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Glossary of Terminology

    Here you will find a glossary of some of the most commonly used terms in laser light shows. You can click on the quick links below to go to a subject area.

Quick links to subject areas:

• Lasers and Optical Devices
• Laser projection equipment
• Projector Sub-systems
• Projector Components
• Laser shows and special effects
• Computer graphics for laser shows
• Scientific and Technical terms
• Safety

Introduction
    This Glossary includes definitions of the most common terms used in the laser display industry (laser light shows). This glossary is based on the ILDA (International Laser Display Association) official glossary of terminology. It is important that laserists all speak the same language so all of the terms defined by ILDA are included herein.
    In addition, other important terms are included here as they also need explanation. Entries in Italics indicate a term that is defined by Laser F/X and is not necessarily an official ILDA term or definition.
    The structure and layout of this glossary has been adapted from the ILDA glossary. Laser F/X gratefully acknowledges the assistance of Patrick Murphy, Chair, ILDA Terminology Standardisation Committee, and the International Laser Display Association who provided the documentation on which most of this glossary is based.

ILDA Glossary Preamble
    The ILDA Laser Glossary is for two audiences. The first is ILDA members' clients, who may be new to laser display technology. The list describes common terms used in sales literature, show preparation, and contractual agreements.
    The second audience is ILDA members themselves. The Glossary standardises concepts which formerly had different names ("cell" and "frame") and sets terms for new products ("PCAOM").

Copyright/Credit
    The information in this glossary (with the exception of italicised entries) is from the International Laser Display Association's Laser Glossary. It was compiled by ILDA's Terminology Standardisation Committee, Patrick Murphy, chairperson. The definitions are copyright ©1993 by the International Laser Display Association and are used with permission.

Lasers and Optical Devices

Air-cooled laser: A laser using fans to force air over the laser tube and through the power supply. Air-cooled lasers have the benefit of needing no water supply, although the fan noise can sometimes be a disadvantage. Usually only small and medium power lasers are air-cooled. Very small lasers, typically helium-neon, need no fans. Although technically they are "air cooled" via convection, the term is usually applied only to fan-forced cooling.

Argon laser: A laser filled with argon gas. It gives off green and blue light. The strongest lines are at 514 nm (green) and 488 nm (blue). Argons range from small 15 milliwatt 110 volt air-cooled models to large 50 watt 440 volt water-cooled systems. Argon lasers are the most common type of light show lasers since they provide unable brightness at a reasonable cost.

Chiller: A unit that recirculates a small volume of refrigerated water through a laser for closed-loop cooling. It includes a compressor and thus can perform more cooling than a recirculator/heat exchanger. [See Also: Recirculator]

Dichroic filters and mirrors: A piece of glass with an optical thin-film coating that transmits certain colours (wavelengths), and reflects the remaining colours. Dichroic filters are used to combine or eliminate specific colours as needed in a laser projector. Dichroic mirrors are used to maximise the amount of light reflected from a laser of a particular wavelength. Dichroics should be handled with care to prevent damage to the coating. [See Also: Colour box]

Diffraction grating; A material, usually flexible plastic, containing microscopic lines (gratings) that break up light passing through it. They are used with lasers to create special beam or graphic effects. Shining a laser image through diffraction gratings produce multiple copies of the original image. The most common grating produces a grid image. Other types of diffraction gratings produce a linear, circular, or abstract patterns.
Most gratings are used in the laser projector, but some pop music light shows distribute inexpensive cardboard glasses with grid diffraction gratings to the audience.

Diode laser: A semiconductor similar to an LED (light-emitting diode) but which produces coherent light. Diode lasers are small and efficient, which has led to their use in compact disc players and pen-type laser pointers.
Currently, diode lasers are too dim or expensive for most light show uses. This is likely to change over the next few years. [See Also: Solid-state laser]

Fibre optic cable: Flexible glass or plastic strands made into a cable, used to carry light from one place to another. There are two main types; Step index and Graded index fibre. Within these two main types there are two further subgroups:

• Transmission fibres carry the beam with as little loss as possible. They are used to transmit the laser's light to remotely located projection devices.

• Display fibres [also knows as sideglow fibres] have no cable jacket, so some light scatters out the side of the strands. The strands themselves look like microscopic neon tubing and become a special effect, such as a laser-lit "whip" or a glowing "rope" wrapped around objects.

Front-surface mirror: A piece of glass with an exposed broadband reflective optical coating. In a conventional back-surface mirror, the reflective coating is protected with a heavy paint-like substance; reflections are seen through the glass. In a front-surface mirror, light does not have to travel through the glass to reach the coating.
Mirrors used in laser work are almost always front-surface, as they can reflect up to 99.8% of the incident light and they have no secondary reflections. Front-surface mirrors require greater care in handling to prevent damage to the coating. [See Also: Mirror]

Head::

1. A laser tube enclosed in a case (laser head): the laser head (as opposed to the laser power supply).
2. A set of X-Y scanners which can produce laser graphics (projector head). A projector may have a number of heads. For example, a four-head projector can produce four different sets of graphics simultaneously.

Heat exchanger: A cooling unit that recirculates a small volume of water in a closed-loop; sometimes used with water-cooled lasers. Hot water from the laser is cooled by water-to-water or water-to-air heat transfer. There is no active refrigeration, as in a chiller. [See Also: Recirculator]

Helium-neon laser (HeNe): A laser filled with a helium/neon gas mixture. Most produce red-orange light having a wavelength of 633 nanometers. HeNe's are low powered, in the 0.5 to 50 milliwatt range. Most run on 110 volts, come with built-in power supplies, and need no special cooling.
Some HeNe's are made to produce orange (612 nm), yellow (594 nm) and green (543 nm) light. These have less power than a red HeNe of the same tube size and input power.
Helium-neon lasers are the most common type of gas laser. Because of their relatively low power and cost, they are used only for small-scale light shows and studio monitors.

