighting board control preset, or a sound desk.

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 H

H.320
An international standard designed to bring interoperability to videoconferencing.

HALF DUPLEX
A system capable of transmitting and receiving signals in one direction at a time.

HALOGEN CYCLE
Chemical process occurring in Tungsten Halogen lamps which makes them possible. During the lamps life, Tungsten evaporates from the filament, and would normally deposit itself on the glass wall of a Tungsten lamp, causing it to blacken, and causing the output of the lamp to reduce until it finally blew. In a Tungsten Halogen lamp, the Tungsten combines with the Halogen gas elements present in the lamp envelope and is re-deposited back onto the filament. This process needs a very high temperature to operate, so Tungsten Halogen lamps are able to be a lot smaller, and run a lot hotter, than their Tungsten equivalents. See also Tungsten Halogen.

HALOGEN LAMPS
Used in some low priced data projectors, and most Ohp's, these lamps last about 40 hours, with consistent output throughout their life. Although halogens look very white compared to a normal incandescent lamp, they are not as white as metal halide units.

HAAS EFFECT
Refers to the condition of the human auditory system that permits a listener to merge all the information arriving in the first 20 milliseconds as a single event. This is sometimes called the precedent effect.

HARMONIC
One of a series of sounds, each of which has a frequency which is an integral multiple of some fundamental frequency.

HARD WIRED REMOTE
Generally a remote control is wireless, and uses infra-red transmitter. There are situations where this is not practical: including large rooms where the speaker is 35 ft or more from the projector; Rear projection, where the screen will pass some signal, but normally has the presenter pretty much tied down. Also, the presenter has to point the remote "at" the projector which often means turning away from the audience. A couple of projectors offer wireless remotes that will accept a cable (hard wiring) back to the projector, assuring range and signal getting though.

HAZER
See SMOKE MACHINE.

HDTV
The new High-Definition Television standard displays and broadcasts signals that use many more scan lines than normal television and a 16:9 aspect ratio rather than the old 4:3 aspect ratio of a standard TV set. HDTV also refers to the televisions that pick up these signals. HDTV does not degrade through many generations of editing; the final broadcast is crystal clear. The resulting picture is up to five times sharper than that of today's sets, with CD-quality sound.

HEADS ON STAGE
A shouted warning (often just "Heads !") for staff to be aware of activity above them. Also used when an object is being dropped from above.

HEADSET

1. General term for theatre communication equipment.
2. A headphone and microphone combination used in such communications systems with a beltpack.
3. See also CANS.

HEADROOM
The difference between the average operating power level of an amplifier circuit and the point at which clipping or severe distortion occurs.

HEARING
The human hearing system is very well designed. It has a dynamic range of over 120 dB. Contemporary digital recording techniques can only achieve a dynamic range of about 90 dB. The typical threshold of pain is around 140 dB, with discomfort starting around a sound level of 118 dB. The normal hearing range is considered to be 15 Hz to 20 kHz. The typical Hi-Fi specification range is 20 Hz to 20 kHz. Typically, however, the average person cannot hear 20Hz. Sound reinforcement specifications reflect 50 Hz to 15 kHz (sometimes 40 Hz). Interestingly enough, this just happens to be the FCC limits on FM radio. The typical telephone has a frequency response of 400 Hz to 4 kHz. The human ear does not hear all frequencies at the same intensity. It's less sensitive at both the lower and upper ends of the frequency spectrum, and this characteristic varies with both age and sex. The amount of sensitivity is also a function of sound pressure level. The greatest intensity variations occur at very low sound pressure levels. The curve is relatively flat at sound pressures of 90 dB or so (Fletcher-Munson). The decibel is used in acoustic measurements because the human ear responds to the intensity of sound in approximately a logarithmic manner.
Sensitivity: Only 5% of people can hear a 1 dB difference in level (60 dB, 1 kHz); about 50% of people can hear a 2 dB change; and almost everyone can hear a 3 dB change. This means that when looking at equipment specifications, 1 dB frequency response specs are good; 3 dB specs are fair.
Relative level: 50% of people say about a 7.5 dB increase in level seems twice as loud . . . some as low as 5 dB, and some as high as 10 dB. This test is very level and frequency sensitive. Higher sound levels produce lower numbers and frequencies below 1 kHz and above 5 kHz yield higher numbers. The rule of thumb is 10dB. Applied to sound systems, to achieve output levels that appear to be twice as loud the system must produce over eight times more output power!

HEATSHIELD
Made by Rosco, Heat Shield is a special clear gel which when placed between a lamp and a coloured gel, dissipates a large amount of heat to give the gel a longer life. There must be an air gap between the Heat Shield and the gel, or it will not be effective.

HERTZ (HZ)
A unit of measurement, previously referred to as cycles per second used to indicate the frequency of sound or electrical wave. A unit of motion referenced to a time period of one second. The frequency of a vibration or oscillation in units per second.

HIGH GAIN SCREEN
A screen that uses one of many methods to collect light and reflect it back to the audience. This dramatically increase the brightness of the image over a white wall or semi-matte screen. Technologies used include curved screens, special metal foil screens (some polarized), and certain glass beaded screens. Prices and performance vary tremendously, but attention to the screen can make a big difference, particularly in "tough" environments such as trade shows.

HIGH HAT
See TOP HAT.

HIGH PASS
All signals above a given crossover frequency.

HIGH Z OR HIGH IMPEDANCE
Any resistance to AC voltage or current generally greater than 2,000 Ohms.

HIGHEST TAKES PRECEDENCE*
Abbreviated to HTP, this is the standard by which some lighting desks operate. If there is more than one control on the desk affecting a particular channel, then the highest level of the controls will take priority and affect the output of the desk and the dimmers. This system is universal on manual lighting desks, but there are problems with the control of moving lights, scrollers etc.

HISS
Audio-frequency noise having subjective characteristics analogous to prolonged sibilant sounds.

HMI (HYDRAGYRUM MEDIUM ARC-LENGTH IODIDE)
A mercury-halide discharge lamp with a colour temperature of 5600K (daylight).

HOOK CLAMP
A clamp with a wing bolt for hanging a lantern on a horizontal lighting bar. Introduced in the UK in 1959 by Strand, replacing the 2-bolt and 2-nut L Clamp. See SAFETY CHAIN and BOOM ARM.

HOOK UP
A Hook Up is paperwork generated by the Lighting Designer for a show. It lists connections or layouts between number systems. For example, a Channel Hook Up lists the channel numbers used on the lighting plan alongside the dimmer numbers into which they're connected, and a brief text description of that channels function.

HORIZONTAL FREQUENCY
This indicates how long it takes to scan each of the horizontal lines that make up the display. The unit of measurement is kilohertz (kHz). It is directly related to the number of lines and the vertical refresh (frequency) so that the higher the vertical refresh or the number of lines, the higher the horizontal frequency required. Illuminance

HOUSE LIGHTS
The auditorium lighting which is commonly faded out when the performance starts.

HTI
See DISCHARGE LAMP.

HTP
See HIGHEST TAKES PRECEDENCE.

HUM
An electrical disturbance that can occur in sound equipment due to the frequency of the power distribution system or any number of its harmonics. Our power line frequency in the U.S. is 60 Hz. Hum can occur at 60 Hz, 120 Hz, 180 Hz, 240 Hz, etc.

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 I

IM (INTERMODULATION DISTORTION)
Nonlinear distortion characterized by the appearance of output frequencies equal to the sums and differences of integral multiples of the input frequency components. For instance, an amplifier with high IM distortion amplifying two frequencies of 100 Hz and 2,000 Hz would also generate distortion components of 1,900 Hz and 2,100 Hz.

IMPEDANCE
The total opposition to alternating current flow presented by a circuit. The resistance to the flow of alternating current in an electrical circuit, generally categorized as either "high" or "low", but always expressed in ohms. Commonly used to rate electrical input and output characteristics of components so that a proper "match" can be made when interconnecting two or more devices, such as a microphone, loudspeaker or amplifier.

IMPEDANCE MATCH
The condition in which the external impedance of a connected load is equal to the internal impedance of the source, thereby giving maximum transfer of energy from source to load, minimum reflection, and minimum distortion.

IMPULSE
A type of signal that switches on and off as opposed to remaining in a steady state like a continuous sine wave. Music is more impulsive in nature than it is steady state.

INCANDESCENT
Light source consisting of a metal filament (Tungsten) which glows white hot when current is passed through. See also DISCHARGE LAMP.

INDEPENDENT

1. An electrical power supply that is totally separate from the stage lighting control. Used for testing lanterns prior to connection to the lighting system and also for powering non-lighting equipment on stage and working lights. See NON DIM.
2. A channel within the stage lighting control which has been temporarily switched to become independent from the rest of the channels which remain under the control of the operator.

INDUCTANCE
That property of an electric circuit or of two neighboring circuits whereby an electromotive force is generated (by the process of electromagnetic induction) in one circuit by a change in itself or in the other.

INDUCTOR
A coil of wire used to create an impedance whose reactive component is low, therefore offering low resistance at low frequencies and high resistance at high frequencies. An inductor passes low frequencies and attenuates or rolls off high frequencies.

INFINITE BAFFLE
A baffle that effectively prevents all of the loudspeaker's rear sound waves from interfering with its front waves.

INFRA-RED
Invisible part of the electromagnetic spectrum with a longer wavelength than visible light. Infra-red remote controls are used for lighting desks and practicals. An infra-red-sensitive CCTV camera can pick up body heat activity even in a "blackout".

INFRA-RED REMOTE
The traditional remote control, it transmits infra-red, just as a television remote. Typical range is limited to 30 - 35 feet. Infra-red requires line of site or a bounce off of a hard surface. The presenter must pay attention to where the remote is pointed. Some projectors have an IR sensor in both the front and rear of the projector, which can help. When working at or near the maximum distance, pointing right at the receiver is necessary. Remember "line of sight" - a person's head, directly between your remote and the projector may be enough to render it unusable. FM (radio frequency remote mousing systems, by comparison, have two distinct advantages, no line of sight requirement, and longer range.

INHIBIT

1. A way of taking control of a rogue lantern (or lanterns) at the lighting desk during the operation of a show and removing them from any further lighting states, until the inhibit is removed. Can also be used for removing the front of house lighting from a curtain call state.
2. See also SUBMASTER.

INHIBITIVE SUBMASTER
See SUBMASTER.

INSERT

1. An additional route into a sound desk.
2. An extra lighting state added into the sequence later. See POINT CUE.

INSTALLATION

1. An electrical system in a particular building (e.g. "the stage lighting installation was tested last year")
2. A piece of art designed to transform a particular room or building into something other than a room in an art gallery. Installations often use complex audio-visual equipment and can be intensely immersive experiences. As with all art, they can also be rubbish.

INSTRUMENT
Same as a LANTERN or LUMINAIRE in the UK/Europe.

INTELLIGENT LIGHT
See MOVING LIGHT.

INTERLACED
Method of significantly increasing data densities at conventional horizontal scan rates. Half the image is refreshed (every other scan line) to produce a field. Two fields are refreshed at rates of 87 Hz forming one 43.5 Hz frame. Causes flicker on CRT displays.

INTERNATIONAL POWER SUPPLY
A unit that can operate under international selection of power requirements. The specs of units vary widely, but the minimum is 105-230 volts, and 50-60 cycles AC (alternating current). If you see a specification like 110v, 220v instead of a range, those ratings are usually +/- a given percent such as 10%. Some units are "self-switching" they will automatically switch to whatever power source you plug it into. Others will have to be switched (internally or externally to accommodate a different voltage or cycle range.

INTEROPERABILITY
A state of compatibility between videoconferencing units that may support differing levels of compatibility.

