halation

Halation is a visual artifact in photography and motion picture film in which light appears to glow beyond the edges of its bright source, producing a halo around high-contrast areas. It arises when light from a highlight penetrates the light-sensitive layers and is scattered or reflected within the film stock or backing material, then re-emerges, softening boundaries and reducing local contrast. The effect is most noticeable around bright subjects set against darker surroundings and can be influenced by development, exposure, and the specific materials used.

Mechanically, halation occurs when light passes through the emulsion and the clear base and is reflected from the underlying support or back layer, then travels back through the emulsion. If the backing layer efficiently absorbs stray light, the halo is reduced; older or experimental stocks without strong anti-halation properties are more prone to halation. In some historical film stocks, especially in early cinema and certain photographic processes, halation contributed a visible glow that some artists valued for its dreamlike quality.

Mitigation and variation: many traditional film stocks incorporated an anti-halation backing to absorb stray light, thereby suppressing halos. Some photographers and filmmakers either avoid stocks with significant halation or use processing methods to minimize it. In modern digital imaging, halation is not a physical phenomenon, though film-like halation can be simulated in post-production or by emulating vintage stock aesthetics. Halation remains a noted term in discussions of film history and vintage photography, often contrasted with sharpness, contrast, and bloom.