Krypton laser: A laser filled primarily with krypton gas. When used with "all-line" or "white" optics, it produces red, yellow, green and blue light. A "red-only" krypton laser uses with specially tailored optics to output a very strong red line at 647 nm.
Kryptons are similar to argons (the same tube design can be used for both). However, krypton gas produce less light (output power) than an equivalent volume of argon gas. Krypton lasers are primarily used when a powerful red light is needed.

Laser: A device which produces a coherent beam of light. The beam remains parallel for long distances and contains one or more extremely pure colours.
Light show lasers are usually gas-filled tubes using high voltage current to ionise the gas (cause the gas to glow). Mirrors at each end of the tube help amplify a process called "stimulated emission". Most of the stimulated emission light travels between the two mirrors; between 1% and 4% comes out of one of the mirrors to create the beam of light that we see.
The gas used determines the colour (or colours) of the beam. Gas lasers remain the overwhelming choice for display applications. The four main types used are a helium-neon mixture, argon, krypton, and an argon-krypton "mixed gas" mixture.
"Laser" is an acronym derived from "Light Amplification by Stimulated Emission of Radiation". [See Also: Head; Power supply; Helium-neon laser; Argon laser; Krypton laser; Mixed gas laser; Diode laser; Solid-state laser]

Mirror:

1. Large mirrors (bounce mirrors) are used to reflect the beam coming from a projector, forming a beam sculpture or matrix in the air. Usually the mirrors are surrounded by black metal, so light which might miss the mirror is harmlessly blocked or dissipated.
2. Small mirrors mounted on actuators can be activated to reflect the beam out over the audience or onto bounce mirrors to form a beam sculpture or matrix.
3. Small mirrors on precision kinematic mounts are used within projectors as steering mirrors to reflect the beam from place to place inside a projector.
4. Tiny mirrors are used on the shafts of laser scanners, which move the beam at high speeds to form abstracts or graphics.

All of the mirrors above are usually front-surface mirrors.

Mixed gas laser: Many gas lasers contain mixtures of various gases, such as helium and neon. In the laser show industry, however, "mixed gas" usually refers to an argon-krypton mixture used to get a white-light beam containing red, green, and blue lines.
Yellow lines are additionally available for applications such as four-head projectors with one head each for red, yellow, green and blue. [See Also: White-light laser]

Passbank: The transistors (or other devices) inside the exciter (power supply) that are used to regulate the current to the laser head. They are usually mounted on a heatsink that may be air or water cooled. The heatsink is usually called a "cold plate" if it is water cooled.

Power supply: A device converting readily available power, usually standard alternating current, into voltages necessary to energise laser tubes. A power supply is also known as an exciter. It may also include other functions such as monitoring the laser's light output or current draw. A power supply (exciter) is usually closely matched to a particular type of laser tube.
Power supplies for high-power lasers (0.5 watt and above) often require 208 or 440 volts three-phase, and are cooled with flowing water. The exciter (power supply) is one of the two main parts of a laser; the other is the laser head.

Recirculator: A pump used to recycle a small volume of water, found in either a chillier or heat exchanger. The term "recirculator" is sometimes erroneously used to mean either of the more specific devices.

Solid-state laser: A laser where the lasing medium is a solid material such as a ruby rod. These can be optically pumped by a flashlamp or diodes. Solid state lasers also include diode lasers as they use electrically pumped solids to produce light.
Currently, solid-state lasers are too expensive for most light show uses. This may change over the next few years. The most promising solid-state laser uses a material called Nd:YAG, which produces infrared light. This can be `frequency doubled' (up-shifted) to produce up to 60 watts of green light at 532 nm.

Umbilical: A group of hoses and/or cables that connects the laser head to the exciter. The umbilical can be a simple power cable, a number of cables and/or hoses held together or a large diameter flexible plastic pipe (like a vacuum cleaner hose) that contains all of the cables/hoses.

Water-cooled laser: A laser using water to cool the laser tube. Often the laser power supply's electronics are also water-cooled. A gas laser (the type used in most laser display applications) is relatively inefficient. For example, an argon-filled laser producing 10 watts of light requires around 10,000 watts of electricity. In such a laser, water is used to carry off the 9,990 watts of excess energy as heat.
Flow rates of two gallons (9 litres) per minute (or more) are typical. In many areas, the water can simply flow through the laser and down a drain. In water conservation areas, a chiller or heat exchanger is used to recycle the cooling water.
Water supply considerations are always important to laser show producers. Even a brief interruption can cause problems. At best, flow sensors shut down the laser when the supply is inadequate. At worst, an expensive laser tube can overheat and be irreparably damaged.

White-light beam: Broadly, a laser beam which contains a number of different wavelengths (colours) so the beam appears white. If the beam is passed through a prism or diffraction grating, it is separated into individual laser beams, each of a single specific wavelength.
More specifically, a white-light beam ideally contains twice as much red as green and blue light for correct colour balance (see appendix). It can be from a single white-light laser or from two or three lasers whose beams have been combined into a single beam. White-light beams are primarily used in RGB laser projectors.
See the definition of white-light laser for more information on what constitutes an "equal mixture" of light.

White-light laser: Many lasers can produce a number of wavelengths (colours) simultaneously. A white-light laser is designed to give a good balance of red, green and blue wavelengths. Usually the laser is intended for an RGB laser projector. (Some models also deliberately add yellow light for specialised 4-colour projectors.) Most white-light lasers use an argon/krypton gas mixture.
It is somewhat difficult to produce an equal balance of desired colours, and to keep this balance consistent during the lifetime of the laser tube. At present, there are no standards defining the exact wavelengths and colour proportions for a laser to be called "white-light".
In addition, the sought-after colour balance can be defined either as equal amounts on a photometer, or as visually equal amounts. Since the eye is much more sensitive to green, a visually equal or "photopically balanced" laser has roughly five times more power in red and blue than in green. Most white-light lasers today are not photopically balanced. [See Also: Mixed gas laser; White-light beam]

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Laser Projection Equipment

Projector Subsystems
    The following terms are suggested for the subsystems within a projector. These definitions are primarily for use within the context of the entire projector, to help distinguish one system from another.
    These terms have been carefully defined. This is because fine distinctions can become extremely important when specifying the equipment to be provided at a laser show, or when drawing up contracts for installations.

