Gels, are the coloured 'filters' placed in front of the lights found in theatres, so to colour the beam. Gels are referenced and referred to by number or a letter and number combination. The numbers correlate to a specific color. Since the numbers are assigned by the manufacturer, and are not interchangeable between brands, if gels from multiple manufacturers are present in one shop they will be marked with a letter before the number; typically an R for Roscolux, and L for Lee Filters or an A for Apollo. Three of the most common manufacturers of gels, Lee Filters, Roscolux, and Apollo manufacture 'swatches'. These are packaged as small books with samples of all the different gel colors, allowing lighting designers to sample different gels. You can also obtain a color equivalancy chart from the manufacturers that compares the different manufacturers number systems relate to the color of the filter.
Gels were originally made out of gelatine. "Gelatine" was shortened to "gels". How and when this occurred is not known. Today, gels are made from a mylar-like polyester. However, the term "gel" remains in common usage. The more correct term color filter, sometimes shortened to color, is also used to refer to these filters.
When gels were made of gelatine, many lighting rookies were given dusty gels that they were to get clean by rinsing in hot water. When they came back, some came back smiling and others came back horrified, convinced that they had done something wrong.
Gels are normally placed in a "color frame", which is a metal or flame-retardant paper frame that holds the gel. This ensures the gels do not wrinkle from the heat as much as they would otherwise and also that they do not bend and fall out of the light. Metal frames are used in most applications because they are more sturdy, and last longer. Paper frames are sometimes used in positions where the lighting instrument will be placed above the audience. The frames are put into the color frame slot at the front of the lighting instrument to color the light.
During use, gels wrinkle and eventually burn through after being used for some time. This is due to the large amount of heat coming out of the luminare and going into the gel. Gel color determines how quickly the gel wrinkles and 'burns-through', with the rate depending on how much heat is concentrated on the gel. Darker colors, which have a lower rate of transmission, mean that more light is stopped from going through, resulting in more heat, and shorter lifespans.