INTERNALLY WIRED BAR (IWB)
A Scaffolding bar (aluminium) which has a number of sockets (usually 15A) positioned along its length, the wiring for which is contained within the bar. See SIX LAMP BAR.

INPUT OVERLOAD
Distortion produced by too strong a signal from the output of a microphone or other signal source connected to the input of a preamplifier.

INSERTION LOSS
A loss in gain of a system after a component has been added or inserted in the system, usually expressed in decibels.

INVERTED IMAGE
Invert image flips the image from top to bottom to compensate for ceiling mounting a projector upside down. Projectors typically ceiling-mount upside down, because most have "keystone" correction built in to compensate for the distortion created by "pointing up" from the table to the screen. Usual positioning has the projector about even with the bottom of the screen in a "table top position," or even with the top of the screen when ceiling mounted.

IP
Internet Protocol. Allows for videoconferencing over LAN or commercial Internet.

IR COMMUNICATION STANDARD
Many new laptops have an Infra-Red transceiver that follow a recent standard for wireless communicating with peripherals (new laser printers complying with the standard) and networks or desktop systems. If you have a laptop like this, you know the pleasure of walking into a room with a configured laser printer, and printing out documents without having to "plug-in." Only a couple of projectors are available that follow this standard. This allows their remote controls to talk directly to your laptop for remote mousing.

IRIS
Adjustable aperture which, when placed in the gate of a profile lantern, varies the size of a beam of light. Originally, iris diaphragm.

ISDN
Integrated Services Digital Network is an international standard for digital services on the public switched telephone network.

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 J

JACK
A receptacle on a receiver, tape recorder, amplifier or other component into which a mating connector can be plugged.

JUMPER
An adaptor from one type of electrical connector to another. For example, a 13 - 15A jumper has a 13A plug and a 15A socket at either end of a short cable. Also applicable to sound cables.

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 K

KBPS
Kilobits per second. 1 Thousand bits per second.

KELVIN
See COLOUR TEMPERATURE.

KEYSTONE CORRECTION
The innovation of using optics design or other methods to apply a "negative" keystone to the image, which will partially cancel the effects of keystoning. If you aim a projector with keystone correction at a screen with the lens level with the middle of the screen, you will note that the image at the bottom is wider than the top. When the projector is in normal position, pointing upward 10-25 degrees, the resulting image is fairly rectangular.

KEYSTONING
Keystoning is caused when the projected image is not perpendicular to the screen, making the top and bottom of the image different widths for a trapezoid effect.

KEY LIGHT
(Esp. TV & Film lighting) The dominant light source/direction in a naturalistic lighting state. In a sunny drawing room, the key light would be through the window, for a naturalistic exterior scene the direction of the key light could change as the sun progressed across the sky. See FILL LIGHT.

KICKER LIGHT
Lanterns placed to the side of the actor to maximise the sculptural quality of the light are sometimes known as KICKERS.

KILL
To switch off (a light/sound effect); to strike/remove (a prop).

KILOHERTZ
A frequency of one thousand cycles per second (1 kHz).

KILOWATT
1 kilowatt (1kW) is equal to 1000 Watts. The WATT is a measure of electrical power.

KIT
1. General name for equipment.
2. Short for Drum kit.

KVA
Kilo-Volt Amps. Unit of electrical power.

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 L

LADDER
Non-climbable structure in the shape of a ladder from which lanterns can be hung in a vertical "stack".

LAMP
A light bulb is used in domestic situations (i.e. in the home). In the industry, we only use LAMPS. As the saying goes, "Bulbs is what you put in the ground". Example usage: "The lamp in the DSC fresnel has blown". In the TV/Film world, a lamp is called a BUBBLE.

LAMP LIFE EXPECTANCY
The time that the lamp can keep its projected screen lumens higher than 50% of the initial value.

LAMP TRAY
Lower section of the body of a theatre lantern on which the lamp holder is mounted. Some lamp trays are hinged, some are removable from the rest of the body. Modern safety legislation (UK) requires that the lamp tray cannot be opened until power is disconnected.

LAMPY
Theatre / Rock & Roll lighting technician. Distinct from a "Techie" who may also be a carpenter or stage crew member. A lampy only concerns him/herself with lighting.

LAN
Local Area Network. A high volume data transmission signal is returned to the sending device after passing through all or part of a communications link or network.

LANTERN
General term for unit of lighting equipment including spotlight, flood etc. Term now being replaced by the internationally recognised "luminaire" (esp. Europe) or "instrument" in the US.

LASER
Acronym of Light Amplification by Stimulated Emission of Radiation. A very high energy beam of light that remains
virtually parallel throughout its length. Visible in the air only when a haze of smoke or dust is introduced. Great care is required when using lasers as this energy can cause permanent damage to the retina of the eye.

LASER POINTER
A small pen or cigar sized pointer that contains a small battery powered laser. The laser pointer projects a small, red (typically), high intensity beam of light.

LCD
LCD stands for liquid crystal display and comes in many forms, sizes, and resolutions. Its primary purpose is to present a digital image for viewing. A common use of LCDs is as a display on a notebook computer.

LCOS
Liquid Crystal On Silicon. This consists of mirror electrodes on a silicon substrate and transference electrodes on a glass substrate, enclosing vertically aligned liquid crystal. LCOS requires a far smaller light blocking layer that allows it to achieve a much higher aperture ratio (92% as opposed to 40-60% for conventional)

LD / L.D.
Lighting Designer.

LECTRIFLEX

1. Multipin connector for carrying multiple lighting circuits down a multicore cable.
2. Also see SOCAPEX.

LED / L.E.D.
Light Emitting Diode, a semiconductor device that produces visible light when a voltage of a certain polarity and potential is applied to it. LED technology is becoming extremely useful in the areas of architectural lighting and video walls. LED light sources are becoming brighter and cheaper. They are extremely efficient, and give off very little heat, making them ideal for display or architectural work. LED video walls are in use all over the world - they are more efficient and lighter in weight than projection alternatives.

LEKO
A type of ellipsoidal profile spot, much used in the USA. Contraction of the original manufacturers names.

LENS
Optical glass with one or both sides curved, the purpose of which is to direct light by concentrating or dispersing light beams.

LIGHTING BOARD
See BOARD.

LIGHT CURTAIN
A lighting effect which, when an area is diffused with smoke, produces a wall of light. Produced (usually) by a batten of low voltage PAR lamps wired in series. Automated versions are available which have colour changers built-in and are able to tilt up and down. (See DIGITAL LIGHT CURTAIN)

LIGHTING PLAN
A scale drawing detailing the exact location of each lantern used in a production and any other pertinent information (E.g. its dimmer number, focus position and colour number). Often drawn from the theatres' groundplan.

LIGHTING PLOT
The process of recording information about each lighting state either onto paper or into the memory of a computerised lighting board for subsequent playback. (in USA, this term is used for a lighting plan and a lights session is when lighting states are set up.)

LIGHTING STATE
The format of lighting used at a particular point in the production; a lighting "picture".

LIGHTING STENCIL
Plastic stencil containing a range of scale symbols for current lighting equipment. Greatly facilitates the drawing of lighting plans.

LIMELIGHT
An obsolete source of intensely bright light, most recently used in followspots. Derived from a burning jet of oxygen and
hydrogen impinging on a rotatable cylinder of lime.

LIMITER
An electronic circuit used to prevent the amplitude of an electronic waveform from exceeding a specified preset level while maintaining the shape of the waveform at amplitudes less than the preset level.

LINEAR
Having an output that varies in direct proportion to the input.

LINEAR FREQUENCY SCALE
A scale on which each equal length division represents an equal number of Hertz. If you add the equal length division in Hertz to the last division frequency, you get the next frequency in a series.

LINE OUT
An output connection found on mixers, preamps, tape decks, etc. providing an output at a level sufficient to drive the input of a power amplifier.

LINK
See POINT CUE.

LOAD

1. The electrical power rating, in Watts, of the equipment connected to a particular lighting dimmer. 
2. The equipment connected to a dimmer.

LOAD IMPEDANCE
The actual impedance of the load or speaker that a power amplifier is connected to, thus driving a load of a certain impedance.

LOBSTERSCOPE
A mechanical device that reproduces the flashing "motion-freezing" effect of a strobe.

LOGARITHMIC FREQUENCY SCALE
A scale on which each division represent an exponential constant, i.e., each division from a reference point is proportional to its logarithms.

LONG THROW LENS
A lens designed for projection from the back of a room, or rather the back of a large room. Long throw lenses would be used in a projection booth in the back of a theater, etc. A typical long throw lens might have to be 15 to 30 from the screen to project a 120" diagonal image.

LOUDNESS CONTROL
A volume control with special circuitry added to compensate for the normal decreased hearing ability of the human ear at the extreme ends of the audio range when listening to lower sound levels. A typical loudness control boosts the bass frequencies and to a lesser extent, the high frequencies. Sometimes this control is called contour.

LOUDSPEAKER EFFICIENCY
The ratio, expressed in percentage, of signal output to signal input used to state the power needed to drive a loudspeaker. An example: Power output 2 watts; Power input 10 watts; Ratio 2/10=20% efficiency. Efficiency can vary from 2% to as high as 25%.

LOW PASS
All of the frequencies below a given crossover frequency.

LOW SMOKE

1. Smoke that has been chilled as soon as it comes out of the smoke machine. This causes the smoke to lay close to the floor. Use fast dispersing smoke for this effect because when the smoke heats up in the air, it will rise.
2. Also see DRY ICE.

LOW VOLTAGE
Lower voltage lamps give more intense light than mains voltage lamps of the same wattage.

LOW Z OR LOW IMPEDANCE
Any resistance to AC voltage or current flow generally less than 2000 Ohms.

LTP
Latest Takes Precedence.

LUMEN / LUMENS
A standard for measuring light output used for comparing light output. The brightness of projectors is stated in Lumens. However, the rating does not always match the perceived brightness. For example, if one projector uses halogen lamps and another metal-halide, the halogen projector will seem noticeably dimmer, even if the two units rate the same.

LUMINAIRE
The international term for lighting equipment. Not restricted to theatre lighting.

LUX

1. A measure of the level of illumination on a surface (1 lumen spread over 1 metre).
2. A standard for measuring light, numbers provided by manufacturers usually do not provide necessary additional information to compare one product to another.

LX
Short for Electrics. The department in the theatre responsible for stage lighting and sometimes sound and maintenance of the building's electrical equipment.

LX TAPE
See PVC TAPE.

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 M

MAGIC SHEET
See CHEAT SHEET.

MAINTAINED LIGHTING
See SECONDARY LIGHTING.

MANUAL DESK
A lighting desk where the interface between operator and dimmer is a fader, rather than a computer. Many modern manual desks have some memory facilities built in, but there are still a large number of venues with solely manual systems.

MAROON
An electrically detonated pyrotechnic device giving the effect of a loud explosion. Made from gunpowder encased in stout cardboard or string. Must be used within a metal bomb tank. Originally developed in the second half of the last century to simulate the sound of cannon. It was often used to call out the volunteer lifeboat crew in an emergency.

MASK

1. Form of theatre where actors faces are covered with masks.
2. Early word for GOBO.

MASTER

1. An overall control on a lighting control console. The Grand Master takes precedence over all other controls. See SUBMASTER.
2. Main level or gain control for a device, bus or mix on a sound control console.
3. An original (e.g. Master tape, master plan) which should be used only to make a copy from which to work.
4. A Department Head (e.g. Master Carpenter, Master Electrician).

MASTER ELECTRICIAN
See CHIEF ELECTRICIAN.

