Color filter

A color gel or color filter (British spelling: colour gel or colour filter), also known as lighting gel or simply gel, is a transparent colored material that is used in theater, event production, photography, videography and cinematography to color light and for color correction. Modern gels are thin sheets of polycarbonate or polyester, placed in front of a lighting fixture in the path of the beam.

Gels have a limited life, especially in saturated colors (lower light transmission) and shorter wavelength (blues). The color will fade or even melt, depending upon the energy absorption of the color, and the sheet will have to be replaced. In permanent installations and some theatrical uses, colored glass filters or dichroic filters are being used. The main drawbacks are additional expense and a more limited selection.

In Shakesperean theater, red wine was used in a glass container as a light filter. In later days, colored water or silk was used to filter light in the theater. Later, a gelatin base became the material of choice. Gelatin gel was available at least until 1979. The name gel has continued to be used to the present day. Gelatin based color media had no melting point, and the color was cast in the media as opposed to being coated on the surface, both important properties for color media. It would, however, char at high temperature and became brittle once heated, so it was impossible to handle once used in the lighting instrument. By 1945 more heat-tolerant self-extinguishing acetate-based through-dyed materials were being manufactured (marketed as Chromoid then Cinemoid by Strand Electric). In the States Roscolene (acetate) was also developed to deal with these higher output light sources. Though cheaper, the acetate filters fell out of favor with professional organizations since they could not withstand the higher temperatures produced by the 'tungsten halogen lamps' that came into widespread use in the late 1960s. The acetate-based material was replaced by polycarbonates like Roscolar (mylar polycarbonate) and polyester based filters. These materials have superior heat tolerance when compared to acetate-based gels. Many were transparent film with a surface coating. The first dyed polyester "gels" were introduced by Berkey-Colortran in 1969 as Gelatran, the original deep dyed polyester. The Gelatran process is still used today to produce GAMColor (100% of the line) and Roscolux (apx. 30% of the line). Other color manufacturers, such as Lee Filters and Apollo Design Technology use a surface applied dye. (Roscolux is 70% polycarbonate and 30% deep dyed polyester.) Almost every color manufacturer today uses either polycarbonate or polyester to manufacture their "gels". Even today's gels can burn out (to lighten in color starting in the center) easily, rendering them useless. To help combat this, a high temperature (HT) material (polyester having the highest melting point 480 °F (250 °C)), can be used to help prolong the life in high-heat output lighting instruments. As instrument design improves, it has become a selling point on many lights to have as little heat radiating from the front of the fixture as possible to help prevent burn-through, and help keep the stage and actors cooler during performances.

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