Optical filters consist mainly of colorants dissolved in gelatin.
Most Kodak filters are made by dissolving suitable organic dyes in liquid gelatin and coating the proper amount of the solution onto prepared glass. After the coating is dry, the gelatin film is stripped from the support material and coated with lacquer. Each gelatin filter is standardized for spectral transmittance and total transmittance by special instruments which apply an optical form of a limit gauge to these characteristics. The dyes are obtained from a number of sources, and many have been synthesized. Like other dyes, the dyes used in filters may, in time, change under certain conditions of heat and light.