Optics plate: A metal plate drilled and tapped with holes, in a grid configuration for mounting the parts and sub-systems. The optics plate is usually the base on which the projector is built.

Galvo, galvanometer: Refers to the basic galvanometer itself, without an attached mirror mount or mirror.

Scanner: A galvo with an attached mirror. There are often three parts, the galvo, a mirror mount, and a mirror. See scanner for more information on general usage.

Scanner amplifier (scan amp): Usually refers to the scan amp board only. Does not include a power supply or chassis unless this is specifically stated (in sales literature, etc.).

Scanner head (scan head): A pair of scanners in an X-Y mount with mirrors. (Fully enclosed X-Y mounts are usually referred to as heads while an open mount is usually referred to as a `scan stage'. Does not include the scanner amps, although those may be mounted close to the scanner head. Usually does not include any blanking system.

Projector head: Scanner head, plus any special optical effects such as beam switching and lumia, in a finished chassis. Usually used when the laser beam is fed via fibre optic cable to a number of projector heads arranged on a stage or set. Does not include the scanner amps, although those may be mounted inside the projector head. Does not necessarily include a blanking device or system.

Blanking system: The blanking system cannot be assumed to be in either the scanner head nor in the projector head. The blanking system is usually set apart from the scanner head (scanners plus mount). It may also be separated from a fibre-fed projector head, since the beam is often blanked before being launched into the fibre. Therefore, the blanking system should be discussed as a separate subsystem; for example, when specifying number and location of heads.

Laser projector: Laser, scanner head, blanking system, special optical effects, and scanner amps, all in a single chassis. The inclusion of the laser, blanking and scanner-related electronics makes a laser projector more comprehensive than a projector head.

Laser system: A laser projector plus a signal source such as a tape playback unit, computer, or operator console.

Beam table: Optics plate plus special optical effects, usually beam positioning actuators, but also can include lumia and other effects. Does not include the laser. The beam table may also be a sub-system in a complex laser projector.

Beam projector: Beam table plus laser. Usually implies no graphics scanning capability. The beam projector may include a basic X-Y scanning system used to generate scanned beam effects.

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Projector Components

Actuator: A device that allows for the positioning of optical elements in a laser beam or for low-speed beam steering or switching applications. Some examples:

• Mirrors on an actuator arm direct beams to different devices in a projector or out over an audience.
• Dichroic filters on actuators are used in colour boxes.
• A beam block on an actuator is used to create a shutter.
• Actuators are typically a low-speed, heavy-duty versions of an open-loop scanner which allows for variable positions. Typically they are operated in unipolar (in beam; out of beam) mode. Two-position solenoid-type devices can also be used.

Arm: A thin metal piece that is attached to an actuator to hold a mirror (mirror arm) a dichroic filter (dichro arm) or a beamsplitter (splitter arm). Arms are available as a regular arm made of metal or a "low-mass" arm that has holes drilled through it to reduce the weight and provide a slight improvement in speed of movement. Arms that have no optics on them can be used for shuttering or dimming and are called flags.

AOM (acousto-optic modulator): A device used for dimming and blanking a laser beam. It can also be used for colour control. In an AOM, a laser beam is shone through an acousto-optic crystal. By applying an electric signal to the crystal, the beam's intensity (brightness) can be modulated.
For blanking and intensity control, a single AOM is used.
For colour control, three AOMs can be used on separate red, green and blue beams. Using additional optics, the beams are recombined to form a single beam whose colour depends on the AOMs' settings. (For easier colour control, more specialised PCAOMs are used. These require only one crystal to modulate a white-light beam, and they need no recombination optics.)
The primary advantage of an AOM is its speed. It can be modulated fast enough to control the intensity or colour of single points within a complex scanned image. However, it is more complex to interface with slower-reacting scanners, since timing adjustments must be made. [See Also: Blanking; Scanner blanking and colour PCAOM]

Barrel distortion: An image distortion caused by projecting onto a convex surface. For example, a grid projected on the outside of a dome has outwardly curved edges, where the centres curve out and the corners pull in. Single-axis barrel distortion occurs when projecting on the outside of a cylinder.
The distortion can be alleviated using optical elements, electronic devices or software to counter-distort the image.
The opposite of barrel distortion is called pincushioning.

Beam splitter: A device which transmits part of a laser beam and reflects the other part. Usually, a beam splitter is a piece of glass with optical coatings; the type of coating determines the ratio between transmission and reflection.
Using beam splitters, a laser's beam is divided and sent to two or more devices simultaneously.

Beam splitter ratios: The most common ratios in use are 50/50 and T70/R30 beam splitters. The T value refers to the percentage of light transmitted while the R value refers to the percentage of light reflected by the optic. (The T and R designations are not used in a 50/50 splitter.)

Blanking: The technique of turning the laser beam on and off with precise control (as opposed to chopping). For scanned graphics, blanking allows images to have disconnected sections where the beam is hidden.
Blanking can be digital (on/off) or analogue (continuous intensity control). The same techniques used for blanking control can be used to control intensity of red, green and blue beams, for colour mixing. [See Also: Acousto-optic modulator; Scanner blanking and colour]

Chopping: The technique of turning the laser beam on and off at a regular rate (as opposed to blanking). High-speed chopping gives a "dotted line" effect. If used with densely scanned images, chopping creates areas of light and dark which shift as the chopping rate changes. [See Also: Colour modulation]

Colour box: An informal term for a projector subsystem using three dichroic filters which pass cyan, magenta or yellow light. Actuators move the filters into a white-light beam. This provides a subtractive colour blend, giving eight possible colours (red, green, blue, yellow, magenta, cyan, white and "black").
For example, cyan (which passes green and blue) plus magenta (which passes red and blue) results in a blue beam (the only colour passed by both).
A colour box system usually colours the entire image (frame by frame colour control), since actuators are relatively slow. This contrasts with scanner or PCAOM techniques which can colour different sections of an image (point by point colour control).

