The basic idea of film is to capture patterns of light using special chemicals. The camera briefly exposes the film to the light coming from a scene (typically for a small fraction of a second), and where the light hits the film, it starts off a chemical reaction.
The instant-camera developing process combines colors in the same basic way as slide film, but the developing chemicals are already present in the film itself. In the next section, we’ll see how the developers are combined with the color layers to form the picture.
Before you take the picture, the reagent material is all collected in a blob at the border of the plastic film sheet, away from the light-sensitive material. This keeps the film from developing before it has been exposed. After you snap the picture, the film sheet passes out of the camera, through a pair of rollers.
The rollers spread the reagent material out into the middle of the film sheet, just like a rolling pin spreading out dough. When the reagent is spread in between the image layer and the light-sensitive layers, it reacts with the other chemical layers in the film. The opacifier material stops light from filtering onto the layers below, so the film isn’t fully exposed before it is developed.
The reagent chemicals move downward through the layers, changing the exposed particles in each layer into metallic silver. The chemicals then dissolve the developer dye so it begins to diffuse up toward the image layer. The metallic silver areas at each layer — the grains that were exposed to light — grab the dyes so they stop moving up. Only the dyes from the unexposed layers will move up to the image layer.