MAXIMUM DISTANCE
Sometimes Maximum Distance refers to the distance from the screen that a projector can focus the image. Most of the time, however, it is the manufacturer's opinion of how far from a screen the projector can be to cast an image that is useable (bright enough) in a fully darkened room. Generally this is very subjective. One projector might quote a distance that allows them to produce a 25ft diagonal image, while a brighter projector might quote a distance that only equates to a 20ft image. Beware!

MAXIMUM IMAGE SIZE
The largest image a projector can throw in a darkened room. This is usually limited by focal range of the optics.

MBI
Metal Halide discharge lamp. See also DISCHARGE LAMP.

MBPS
Megabits per second. 1 Million bits per second.

MCB
Minature Circuit Breaker. Up to 63A (UK). See FUSE.

MCCB
Moulded Case Circuit Breaker (over 63A - UK). See FUSE.

MEMORY BOARD
An electronic storage device which enables recording and subsequent "playback" of lighting states.

METAL HALIDE LAMP
The type of lamp used in many medium and all high-end portable projectors. These lamps typically have a "half-life" of 1000-2000 hours. That is they slowly lose intensity (brightness) as they are used, and at the "half-life" point, they are half as bright as when new. These lamps output a very "hot" temperature light, similar to mercury vapor lamps used in street lights. Their whites are "extremely" white (with slight bluish cast) and make Halogen lamp's whites look very yellowish by comparison.

MICROPHONE
A microphone is a transducer that changes acoustical energy (sound) into electrical energy.

MINIMUM DISTANCE
The closest position that a projector can focus an image onto a screen.

MIMIC
The VDU associated with most medium and large lighting desks has a detailed mimic of the level of all dimmers and other associated information.

MIRROR BALL
A lighting effect popular in discos, ballrooms etc. A large plastic ball covered with small mirror pieces. When a spotlight (usually a PINSPOT) is focused onto the ball, specks of light are thrown around the room. Usually motorised to rotate.

MIXER
A device in sound reinforcement that has two or more signal inputs and a common signal output. Used to combine separate audio signals in desired proportions to produce an output audio signal.

ML
Short for MOVING LIGHTS.

MODE
Another word for room resonance. When sound energy is restricted by boundaries (such as walls, floor, and ceiling) waves are developed at certain frequencies or wavelengths that are integers of the distance between the room boundaries. Room modes or resonances cause standing waves because once the wave is generated it stands there, i.e., the positive pressure peaks (anti-nodes) and negative pressure troughs (nodes) stay stationary within the boundaries.

MONITOR

1. A loudspeaker or system of loudspeakers that permits the performer to evaluate or monitor his sound alone or in conjunction with other sounds that may be desired and is mixed to the listeners preference by means of a separate monitor or reference mix.
2. A video display screen (not normally able to receive broadcast TV pictures) used with a CCTV system or a computer.

MONO
Monophonic Sound - Sound produced by a system in which one or more microphones feed a single signal to an amplifier(s) whose output is coupled to one or more loudspeakers.

MOON BOX
Lighting effect. A large shallow circular box with calico cloth on one face and low wattage lamps arranged on the back. Can be flown behind a gauze or thin cyclorama to give the effect of the moon rising.

MOTORISED ZOOM
A zoom lens with the zoom in and out controlled by a motor usually adjusted from the projector's control panel and also the remote control.

MOVING HEAD
See MOVING LIGHT.

MOVING LIGHT
Remotely controllable "intelligent" lighting instrument. Each instrument is capable of a massive variety of effects which are operated "live" via a moving light control desk, or can be pre-programmed by a standard memory lighting desk. The instruments require a power supply and a data cable (normally carrying DMX512 signal from the control desk). There are broadly two types:

1. Moving Head: A luminaire is mounted on a moving yoke.
2. Moving Mirror: A stationary luminaire directs light onto a motorized mirror.

Both types have in common:

1. A discharge (non-dimmable) light source
2. A dimming shutter
3. Motorized rotating colour wheels. Some offer colour mixing using graduated red, green and blue wheels or prisms.
4. Profile versions have motorized gobo wheels with rotation.
5. Strobing effects and adjustable iris. Some also have shutters.
   The term "intelligent" is used as the instrument has a processor chip and electronics built into it, not because it's able to interpret the designer's artistic intent! It can be incredibly frustrating trying to get moving lights to behave exactly as required in a dramatic situation. Musicals and live music performances are more forgiving.

Moving Head lanterns are sometimes known as NODDING BUCKETS, Moving Mirrors are sometimes known as WIGGLIES or SCANNERS.

MOVING MIRROR
See MOVING LIGHT.

MR16
A 12 Volt lamp dichroic lamp commonly used in place of a Par 16 lamp in BIRDIES. See BIRDIE.

MSDS
Material Safety Data Sheet. Form available from manufacturers of, for example, smoke fluids. Lists any hazardous ingredients and other safety-related data about the product.

MSR
(Medium Source Rare earth) High efficiency discharge lamp with a high colour temperature (approx 5600°K). Provides around 50% more light output than a incandescent lamp of the same wattage.

MULTI
Short for MULTICORE.

MULTICORE
A flexible electrical cable composed of several well-insulated cores covered in a strong PVC or rubber covering. Enables a number of different circuits to be carried down one piece of cable. Both lighting and sound multicores are available. Sometimes known as a Multi or Snake.

MULTIMETER
Also called a Volt-Ohm-Meter (VOM). A measuring instrument that can measure different ranges of voltage, current, and resistance. A multimeter can have an analog needle indicator or a digital read out.

MULTIPAIR CABLE
See MULTICORE.

MULTIPLEXED (MUX) SIGNAL
All modern lighting desks use this serial form of communication with dimmers. All the information from the desk is transmitted along a single pair of cables to the dimmer where a de-multiplexing unit (demux box) decodes the string of data and passes the correct piece of information to the correct dimmer.
The industry standard protocol (language/standard) for multiplexing is the digital USITT DMX512 (introduced in 1986, based on RS485 data protocol). However, new protocols are continually being added to keep up with more demanding equipment.
SMX is a communications protocol which enables digital dimmers to "report back" to the desk on any faults (eg blown lamps).
RDM (Remote Device Management) is an emerging upgrade to DMX512 which will include bi-directional communication between controller and device.
DMX512-A (officially ANSI E1.11) is a new standard under development at ESTA which is backwards compatible with DMX512 but has stricter safety parameters and offers some upgrades of functionality.

MULTIPLEXER (MUX)
A device allowing two or more signals to pass over and share a common transmission path simultaneously.

MUSIC POWER
This is a power rating generally applied to high fidelity amplifiers for tones of short duration. It takes into account the fact that most amplifiers can produce a greater amount of power in short bursts than they can continuously. The rationale is that music is made up of such bursts rather than sustained single frequencies. It is higher than continuous power ratings for the same amplifiers. It is measured at a signal frequency of 1000 Hz for a specified distortion.

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 N

NEON

1. A type of discharge lighting generated by a high voltage across two oppositely charged electrodes at opposite ends of a long, thin glass tube filled with neon gas. As the electrical charge flows between the electrodes, electrons collide with neon atoms causing them to give off energy in the form of visible light. Different colours can be obtained by mixing other gases, or by using fluorescent coatings. Mostly used for advertising signs - the glass tube is bent to form letters.
2. A small mains voltage indicator lamp.

NETWORK

1. An interconnected group of nodes.
2. A series of points, nodes, or stations connected by communications channels; the collection of equipment through which connections are made between data stations.

NEUTRAL DENSITY FILTER
(ND) Lighting filter which reduces the brightness/intensity of a light source without changing its colour value. Used extensively in TV/film for reducing the intensity of discharge lamps or natural light sources (e.g. windows). Rarely used in theatre as dimmers fulfil a similar function (although as incandescent lamps are dimmed, the colour temperature gets warmer).

NOISE
Any extraneous sound or signal that intrudes into the original as a result of environmental noise, distortion, hum, or defective parts in the equipment.

NON DIM
Channels, usually controlled from the lighting desk which are switched, rather than dimmed. This enables motors, slide projectors, smoke machines etc to be controlled from the lighting desk.

NOTCH FILTER
A band rejection filter that produces a sharp notch in the frequency response of a system, thus reducing the gain or amplitude of a narrow band of frequencies centred on a given frequency.

NTSC
The United States broadcast standard for video and broadcasting. An older standard and lower resolution than systems used in most of the world.

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 O

OCTAVE
The interval between any two frequencies having a ratio of 2 to 1. Example: "A 440" is an octave above "A 220".

OFFSTAGE

1. A movement towards the nearest side of the stage from the centre. (e.g. "Focus that spot offstage a bit please")
2. The area out of sight of the audience (e.g. "Get that donkey offstage!")

OHM
The unit of electrical resistance, equal to the resistance through which a current of one ampere will flow when there is a potential difference of one volt across it. Ohm is the unit of measure used to express opposition to current flow. Every wire or part through which electricity passes has some resistance to that passage.

OHP
See OVERHEAD PROJECTOR

OMNIDIRECTIONAL
A term usually applied to microphones to refer to uniform pickup of sound from all directions.

ONSTAGE
A movement towards the centre of the stage from the sides.

OPEN WHITE
(o/w) Lighting with no colour filter.

OSCILLOSCOPE
A test instrument that shows a picture of electrical waveforms by means of a cathode ray tube. An oscilloscope is calibrated to allow measurement of the instantaneous values and waveforms of electrical signals that are changing rapidly or varying as a function of voltage or time. Also known as a Scope.

OSD
On Screen Display.

OUTPUT IMPEDANCE
The internal output impedance of an amplifier presented by the amplifier to the load. (not to be confused with load impedance.)

OUTPUT POWER
The power delivered by a system to its load. (i.e. an amplifier driving a loudspeaker system)

OUTRIGGER
An extendible leg to increase the stability of access equipment (eg Tallescope, Scaffold tower).

OVERHEAD PROJECTOR (OHP)
A device consisting of a light source, a transmissive or reflective platform, and a focusable lens assembly. An OHP is designed to project images from tranparencies onto a screen. LCD projection panels are designed to be used with transmissive OHPs and work best with OHPs that produce at least 3,000 lumens. Since 5% to 10% of the light that shines through an LCD panel gets onto the screen, a 3000 lumen OHP will produce an image of 150 to 300 lumens. Transmissive OHPs are fairly bulky (bigger than many projectors). Reflective OHPs are fairly portable but are not useful with LCD projection panels.

OVERTONE HARMONIC
Multiples of frequency of a fundamental waveform.

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 P

PAD
A fixed passive network which reduces the electrical level of an audio signal. An attenuator.

PACKET
An ordered group of data and control signals transmitted through a network as a subset of a larger message.

PAGE

1. Some theatre announcement systems use the term "PAGE" to mean making a call (e.g. "Can you page Simon to come to the fly floor")
2. A way of increasing the functionality of a control on a lighting desk. For example, most computerised lighting desks with SUBMASTERS will allow you to store more than one lighting state in each submaster. Each group of submasters is given a page number which is used to select which set you want to use.
3. Also see SUBMASTER.

PAIRING
Connecting more than one lantern to one power outlet via an adaptor or splitter, or more than one speaker to one amplifier channel.

PAL
A European and international broadcast standard for video and broadcasting. Higher resolution than NTSC.

PAN

1. A control on a mixing desk which allows the operator to position the channel's output in the final stereo image (L - R).
2. A horizontal (side-side) movement of a camera or a moving light. See also TILT.