Galvo, galvanometer: A limited excursion motor whose torque is directly proportional to the current. When current is applied, the galvo's shaft rotates through part of a circle. When current is removed, the shaft returns to the rest position. Examples include the ammeter in an automobile or the needle-style VU meter in audio equipment.
Galvo-based scanners, such as General Scanning model G-120 or Cambridge Technology model 6800, are often used in laser displays. Strictly speaking, the galvo is the motor device only, while a scanner includes the galvo, mirror mount, and mirror.
There are three types of galvos: moving iron (GS G-120), moving magnet (CT 6800), and moving coil (not used in laser light shows). [See Also: Scanner]

Flag: An arm that is attached to a galvo or actuator that is used to shutter (turn on or off) a laser beam. If it mounted to a variable position device such as a galvo, the flag can be gradually inserted into the beam to reduce its diameter and apparent brightness.

Keystoning: The distortion caused when an image is projected onto a screen from off-centre. For example, a projector aimed up at a screen produces a wide image at top and a narrow image at bottom. (This shape is like that of the "keystone" at the top of an arch.)
Solutions to keystoning include repositioning the projector, or using optical elements, electronic devices or software to counter-distort the image.

Kinematic mount: A mount designed for holding mirrors, dicros or other optics that features two (or more) adjustments for accurately positioning the beam deflected from the optic. Examples are the popular MM1 type mounts and the end plates that hold the optics on a laser head.

Mask: A device used to obstruct the laser beam so it is blocked from undesired areas. The mask is usually placed at the final output aperture of the laser projector.
Masks are used for aesthetic reasons, to keep light from going off a screen, and as a safety feature, to ensure lasers cannot reach the audience in case of equipment malfunction. This is also called a beam block.

Mirror mount: A tiny machined metal piece used to mount laser scanner mirrors to the galvanometer scanner shafts. Usually the mirror is glued to the mount; the mount has a set screw to hold it to the shaft.
The mount can be eliminated by gluing the mirror directly to the shaft. This reduces the weight moved by the scanner, increasing speed. But it makes it more difficult to replace scanner mirrors if one should break.

PCAOM (polychromatic acousto-optic modulator): A device used to control the colours in a white-light beam, to produce a final desired colour. A PCAOM is a complex type of acousto-optic modulator. It can control the intensity of not one, but a number of wavelengths (colours) simultaneously using a single crystal.
Special driver electronics are employed to do this. The more channels in the driver, the more individual laser colours (lines) that can be controlled. Typically, four to eight channels are available.
A PCAOM is simple to incorporate in a projector, compared with other methods of colour selection, such as using three separate AOMs to control red, green and blue components. A significant PCAOM advantage is that the output beam remains colour converged; this eliminates additional optics needed to recombine separate colours into one beam. A PCAOM provides both blanking and colour control in a single device.
An advantage it shares with AOMs is speed. It is fast enough to control the intensity or colour of single points within a scanned image.

Usage note: PCAOM and AOTF: Some people may incorrectly use the term "AOTF" (acousto-optic tuneable filter) to refer to PCAOMs. Both devices change the colour of an input light source. However, an AOTF controls only one wavelength at a time; that wavelength is tuneable. A PCAOM controls many wavelengths simultaneously; those wavelengths are fixed by the driver. Only PCAOMs are suitable for colour control of multi-wavelength white-light laser beams.
Some manufacturers may use "PCM". However, to avoid confusion with "pulse code modulation" (a common method of storing digital data in an analogue format), ILDA recommends the more descriptive and accurate acronym "PCAOM."

Pincushioning: A distortion caused by projecting onto a concave surface. For example, a grid projected on the inside of a dome has inwardly curved edges, where the corners stick out and the centres curve inward. Single-axis pincushion distortion occurs when projecting on the inside of a cylinder.
The distortion can be alleviated using optical elements, electronic devices or software to counter-distort the image. The opposite of pincushioning is barrel distortion.

RGB laser projector: A laser projector whose colour system can independently control the amount of red, green and blue light. These three components are combined to produce the final beam. This technique provides a wide range of colours. RGB projectors can use different methods, including scanner colour, AOMs and PCAOMs.
The term should not be used for colour boxes or other methods which provide only eight basic colours. An RGB projector implies a wide range of colours due to being able to vary how much red, green and blue is used. The term also implies high-speed colour control, so different parts of a scanned image can have different colours (point by point colour control).

Scanner: Any device which moves a beam back and forth. This can include polygonal faceted scanners, acousto-optic deflectors, and galvanometers with mirrors.
In laser display, "scanner" usually refers to a galvanometer which rotates a shaft back and forth through part of a circle, rather than spinning continuously. A small mirror (approximately 5 x 8 mm) is attached to the shaft.
Two scanners are needed to draw laser graphics. The scanners are arranged so the beam reflects first off one mirror, then off the other. The first mirror moves the beam horizontally, the second moves it vertically. This arrangement means the scanners can position the beam anywhere within a square area.
The galvo shaft moves in response to an electrical current. By repeatedly following the same path at high speeds, the illusion of a single, fixed image is created.
Because of the mirror's mass, a scanner is limited in how fast it can move while still remaining accurate. Complex scanner amplifier circuits are used to get maximum performance. [See Also: Vector graphics]

Scanner amplifier (Scan amp): An electronic device which conditions a signal from a computer or other source, and makes it compatible with scanners.
Closed-loop scanner amplifiers are used with position-detecting scanners; the amplifier controls the scanner based on position feedback signals. Open-loop scanner amplifiers are used with non-position detecting scanners.
The more expensive closed-loop scanner systems are usually required for complex representational graphics such as logos.

