Instant camera film is remarkable because it has its own built-in developing studio. To understand how this works, you need to understand the basics of traditional photographic film. Film is a plastic base coated with particles of silver compound that are sensitive to light. Black-and-white film has one layer of silver compound, while color film has three layers -- the top layer is sensitive to blue light, the next layer is sensitive to green and the bottom layer is sensitive to red. When you expose the film, the sensitive grains at each layer react to light of that color, forming metallic silver at that layer. This gives you a chemical record of the light and color pattern.
To turn this into a picture, you have to develop the film. One developer chemical turns the exposed particles into metallic silver. The film is then treated with three different dye developers containing dye couplers. The three dye colors are cyan (a combination of green and blue light), magenta (a combination of red and blue light) and yellow (a combination of green and red light). Each of these dye coupler types react with one of the color layers in the film. In ordinary print film, the dye couplers attach to particles that have been exposed. In color slide film, the dye couplers attach to the non-exposed areas. Developed color film has a negative image -- the colors appear opposite of the colors in the original scene. In slide film, the two dyes that attach to the unexposed area combine to form the color captured at the exposed layer. For example, if the green layer is exposed, yellow and cyan dye will attach on either side of the green layer, but the magenta dye will not attach at the green layer. The yellow and cyan combine to form green.
The instant camera developing process combines colors the same basic way as slide film. It has the same layers of light-sensitive grains as traditional film, all arranged on a plastic sheet. The film also contains several additional layers, however. These layers contain all the necessary chemicals for the development process. Underneath each color layer, there is a developer layer containing dye couplers. All these layers sit on top of a black base layer, and underneath the image layer, timing layer and acid layer. This arrangement is essentially a chemical chain reaction waiting to be set in motion.
Instant camera film has the entire developing process arranged in chemical layers. When the reagent enters above the light sensitive layers, it gets the process going.
The component that gets the developing process going is the reagent (as in re-agent), a mix of opacifiers, alkali, white pigment and other elements. The reagent sits in a layer just above the light-sensitive layers and just below the image layer. Before you take the picture, the reagent material is all collected in a blob at the border of the plastic 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. For example, if the green layer was exposed, no magenta dye will make it to the image layer, but cyan and yellow will. These colors combine to create a translucent green film on the image surface. Light reflecting off the white pigment in the reagent shines through these color layers, the same way light from a bulb shines through a slide.
At the same time these reagent chemicals are working down through the light sensitive layers, other reagent chemicals are working through the upper film layers. The acid layer in the film reacts with the alkali and opacifiers in the reagent, making the opacifiers become clear. This lets you see the image below. The timing layer slows the reagent down on its path to the acid layer, to give the film time to develop before it is exposed to light.
When you watch the image in a photo film come into view, you're actually seeing this final chemical reaction. The image is already developed underneath -- you're just watching the acid layer clear up the opacifiers in the reagent so the image becomes visible.
Here are some interesting links: