Halation

halation [hā′lā·shən]

(electronics)

An area of glow surrounding a bright spot on a fluorescent screen, due to scattering by the phosphor or to multiple reflections at front and back surfaces of the glass faceplate.

(optics)

A halo on a photographic image of a bright object caused by light reflected from the back of the film or plate.

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Halation 

additional, uneven blackening (in black-and-white photography) or coloring (in color photography) that appears near the edges of the image on developed photographic materials. The most noticeable halation effect occurs in the photography of objects that are very bright or that have strong contrast. The halation effect decreases the gamma of photographic materials.

The main mechanism of the halation effect is similar to the mechanism of formation of an optical halo (see). The scattering of radiation in the light-sensitive emulsion causes the appearance of “flare spots” in the areas that were not directly illuminated by the incident light rays. The scattering occurs in the microcrystals of silver halides contained in the emulsion. Another cause of halation is the reflection of radiation that has passed through the emulsion; such radiation may be reflected by the back boundary of the emulsion or by the substrate. In modern photographic materials one side of the substrate is coated with an absorbent (antihalation) layer to reduce such reflection.