Let’s get technical for a second to explain in simplified terms how halation occurs when shooting on film.

When light hits film stock it travels through the blue, green and red layers and the light is absorbed. When these 3 layers are combined, they create a color negative. When you print them to a positive you get the footage we are used to seeing.

The last layer is the anti-halation backing and it’s purpose is to absorb any light that penetrates the red layer. When the light is really bright, the anti-halation backing can’t absorb it all, and some of the light is reflected back onto the red layer.

The result of that reflected light can be seen in very bright parts of the shot with a reddish glow, usually around light sources, like lamps, car headlights, backlit windows or the sun.