PANEL
Also known as a projection panel, LCD projection panel, or plate. The panel is the predecessor of today's projectors. It is slightly larger and heavier than a notebook computer and the LCD it uses to produce an image is very similar to that of the notebook computer. Because panels lack their own light source, they are designed to sit on top of a transmissive overhead projector (OHP). (See the definition of Overhead Projector for lumen performance.) Because of its small size, low cost, and versatility, panels have been a popular solution for education applications where an OHP is frequently available in the classroom for other instructional purposes. A few products have been built that integrated the panel and the OHP. These were some of the earliest projectors.

PANELLINK
An all digital interface used to transmit computer video from a PC/Notebook to a projector. Supports resolutions from 640x480(VGA) up to 1600x1200(UXGA). This digital interface might someday replace the analog VGA interface typically used to connect projectors to computers.

PAR

1. Short for Parabolic Aluminised Reflector lamp. A lamp containing a filament, reflector and lens in one sealed unit. Used in PARCANs to produce a high intensity narrow beam of light. Par lamps are available in many different sizes and powers. Par sizes available include 16, 20, 36, 38, 56 and 64. (The number refers to the diameter of the lens, in eighths of an inch). The most common for theatre use are Par 64s rated at 1000W (1kW), although other wattages are available. 110V Par lamps are often used in large UK venues or for touring due to the increased light output. Because the current is greater, the lower voltage lamps have smaller thicker filaments which give a more focussed beam than the thinner 240V filaments.
2. In the film business, PAR lamps are known as "bird's eyes" after the alleged inventor Clarence Birdseye.

PAR BAR

1. A bar pre-rigged with a number of Parcans is.
2. See also SIX LAMP BAR.

PARABOLIC REFLECTOR
See PAR.

PARALLEL

1. The folding frame that forms the base of a readily portable platform.
2. The opposite of SERIES when referring to wiring two loads into one outlet.
3. An electric circuit in which the elements or components are connected between two points with one of the two ends of each component connected to each point. The two loads share the available current, but are both given the same voltage.

PARAMETRIC
A type of equalization circuit that has three variable parameters; frequency, cut or boost bandwidth, and Q (the width of the filter).

PARCAN

1. Type of lantern which produces an intense beam of light, ideally suited to "punching" through strong colours, or for special effect. The Parcan is literally a cylinder of metal (the "can") within which sits the PAR lamp (PAR stands for Parabolic Aluminised Reflector) which consists of the bulb, a reflector and a lens in a sealed unit. The Parcan first appeared in the early 1970's in the Rock concert industry due to the intensity of the beam, and the light weight and near indestructibility of the lantern. The lens of the lamp is either clear (which produces a narrow beam), frosted (medium) or stippled (wide).
2. Also see PAR and FLOORCAN.

PASSIVE
An electronic circuit composed of passive elements, such as resistors, inductors, or capacitors, without any active elements, such as vacuum tubes or transistors generally resulting in a signal loss.

PASSIVE MATRIX LCD
The original LCDs, these are controlled by a single processing system, for the whole screen, unlike active and poly-si, which have descrete circuits for each "pixel." This results in a panel with terrible colour dynamics and contrast (typically 15:1). They are also incredibly slow: On passive laptop computers, the cursor (or anything else) moving on the screen, goes invisible until you stop moving it (submarining) Only one or two projectors use any type of passive matrix display.

PATCH

1. (verb) The act of plugging a lantern into a dimmer (e.g. "Can you patch circuit 12 into dimmer 18 please").
2. (noun) The system for connecting lanterns to dimmers (The Patch).
3. The term also applies to sound - a PATCH BAY is used to connect outboard equipment into the sound desk and to connect sound desk outputs to amplifiers, and amplifiers to speakers.

PATCH PANEL

1. A board consisting of rows of sockets into which plugs can be connected to route sound signals or power for lighting circuits. Some American systems use a Pin Plug patching system.
2. See also PATCH.

PATCHING
1. To cross-connect lighting circuits around the stage area to a chosen dimmer. Connecting lanterns to dimmers.
2. Using a cross-connect panel which enables any stage lighting channels to the control desk to control any dimmer or group of dimmers. Some large lighting boards have the facility for soft patching - a totally electronic way of patching. Some Rock Desks have a pin patch which allows groups of dimmers to be allocated to a particular control channel. Also applies to routing of sound signals.

PATTERN
See GOBO.
See FURGO.

PBX
Private Branch Exchange.

PC
Originally short for plano-convex - the basic lans shape of many lanterns / projectors. Now short for Prism- or Pebble-convex: a type of lens with a pebbled flat surface which gives a slightly harder edge than a Fresnel, but not as hard as a Profile. PC refers to a lantern with a PC lens.

PEAK
The maximum instantaneous value of a signal amplitude.

PEAKING
A term used to indicate an increase in gain of a frequency or band of frequencies when equalizing an audio signal.

PEAK LIMITER
A device which automatically limits the level of its output signal to approximate a preset maximum value by reducing its gain when the signal exceeds a preset value.

PEAK-TO-PEAK
Amplitude of an alternating voltage measured from negative peak to positive peak.

PEBBLE SPOT
See PC.

PEPPER'S GHOST
Trick used to make a ghost appear on stage next to an actor. A sheet of glass is hung across the front of the stage so that the image of an actor standing in the orchestra pit appears to float on stage. First shown at the Royal Polytechnic Institution in London by J.H.Pepper on Christmas Eve, 1862. Following many subsequent events, Charles Dickens used it during readings of The Haunted Man. Several plays were written specially to use the effect around 1863, but the long-term future of the effect was limited by the fact that the ghost couldn't speak. Peppers Ghost is now used to great effect in smaller scale applications like the Haunted Mansion in Disney theme parks.

PERCENT
Dimmer levels are expressed as a percentage of the supply voltage. (100% = 230 - 240 Volts(UK)).

PERCHES

1. Lighting positions (often on platforms) at each side of the stage, immediately behind the proscenium.
2. Some theatres use the term for vertical boom positions in front of the proscenium in the house.

PFL
Pre Fade Listen. This describes a button on a mixing console that permits a channel or sub to be monitored before that channel or sub's level control or fader. This allows an operator to listen to a channel without that channel being heard by the audience.

PHASE
Phase is the time interval between two related events. Two signals are in phase when they reproduce the same sound or signal simultaneously; they are out of phase to the extent that one leads or lags behind the other in time. A signal is said to be in phase with another when the voltage and current amplitudes begin at the same time and move in the same direction.

PHASE CANCELLATION
Signals that are out of phase will cancel one another according to the difference in phase in degree. A transducer (speaker or microphone) wired out of phase with another will result in reduced output from both; instead of their combined outputs adding, they will subtract due to phase cancellation.

PHASES
Electricity is generated and supplied to large installations as three separate supplies, known as phases.

PHOSPHORESCENCE
The property of some materials that can store light energy and glow in the dark.

PHOTOFLOOD
A lamp used by photographers which gives a bright white light. Because it has a thin filament, it gives a good flash effect (eg lightning), but has a relatively short life, so should not be left on for any length of time.

PICK-UP

1. Device which, when attached to an acoustic musical instrument, converts sound vibrations into an electrical signal.
2. A way of describing the directional sensitivity of a microphone. An Omnidirectional microphone has equal pick-up from all around, a Cardoid microphone is more sensitive from the front, a Hypercardoid has very strong directionality from the front. A figure-of eight microphone picks up front and rear, but rejects sound from the sides.
3. The action of turning a followspot on a performer. (e.g. "that was a good pick-up", "your next pick-up is downstage left"). A BLIND PICKUP is on a moving performer and requires good hand-eye co-ordination. A SET PICKUP is on a specific area, is preset, and is made on a cue from the stage manager. A SIGHT PICKUP is made visually by the operator to a preset position.

PIEZO-ELECTRIC
Having the ability to generate a voltage when mechanical force is applied; or to produce a mechanical force when a voltage is applied, as in a piezo-electric crystal.

PILE-ON SUBMASTER
See SUBMASTER.

PINCUSHION (PILLOWING)
An inward bowing of the video image. Some projectors experience a slight amount of pincushion distortion. There are guidelines on what the specifications are for each model. The pincushion changes according to the size of the image. Pincushion is similar to bowing or barrel distortion.

PIN PATCH
See PATCHING.

PIN PLUG
See PATCHING.

PIN SPOT

1. A lantern focused very tightly on a small area (eg an actors head)
2. A luminaire used widely in disco installations, consisting of a low voltage (6V) Par 36 lamp (lamp code is 4515) with a very narrow beam in a metal case with built in transformer.
3. Pin Spots are ideal for lighting MIRRORBALLs.

PIP
Picture In Picture.

PIXELS
The smallest element of a display that has the full-colour capacity of the display.

PLAN
A scale drawing showing a piece of scenery, lighting layout etc from above. Lighting plans are usually drawn onto the theatre's groundplan.

PLASA / P.L.A.S.A.
Professional Lighting and Sound Association (UK). See ESTA for the American equivalent.

PLAYBACK
The part of a computerised lighting control desk which enables the operator to recall cues from the electronic memory.

PLOT

1. List of preparations and actions required of technical crews during the performance (eg Sound Plot = list of sound cues and levels in running order.) In the US, the term plot refers to a plan. (eg Light Plot = scale plan showing lighting instruments). See also RUNNING PLOT, STATE PLOT.
2. The basic story thread running through a performance / play which gives the reason for the character's actions.

PLOTTING SESSION
Time during which the plot for each department is prepared (eg Lighting Plotting session)

PND
Plug and Display.

PNP
Plug and Play.

POINT CUE
A cue inserted during / after plotting between two existing cues. (eg 8.5 is inserted between cues 8 and 9). Most computer lighting desks have the ability to either insert an additional cue in a sequence, or to link to another cue out of the sequence, and then link back again. Inserting cues into a plotted sequence on a manual lighting desk is more awkward, because it is a running plot (where only the changes between cues are noted down). Sound cues within a sequence should have lettered cues (e.g. 8A is a fade up of Cue 8 and 8B is the fade out).

POINT SOURCE
A source of acoustic sound waves having definite position but no extension into space. A point source is an ideal; an imaginary single point in space. This imaginary point source provides a good approximation for distances from the point source that are much larger compared to the actual dimensions of the source. A cluster or array of horns and loudspeakers is positioned using this imaginary point in space as a reference for the actual source of the sound. Properly configured the array will perform as a single or point source of sound.

POLARITY
The quality of having opposite poles. In electro-magnetic-mechanical systems, some form of potential is referenced to one of two poles with different (usually opposite) characteristics, such as one which has opposite charges or electrical potentials, or opposite magnetic poles.

POLE OPERATION
A mechanical means whereby pan (horizontal rotation), tilt (up and down) and focus of a lantern may be adjusted by a pole from floor level. Commonly used in TV & Film studios where fast resetting of positions is necessary.

POLY-SI (SILICON) LCD
A popular LCD technology for the top of the line LCD projectors. Monochrome Poly-Si LCDs are typically placed in each of the three colour light paths inside a projector, one each for Red, Green, and Blue. This results in increased colour saturation, with contrast ratios above 200:1. Poly-Si technology is also a bit faster than the Active Matrix TFT, for smooth video and multimedia.

PORT
The physical interface to a computer multiplexer, for connection of terminals and modems.

POST
(after) on a mixer; post indicates that the function is derived after the channel slider or gain control.

POWER
Electrical energy, measured in watts, such as the current from an amplifier used to drive a loudspeaker. Power in watts is expressed as W = V2/R

POWER AMPLIFIER
The final active stage of the audio chain, designed to deliver maximum power to the load or speaker impedance for a given percent of distortion.

POWERCON
Power connector with the same body design as the SPEAKON speaker connector. Manufactured by Neutrik. Used extensively on lighting patch panels (at least in the UK) due to its smaller physical size.