Scanner blanking and colour: A method of blanking or colouring laser beams, using scanners. A scanner is set up using a mirror or small beam-blocking arm (flag) on its shaft. As the shaft rotates, the beam is reflected or blocked so it does not reach the scanners.
Often, a complex beam path is used to make an "optical lever", as simple blocking methods may not be fast enough or may cause unwanted beam movement.
A blanking scanner on a single beam provides intensity control for that colour. To get coloured images, three scanners can be used on red, green and blue beams. The beams are recombined to form a single beam whose colour depends on the scanner settings.
The primary advantage of scanner blanking and colour is that it is easier to interface with beam-positioning scanners -- all devices react at the same rate. However, AOM's and PCAOM's can turn on and off faster than scanners. This provides a wider range of effects (such as smaller "dotted lines") and it does not require as many control points where the scan must pause to wait for the scanner to fully blank. [See Also: Scanner]

Shear: A distortion where one axis is at an angle while the other is correctly straight. The resulting image is slanted. For example, horizontal shear causes normal text to look like italic type.
True shear is a result of not mounting the X and Y scanners exactly perpendicular to each other. (Apparent shear, caused by being at an angle to a projection surface, is really due to keystoning.)
Solutions to shear include repositioning the scanners or using optical elements or electronic devices to counter-distort the image.

Shutter: An actuator (or solenoid) used to block the laser's beam. Usually positioned between the laser and the laser projector.
When activated, the shutter opens, letting the beam pass. This arrangement ensures that the beam will be safely blocked if power to the shutter should fail. [See Also: blanking]

Throw: The distance between the laser projector and the projection surface. Too short a throw means that the scanners may not be able to cover the entire screen. Too long a throw means that the beam may diverge too far, or that special effect graphics such as lumia may be too dim.
Generally, laser projectors give longer effective throws than conventional slide, motion picture or television projectors. Also, the beam remains "in focus" anywhere throughout its travel, unlike images from conventional lensed projectors.

X-Y mount: A machined piece of metal which holds the scanners in the correct position relative to each other, so the beam can scan first off the horizontal (X) scanner, then off the vertical (Y) scanner. The mount also serves as a heat sink to carry away heat generated by the scanners' operation. A fully enclosed X-Y mount is known as a scan head while an open frame type is often referred to as a scan stage.

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Laser Shows and Special Effects

Abstract: Laser-projected images which are usually non-representational (as opposed to graphics). This implies using synthesisers or other techniques which do not afford point-by-point position control of the beam, and usually implies using scanners to create the abstract image. Usually excludes lumia, diffraction gratings, or similar non-scanner (optical) techniques.

Beam effects: Using the laser's beam as a sculptural element in space (as opposed to shining it on a screen to create graphics). The beam can be static or kinetic (dynamic or moving). If static, it is usually reflected off a series of widely spaced mirrors to create a "beam sculpture" or "beam matrix".
If kinetic, it can be shot directly into space (over an audience), or can be directed to different groups of mirrors to create a beam sequence. A common technique is to scan the beam through theatrical smoke; if a line is scanned you see a fan of laser light, if a circle is scanned you see a cone.
These effects require high power lasers and often require smoke, dim lighting and other environmental controls.

Beam sequence: A beam effect where beams are directed to different mirrors in quick sequence. This gives a Star Wars-type impression of shooting beams bouncing around the performance arena.

Beam sequencer : A hardware device that generates an analogue stairstep voltage. When fed to the X input of a scanning system, the beam steps through the air producing beam sequences in various patterns.

Colour modulation, colour mod: Rapidly varying the colour of the laser beam. In beam effects, colour mod can be used on the entire beam matrix or can be sequenced in individual beams. In graphics effects, colour appears to "chase" through an image.
A classic light show effect is to project a dense spiral pattern, then use colour mod to create shifting zones of colours pulsating through the image. [See Also: Chopping]

Cone: A beam effect where the beam is rapidly scanned to enclose space. For example, by scanning a circle, the effect looks like a cone of light emanating from the projector location. Other shapes can be scanned; for example, a square produces a pyramid "cone".
Theatrical fog or smoke is often used to make the scanned light more visible. [See Also: Beam effects]

Cone maker: A device usually consisting of a high speed motor with a small mirror on an eccentric mount attached to its shaft. When targeted by a static laser beam, the mirror spins the beam in a circle producing a cone of light. [See Also: Cone]

Fan: A beam effect where the beam is rapidly scanned from side-to-side, usually through theatrical smoke. If the scanning is smooth, the audience sees a flat plane of light (Sometimes called a `sheet scan'). If the beam scans discrete positions, the audience sees a ribbed fan of light. The fan can be rotated and translated to produce a moving fan effect.
Theatrical fog or smoke is often used to make the scanned light more visible. [See Also: Beam effects]

Graphics: Laser-projected designs, usually representational (as opposed to abstract) such as logos or drawings. Graphics implies point-by-point control of the beam position, which in turn implies computer control of laser scanners. A computer can generate non-representational designs, but these are normally classed as graphics rather than abstracts.

Hologram: A light-sensitive film which captures and plays back light wave interference patterns. One of the most striking results is the true three-dimensional nature of the recreated holographic image.
Image holograms are not usually used in laser display technology. They are mentioned here because of a frequent misconception that laser shows can present "floating" 3D holographic projections.
Although a holographic image can appear to be floating in front of the film plate, the viewer must always be looking at or through the hologram. Viewing angles are usually limited to one or two people at a time, and image size is usually one cubic meter or less. At present, there are no free-floating "Princess Leia" Star Wars-type holograms.
Note: the term is hologram ("a laser recording of interference patterns"), not holograph ("a document wholly hand-written by the person under whose name it appears").

Laser light show: A presentation where laser light is the primary attraction (as opposed to laser special effects). The four main elements of a laser light show are: abstracts, graphics, lumia (optical) and beam effects. These may be present in various combinations.
A laser show is usually set to music, often with other effects such as theatrical lighting and pyrotechnics. Musical shows are usually divided into a number of song-length modules.

Laser special effects: Any use of lasers where the laser is not the primary attraction (as opposed to laser light show). For example, a rock band may use complex laser lighting effects comparable to standalone laser light shows. However, because the band itself is the primary attraction, the lasers are considered supporting special effects.

Laserist: The person who performs a live laser show, especially in a planetarium environment. Can also be used for the person who designs and choreographs a pre-recorded show.

Laserium ®: A registered trademark of Laser Images Inc., referring to their laser light shows. Sometimes used incorrectly as a synonym for any planetarium-based laser light show.

