Light is controlled and directed on stage using various specialized fixtures. In the US the theatrical name for a lighting fixture is an "instrument". The are also sometimes called "luminaires" but that is a more European term.
The part of an instrument that actually makes light is called a lamp. This is often mis-named a "bulb" but the bulb is just the glass part. Most theatre lamps are "incandescent"; a piece of tungsten wire is heated to white hot by running electricity through it. If this happens in air, the wire burns out immediately, so the filament is placed inside a glass bulb or envelope, and the bulb is filled with an inert gas, like argon, to prevent the filament from failing immediately. If the gas used is a halogen gas like iodine or bromine a reaction occurs which greatly lengthens the life of the lamp. These "tungsten-halogen" lamps are universal in theatre for fixtures that use incandescent lamps.
Instruments like follow spots and most moving lights need a brighter source than incandescent lamps usually provide, so various tungsten short-arc lamps are used instead. These lamps use tungsten electrodes to create the electric arc inside a high pressure glass lamp filled with a special gas. The gas slows electrode erosion so that the lamps last a long while. They are very bright and very white at a much lower wattage than the equivalent incandescent lamp, but they cannot be electrically dimmed.
Lamps are usually rated by color temperature of the light produced. Color temperature is a measure of how "white" a light is, measured in degrees Kelvin. Kelvin degrees are the same "size" as Celsius, but are measured from "absolute zero". Typical incandescent lighting is very red, about 2500 K. Stage lights are whiter, about 3200 K, and Video lighting is whiter still, about 3600 K., and arc lamps are usually higher yet.
Incandescent fixtures are rapidly being replacing by fixtures using high-output LEDs as the light source. LEDs are much more energy efficient, and can be built as "color mixing" units. LED arrays allowing the color output to be created by mixing the output of multiple LEDs. LED fixtures using arrays do not work well with profiles projecting gobo patterns. Such units often use a single "white" LED to better work with the optics of the fixture.
LEDs have a relatively long life, provided they are not allowed to overheat. LEDs can fail quickly if not properly ventilated. LEDs cannot be easily replaced when they fail or if they grow dim as they age (which they will do). Also, LED technology is still being developed, and fixture models are discontinued and replaced very quickly by manufacturers. This generally means an entire fixture must be replaced when the LED source fails.
Lighting instruments are colored using various types of color media. Most come in sheets which are cut to size, inserted in metal frames, and put in front of the lens to color the beam. Most work by absorbing some colors in the light; the perceived color is what has NOT been absorbed or "subtracted" from the light. Comes in several forms:
Glass media: available as either custom glass sheets, usually cut into strips, or as Rondels, shallow glass "bowls" of colored glass. Permanent, but breakable, and only available in a few colors.
Colored sheets: a clear base is colored with dyes to make the color, short or long life depending on the base used. The sheets are numbered by color by the manufacturer. Since there are so many similar colors available, the FIRST thing to do upon receiving an order is to number each sheet with a china marker or wax pencil, even if the manufacturer has also printed the number on it. The sheets are then cut fit the metal color frames, and each cut piece should also be marked with the color number.
Dichroic filters: dichroics are a very thin layer of metal oxides is deposited onto glass, then placed in the beam of the light. Dichroics change color by reflection rather than absorption. When the filter is tilted, some colors of light are reflected to the side and away from the opening. The rest pass through and to the stage. Which colors are reflected changes with the angle, which means that as you tilt the filter, you change the color that passes through. Used primarily in some moving lights. Permanent but very expensive.
Instruments fall into several categories, but all are either floodlights or spotlights.
Floodlights are not quite as common as they use to be, but are sometimes encountered. Since they have no lens, they are basically a reflector and a lamp, and a way to put color media in front of it. Floods are primarily designated according to the shape of the reflector used.
Ellipsoidal Reflector Floods (ERF or "Scoops"): uses an ellipsoid (a 3D ellipse) as a reflector. Efficient at covering a large surface area, like a drop or cyc, from a fairly close distance.
Striplights: Basically a row of small floods arranged in either three or four circuits so that every third or fourth lamp is on the same circuit. All lights on the same circuit are given the same color, usually Red, Green, and Blue, the primary colors of light, for additive color mixing of light. Amber is also often used, since it is a difficult secondary color to make.
Older striplights use spherical reflectors, R-lamps, or a lineal reflector parabolic in one plane and flat in the other. Often colored using roundels, curved bowls of colored glass placed over the lamps and reflectors in a repeating RGBA array. There are also striplights that use MR-16 high intensity lamps to make a bright but compact strip. Modern striplights use multicolored LEDs to mix the desired color. Striplights are used to light large flat surfaces from close range, such as drops and cycs.