POWER ZOOM
A zoom lens with the zoom in and out controlled by a motor, usually adjusted from the projector's control panel and also the remote control.

PRACTICAL
Any object which appears to do onstage the same job it would do in life, or any working apparatus (eg light switch or tap). Light fittings which have to light up on the set are called Practicals.

PRE
(before) On a mixer, pre fade listen (PFL) indicates that the function is derived before the channel slider or gain control pre monitor send, the monitor send is before and independent of the channel slider or gain control. A pre monitor send is usually pre channel EQ also.

PREAMP (PREAMPLIFIER)
An amplifier whose primary function is boosting or amplifying the output of a low level audio-frequency source, (such as a microphone), so that the signal may be further processed without appreciable degradation of the signal-to-noise ratio of the system. An amplifier which increases electrical signals from a microphone or other instrument to a level usable by a power amplifier. Preamp levels are approximately .1 volt.

PREAMP OUT
A means of obtaining an output signal from the preamplifier of a channel of a mixer or musical instrument amplifier. The preamp out is actually a line level signal or 1 volt.

PREHEAT / PRE-HEAT

1. Smoother lighting builds from zero are achieved when a lamp filament has been warmed (at approx 15%) in the previous state.
2. Preheating lamps MAY prolong the life of the lamp by reducing the thermal "shock" of going to 100% instantly. It's good practice to preheat lamps where possible, and some computerised lighting desks provide this function at the push of a button.
3. See also RIG CHECK.

PRESET

1. Anything in position before the beginning of a scene or act (eg Props placed on stage before the performance, lighting state on stage as the audience are entering.)
2. An indepenently controllable section of a manual lighting board which allows the setting up of a lighting state before it is needed. Each preset has a master fader which selects the maximum level of dimmers controlled by that preset.

PREVIEW
A function on some memory lighting control desks with video mimics. Preview enables the operator to see the levels of dimmers and other information in a lighting state other than that on stage.

PRI
Primary Rate Interface is the combined signal-carrying capacity of 23 B-Channels (Bearer Channels) operating at 64 kbps and one D-Channel (Data Channel) also functioning at 64 kbps. The total capacity is equal to that of 1 DS-1 circuit.

PRIMARY COLOURS
The primary additive colours of light are red, green and blue, and the subtractive colours are cyan, magenta and yellow.

PRODUCTION DESK
Table in the auditorium at which director/designer etc sit during rehearsals (especially technical rehearsals). Usually has its own lighting and communications facilities.

PROFILE

1. A type of lantern with at least one plano-convex lens which projects the outline of any chosen shape placed in its gate, sometimes with a variable degree of hardness/softness. Profiles include four beam-shaping metal shutters, a gate to take an iris or gobo and an adjustment to make the beam smooth and even ("flat") or hot in the centre ("peaky"). See Bifocal Spot, Zoom Profile.
2. Shaped piece of scenery added to the edge of a flat instead of a straight edge. Also known as a cutout.

PROGRAM
This usually refers to a signal source composed of music and/or speech as opposed to sine wave or noise.

PROGRAM LEVEL
The level of program material in an audio system expressed in VU.

PROJECTION DISTANCE
The distance between the projector lens and the screen measured in linear units (e.g., metres, feet, or inches).

PROJECTION PANEL
The panel is the predecessor of today's projectors. It is slightly larger and heavier than a notebook computer and the LCD it uses to produce an image is very similar to that of the notebook computer. Because panels lack their own light source, they are designed to sit on top of a transmitting overhead projector (OHP).

PROJECTION

1. Slides are used to project still archive images or textures. Libraries of slides contain images for every occasion. Kodak Carousel projectors are the industry standard, and some types can be linked to a controller to perform complex dissolves and fades from one projector to another. More powerful projectors are available using very intense discharge sources and large format glass slides to produce a massive image.
2. Lighting effects: Moving cloud / rain / fire effects can be achieved using a powerful lantern known as an effects projector with a motorised glass disc painted with the required effect. An objective lens is required in front of the disc to focus the image. See Effects.
3. Gobos: See GOBO.
4. Film: 35mm film projection is common in many theatres as a device for keeping the building open to the public when productions are in preparation. 16mm film projection is used in smaller venues. Film projection can, of course, also be integrated into a performance.
5. Video: Video projection is now being used to bring television pictures to the large screen. Cheaper than using film, but the image quality is not as good. Video projection equipment can also be more difficult to set up. Images can be front projected or back/rear projected depending on the amount of space and the effect required. For example, if actors are required to walk in front of the screen and not have the image appearing on them, back projection is the only answer.
6. See SCREEN.

PROJECTOR
A projector is a device that integrates a light source, optics system, electronics and display(s) for the purpose of projecting an image from a computer or video device onto a wall or screen for large image viewing. There are hundereds of products available in the market and they are differentiated by their resolution, performance and features. These devices attach to a computer or video device as you would connect a monitor.

PROTOCOL

1. A formal set of conventions governing the formatting and relative timing of message exchange between two communicating systems.
2. See MULTIPLEX (MUX) SIGNAL.

PROXIMITY EFFECT
The boost in low-frequency response when a unidirectional or proximity effect microphone is used close to a sound source.

PSTN
Public Switched Telephone Network. The telecommunications network commonly accessed by ordinary telephones, key systems, PBX trunks, and data equipment.

PSU
Power Supply Unit.

PULL
The amount of electrical power required by a touring show in a venue.

PVC TAPE
Plastic insulating tape used for taping cables to bars and for securing coiled cables. Neater and cheaper than Gaffa tape. Also known as LX tape.

PYROTECHNICS
(Pyro) Chemical explosive or flammable firework effects with a manual detonation. Usually electrically fired with special designed fail-safe equipment. There are many different variations of pyrotechnic effects available. The categories are as follows:

1. Theatrical Flash - a flash and a cloud of smoke
2. Maroon: produces a very loud bang. Must only be detonated inside a bomb tank covered with a protective mesh.
3. Gerb: version of the Roman Candle firework, throwing a shower of sparks into the air. Possibly named from the French "Gerbe" meaning a sheaf of wheat, due to it's shape.
   All pyrotechnics should be used with close reference to local licensing laws, and the manufacturer's instructions. Professional advice should be sought before the first use of effects. Some territories only permit licenced pyrotechnicians to use these devices.

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 Q

Q
The directivity factor of a transducer (loudspeaker) used for sound emission. The higher the value of Q the more directional the speaker.

QUARTZ HALOGEN
See TUNGSTEN HALOGEN.

QXGA
QXGA is used to define a specific display resolution. Resolution is defined by the number of individual dots that a display uses to create an image. These dots are called pixels. A QXGA display has 2048 horizontal pixels and 1536 vertical pixels giving a total display resolution of 3,145,728 individual pixels that are used to compose the image delivered by a projector. A QXGA display has 4 times the resolution of an XGA display.

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R

RADIO CONTROL
System whereby battery-powered practicals / props on stage can be controllable from offstage with no connecting leads.

RCD
Residual Current Device. Protects the user against short circuit (earth faults) and earth leakage caused by damaged cable or faulty equipment. A RCBO is a combined MCB and RCD, protecting against earth leakage/short circuit and overload. Known as a GFI (Ground Fault Interruptor) in the USA.

RDM
Remote Device Management. New lighting control protocol (officially ANSI E1.20 standard) currently under development at ESTA, which allows two-way communication over standard DMX512 cable. See also MULTIPLEXED SIGNAL.

RE-RIG
To change the lighting rig after the last performance of one show to the positions for the next show.

REACTANCE
A resistive like property that offers opposition to electron flow in an alternating current (AC) circuit. There are two types of reactance; capacitive reactance (XC) and inductive reactance (XL). Reactance varies with frequency.

REAL TIME ANALYZER (RTA)
An electronic instrument used to measure the combined response of an audio system and the room in which the system is operating.

REAR SCREEN PROJECTION
Using an opaque screen, the projector is placed behind the screen, invisible to the audience. The image projects onto the screen which the audience sees on the other side. Good rear projection screens actually produce brighter images than some standard screens. A projector with a short throw lens is typically used to accommodate the lack of space behind the screen. Since the projector can be placed at the ideal height, without blocking anyone's view, keystoning is not a problem. Some mid-room projectors have available 3rd party short throw lenses. Since the image is projected through the screen, the image must be reversed.
Also See PROJECTION

REFERENCE LEVELS

1. O dB - In the measurement of SPL or Sound Pressure Level, 0 dB is referenced to the threshold of hearing or auditory perception of a tone of 1000 cycles (hertz) per second (1 kHz). 0 dB must always be referenced to some base of measurement. In gain functions 0 dB is unity gain (1).
2. 3 dB - The amount of SPL gained by doubling the power to a speaker. Also the amount gained by doubling the number of speakers.
3. +/- 3 dB - Plus or minus 3 dB as used to express a measurement of frequency response indicates that the response will be no more than +3 dB and no less than -3 dB below a given reference. It is actually a 6 dB window. The Response of 60 Hz to 14 kHz +/-3 dB means that within the bandwidth of sixty cycles per second to fourteen thousand cycles per second, no frequency is +3 dB more nor -3 dB less than a specified reference frequency.
4. 3 dB DOWN (-3 dB) - The point at which a measured power level is 3 dB below the specified level. In an electronic crossover, the point (frequency) at which the high pass signal is -3 dB down in response or power level is considered the crossover point (frequency).
5. -6 dB - The amount of loss in SPL as you double the distance away from a sound source.
6. dBm - A decibel scale referenced to 0 dBm = 1 milliwatt of power into 600 Ohms or .775 volts RMS across 600 ohms.
7. dBu - Primarily a British term for gain referenced to 0 dBu = .775 volts RMS.
8. dBV - A decibel scale referenced to 1 volt RMS; 0 dBV = 1 volt.
9. dBW - A term for power gain referenced to 0 dBW = 1 Watt.

REFERENCE VOLUME
The volume which yields a reading of 0 VU on a standard volume indicator.

REFLECTION
The bouncing back or return of sound waves from walls or other obstacles which they strike.

REFRACTION
A change in direction or bending of the propagation of a sound wave when it passes from one medium to another in which the velocity of sound is different.

REFRESH RATES
An ergonomic issue that is directly related to long-term ease of use. A higher refresh rate translates to a more "flicker" free display on a CRT. Bandwidth and horizontal and vertical scanning rates depict a projector's ability to provide a higher resolution and refresh rate.

REGENERATION
In audio, regeneration is another word for feedback; when something regenerates it continues or sustains itself as an oscillation. When an electronics engineer designs an oscillator, he takes the output of a gain stage and feeds it back into the input through a tank circuit (an RC, resistor and capacitor or an LC, inductor and capacitor combination); when the circuit is turned on it begins to regenerate or oscillate at a specific frequency determined by the value of the RC or LC combination. When the output of a loudspeaker in a sound reinforcement system is able to get back into a microphone or sound system input, at some level and resonant frequency, the system is going to go into regeneration or feedback oscillation (squeal).

RESISTANCE
Opposition to the flow of electrical current. Measured in ohms.

RESISTANCE DIMMER
A now obsolete method of dimming which decreases the current available to the load by introducing a variable resistance between supply and load. The excess current is converted into heat. Based around a rheostat.

RESISTOR
An electronic component designed to have a definite amount of resistance; used in circuits to limit current flow or to provide a voltage drop.

RESOLUTION

1. The point during a drama when the plotline reaches a conclusion, and conflict is resolved.
2. The quality of a sound sample is measured by the sample rate (e.g. 44.1kHz is CD quality sample rate) and the resolution (either 8 bit or 16 bit normally).
3. A measure of the quality of a video display. The density of lines and dots per line, which make up a visual image. Usually, the higher the numbers, the sharper and more detailed the picture will be. In terms of DTV, maximum resolution refers to the number of horizontal scanning lines multiplied by the total number of pixels per line, called pixel density.

