How photography film records light
A strip of photographic film is a piece of plastic coated with many layers of chemicals on one side, often up to twenty. The layers contain tiny grains of silver halide crystals. When manufactured, these crystals are sensitized to light – this means that as soon as the crystals are exposed to light, a photochemical reaction happens and they record an image. Just like digital camera sensors (see below) the imaging layers of photographic film detect photons.
However, the silver halide crystals are only sensitive to the color blue, so spectral sensitizers are added to make the film more sensitive to other colors of light, such as red and green. Other filters are added above the film’s imaging layers for a similar reason.
It is important to keep photographic film in absolute darkness. As soon as it is unintentionally exposed to light, the recorded images will be spoiled as the new light is captured.