antihalation backing[‚an·tē·hə′lā·shən ′bak·iŋ]
a nonphotosensitive layer applied to the back of a motion-picture or still photographic film support or a photographic plate; it may sometimes be applied between an emulsion and a support. The necessity of an antihalation backing arises during the photographing of very bright subjects, when light rays may pass through the emulsion and reflect back from the surface of the support. This produces halation, a secondary image that washes out the primary image. Use of an anithalation backing makes it possible to eliminate the formation of halation.
The antihalation backing is usually a gelatin dyed red, green, or gray, depending on the type of photographic emulsion used. An emulsion with low sensitivity is sometimes used to achieve the same effect; this emulsion does not take part in the formation of the image. The antihalation backing is bleached out during film processing.