Halation

Glossary Terms

Halation is a phenomenon that occurs within film during exposure. Light scattered as it passes through the emulsion reaches the back of the film, and some of it is reflected off the rear surface of the film support, back into the emulsion. This leads to visible haloes around bright point objects in the image, and along bright edges. Detail is lost in these areas, and the effect is generally considered undesirable. Many film manufacturers counter halation by coating the film base material, below the emulsion layer, with a gelatine layer containing anti-halation dye; a dark-coloured dye that serves to absorb much of the light that reaches the back of the emulsion layer. This dye is water-soluble, and washes out of the film during development without affecting the developer, though it colours the solution.