Note:  Do not rely on this information. It is very old.

Halation

Halation. In the photography of object or landscapes in which very bright parts are present, the high lights may frequently appear blurred and surrounded by a halo. This appearance is known as "halation," and is caused by the reflection, from the back of the glass, of light which has penetrated the film. It is often seen around windows in photographs of interiors, and in the case of dark objects, as tree trunks, branches, etc., standing out upon a bright sky. Halation may be more or less completely prevented by various devices: as (1) backing the plate with a black (or yellow) varnish, so that the reflection of light (or actinic light) becomes practically nil, and washing away the varnish before development; (2) by the use of thick opaque films; (3) by the use of films supported upon transparent paper, etc., instead of glass. Varieties of plates intended especially to obviate halation are also obtainable in the market.