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A source Four ERS, gelled a light yellow color. The gel is held in a specially-mounted frame at the end of the barrel of the fixture, and the frame is itself slotted into holders.

A color gel or color filter (UK colour gel or colour filter), or a lighting gel or simply gel, is a transparent colored material that is used in theatre, 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. 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 theatre, red wine was used, in a glass container ,as a colorant in lighting. In later days, colored water or silk was used to color light in the theatre. Later, gelatin became the material of choice. More heat-tolerant self-extinguishing acetate-based materials (marketed as Geltran, Cinemoid and Roscolene) were developed to deal with higher output light sources. This material fell out of favour, since it could not withstand the higher temperatures produced by the tungsten halogen lamps that came into widespread use in the late 1960s. The name gel however has continued to be used to the present day. 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) gel 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 is possible to help prevent burn-through, and help keep the stage and actors cooler during performances.

A Motorised Color Gel

Gels are typically available in single 20×24-inch (500×600 mm) sheets cut from rolls 24" or 48" (600 mm or 1200 mm) wide and 50' (16 m) long, which are then cut down to the appropriate size before use. The size originates from the gelatin days: it is the same as a standard baker's sheet, which was used to cast the sheets.

Similar colors may vary between different companies' formulations - for example, many have a color named "bastard amber", but the transmitted color spectrum may be different. For this reason, gel colors are not referred to by name, but instead by a code consisting of a letter and number combination. For example, G841 is a dark blue made by Great American Market (GAM), and R02 is a light amber made by Rosco and L216 is a diffusion filter made by Lee Filters.

Manufacturers produce swatch books, which contain a small piece of each color available, adjacent to its color code, to simplify ordering. Swatch books enable designers and technicians to have a true representation of the manufacturers' range of color.

Most designers chose a color palette of 15-30 colors that they like and use for generic applications because it is financially and logistically difficult to have access to all colors for a single show.

There are also gels for color correction, such as CTB and CTO, which alter or correct the color temperature to be higher with a light blue, or lower with a light orange, respectively. A "half" CTB or CTO is very light, for minor corrections.

"ND" is a neutral-density grey, meaning it affects all colors evenly and reduces the light intensity neutrally with respect to wavelength.

Most ranges of gels also include non-colored media, such as a variety of diffusion and directional "silk" materials to produce special lighting effects. "Opal" for example is an opalescent or translucent diffusion filter.

It is common for a gel manufacturer to publish the transmission percentage (amount of light that is not absorbed) or even the transmission curve in the swatch book and catalogs. A low transmission gel will produce relatively little light on stage, but will cast a much more distinct color than a high transmission gel.

Occasionally, as a hazing for new lighting technicians (usually for those who have never worked with such lighting), an old gelatin color gel can be given to an apprentice to clean (sometimes to be told that it is the only bottle they have and must be careful). They are told to wash the gel with water, only to have the old gelatin sheet dissolve in their hands much to the inside entertainment of their colleagues upon their return.

Manufacturers of gel include:

• Apollo
• Great American Market (GAM)
• Lee Filters
• Rosco International