There are various types of color media used in theatre. Color media refers to a glass or plastic (gelatin based filters are now obsolete) filter used to alter the color of light emitted from a fixture.
The most common color media used in stage lighting is commonly referred to as gel. Originally Gel was made of dyed gelatin but today it is made from acetate, polyester, polycarbonate, or other plastics. Lighting instruments generally have a set of frames for color media; they vary in size depending on the unit. Gel is cut from a larger sheet to fit in frames that slide into the holders on a fixture. If a fixture does not have frames the gel can be taped or clothes-pinned or binder-clipped to the fixture. Gel is a very flexible media which makes it ideal for use in color scrollers.
Gels work by only allowing a specific color to pass through them. Because of this, they are most effective when used on fixtures whose lamps emit a white light that is made up of generally equal power along all wavelengths of the spectrum. All the wavelengths that cannot pass through the gel are converted into heat. Because of this, any color gel with a low transmission rate or a saturated color has the potential of burning through. This is when the heat causes the gel to lose its color or literally start to melt.
Dichroic filters are glass filters that are coated with a special coating that allows certain wavelengths of light to pass through it while reflecting the rest. This type of filter generally has a longer life-span than gel and is claimed capable of producing richer colors. Since this media is rigid it cannot be used in color scrollers [EDIT: Rosco now has a DichroFilm available], but there are color changing devices that do make use of dichroic filters.
Dichroic filters are available in a variety of sizes, generally either in rounds or in squares. Though not available in every color offered by manufacturers as gel colors, there are many available including colors that are not offered in gels. Colors like "Woods Glass" which only transmits UV wavelengths are made for producing black light effects or filtering the UV energy from projected light. The reflectors in many modern ERS fixtures like the ETC Source Four use dichroic reflectors to remove some of the invisible wavelengths from the beam, which effectively cools the beam of light.
Dichroics are often used in intelligent fixtures and devices that feature CMY Color Mixing. In this case the dichroic coating is usually applied in a gradient to clear or in a "toothed" pattern to vary the density of color. The static color wheels in most intelligent fixtures also generally use dichroic filters as they have to be able to withstand the high temperatures inside the fixture.
Other types of color media exist, such as rondels by Kopp Glass, plain glass: Devon Filters, or rigid polycarbonate plastic: SpecialFX®. One of the other common ways to color light is to use lamp dip. Lamp dips are usually not intended for high wattage lamps, and are used simply by dipping the lamp in a liquid that coats the envelope with color. It is often used to color standard household lamps for use as backstage run lights.
Rosco, Lee, GAM, and Apollo (in chronological and market-share order) are the major color media manufacturers at this time. In the US you can generally purchase gel in sheets of 20"x24" or in rolls of 24"x25'. Some of the manufacturers also offer some of their gels in versions that are designed to withstand high temperatures. Apollo even offers gel in pre-perforated sheets so that the user does not need to use a cutting tool to cut the gel to the correct size.
A fairly comprehensive cross reference chart: TLS,Inc. tools - Gel Cross Ref.
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