Inside a Roll of Film
If you were to open a 35-mm cartridge of color print film, you would find a long strip of plastic that has coatings on each side. The heart of the film is called the base, and it starts as a transparent plastic material (celluloid) that is 4 thousandths to 7 thousandths of an inch (0.025 mm) thick. The back side of the film (usually shiny) has various coatings that are important to the physical handling of the film in manufacture and in processing.
It is the other side of the film that we are most interested in, because this is where the photochemistry happens. There may be 20 or more individual layers coated here that are collectively less than one thousandth of an inch thick. The majority of this thickness is taken up by a very special binder that holds the imaging components together. It is a marvelous, and ubiquitous material called gelatin. A specially purified version of edible gelatin is used for photography -- yes, the same thing that makes Jell-O jiggly holds film together, and has done so for more than 100 years! Gelatin comes from animal hides and bones. Thus, there is an important link between a cow, a hamburger and a roll of film that you might not have appreciated.
Some of the layers coated on the transparent film do not form images. They are there to filter light, or to control the chemical reactions in the processing steps. The imaging layers contain sub-micron sized grains of silver-halide crystals that act as the photon detectors. These crystals are the heart of photographic film. They undergo a photochemical reaction when they are exposed to various forms of electromagnetic radiation -- light. In addition to visible light, the silver-halide grains can be sensitized to infrared radiation.
Silver-halide grains are manufactured by combining silver-nitrate and halide salts (chloride, bromide and iodide) in complex ways that result in a range of crystal sizes, shapes and compositions. These primitive grains are then chemically modified on their surface to increase their light sensitivity.
The unmodified grains are only sensitive to the blue portion of the spectrum, and they are not very useful in camera film. Organic molecules known as spectral sensitizers are added to the surface of the grains to make them more sensitive to blue, green and red light. These molecules must adsorb (attach) to the grain surface and transfer the energy from a red, green, or blue photon to the silver-halide crystal as a photo-electron. Other chemicals are added internally to the grain during its growth process, or on the surface of the grain. These chemicals affect the light sensitivity of the grain, also known as its photographic speed (ISO or ASA rating).