All incandescent lamps work by sending an electric current through a metal wire called a filament. The metal resists the passage of current, and in so doing it gives off heat, and if it gets hot enough it gives off light. The wire is placed in a transparent glass envelope in order to keep out oxygen; otherwise it would just burn. Originally there was a vacuum inside. However if the filament is hot enough to give out light, eventually due to heat it will evaporate, so the bulb is filled with a gas which conducts away some of the heat and slows down evaporation. When enough bits of filament evaporate from the same part of the wire, a gap is formed in the wire and it can't conduct the current, and so the lamp burns out.

Household incandescent bulbs usually have a pear shape, and the filament usually forms part of a circle, so that it can emit light in all directions and flood the room. The bulb is filled with an inert gas such as argon. In theater lighting we usually need the light to be focused in one direction only, so that it goes to the reflector and the lens, and not to the sides, so theater lamps were manufactured with the filament in the shape of a plane, parallel to the lens and reflector planes. If you look closely at the filament you'll see that it's not a simple wire but rather a coil. In fact, it's a coil which is then coiled round itself, to make it burn hotter, and the coiled wire is then put in a plane shape. Household lamps might have a rated average lifetime of about 800 hours, but in the theater lifetime is less important than intensity, so theater lamps are made to run at a hotter temperature, even though that means a shorter lifetime, generally about 200 hours.

Nearly all theater incandescent lamps today are of this kind. Regular incandescent bulbs burn out because the heat causes bits of filament evaporate (though this is slowed down by gas pressure). Tungsten halogen lamps, on the other hand, are filled not with inert gas but rather with one of the halogen gases — iodine or bromine (Indeed, in the early days of this technology, tungsten halogen lamps were referred to as "quartz iodine", because of the iodine gas and the quartz used in the glass bulb. Even today, some people refer to them as "quartz.") When a bit of filament evaporates, it combines with molecules of the gas to form a compound, eventually molecules of the compound are drawn back to the filament and the bits of filament then redeposit on the wire. Why doesn't this lead to eternal life for the bulb? Because the bits of filament don't necessarily go back to the same place they came from, and eventually there will still be a gap in the wire...and the lamp burns out. Still, lamp life is far longer than that of the traditional incandescent bulbs.

However - as I said, lifetime is not as important for theater lamps. So rather than giving the new lamps longer life, they were made to give brighter and whiter light and a more compact light source, which makes for far more accurate optics. Mirrors and lenses need a point source to be absolutely precise, and the smaller the light source, the closer it is to a point, and the more light can be utilized efficiently. In addition, a Fresnel or PC with a physically smaller lamp will have a larger range of beam size, because there's more room to move the lamp further from or closer to the lens. So the lamps are made much smaller, at the expense of the extra lifetime.

Tungten halogen lamps are also made to burn hotter than do traditional incandescents. Since ordinary glass would not withstand the greater heat,the envelope is quartz. If the bulb is touched directly, and not with paper, cloth, or protective glove, skin oil is deposited on the quartz and when the bulb heats up, that part of the glass will expand into a bubble and eventually crack. If touched, the bulb should be cleaned with alcohol and allowed to dry before the lamp is used.

All incandescent lights are dimmed by reducing the current that goes through the wire. Dimming has a significant effect on lamp life: a 5% reduction in voltage will double lamp life, while reducing light output by about one fifth.

Like incandescents, discharge lamps are filled wih gas, but they do not have a filament. If a high voltage is applied to the gas, electrons are knocked off of the atoms of the gas. The gas is then full of positive ions and electrons, which are attracted to electrodes at the opposite ends of the lamp. After the initial push of the high voltage, the gas will keep on conducting current because the moving charged particles then bang into other atoms and knock electrons off of them, too.

In some cases when they bang into an atom they aren't going fast enough to knock off its electrons, but the "banging" gives energy to the atom, thus raising its outer electrons to a higher level. As those electrons fall back down to the original level, they emit that extra energy in the form of light. The color of the light depends on the type of gas (specifically it depends on the size of the energy gap in the gas). Discharge lamps often are filled with a mixture of gases and the dominant one is responsible for the color.

Discharge lamps can be low or high pressure. In low pressure lamps the charged particles move a relatively long distance before banging into another atom, meantime building up enough speed so that when they do bang into it, they knock off electrons. In high pressure lamps there are more collisions and so the gas gets hotter. This causes electrons to be excited to higher energy levels before being knocked off at higher energy levels, changing the color of the gas.

Light from discharge lamps does not have a continuous spectrum like incandescent lamps, but rather the light is produced at discrete bands of the spectrum.

There are various types of discharge lamps, characterized by the type of gas that fills them:

• Fluorescent:

  This is the most common type of discharge lamp, found in many homes and offices. Household fluorescents are filled with mercury at low pressure, which produces invisible ultra-violet light. The inside of the tube is coated with phosphor, which converts the UV to visible light. Fluorescent lamps are available in different shades of white (daylight, cool white, warm white) whose different colors are due to the combination of phosphors on the envelope. They were originally (and still are) manufactured as tubes, but today, compact fluorescents ("CFL") are also common and are found with a screw base identical to the household incandescent. Fluorescent light is generally cooler (in color) than incandescent light.

