Jenn Nelson-Kelley
Technical Production
Lamps
Different types of lamps (the technical term meaning light bulb) emit different types of light. There are many different types of lamps, and they can work many different ways. The four types of lamps most often used in technical production are Incandescent Lamps, Tungsten-Halogen Lamps, Arc Lamps, and Fluorescent Lamps. Increasingly, LED’s are being incorporated into stage fixtures.
Incandescent Lamps
Incandescent lamps, such as the ones you use in your house, have tungsten filaments sealed into a bulb that is filled with inert gas. The inert gas is mostly non-reactive, but contains some oxygen. The bulb, or envelope, is made of pyrex or synthetic quartz. When electricity passes through the filament, it heats up until it glows. The glowing filament is what produces light. When the filament heats up enough to glow, however, the filament starts to decompose, releasing tungsten particles into the gas surrounding it. Over time, the filament will give off particles until it finally becomes so weak it breaks. Because the light is created by the burning filament, the light produced is red or orange in color, with very little blue light emitted.
Tungsten-Halogen Lamps
Tungsten-Halogen lamps, the type we use most often in stage lighting, are very much like incandescent lamps. However, while the filament is made of tungsten, the bulb is filled with halogen. Halogen is a chemically active gas, as opposed to an inert gas. When the filament releases particles, these particles combine with the halogen to form a compound that is attracted back to the filament. The tungsten particles rejoin the filament, and the halogen is released back into the bulb’s “atmosphere.” This configuration allows us to push more wattage into the lamp, while making it last longer. Our average stage lamp is 575 watts. TV/Film lights can be as much as 20Kw, or 20,000 watts. The envelope is always made of synthetic quartz, in order to withstand the large amount of heat necessary for the chemical reaction between the filament and the halogen. You must never touch the envelope on these lamps. The oils on your fingers will stick to the quartz, and cause uneven heating inside the envelope, making the lamp burn out very quickly. These incandescent, filament type lamps often emit lots of amber light, but not much blue.
Arc Lamps
Arc lights, such as xenon or carbon arc lamps, have no filament. Instead, an electrical arc that jumps between two electrodes is the light source. In an oxygen rich environment, the electrodes deteriorate quickly. But, if the electrodes are encased in an envelope filled with a non-corroding gas, (such as xenon or metal halide,) they will last for a long time. Arc sources require a constant amount of electricity, and cannot be dimmed. For this reason, Arc lamps are mostly used either in fixtures with built in, mechanical “dousers” (such as followspots) or for TV/Film. These types of lamps emit a light with a blue cast.
Fluorescent lights
A fluorescent light is usually a long, straight tube that produces white light. You see fluorescent lights in offices, stores and some home fixtures, as well as in fixtures used for television and movie lighting.Inside a fluorescent light is Argon, with a small amount of Mercury gas. When ionized by the cathode inside the envelope, this gas emits ultraviolet light. Human eyes cannot see ultraviolet light. Therefore, the inside of a fluorescent light is coated with a phosphor. A phosphor is a substance that can accept energy in one form and emit the energy in the form of visible light. In a fluorescent lamp, the phosphor accepts the energy of ultraviolet light and emits visible light. The light we see from a fluorescent tube is the light given off by the phosphor that coats the inside of the tube (the phosphor fluoresces when energized, hence the name.) Fluorescent lights do not emit true white light, but rather a greenish-white, because the phosphors generate very little red light. If the light appears to be any other color, the outside of the envelope has been colored. This is usually done by the manufacturer, but gel tubes can be used for the same purpose.
LED Fixtures
LED’s are “solid state;” the have a single component embedded in a plastic shell. LED stands for Light Emitted Diode; a diode only allows electricity to flow one way, and lights up when that occurs. LED’s emit a very specific range of wavelengths. As a result, they have a very specific color that varies wildly, and may “miss” certain wavelengths altogether (particularly amber.) A good quality RGB fixture, either with separate red, green, and blue diodes or a combo diode where all three are embedded in the plastic shell, should be able to produce nearly the full spectrum.
Color Temperature
The subtle differences in color produced by the different lamps are generally referred to as “color temperature,” and are measured in Kelvins. A xenon lamp has a color temperature of around 6,000 degrees Kelvin, fluorescent is at 4,000 K, and incandescent about 3,200. By comparison, sunshine measures at around 5,700. Obviously, the filament is not actually burning at 3,200 degrees Kelvin. What this number refers to is the surface temperature of a star producing an equivalent color. This is where things get kind of weird. When an object is heated, it gives off what is known as "black body" radiation. A star with a surface temprature of 3000 Kelvin would burn red, emitting mostly red light. A slightly hotter star, with a temperature of 3,200 Kelvin, would emit an amber light. Our sun has an estimated surface temperature of around 5,700 Kelvin, and defines our perception of white. Stars burning at 6,500 Kelvin or greater appear blue. To recap, the colder a star burns, the more orangey-red it is, and the hotter it burns, the more blue it is. That’s the basic idea behind color temperature. The lower the color temperature, the more orange the light emitted will be. The higher the color temperature, the more blue the light emitted will be.
Amber Drift
Another topic related to color temperature is Amber Drift. As you dim a tungsten-halogen lamp, the color temperature lowers. This makes the light from the fixture look more amber at 50% intensity than it does when it’s at full. This phenomenon is called amber drift, because the color temperature drifts closer and closer to amber as you reduce the intensity of the lamp. Amber drift can significantly change the effect of the gels being used. For instance, Roscolux 51, surprise pink, is actually a pale lavender. When the fixture with this gel is turned on at full, the light is a pale, blue-lavender color. However, when the fixture is at around 50%, the light becomes a warm pink. The reason is that the blue in the filter blocks some of the amber from the amber drift, causing only the red (pink is light red) to pass through.