In the last section, we saw that instant camera film has three layers that are sensitive to different colors of light. Underneath each color layer, there is a developer layer containing dye couplers. All of these layers sit on top of a black base layer, and underneath the image layer, the timing layer and the acid layer. This arrangement is a chemical chain reaction waiting to be set in motion.
The component that gets the reaction going is the reagent (as in re-agent). The reagent is a mix of opacifiers (light-blockers), alkali (acid neutralizers), white pigment and other elements. It sits 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 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. (In another configuration, often used by professional photographers, the reagent and developer are coated on a separate sheet which is pressed up against the film sheet for a set amount of time.)
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 is 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 that these reagent chemicals are working down through the light-sensitive layers, other reagent chemicals are working through the film layers above. The acid layer in the film reacts with the alkali and opacifiers in the reagent, making the opacifiers become clear. This is what finally makes the image visible. The timing layer slows the reagent down on its path to the acid layer, giving the film time to develop before it is exposed to light.
One of the coolest things about instant photography, watching the image slowly come together, is caused by this final chemical reaction. The image is already fully developed underneath, but the opacifiers clearing up creates the illusion that it is forming right before your eyes.
For more information about instant film and photography in general, check out the links below.