The spreading of light through a photographic emulsion, which results in the recorded image of a star being larger than the actual image. The more light that falls on the emulsion, the larger the recorded image. Halation occurs because the light diffuses through the grains of the emulsion, and reflects off the surface of the glass or film on which the emulsion is coated. It has a practical use in that it allows a star's brightness to be found by measuring the size of its image.
Subjects: Astronomy and Astrophysics.