The spread of actinic light due to reflection within the emulsion or from the base surfaces, which causes unwanted exposure, generally in the form of haloes round the images of bright objects.

To combat halation, film manufacturers treat the base of the film to reduce or nullify reflections. grey base is a film base permanently dyed a low density grey colour, this reduces reflection back into the emulsion and light-piping within the base.

Another way is to introduce an anti-halation layer between base and the emulsion which has the same effect as a dyed base; or the coat the rear surface of the base with an anti-halation layer.