RESONANCE
A tendency of mechanical parts, loudspeaker cone, enclosure panels or electrical circuits to vibrate at or emphasize one particular frequency, every time that frequency, or one near it, occurs.

RESPONSE
The range of frequencies to which an amplifier or speaker will respond, and the relative amplitude or intensity with which these frequencies are reproduced.

RESTORE
A cue to resume or return to any previous state, setting or function. (e.g. "at the end of the dance number we restore to a warm general cover").

RETROFIT
A modification that can be made to an existing piece of equipment after purchase to bring it up to date.

RETURN
An input, typically found on a mixing console, used to patch a signal returning to a particular BUS after having been further processed, such as an echo or effects return.

REVERB (REVERBERATION, ACOUSTICAL)
The prolongation of sound at a given point after direct sound from the source has ceased, due to such causes as reflection from physical boundaries. (Electro-mechanical) An electro-mechanical device usually employing springs which randomly reflect as great amount of sound as possible, therefore simulating natural reverberation. (Digital Reverb) An electronic reverberation effects processor that uses digital electronics to introduce the multiple delay paths.

REVERBERATION
The sustaining of acoustical energy in a room after the reception of the direct field (the sound coming directly from the source) ceases in producing sound. Reverberation is caused by the reflections and scattering of sound energy from the boundary surfaces of the room.

REVERSE IMAGE
Reverse image is a feature found on most projectors which flips the image horizontally. When used in a normal forward projection environment, text, graphics, etc, are backwards. Reverse image is used for rear projection.

RFU
Remote Focus Unit. Name used by ETC for a remote control for the lighting desk. Same as RIGGERS CONTROL.

RGB
Red, Green, Blue; the standard type of monitor used with computers. Examples of usage: RGB input or output often referred to as computer input or output.

RIG

1. The construction or arrangement of lighting equipment for a particular production.(noun)
2. Installing lighting, sound equipment and scenery etc for a particular show.(verb)

RIG CHECK
(also known as LAMP CHECK) The process of checking all lanterns in the rig are working correctly prior to each performance. Should be done daily in sufficient time to change a lamp if necessary.
Most venues do not have the electrical capacity to run all lanterns at full for a rig check, so a level of 25% is normally used.
N.B. this is NOT the same as preheating - a rig check happens before each performance and involves the whole rig, and preheating involves individual lanterns/dimmers and happens before a cue in which that lantern appears.

RIGGERS CONTROL
A remote control for a lighting desk which enables dimmer channels to be called up from the stage when rigging or focusing. Usually battery powered, sometimes with infra-red (cordless) control. A Designers Control allows whole memories to be called up and/or played back, as well as individual dimmers.

RING MODE
A tone or frequency sounded in a room with a live sound reinforcement system prior to the system breaking into feedback. A ring mode lies just below the threshold of feedback.

RMS (ROOT MEAN SQUARE VALUE)
The square root of the time average of the square of a quantity; for a periodic quantity the average is taken over one complete cycle. RMS voltage is .707 times the peak voltage of a sine wave.

ROLL OFF
A signal is rolled off when it is attenuated or reduced in level above (high pass roll off) or below (low pass roll off) a certain frequency. The amount of roll off is rated at so many decibels per octave. A signal that is rolled off below 100 Hz at a rate of 18 dB/octave would be reduced in level or attenuated -18 dB at 50 Hz, -36 dB at 25 Hz, etc.

RT60 (REVERBERATION TIME)
The time required for sound to drop to - 60 dB in level once the source of sound has been stopped. The -60 dB is below that of the measured level while the sound system was on and after any initial transients or fluctuations settled.

RUNNING PLOT
A plot giving details of the changes between cues, as distinct from a state plot which gives the whole state of the system at any time. For example, a lighting plot on a manual board is normally a running plot. It is difficult to start a running plot half way through; often the operator has to go back to the beginning and work through until the required point is reached. However, it contains the minimum information necessary to perform the cues, and is therefore more efficient on a manual lighting desk or complex sound setup.

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S

S-VIDEO
A video transmission standard that uses a 4 pin mini-DIN connector to send video information on two signal wires called luminance(brightness, Y) and chrominance(colour, C). S-Video is also referred to as Y/C. A composite signal, typically found coming out of an RCA jack on the back of most VCRs has the Y and C information combined into one signal. The advantage of having luminance and chrominance separated is that a comb filter is not needed inside the video projector to separate the composite signal into the luminance and chrominance signals. A comb-filter can reduce the sharpness of your video image.

SAFETY CHAIN
Chain or wire fixed around lantern and lighting bar or boom to prevent danger in the event of failure of the primary support (eg Hook Clamp). A requirement of most licensing authorities. In the UK, current legislation

SATURATION RIG
An arrangement of lanterns in which to maximum number of spotlights is placed in every possible position.

SCANNER
General name for a moving mirror lantern, especially those used in discos, rather than the more flexible units used in theatre.

SCENE

1. A pre-programmed lighting state which can be faded in and out during a one night show when there's no time for plotting.
2. A subdivision of a play.

SCREEN (PROJECTION)
Many types of projection screen are available. Some are multi-purpose, some only for front projection, some only for back projection. If a screen is not self-supporting, it often has eyelets around the outside edge which are used to "lace" the screen onto a larger frame. See also PROJECTION.

SCRIM

1. A coarse gauze
2. A fine metal mesh used to reduce the intensity of light from TV lanterns without affecting colour temperature.

SCROLLER
See COLOUR CHANGER.

SECAM
A French and international broadcast standard for video and broadcasting. Higher resolution than NTSC.

SECONDARY LIGHTING
Separately powered lighting system for use throughout the building in the event of failure of the primary system. Usually battery powered. Maintained lighting is on all the time, regardless of changes in the stage lighting, and is battery backed-up.
Non-maintained systems only light in the event of power failure or an alarm condition.
Secondary lighting systems should be regularly checked by an electrician to ensure they operate correctly.

SEMIOTICS
The study of signs - many conventions in lighting design rely on signs (blue must be night-time, red is evil etc.)

SEND
An output used to patch a signal from a channel or bus of a mixer to an external signal processor such as an echo or digital delay.

SENSITIVITY
The minimum input signal required to produce a specified level of output. In an amplifier, the input sensitivity is the amount of voltage at the input necessary to drive the amplifier to its rated power output. Loudspeaker sensitivity is the power level necessary to produce a stated SPL at a given distance from the loudspeaker, usually rated at 1 watt 1 meter.

SERIES
An arrangement of circuit components, end-to-end, to form a single path for current.

SERIES-PARALLEL
A circuit in which some of the components or elements are connected in parallel, and one or more of these parallel combinations are in series with other components of the circuit.

SERIES SPLITTER
An adaptor consisting of a plug and two sockets wired in series. Enables two identical 110 Volt loads to be safely run from a 240 Volt supply (UK).

SHELVING
A type of equalization circuit that has a shelf-like characteristic at the upper or lower ends of the spectrum. A shelving EQ at 15 kHz, in the boost position, would increase the high frequencies up to 15 kHz where it would shelf.

SHIFTER
The common Australian term for AJ's, c-wrenches, spanners etc.

SHIN BUSTER
The lowest lantern on a lighting boom. Named because of the proximity of sharp parts of the lantern to the flesh of the lower leg.

SHORT NOSE
Normally refers to a Short-Nose Parcan - a lighting instrument that uses a normal size PAR lamp, but has been shortened to either make it less obtrusive, or to get a wider beam angle.

SHORT THROW LENS
A lens designed to project the largest possible image from short distance. Most front room projectors use a short throw lens. They are often required for rear projection, where the depth behind the screen is limited. A typical short throw lens might produce a diagonal image size of 10 ft, from a distance of 7 to 10 ft.

SHUTTER
Part of a profile lantern. Metal blade which can be used to shape the edge of the beam. Shutters (normally four) are located in the gate at the centre of the lantern. Similar in effect to barndoors on a Fresnel or PC lantern, but a lot more flexible.

SIGHTS
A pair of metal rings attached to the side or top of a followspot which enables the operator to accurately line up the beam (by looking down the length of the followspot through the rings) before turning it on. See GHOSTING.

SIGNAL-TO-NOISE RATIO (S/N)
The ratio of the amplitude or level of a desired signal at any point to the amplitude or level of noise at the same point.

SILHOUETTE
To light the cyclorama or a piece of upstage set in such a way that the actors are cast into shadow. Can be a very dramatic effect.

SILK
A special type of diffusion frost filter which stretches the light in one direction. Especially useful for lighting large cycloramas with a limited number of lanterns, or for lighting an elongated object (eg a staircase) with one lantern.

SINE WAVE
A wave whose amplitude varies as the mathematical sine of a linear function of time, also known as the sinusoidal wave.

SIX LAMP BAR
An internally-wired lighting bar, designed for touring, with six socket outlets terminated in a multi-way connector (e.g. Socapex or Weiland). Often pre-rigged with lanterns (eg Parcans). Stored in Meatracks. A bar pre-rigged with Parcans is sometimes known as a PAR BAR.

SLAPBACK ECHO
A echo caused from reflections off the rear wall in many auditoriums. Such echo can reduce the intelligibility of a sound reinforcement system, as well as distract speakers and performers on stage. Severe slapback echo is very distracting for a musician, as it can cause confusion that makes following that beat in time extremely difficult. A professional high quality stage monitor system can mask some of the slapback echo to a certain extent by providing more direct sound from the monitor speaker in the first 25 milliseconds which the performer perceives through auditory fusion as more direct field, thus increasing the ratio of the level of the direct field to that of the slapback echo.

SLEW LIMITING
The failure of the amplifier's output to move as fast, voltage-wise, as the input would have it move.

SLEW RATE
Refers to the ability of an amplifier's output to accurately reflect the input waveforms' rise time transients. An amplifier is said to have a slew rate of so many volts per microsecond. A slew rate of 20 volts per microsecond (20 V/U sec.) means that the amplifier is capable of swinging 20 volts positive or negative in the period of one microsecond.

SLIDE
See PROJECTION.

SMOKE
See SMOKE MACHINE

SMOKE DETECTORS
Many theatre buildings have complex fire alarm systems installed. Some theatre spaces have smoke detectors in them, which trigger a fire alarm when the space fills with smoke. The use of SMOKE MACHINES in these spaces can (and does) result in expensive call-outs of the fire department and evacuated auditoria.
There are special heat-sensitive detectors called RATE OF RISE detectors which trigger a fire alarm when the temperature rises faster than it should normally. Properly calibrated (and regularly tested) these can be as effective than the smoke detectors (which work by "seeing" smoke particles in the air). If it's not possible to get Rate of Rise detectors installed in your theatre space instead of smoke detectors, you may be able (subject to local building regulations and local fire department advice) to isolate the smoke detectors for the duration of the performance when you use smoke effects. Properly designed alarm systems incorporate timed isolation, so that smoke detectors are only off for a specific period, and automatically come on after that period.

SMOKE MACHINE
A Smoke Machine or Fogger is an electrically powered unit which produces clouds of white non-toxic fog (available in different flavours/smells) by the vaporisation of mineral oil. It is specially designed for theatre & film use. A Haze machine, Hazer or Diffusion Fogger is used to produce an atmospheric haze, rather than clouds of smoke, and is used by many lighting designers to reveal airborne light beams. The first smoke machines came onto the market in the late 1970's.
It's essential to know whether your venue uses SMOKE DETECTORS on the fire alarm system. See that entry for more information.