Lumia: A gauze-like laser effect produced by shining a laser beam through distorting material such as rippled glass or plastic. Lumia are often composed of fine parallel lines of light and dark, and they show the characteristic speckle of coherent light. There are many different lumia effects, depending on the type of distorting material.

Module (laser show): A song-length segment of a laser light show. A module is the smallest unit of music-plus-lasers which can artistically stand on its own.
A typical planetarium-type laser show is usually made up entirely of different modules.

Three-dimensional (3D) laser effects: In laser effects, this can refer to beam effects or multiple scrim techniques. (In laser graphics, "3D" usually refers to either a 3D graphics database or to stereoscopic projection systems.)
Laser beam effects are inherently 3D, filling space with static or kinetic beams. However, these cannot create space-filling objects. Science-fiction films have popularised the misconception that lasers can project large free-standing 3D images. At present, there is no technique which can do this without the image being enclosed in a small volume.
One interesting simulation is to project laser graphics through partially translucent screens; a series of these screens can give a floating mid-air effect. This is not true 3D as all images are identical, and merely increase in size with increased projection distance. [See Also: Three-dimensional laser graphics; Hologram]

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Computer Graphics for Laser Shows

Anchor points: Additional points placed to help slow the laser beam. Anchor points are usually added at corners and tight curves, to help the scanners accurately follow the desired path. [See Also: Velocity points]

Animation: A sequence of frames where each frame is slightly different, giving the illusion of motion. "Animation" refers to the complete group including key frames and tweens.
These can be precomputed, such as traditional hand-drawn (character) animations. Or they can be computed in real-time; for example, the single frame of a logo may grow in size and rotate to produce the animation. [See Also: Key frames; Tweens]

Character animation: An animation where a human, animal or cartoon character is the main focus of interest (performing an action). Usually these are created by hand drawing or digitising a number of sequential frames (occasionally referred to as a sequential frame animation the technical description for the process).

Object animation: An animation where the action is produced by computer manipulation of a frame or 3D object. Object animations can be pre-computed and stored as a series of frames in memory or created by manipulating a single frame in real-time.

Artware: Frames and animations used in creating laser modules and shows. Artware can be created by the end user, but the term usually implies "clip art" sold or traded by a third party. This term is preferred over "clip art". [See Also: Showware]

Blanked points: Those points in a computer-controlled graphic that are turned off by the blanking device so they cannot be seen.

Cel or cell: Refers to a single drawing in a character animation created on paper or acetate. [See Frame]

Co-ordinates: The placement of a point in Cartesian space; used in laser graphics software. A point at (5,10) is located five units to the right of, and ten units above the origin (0,0). [See Also: Point]

Depth cueing: An effect in 3D graphics where lines appearing further from the viewer are dimmed. This helps enhance the 3D illusion for wireframe images such as those used with lasers.

Erase: A special effect used in computer-controlled laser graphics. A design disappears point-by-point, as if being erased. The opposite effect is called write out.

Frame: A sequence of points forming a single, fixed drawing or design. It is analogous to a motion picture frame. The alternative terms "cel" or "cell" are sometimes seen but refer to a single drawing on paper or acetate.

Key frames: Frames containing the start and end points of action in an animated scene. Intermediate positions are shown in the tween ("in-between") frames. In computer animation, key frames are hand drawn and the computer calculates tweens. In hand animation, the key frames are drawn by the primary animator, while tweens may be drawn by an assistant.

Module (computer graphics):

1. In laser light shows, a module is a song-length segment. It is the smallest unit of music-plus-lasers which can artistically stand on its own. A traditional planetarium-based laser show is usually made up entirely of different modules.
2. In laser computer graphics, a module is a complete set of frames, animations and scenes which can be played back by a user or operator. A laser show can be a single module, but is often a series of modules which have been arranged in sequence.

The terms "scene" and "module" express a similar idea: a group of related frames and animations. They differ in that a scene is more of an artistic concept, while a module is more of a technical concept.

Point: The smallest object which can be manipulated by laser graphics software. Usually includes X (horizontal) and Y (vertical) co-ordinates, and visibility (on or off) and/or colour. Can also include Z (depth), size, shape and other attributes.
Computer controlled graphics work by rapidly moving the laser beam from one point to another. The software shows a sequence of points to form a "connect the dots" drawing. [See Also: Co-ordinates; Point number]

Point number: The sequence number of a particular point. For example, "point number 1" refers to the first set of co-ordinates in a point output list. [See Also: Point]

Point-plot frames: The traditional laser frame is a connect the dots drawing where every point needed to draw the image is stored as part of the frame.
This is in contrast to a vector-oriented frame.

Precomputed animation: In a computer graphics system, a frame sequence which has been hand-drawn, or calculated in a longer time than it takes to play back the animation.
As a consequence, every frame is stored in the computer memory, hard drive or similar device. Playback is done by recalling each frame in turn.
This is the opposite of real-time animation.

Raster graphics: Images created by constantly scanning from side to side and up and down -- examples include television and computer monitors.
Most laser projectors do not use raster graphics. This is because a raster graphic spreads the beam's power over the entire screen, making the beam visible only at certain times (as with television). The relatively few laser raster (laser TV) projectors available use high-powered lasers to overcome this problem.
An advantage of laser raster projectors is that most can be used to project video, making them powerful television projectors.
Non-video laser raster projectors are used to project moving words, giving a look similar to that of a "Times Square" moving lights signboard. Raster graphics are contrasted with vector graphics.

Real-time animation: In a computer graphics system, a frame sequence which is developed from a source frame in "real time". The manipulations or calculations are completed in less time than it takes to draw the source frame.
As a consequence, the source frame plus the manipulation instructions are stored in the computer memory, hard drive, or similar device. Playback is done during the presentation, by recalling the source frame and applying the manipulations. This is the opposite of precomputed animation.

Retrace: A laser-drawn graphic consists of a series of connect-the-dot points. When all points have been drawn, the laser must return to the first point, to "re-trace" the graphic. The line from last point to first point is the retrace line.
Usually, retrace lines are undesired. They can be eliminated by drawing the graphic so the first and last points overlap, or by using a blanking device to block the laser beam during the retrace.