  Dimming: Fluorescent lamps can be dimmed, but generally a special circuit needs to be added in series with the dimmer. The basic idea is that the current going through the lamp can only be reduced after the arc has been struck (that is, the first push of current has caused ionization to begin). This used to involve preheat transformers, but today, electronic control makes it easier. Sometimes it's possible to connect a fluorescent to a regular dimmer with a "ghost" load (a conventional incandescent located in an unobtrusive place). Before the show the dimmer is brought sharply up to full, and afterwards not allowed to go below about 10% of maximum intensity, so that the gas remains warm. How well this works depends on the particular dimmer circuit, and it's better, if possible, to use a specialized fluorescent dimmer.

• Sodium Vapor:

  Sodium vapor lamps give off yellow light, and come in two types: high pressure (HPS) and low pressure (LPS). Low pressure sodium gives off an almost completely monochromatic light! This means that the only colors you will see under such light are white and yellow. This can be used to fascinating effect. I once saw a performance of the Sleeping Beauty where at the ball, after the witch cast her spell, incandescent lighting with the same tint as sodium was slowly replaced by low pressure sodium lamps. It looked as if the colors just drained out of all the costumes as the spell did its work.

• Mercury vapor:

  Mercury is often used with a mixture of gases, for example metal halide lamps contain mercury along with halides (compounds of metals with bromine or iodine). All these lamps have a bluish color, which is modified by the other gases in the mixture. Generally lamps with initials ("HMI", "CID", "CSI") are of this type. These discharge lamps are used in most high intensity follow spots. Like most discharge lamps, they can't be electrically dimmed. In the theater they are generally dimmed mechanically, with a "venetian blind" type shutter in front of the instrument lens, or an internal shutter in some follow spots.

  HMI Fresnels originally used in film are now used in theater, dance, and opera when a very powerful single source is desired.

• Xenon:

  Extremely powerful follow spots, such as the Strong Super Trouper, have Xenon lamps. These are always at high pressure, give off a bluish white light, and, because of the high pressure, extreme care must be used when handling the lamp. Xenon lamps for follow spots are usually supplied with protective goggles and plastic gloves to be used when changing the bulb, and the lamps should never be changed or otherwise handled by untrained personnel.

• Neon:

  Commercial neon lights, used in signs and advertising, use a very high voltage of several thousand volts. They aren't all actually filled with neon gas; the red ones have neon gas, yellow have helium, blue have mercury. There is a related lamp, also called "neon", which is very small, and used as an indicator light, for control panels on boilers for example, and as circuit testers; this is actually called a neon glow lamp and contains neon gas at low pressure. In theory, dimming is the same as with other discharge lamps, but in practice I've connected them to a regular AC theater dimmer and it worked.

LED stands for "Light Emitting Diode". This is a semiconductor device which emits light when a current flows through it. The general idea is that as electrons move through the semiconductor material, they will suddenly lose energy, due to the structure of the material. These energy losses happen in jumps: a certain packet of energy will be lost at one jump, and the energy thus released emerges as light. The exact amount of energy determines the color of the light. Each LED will give off light in a specific color, because it is constructed so that the electron jumps will contain just that amount of energy. Today LED bulbs are available which give off multiple colors, but these are actually tiny envelopes containing several different LEDs, most generally red, green, and blue which then combine to make other colors.

White LEDs are actually blue, with phosphor on the inside of the envelope converting the blue light to white. LEDs were first used in theater for decorative purposes (star curtains, signs); however, in recent years there has been a huge increase in efficiency, so that today they are bright enough to be used in lighting instruments. Dimming is done with an electronic unit which connects to a DMX cable to a lighting computer, where the LEDs can be called up as channels (for instance, Channel 101 might be red, Channel 102, green, and Channel 103, blue) or as spots in lighting consoles for computerized lights.

Instruments using LEDs today include:

• Par LEDs: theater units containing between 18 and about 40 LEDs, generally in red, blue and green, but often with amber and/or white also included. These might use only 40 Watts of electricity and be as bright as a 500W Fresnel.

• Wash units: very useful for lighting scenery and as footlights.

• LED Ellipsoidals: available in multicolored units (for color mixing) or in white (desireable mainly for their low power consumption and long lamp life).

Dimming is not as smooth as with incandescent lights, but it's improving all the time. There are also moving lights which use LEDs. These fixtures are also discussed in our chapter on Lighting Fixtures.

LEDs are used today in lighting homes, buildings, streets, and many other applications because of their extremely low power consumption and extremely long lamp life; average life of an LED may be 10,000 hours, which is very convenient in places where it's difficult to change a lamp. This is not very important for theater; however the low power consumption can be useful. The colors themselves are wonderfully pure, and LEDs are increasingly used to replace units with color scrollers.