Anti-halation backing

An anti-halation backing is a layer found in many photographic films—and almost all film intended for motion picture cameras—usually a coating on the back of the film base, though it is sometimes incorporated between the light-sensitive emulsion and the base.

This prevents a halo-like effect (halation) from forming around bright points or edges in the image.

[citation needed] The notable exception was Kodak's Kodachrome, which incorporated such a backing to aid with a very sensitive innermost layer.

[3] Halation is one of the properties unique to analog film stock[4] and isn't found in digital footage unless modified in post-production.

Despite anti-halation backing, most film stock still renders a slight red halo around the brightest elements in a picture, where the incoming light is so strong that it cannot fully be absorbed by the anti-halation backing, and instead is scattered back into the red layer of the stock, creating additional, halo-like exposure in that particular layer, before it gets fully absorbed.

layers of film and light interacting
Anatomy of the halation effect: the incoming light penetrates the blue, green, and red layers of the film stock (in that order), then are typically absorbed by the anti-halation backing. The strongest rays are not completely absorbed, and bounce back into the red layer and create halation. [ 1 ]
Digital "halation" before and after, to illustrate what an image looks like without and with halation