Far-cycs and Cyc floods: Four floodlights arranged in a rectangular array. Each individual bay is one a separate circuit. Used in groups to light drops and cycs. Similar to strips in that four colors are additively mixed to make other colors, but since they use sheets of color media, they can easily be other than the primary colors usually found it striplights.
Beam projectors: Floods using a parabolic reflector, which project a very compact parallel beam similar to a narrow spotlight. They were replaced by PAR-cans in the US, and then by LED spot fixtures.
PAR Units (PAR-cans): basically a self-contained Parabolic Aluminized Reflector and lamp combination with some rudimentary lensing built into the front to shape the beam, and placed in a tin-can to contain some of the spill. In many ways just an updated beam projector. PAR cans are quite efficient compared to other incandescent fixtures and were very popular for concert work. PARs have been updated again by making the lamp replaceable, as in a Source 4 PAR and the Altman Star PAR. In some ways they sort of recreate the Beam Projector but with PAR's lensing added.
Lenses in striplights are used to gather and control the light produced by the lamp. Like floods, the different types are defined by their reflector as well as their lenses.
PC Spots- an obsolete type, but worth examining as the prototype theatre spotlight. A lamp with a spherical reflector in back and a Plano-Convex lens on the front. A PC lens is a flat plane (the Plano- part) on one side and Convex (curved out) on the other. A very important theatre lens type and the basis of most lenses. Adjustable from flood to spot like a Fresnel (see below).
Fresnel- a modified PC-Spot, with a spherical reflector like a PC, but useing a Fresnel Lens instead. A Fresnel lens is planar on the back, but it cut away on the curved side in concentric rings. The result has the same curve as a PC lens, but removes most of the glass between the surfaces, making the lens thinner and lighter, and less prone to breaking from the heat. A 1/2" thick Fresnel lens is about the equivalent of a 2" thick PC lens. However, to keep the rings from showing up in the projected image, Fresnel lenses usually have a dappling added to the planar side as built-in diffusion.
The lamp and reflector are fixed on a moveable carriage. When the lamp is moved forward near the lens, the projected pool of light gets larger ("Flood" position). When the lamp is moved away from the lens, the pool of light gets smaller ("Spot" position).
Ellipsoidal Reflector Spotlight (ERS, Ellipsoidal, Leko, Source4)- uses an ellipsoidal reflector with a PC lens or lenses in front. ERS are the most efficient spotlight type; an ERS can illuminate efficiently from a distance, so is the best instrument for FOH (Front of House) lighting, from the Beams, Coves, and Bacony Fronts positions. The ERS has been updated with an LED engine as a light source, but the incandescent versions are still workhorses for long-throw illumination.
Ellipsoidal reflectors are unique in that they have TWO focal points. When the lamp is placed at one focal point, the light beam can be shaped at the other focal point with shutters, irises, and patterns called "gobos" or "cookies". This second focal point position is called the "gate".
The lens is usually one or two PC lenses. Using two lenses makes a combined focal length shorter than either lens separately. The Focal Point is the point where a lamp filament will project parallel beams through the lens.
Effectively, Ellipsoidals with a long focal length will project the same size beam on stage from a long distance (called "throw") as a short focal length will at a short distance.
Lenses are designated by diameter and focal length. A 6x9 lens has a 6 inch diameter and a 9 inch focal length. A 6x12 lens has a 6 inch diameter and a 12 inch focal length. The shorter the focal length, the wider the beam, so a 6x9 has a wider beam than a 6x12. Similarly, an 8x9 has an 8 inch diameter and a 9 inch focal length. Generally speaking, larger lenses are brighter because they gather more of the light.
This system is not especially intuitive. To help clear up the confusion (and because most of the world is metric), modern fixtures label instruments by field angle (beam spread) rather than by lens dia. and focal length. Thus a "Source 4- 26 degree" is an ERS made by Electronic Theatre Controls (ETC™) with a beam spread of 26o. The lense diameter is immaterial.
Moving lights have become quite common, sometimes replacing all "conventional" fixtures on a show. They are expensive compared to "conventionals", but one "mover" can replace several individual units. They are much quicker to focus from a console & can be "re-focused" cue to cue. They require a power connection like any other fixture AND a data line for the remote control signal. The control data may be in a proprietary language, but most moving lights are run using DMX-512 as the protocol (see Dimmers).
