Glossary of Terms - C

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.


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

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.

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

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


    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.
    See also WYSIWYG.

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.


    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!").
    Any headphones
    Short for PARCANs.

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.


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

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

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.


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

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).

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.


    Opposite of Build; a smooth diminishment of light or sound level.

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).

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

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.

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

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

(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.

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.


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

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

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

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

Invaluable knot that every technician should know.

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

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.

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.

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.


    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.

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.


    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.
    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.
    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.
    Magazine: Manual semaphore-type device used on the front of a followspot.

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

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.

(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.

Combining the effects of two or more lighting gels:

    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.
    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).


    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.


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


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.

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

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

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.

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.

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.

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.

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.

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

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

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.

The transmission of two or more simultaneous images.

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

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

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

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.

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

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

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).

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.

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.

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

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.

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.

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

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.

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

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.

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.

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.

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

(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.

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).

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.

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.

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

There is a standard sequence for giving verbal cues:

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

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).

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

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.

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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