In photography and cinematography, halation refers to the spreading of light beyond its proper boundaries. It creates a soft, glowing effect around the brightest parts of an image.
The word halation comes from the word halo, a glowing circle of light shown around the head of an angel, saint, or other holy person in many paintings. Halation is usually associated with analog film. When you take a photo, light passes through the film's light-sensitive layer. Some of the light might go all the way through the film and reflect back, scattering into a slightly larger area. It might create a reddish or orange glow in color film, or a soft white blur in black-and-white film. While halation was originally seen as a flaw, today, many photographers and filmmakers like the artistic, dreamy feel of halation. They often try to imitate the effect digitally.