Difficulty is sometimes experienced in photographing scenes in which bright lights appear, due to the halo which surrounds such objects in the picture. This halo is due to two causes:
1. irradiation, or the spreading of light within the emulsion; and
2. halation which is produced by reflection of light from the rear surface of the base back into the emulsion.
The halo around bright objects in the picture, so far as it is due to halation, may be improved or prevented entirely by backing the emulsion support with a light absorbing substance which will prevent the reflection of light back into the emulsion.
For motion picture work a soluble backing in the rear of the film was not desirable, and therefore, in 1931, the Eastman Kodak company introduced grey-based film, in which the backing is an integral part of the film base and involves no danger in processing. The increased density of the negative (approximately 0.25) is readily compensated for by increasing the printing exposure.
British Standard Glossary of Terms used in the Motion Picture Industry