Modern units can change intensity, focus, color, gobo, size, and sharpness and other characteristics. These are referred to as "Attributes", and each takes one or more channels of control from the control board to operate. As far as the board is concerned, the state of each attribute is treated as a "dimmer" and the dimmer level is translated into an attribute state or position. There may be as few as 5 or 6 attributes or as many as 32 attributes per unit, depending on the complexity of the moving light.
There are two broad categories of Moving lights:
Moving body types have a body held by a motorized yoke. Stepper motors in the base are used to pivot the yoke to control "Pan", the horizontal motion, and stepper motors in the arms of the yoke control "Tilt", the vertical movement of the beam. Inside the unit are many small motors which can rotate controls to iris the beam, spin one or even two disks full of gobos into place, perhaps rotate the gobos, change colors, add diffusion effects, focus the beam for sharp or soft, and even zoom in and out to change the image size. Examples include units from Vari-light, some Martins, Icons, Clay-Pakys, and some High-End units.
Some are units that are basically motorized fresnels (Robo-colors), PARS (VL-5s), and zoom ellipsoidals (VL-6 and VL-7). Also common are motorized LED arrays.
Moving mirror types use a body which is fixed in position, but which directs the light beam onto a mirror attached to a stepper motor. The light is directed by changing the angle of the mirror to redirect the angle the light is reflected from it. All the other controls, the motorized iris, the gobo and color wheels, the focus and zoom motors are inside the non-moving body, and alter the beam before it gets to the mirror..There are advantages to each method. Both types are heavy because of all the stepper motors.
Early on, moving mirror designs were more common, as they were less prone to breakdown. However, as light sources became smaller (and as LEDs became a dominant light source), the moving light fixture became predominant These days, nearly all fixtures are of moving head design.Automated Color change is handled in various ways.
LEDs: Modern fixtures generally use an array of LEDs to provide color and beam spread. Units are smaller and brighter, and can deliver efficiency in a moving head format. Moving mirror is no longer necessary. As a result, most modern fixtures use LED's in a moving head configuration.
Follow Spots spots are specialized instruments meant for manually following a performer around the space to keep them lit. They often have a parabolic reflector to project the light through the lenses, and a gate with irises, shutters to shape the beam. The lenses may also be adjustable to increase and decrease the size of the projected spot field.
Follow Spots use a variety of light sources. Small spots may use incandescent lamps. Bigger, brighter spots use HMI, HTI or Xenon arc lamps. These are short-arc lamps using tungsten electrodes to create the arch inside a high pressure glass lamp filled with a special gas. They will last many hours before they must be replaced. HOWEVER, if they are not cooled properly before being re-lit, the life will be drastically shortened. Therefore, once an HMI, HTI or Xenon lamp is "struck" or turned on, leave it run until the performance is over, and let it cool AT LEAST 10 minutes before re-lighting.
Older spotlight used a carbon arc to create light. They are no longer made, and are actually illegal in most localities. Essentially a carbon arc is an arc welder in a box, fitted with a reflector and a set of lenses. Carbon arcs incorporate a transformer to regulate the arc, usually built into the base. AC carbon arcs used two identical carbons for creating the arc, while DC spotlights used a larger Positive carbon and a smaller Negative carbon, and used a rectifier to convert AC to DC for the arc. Keeping the arc running properly takes practice. As the carbons are burned up by the arc, a motor is used to slowly feed the carbons into the flame, hopefully at the same rate they are consumed. A set of carbons will last from 45 minutes to a little over an hour; then a new set must inserted.
Neither type of arc can be dimmed electrically. They go out if the voltage drops lower than about 80% of full. Therefore, all arc lights must use mechanical fader to dim the light out.
Strong Followspots was a popular brand of spotlight. There are a number of models depending on the "throw" distance involved.
Strong spots typically had two or three control levers on top. The front lever is the Iris, which mechanically reduces the size of the round beam. The middle lever is the Chopper, a shutter that cuts off the top and bottom of the beam. The rear lever if the Douser, which mechanically fades the beam. Followspots are usually an arc light of some sort, which cannot be electrically faded, but must be mechanically faded. There is also a lever on the side, the Trombone, which adjusts the size of the beam field optically, varying it from "normal" to very large.
Lycian Spotlights are another common brand of follow spot. Lycians have similar controls be they are located in different places on the spot. The Iris is typically a lever located on the side rear of the lamp head. The douser is a small lever located at the front of the barrel and at the rear, and the chopper of a twist lever at the back, OR the controls mat be located on top like those of Strong lights. The design seem to vary more model to model and depending on when it was made.