Scene: A sequence of frames and animations which are lyrically, thematically or temporally related to each other.
A lyrically related scene could illustrate the verse or chorus of a song. A thematically related scene could illustrate concepts such as love or events such as boy-meets-girl. A temporally related scene is what we think of as a movie scene: depicting events taking place at a single location during a short time period.

Showware: All or some of the parts used to create a module or show, such as scripts, computer graphics command programs, or taped audio and control signals. Showware can be created by the end user, but the term usually implies sale or trade by a third party. [See Also: Artware]

Three-dimensional (3D) laser graphics: In computer laser graphics, usually refers to either a 3D graphics database or a stereoscopic projection system. Can also be applied to 3D volumetric techniques.

1. 3D database. Referring to a system of storing height, width and depth information of an object. A 3D computer graphics system can then display a 2D projection of the 3D object drawn from any angle. The resulting projection seems to have depth due to the perspective drawing cues, especially if it rotates or moves.
2. 3D stereoscopic. Referring to a method of presenting a viewer with two simultaneous images, a slightly different one for each eye. The brain interprets the parallax cues from the views as if it were looking at a 3D object or scene. (Other non-stereoscopic cues, such as perspective drawing or rotation of a 3D database object, may be simultaneously employed to enhance the effect.) Special glasses are usually used to ensure that each eye is presented with a unique view.
3. 3D volumetric. Referring to methods which present a 3D image within a volume. This is true 3D in that no glasses are required and the image truly occupies a volume in space. A common technique uses a rotating screen synchronised to the laser output. Usually the 3D volume is small, so volumetric laser displays are currently used only in research and technical applications. [See Also: Three-dimensional laser effects]

Tweens: Individual frames which are part of an animation. The term comes from "in-between". The complete set of tweens plus key frames is an animation. [See Also: Animation; Key frames]

Vector frames: Vector frames store only the points needed for the beginning and ends of lines and angle changes within lines thus they are very compact. Since the scanners need every point along each line and curve to draw the image, additional points between the vector points are generated in real-time by the computer system at playback.
Vector frames are contrasted with point-plot frames.

Vector graphics: Images created by moving from point to point -- examples include handwriting and computer plotters. Most laser graphics are vector graphics.
To produce a laser graphic, a computer is first used to draw a series of connect-the-dot points. These are translated to electrical signals and are sent to the scanners. At the projection screen, the moving beam travels from point to point at high speeds, smoothing out the connect-the-dot points. This creates the illusion of a fixed, non-moving image.
One limitation is the number of points which can be drawn before the fast-moving image begins to flicker. With lasers, the practical limit depends on the speed of the scanner in use to project the image.
Vector graphics are contrasted with raster graphics.

Velocity points (velocity control points): Additional points placed to keep the laser beam scan velocity constant during long excursions. While anchor and velocity points express similar concepts, there are important differences.
Anchor points are usually repeated visible points placed to "nail down" the laser beam at sharp turns. Velocity points are usually widely spaced blanked points used to prevent large, uncontrolled jumps as the scanner draws. [See Also: Anchor points]

Write out: A special effect used in computer laser graphics. The laser appears to "write out" a design such as a signature. The opposite effect is called erase.

X axis: Refers to horizontal (left-and-right) movement of the laser beam. In the ILDA Image Data Transfer format, negative values are to the left, positive values are to the right. [See Also: Co-ordinates]

Y axis: Refers to vertical (up-and-down) movement of the laser beam. In the ILDA Image Data Transfer format, negative values are down, positive values are up. [See Also: Co-ordinates]

Z axis: Refers to movement in front of or behind the viewing plane. (Used primarily with laser graphics software which uses a three-dimensional database.) In the ILDA Image Data Transfer format, negative values are to the rear (away from the viewer), positive values are to the front. [See Also: Co-ordinates]

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Scientific and Technical Terms

Aberration: A defect in a optical system that can degrade or distort the light passing through it. For example, a flaw in a beam telescope lens that may distort the laser beam.

Beamwidth: The width or diameter of the beam. This is important for laserists to know as you don't want a beam that will overfill your scanning mirrors. The scientific definition is the linear width of the beam specified as the region where the beam intensity falls within a predetermined limit.

Brightness: An objective measure of how powerful light is over a specific area. (Specifically, watts per unit area per unit solid angle.)
Two lasers can be equal in power; the one with the smaller divergence has higher brightness, since the light is concentrated into a smaller area. [See Also: Luminance]

Chromatic aberration: An aberration occurring in lenses resulting from the normal increase in refractive index of all common materials towards the blue end of the spectrum. Essentially the lens focuses the red and blue light in slightly different planes.

Coherence: A laser produces coherent light; conventional light sources produce incoherent light. Coherent light waves all travel the same direction (spatial coherence) at the same frequency and in phase (temporal coherence).
This gives rise to the very narrow beam and intense, pure light that characterises laser. Coherence also produces shimmering laser speckle and the web-like effects of lumia.

Colour: A subjective perception of light wavelengths. One of the attractions of lasers is the intense, pure colours they produce. This is because they emit specific, narrow wavelengths of light, which we see as saturated colours. Scientifically, colour is the perceived wavelength of light.]
Different lasers produce different wavelengths or lines, depending on the type of lasing media. Most light-show lasers use gas as the lasing media. Only a few, well-defined lines are emitted by gas lasers with sufficient power to be useful in light shows. These colours cannot be filtered or otherwise modified to produce intermediate colours.
This means that only a relatively small number of naturally occurring laser lines are available. To get a wider range of colours, different lines are mixed together using various techniques. Most commonly, red, green and blue lines are individually controlled and combined into a single beam which theoretically can be any desired colour. [See Also: Line; Wavelength]

Divergence: An objective measure of the amount that a laser beam spreads as it leaves the laser head.
High divergence produces larger spot sizes, which is undesired. Divergence can be a problem when the throw is long. Arrangements of lenses are usually used to minimise divergence in these cases.

