A transparent material having the ability to absorb certain wavelengths of light and transmit others (1983). A transparent material (glass or plastic) fitted to a lens that alters the characteristics of light passing through it, most commonly in colour.
Can also consist of neutral density to control light during printing.
In colour film, there is often a thin, uniform, layer that is coated above or below the emulsion to serve as a light filter;it controls the spectral quality of the light reaching the emulsion below.
Happe, L. B., 1983, Your Film and the Lab, 2nd Edition, Media Manuals, Focal Press, London, Boston