SNAKE
A multiconductor shielded input cable which allows a single run between the microphones and the audio mixing console. Snakes often provide "return" wire pairs to permit the mixer outputs to be sent to amplifiers located on stage.

SNAP
A lighting or sound cue with no fade time - the cue happens instantly.

SNOOT
See TOP HAT.

SOCAPEX
A multipin connector which can carry a series of lighting or sound circuits. Very robust and designed for touring. Available in 19 pin (6 circuits) and 37 pin (12 circuits) configurations. Sometimes shortened to SOCA.

SOFT LIGHT
Asymmetric flood light used as a fill light in TV studios to eliminate shadows and balance the key light.

SOFT PATCH
See PATCHING.

SOLO

1. On a sound desk, the solo button on each input channel silences all other inputs so that channel alone can be heard. Dangerous to use during a show, but can be useful for fault-finding or testing equipment
2. On a lighting desk, SOLO mode kills all other channels except the single dimmer you're working with. Again, can be useful for identifying a channel in a large rig, but can be dangerous during a show. Some desks allow you to assign flash buttons to SOLO mode which will turn off all channels except those loaded into that flash button or submaster.

SOUND
A pressure wave motion propagated in an elastic medium (air) producing an auditory sensation in the ear by the change of pressure at the ear. Sound waves are produced by a vibrating body in contact with air.

SOUND LEVEL METER
The instrument used to measure noise and sound pressure levels (SPL), calibrated in decibels.

SOUND FIELDS

1. Direct Field - the sound that emanates directly from a sound source or loudspeaker.
2. Indirect Field - that sound perceived from behind a speaker system, i.e. when no direct field is present.
3. Near Direct Field - the sound field that is generated close to the source or loudspeaker. Generally considered that sound field that is within a distance from the source of less than two wavelengths.
4. Far Direct Field - the sound field that is perceived at a distance from the source loudspeaker that is greater than 2 wave lengths.
5. Free Field - that portion of the direct field of a sound source or loudspeaker that is reflection free or not yet affected by boundaries such as walls or ceiling (such as may be encountered with an outdoor sound system).
6. Reverberant Field - that sound field beyond critical distance where most of the energy arriving at the listener is in the form of reflected energy off the room's boundaries.

SOUND TO LIGHT
A facility which can link the effects panel on a lighting board to an audio input which detects treble, mid and bass beats, and can flash lights or trigger effects in time to those beats. First used when electronics allowed it cheaply in the late 1960's.

SPARKY
See LAMP.

SPECIAL
A lantern within the lighting rig which is required for a specific moment or effect within the performance, and is not part of the general cover lighting. See GENERAL COVER.

SPECTRUM
Refers to a particular band of frequencies. The normal acoustic sound spectrum is the range of human auditory perception (20 Hz to 20,000 Hz). There is also a Subsonic spectrum (considered to be below about 40 Hz) and an Ultrasonic audio spectrum (above 20,000 Hz).

SPECTRUM ANALYZER
See Real Time Analyzer (RTA)

SPIDER
Adaptor to connect many lanterns to one multicore cable. Consists of multipin connector (typically Socapex or Lectriflex), short length of cable, then a number of sockets related to the number of circuits in the cable.

SPIGOT
An adaptor screwed onto the hanging bolt of an instrument to enable it to be used on an upright stand.

SPILL
Unwanted light onstage.

SPILL RINGS
Concentric rings attached to the front of a beamlight (eg Strand Pageant) to contain spill.

SPL (SOUND PRESSURE LEVEL)
The level or intensity at a point in a sound field (loudness). The deviation above and below normal atmospheric pressure. The unit of measurement of Sound Pressure... the microbar. One microbar is equal to the sound pressure of 1 dyne per square centimeter, which is a sound level of 74 dB above the threshold of hearing (0.0002 microbar.) It is also equal to approximately one-millionth of normal atmospheric pressure. Sound pressure levels are stated in decibels as follows: Where P is the RMS sound pressure in microbars, and the reference is the threshold of hearing of 0.0002 microbars (50% of young men, 1 to 4 kHz).

SPLITTER

1. A box which has one signal input and has two or more individual outputs available for that signal. Used to connect one signal source to multiple other devices.
2. See also ADAPTOR.

SPOT SEAT
Chair for suspending followspot operator above a stage / auditorium. Normally rigged on a truss system. The operator gets to the seat up a wire rope ladder, and is strapped into the seat. He or she will normally wear a harness when getting to the chair for extra safety. The seat itself is an adapted car "bucket" seat.

SPOTLIGHT
General term for any lantern with a lens system. See FRESNEL, PC, PROFILE.

SRGB
A new industry standard developed to ensure standard, uniform colour reproduction regardless of the type of display used. sRGB colour profile technology uses colour coordinates common to all display technologies. As a result, it eliminates the hue variations that occur between different display systems. 16:9.

STAGE BOX
A connection box at the end of a lighting or sound multicore cable.

STAGE ELECTRICIAN
Member of the electrics staff whose responsibility it is to set or clear electrics equipment during scene changes. May also carry out colour changes on booms etc.

STAGE LEFT / RIGHT
Left/ Right as seen from the Actor's point of view on stage. (ie Stage Left is the right side of the stage when looking from the auditorium.)
Stage Right = OP (Opposite Prompt) French: CotÚ Jardin, Netherlands: Toneel Links (translates to Stage Left!)
Stage Left = PS (Prompt Side) French: CotÚ Cour, Netherlands: Toneel Rechts (translates to Stage Right!).

STANDING WAVE

1. Standing waves occur in rooms because of the boundaries. A standing wave is a soundwave that once excited, stands there, i.e., the positive air pressure peaks (antinodes) and negative air pressure troughs (nodes) remain in the same position within the room's boundaries. Also known as a stationary wave.
2. See also MODES.

STATE
In lighting terms, a lighting "picture" ; each lighting cue results in a different state (or a modified state).

STEEL

1. Refers to a steel blue / pale blue lighting gel. (Lee 117) (e.g. "Use the Steel General Cover for the scene in the castle"). See also STRAW.
2. Many set construction now uses steel frames with timber cladding. Steel is stronger and lighter weight compared to timber of the same size.
3. Generic term used for a plain wire rope sling. Also used when referring to roof structural steel and individual steel beams or scaffold materials and so on.

STEP

1. A control on some lighting effects boards which enables the operator to "step" through a chase effect in time to music etc.
2. Each separate component of a lighting effect is called a step. A chase effect with four channels flashing on will have four steps.

STEREO
In a sound reproducing system, stereo refers to the use of two separate signal processing channels driving two separate power amplifiers, which in turn power two separate speaker systems. However, most times in sound reinforcement, a stereo mixer is employed to drive a mono (single channel) system in order to allow separate mixes of the program (such as speaking mics, instrurnents, vocal mics, etc).

STRAW
Refers to a pale yellow lighting gel. (e.g. Use the Straw General Cover for the garden scene). See also STEEL.

STRIPLIGHT

1. A thin linear filament lamp similar to an Architectural, but having contacts at the ends of the lamp. Available clear or opaque.
2. See also BATTEN.

STROBE
Device giving a fast series of very short intense light flashes which can have the effect of making action appear intermittent. Because strobe lighting can trigger an epileptic attack in sufferers, the use of a strobe must be communicated to the audience before the performance begins. Strobes should be synchronised so that they operate outside the dangerous frequency band 4 to 50 flashes per second. (i.e. a strobe should operate at less than 4 flashes per second, or more than 50 flashes per second). If the effect is momentary, this rule may be relaxed. Strobes must never be used in public areas where there are changes of level or steps. Always seek the advice of the licensing authority if you are in any doubt about the safety of strobe effects.

SUBMASTER

1. Fader on a lighting desk which can have a lighting state recorded onto it for additional control, or to use when manually mixing lighting states for music concerts or one-off events.Lighting desks normally have a series of submasters (12 or 24 are common) which can have states, cues or effects loaded onto them. Some desks can have submasters set to inhibit the main output (known as INHIBITIVE SUBMASTER. For example, the FOH lighting can be loaded onto a submaster which is then brought down as the house tabs are flown in between curtain calls to block light spill onto the tabs). Multiple cues can be recorded onto some submasters through the use of pages. A PILE-ON submaster can be used to add its contents to the existing lighting state. Any number of pile-on submasters can be used in combination to modify a state.
2. A separate audio mixing bus assigned to a group of mixing channels that enables the sound mixer to regulate the level of that group with one control called the Sub-Master.

SUB-MIX
A level control preceding the main (master) level control, that regulates the level of an individual sub-mix.

SUBTRACTIVE COLOUR MIXING
See COLOUR MIXING.

SVGA
SVGA is used to define a specific display resolution. Resolution is defined by the number of individual dots that a display

uses to create an image. These dots are called pixels. An SVGA display has 800 horizontal pixels and 600 vertical pixels giving a total display resolution of 480,000 individual pixels that are used to compose the image delivered by a projector.

SWITCHING JACK
An input or output jack that performs some switching function in addition to providing an input or output for a signal. These jacks disconnect the normal flow of signal and allow for additional patching capabilities such as inserting equalizers and feedback reducers. Most typically these are 1/4" phone jacks, but some equipment manufacturers are now using switching RCA type phono connectors.

SWL
Safe Working Load.

SXGA
SXGA is used to define a specific display resolution. Resolution is defined by the number of individual dots that a display uses to create an image. These dots are called pixels. An SXGA display has 1280 horizontal pixels and 1024 vertical pixels giving a total display resolution of 1,310,720 individual pixels that are used to compose the image delivered by a projector.

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T

T1
A Level 1 digital transmission system operating at 1.536 mbps. Also known as DS-1.

TAB DRESSING
Lighting focused onto the front tabs to before the show starts.

TAILS
Also known as BARE ENDS, TAILS refers to a cable or set of cables with a connector at only one end which is used for connecting a company's equipment directly to the mains supply in a venue. The connection should only be made by a qualified electrician with the power off!

TALLESCOPE
(Trade Name) A retractable alloy vertical ladder on an adjustable wheeled base. The platform at the top is just large enough to hold one person. Used for rigging lanterns, focusing etc. Collapsible enough to fit through a standard doorway. Outriggers are used to stabilise the tower from falling sideways. Two people are used to move and steady the tallescope. Sometimes known as a "TALLEY".

TECHNICAL REHEARSAL
Usually the first time the show is rehearsed in the venue, with lighting, scenery and sound. Costumes are sometimes used where they may cause technical problems (eg Quick changes). Often a very lengthy process. Often abbreviated to the Tech. A DRY TECH is without actors to rehearse the integration of lighting, scenic changes etc. It follows that a WET TECH is a full technical rehearsal with actors and all technical elements, although this term isn't used as often as DRY TECH.

TEMPLATE
See GOBO.

TFT
Thin Film Transistor.

THD (TOTAL HARMONIC DISTORTION)
When a single frequency of specified level is applied to the input of a system, the ratio of the voltage of the fundamental frequency to the voltage of all harmonics observed at the output of the system because of the nonlinearities of the system. THD is expressed in percent.

THROW
Distance between a light source (e.g. lantern or projector) and the actor or object being lit.

THYRISTOR
Also known as an SCR (Silicon Controlled Rectifier). An electronic switch which will pass current when triggered until the current passing through it falls to zero. See also TRIAC.

TILT

1. Up and down (vertical) movement of a lantern, camera or moving light. See also PAN.
2. Feature on pinball machines which detects excessive movement of the case. Only related to theatre in connection with the musical "Tommy".

