As Eric Turnbull, professor of theatre arts and communication studies at Northern Virginia Community College, explains, in the pre-electricity years, stage designers mixed colored liquid in clear bottles to exacting tones before placing the bottles in front of a flaming light source. Though effective, the bottles could heat up, causing the liquid to boil and the bottles to burst.
Sheets of colored glass came into use as the cost of glass declined; while they remain in use today, disadvantages include their weight and cost. Until the mid-1970s, theaters used colored gelatin sheets, leading to the name, "gels." Moisture can easily destroy the cheap and light-weight gels, however.
Today, companies such as Rosco, Apollo, Lee and GAM manufacture and sell plastic gels in large sheets similar to bolts of fabric, cut to order in an exact size and shape. Unlike gelatin or colored light bulbs, you can store gels in a flat file or notebook, and clean them with water. The durability of gels combined with their ease of use, portability and simplicity make them a lasting choice among designers, Turnbull reports.
Visible sunlight or white light is actually made up of different wavelengths. Rain droplets or a prism separate the light into its component colors: red, orange, yellow, green, blue, indigo and violet. Gels, as light filters, allow only certain wavelengths of the color spectrum to pass through. For example, placing a red filter blocks the other wavelengths and allows the red part of the color spectrum to pass through, note Judy Kupferman and Jeffrey E. Salzberg on StageLightingPrimer.com.
"Subtractive mixing" involves gels used in combination or groups; for example, using a yellow and blue gel in front of the same instrument filters out all but the yellow and blue, allowing green light to pass through. Additive mixing occurs when two or beams of light meet on the subject, making it appear lighter or brighter. Adding blue, green and red together on one spot would give you white light.
People use colored gels most frequently with white or near-white incandescent and florescent lighting. Called "lamps" in lighting parlance, single bulbs paired with a lens and socket become a "lighting instrument." Generally, gels go into frames inside the instrument or in front of the lamp, close enough to catch most of the passing beam of light, but far enough away that the gel doesn't scorch, fade or melt. Those using or installing gels should consult the material safety data sheets from the gel manufacturer and instructions from the instrument maker.
According to a 2010 study by Pike Research, LED and other solid state lighting devices will account for over half of commercial, industrial and home lighting by 2020. Leading gel color manufacturers and lighting companies currently market and sell colored, more efficient LED systems. Stage, event and performance lighting designers likely will continue to adopt LED lights, as development and the cost barriers fall with ongoing development, point out Kupferman and Salzberg.