Flicker: A perceptual effect in laser-drawn graphics when the laser cannot complete its path before the eye's persistence of vision sees the image fade. The effect, usually undesired, is that the image is flickering or pulsating.
Solutions include fully darkening the environment, changing the laser's power, speeding up the scanning rate, or reducing the complexity of the graphic traced by the laser.
A related effect occurs when viewing videotaped laser graphics. The laser's scan rate often does not match the TV camera's frame rate. The video image appears to flicker. Videotaped laser graphics generally exhibit much more apparent flicker than the original live laser graphics.

Line: Short for "spectral line", referring to a particular wavelength of light in the visible spectrum. [See Also: Wavelength]

Luminance: A subjective measure of how brightness is perceived by the eye. It depends not only on brightness but on wavelength.
Two lasers can be equal in brightness; but a green beam has higher luminance than red, since the eye is more sensitive to green light.

Milliwatt: One thousand milliwatts equal one watt. Small lasers' beam powers are measured in milliwatts. For example, a 50 mW laser is one-twentieth of a watt; a 500 mW laser is one-half watt. [See Also: Watt]

Speckle: An effect caused by the coherence of laser light. Laser speckle looks like a dense pattern of dark and light areas, which shimmers as you move your head from side to side.
The more spread out the laser light on a surface, the more visible the speckle. For example, it is easier to see speckle when viewing lumia than when viewing graphics. Only laser light exhibits this eerie, beautiful iridescence.

Watt: An objective measure of power; in lasers, usually refers to the optical output power, or strength, of a laser beam. Large light show lasers are generally in the 1 to 20 watt range.
Watts are also used in a more conventional sense, to measure electrical power used by a laser. For example, a 10 W (optical) argon laser consumes around 10,000 W of electrical power.

Wavelength: The distance from the crest of one wave to the next. In lasers, wavelength is important because it determines the perceived colour of the light.
Visible light has wavelengths extending from about 700 nanometers (red) through orange (~600 nm), yellow (~580 nm), green (~550 nm), blue (~450 nm) and violet (~400 nm).
Most gas lasers used in laser display emit many wavelengths, or spectral lines, simultaneously. The lines have different strengths. Usually, only a few lines have significant brightness. For example, an argon laser may produce seven or more lines. The principal lines have wavelengths of 514.5 nm (green) and 488.0 nm (blue); they account for about two-thirds of the laser's total light output.
The terms "wavelength", "line" and "colour" are often heard when discussing laser light colours. Although there are important technical differences between these concepts, in common usage they are often used interchangeably. [See Also: Colour]

For more detailed scientific terminology, visit the Laser Adventure site

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Safety

    The primary hazard of light show lasers is eye damage. Higher power lasers also have some burn and fire hazards. However, no light show laser can do science-fiction type damage or disintegration. Both equipment and show site must be made safe so beams cannot go into the audience. Usually, governmental regulatory agencies must be notified in writing before lasers can be used. Thus far, the regulations have worked well. There have been no reported injuries in the United States from light show lasers since U.S. federal regulations took effect in 1976.

Audience scanning: When laser effects, both scanned beams and abstracts are projected directly onto people in the audience it is called audience scanning. Audience scanning is inherently dangerous since laser beams could be projected directly into a person's eyes causing damage or even blindness. Static beams such as those from a beam table are never projected into the audience since they are usually too powerful.
For this reason, audience scanning is illegal or restricted in most countries. In countries where audience scanning is permitted, there are restrictions on the power levels and duration of the effects -- you must abide by these restrictions for the safety of your audience.

CDRH: The Center for Devices and Radiological Health of the Food and Drug Administration, Department of Health and Human Services, is the principal federal regulatory agency covering U.S. laser displays.
Some states also have their own regulations and regulatory agencies. A complete list is available from the CDRH.

Irradiance: A measure of laser energy density used in safety work; usually measured in watts per square centimetre. Technically irradiance is the radiant flux incident per unit area of a surface. Also called radiant flux density.

MPE (Maximum Permissible Exposure): Used in safety calculations, the MPE is the maximum level of laser radiation that a person may be exposed to without adverse biological effects. The international laser safety standard, IEC-825, defines a short exposure as 2.5 mW per square centimetre. Check with the authorities in your jurisdiction to determine the MPE for your area.

Radiation: The CDRH currently requires higher power U.S. lasers to bear labels reading "Warning: Laser radiation". The word "radiation" refers merely to the laser's light. This is not high-energy ionising "atomic" radiation, which is the more conventional use of the word .
The CDRH is considering changing the wording to the more appropriate "Warning: Laser light."

Restricted zone: The area around a laser projection system that is off limits to members of the public as the laser show does not meet safety guidelines in this area. For example, static beams may be less than three meters above the ground/floor.

RPB: The Radiation Protection Bureau (a division of Health Canada), is the federal government agency which has jurisdiction over laser displays in Canada. Some provinces also have their own regulations and regulatory agencies. A complete list is available from the RPB in Ottawa.

Variance: Any U.S. laser display viewed by the public must follow CDRH regulations. The regulations cover both laser equipment and the performance site. Permission to vary from the regulations is given on a case-by-case basis, in a document called a "variance".
Only very small lasers (5 milliwatts or less) can be used in displays without a variance in it USA. The maximum power usable without a permit depends on the rules in your jurisdiction. Because of the higher power necessary in laser light shows, a variance or other type of permit is almost always required.
All ILDA members doing laser shows in the United States are required to hold a current CDRH variance. ILDA members outside the U.S. are required to follow the regulations of the location in which they provide laser displays.
In other countries, the paperwork or permit required to produce a laser show for a public display has different names. In Canada for example, you must fill in and submit a "laser light show report". The BMRD then sends you a letter or fax acknowledging receipt of your notification.

    For more information, see the Basic Laser Safety section.
    A translation of this glossary into Russian is available at the Laser Action pages.

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[ Introduction | Bibliography | Glossary of Terminology | Laser safety overview | Other applications of lasers | Selected laser related web sites | Basic laser science projects | Intermediate laser science projects | Advanced laser science projects | Illustrations for laser science projects ]