TIMBRE
Timbre is a word that relates to the musical quality of sound. It is the relation of the fundamental frequency to the level and number of the associated harmonics. The human ear can perceive differences in timbre. Two different instruments, such as a saxophone and a flute playing the same note or fundamental at the same loudness, sound different to the listener due to the two instruments different number and level of related harmonics also produced at the same time as the fundamental. The two instruments are said to have a difference in timbre.

TIME
The facility on memory lighting boards for playing back timed fades at the touch of a button.

TOP HAT
Also known as HIGH HAT or SNOOT. Cylinder of metal inserted into colour runners on the front of a parcan or other lantern to limit spill light.

TRANSDUCER
Any device or element which converts an input signal into an output signal of a different form. A transducer changes energy from one form to another. A microphone is a transducer that changes acoustical energy (sound) into electrical energy (voltage). A loudspeaker is a transducer that changes electrical energy into mechanical energy, producing sound or acoustical energy.

TRANSFORMATION
An instant scene change, often effected by exploiting the varying transparency of gauze under different lighting conditions.

TRANSFORMER
An electrical component consisting of multiple turns of wire placed in a common magnetic field (medium) which will transfer electrical energy from one electrical circuit to the next. A transformer will only pass alternating currents (AC) and will not pass direct current (DC). By adjusting turn ratios, a step up or down condition of voltage can be achieved.

TRANSFORMER BALANCED
An input or output that is coupled by means of a transformer in a configuration that makes it balanced or capable of being operated so that the voltages of the two conductors at any transverse plane are equal in voltage and opposite in polarity with respect to ground. A transformer balanced input or output will offer common-mode rejection, which means any common-mode interference signal will not pass through the transformer because it will be cancelled out.

TRANSIENT
Rapidly changing peaks of short duration in the level of sound such as would be produced by a cymbal crash or a rim shot on a snare drum. A wave having a very short or no sustain time.

TRANSIENT DISTORTION
Transient distortion interferes with the ability of an amplifier to accurately follow abrupt changes in volume, such as the sudden burst of sound when an instrument is first played. Minimum transient distortion is vital to clean and crisp overall sound.

TRANSIENT RESPONSE
Ability of an amplifier or loudspeaker to accurately follow abrupt changes, such as the sudden burst of sound generated by an instrument. Good transient response is vital to "clear" or "crisp" overall sound.

TRI-AMP
Separating the audio spectrum into three bands, i.e., high frequencies, mid-band frequencies and low frequencies by means of an electronic crossover and using three separate power amplifiers to amplify the three outputs of the crossover (high pass, mid pass, low pass outputs) driving three separate components of a speaker system; resulting in increased headroom and dynamic range.

TRIAC
(Triode Alternating Current switch) Electronic Semiconductor device which is an integral part of modern dimmers. When a current is applied to a triac, it starts conducting, and continues until the current passing through it falls to zero. Whereas a thyristor can only conduct half of the AC wave, a triac (as long as it's triggered at the appropriate point) will conduct both halves of the wave. See further explanation below.

TRS

1. Tough Rubber Sheath. Jargon for any Rubber-sheathed mains cable.
2.  Tip Ring Sleeve. The three contacts on a stereo jack audio connector.

TRUNKING
Metal or plastic wall-mounted enclosure for cables. Box shaped in cross-section.

TRUNNION ARM
U-shaped bracket between the hook clamp and the main body of a lighting instrument, enabling it to be tilted to any angle.

TRUSS
A framework of alloy bars and triangular cross-bracing (usually of scaffolding diameter) providing a rigid structure, particularly useful for hanging lights where no permanent facility is available. Very often box-shaped in cross section, so known as BOX TRUSS. This type of truss is useful for touring as lanterns / speakers etc can be hung inside the truss which protects them when loading and takes up less space in the truck.

TSS
Telephony System Specification.

TUNGSTEN HALOGEN
A standard tungsten filament lamp loses its brightness in its' lifetime. Tungsten Halogen lamps use a Quartz envelope ("bulb") filled with halogen gas to give an almost constant colour temperature. See HALOGEN CYCLE.

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U

ULTRA VIOLET (UV)
Short wavelength source of light at the end of the visible light electromagnetic spectrum which causes specially treated materials to fluoresce on an otherwise blackened stage. Used for special effect and for lighting onstage technical areas (eg Fly Floors). Ultraviolet sources designed for stage use are known as Black Light sources and have all harmful radiations filtered out.

UNBALANCED CABLE OR LINE
A single conductor cable with a surrounding shield that connects to ground. Such a system is called unbalanced because it cannot be balanced or offer common mode rejection.

UNBALANCED INPUT
An input in which one of the two terminals is at ground potential or connected to the chassis ground.

UNIDIRECTIONAL
This term commonly refers to microphones which pickup sounds predominately from one direction as opposed to all directions (omnidirectional). Unidirectional microphone types include cardioid, super-cardioid and hyper-cardioid. Unidirectional microphones are most often used in sound reinforcement applications because they are generally less prone to feedback than omnidirectional types.

UNIFORMITY
Comparison of the brightness of one area to an adjacent area. In general, the brightest part of the image will normally be in the centre area. When moving out to the edges, the intensity of image will vary in a non-linear function. This means that one corner of the screen will not be the same brightness as another corner of the screen. A typical projector specification may call for up to a 30% difference between the centre area and the corners.

UPLIGHT
Light from below the actors - from a light source on the stage floor. French: contre-plongÚe

UPSTAGE

1. The part of the stage furthest from the audience.
2. When an actor moves upstage of another and causes the victim to turn away from the audience he is "upstaging". Also, an actor drawing attention to himself away from the main action (by moving around, or over-reacting to onstage events) is upstaging.

UXGA
UXGA is used to define a specific display resolution. Resolution is defined by the number of individual dots that a display uses to create an image. These dots are called pixels. A UXGA display has 1600 horizontal pixels and 1200 vertical pixels giving a total display resolution of 1,920,000 individual pixels that are used to compose the image delivered by a projector.

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V

VELOCITY
In audio the velocity or speed of sound is approximately 1130 feet per second. The speed of sound changes slightly with changes in temperature, humidity, and altitude.

VERTICAL FREQUENCY
This indicates how many times per second the graphics adapter can draw all the lines on the screen. A higher vertical frequency or refresh rate will produce less flicker.

VESA
Video Electronics Standards Association. An association whose mission is to promote and develop timely, relevant, open display and display interface standards, ensuring interoperability and encouraging innovation and market growth.

VGA
VGA is used to define a specific display resolution. Resolution is defined by the number of individual dots that a display uses to create an image. These dots are called pixels. A VGA display has 640 horizontal pixels and 480 vertical pixels giving a total display resolution of 307,200 individual pixels that are used to compose the image delivered by a projector.

VISUAL CUE
A cue taken by a technician from the action on stage rather than being cued by the stage manager. Often abbreviated to "Vis".

VOICE COMPRESSION
The conversion of an analog voice signal into a digital signal using minimum bandwidth (16 kbps or less).

VOLTAGE

1. The pressure at which electric current is available. The New Zealand standard voltage is 240 Volts.
2. The scientific name for Voltage is Electromotive Force.

VOLT (VOLTAGE)
Potential difference or electromotive force (EMF).

VOLUME
The intensity or loudness of sound.

VU (VOLUME UNIT)
A unit for expressing the audio frequency power level of a complex electronic waveform such as that corresponding to speech or music. 0 VU is referenced to 1 milliwatt of power.

VU METER
A meter than indicates the audio frequency power level or volume units of a complex electronic waveform.

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W

WASH

1. A lighting cover over the whole stage (e.g. "We'll use the red wash for the hell scene")
2. A lantern which produces a large spread of softly focussed light (e.g. the VL4 and Mac 600 are wash luminaire).

WATER CRACKER
Device which produces a thin haze in the air by "cracking" water droplets.

WATT
A unit of measure of power. The electrical wattage of an amplifier describes the power it can develop to drive a speaker. The greater the voltage capability, the higher the wattage. Amplifier wattage requirements are greatly dependent upon the speakers that will be used, the size of the listening room and average loudness that will be played through the speakers. Wattage is expressed as W =V2/R

WATTAGE
Unit of electrical power derived from the current (or "quantity" of electricity) multipled by the voltage (or "pressure" at which the current is delivered). Stage lighting equipment is rated in Watts (or Kilowatts - 1kW being equal to 1000W). This refers to the amount of power required to light the lamp. A higher wattage lamp requires more power and gives a brighter light output.

WAVELENGTH

1. The distance from one point on a vibrating wave to the same point on the next wave. The lengths of the sound waves (wavelengths) we can hear range from one inch to 40 feet. High frequency sounds have short wavelengths (and are more directional), low frequency sounds have long wavelengths (and are less directional). 
2. In lighting terms, blue light is short wavelength, green is medium and red is long wavelength. Beyond visible light are the short wavelength Ultra Violet light and the long wavelength Infra Red light. Wavelengths of light are measured in Angstroms.

WIGGLY MIRRORS
Slang for so-called "Intelligent" lights with moving mirrors.

WINGS

1. The out of view areas to the sides of the acting area.
2. Scenery standing where the acting area joins these technical areas.

WOOFER
A speaker designed to reproduce bass or low frequencies.

WORKING LIGHTS

1. High wattage lights used in a venue when the stage / auditorium lighting is not on. Used for rehearsals, fit-up, strike and resetting.
2. Low wattage blue lights used to illuminate offstage obstacles and props tables etc. Known as Wing Workers.

WYSIWYG
Acronym of "What You See Is What You Get". Mainly used in the context of a software tool for lighting design and production administration. Capable of stunning 3D rendering of lighting states, and direct connection to a lighting control desk. Enables accurate pre-visualisation of lighting designs and greatly increases the understanding between director / producer and lighting designer in the early stages of a production.

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X

XENON

1. High output discharge lamp commonly used in Strobe lighting. Some followspots also use Xenon lamps. Xenon lamps have colour temperature of between 5600 - 6500°K.
2. Also see DISCHARGE LAMP.

XFADE
See CROSS FADE.

XGA
XGA is used to define a specific display resolution. Resolution is defined by the number of individual dots that a display uses to create an image. These dots are called pixels. An XGA display has 1020 horizontal pixels and 768 vertical pixels giving a total display resolution of 783,360 individual pixels that are used to compose the image delivered by a projector.

XLR
Multipin metallic audio connector. (3 pin for normal sound use, 5 pin for DMX, Colour Scrollers etc). Sometimes called Cannons after the original manufacturer. See balanced line. The UK standard for wiring the 3 pin connector is as follows : Pin 1 (Screen), Pin 2 (+ve / "hot"), Pin 3 (-ve, "cold"). (Xternal, Live, Return).

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Y

YO-YO
A device used for remotely moving a gobo in one plane whilst it is in the lantern. Gives the effect of a lateral movement (door opens, train passes etc.)

YOKE
The TRUNNION ARM of a lantern.

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 Z

ZOOM LENS
A lens with a variable focal length. You can adjust the size of the image on a screen by adjusting the zoom lens, rather than making the projector closer to or further from the screen.

ZOOM LENS RATIO
The ratio between the smallest and largest image that a lens can project from a fixed distance. For example, a 1.4:1 zoom lens ratio means that a 10 foot image without zoom would become a 14 foot image with full zoom. Conversely, a 10 foot diagonal image at 15 feet with no zoom would still be a 10 foot image at 21 feet at maximum zoom (15 x 1.4 = 21 feet). A zoom lens is "not as bright" as a fixed lens, and the higher the ratio, the less light output.

ZOOM PROFILE
A type of profile lantern with two lenses enabling the adjustment of both size of the beam and whether it's hard or soft focus. Essential when working with gobos.

ZOOMSPOT
See Zoom Profile

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