Early photographic film could only see blue light
Untreated silver halide crystals, the light-catching heart of photographic film, respond only to the blue end of the spectrum, so early photographs rendered some colours strangely. Chemists fixed this by coating the crystals with sensitising dyes, first extending them to green and eventually to every visible colour.
Film is a transparent base coated with gelatin holding microscopic silver halide crystals, and the size and character of those crystals set its sensitivity, contrast and sharpness. Left in light, the emulsion slowly darkens on its own, but far too slowly to be useful. Instead a camera gives a brief exposure that causes only a tiny chemical change in each crystal, in proportion to the light it absorbed. That creates an invisible latent image, which developing chemicals then turn into a visible one. Besides light, most films also react to ultraviolet, X-rays, gamma rays and energetic particles.
The dye breakthrough produced orthochromatic film, sensitive to blue and green, and then panchromatic film, which records all visible colours as greys roughly matching how bright they look to us. Similar tricks make special films sensitive to infrared.
Black-and-white film usually has a single layer; development turns exposed crystals into metallic silver, which blocks light and forms the dark areas of the negative. Colour film stacks at least three layers. Blue-sensitive emulsion sits on top, a yellow filter beneath it blocks leftover blue, and lower layers record green and red. As the silver develops, by-products react with colour couplers to form dye clouds in proportion to the exposure. The silver is then bleached and dissolved away by a fixer, leaving only dyes. Later films such as Kodacolor II packed up to 12 layers with more than 20 chemicals in each.
Print film yields a negative with tones and colours reversed, plus an orange mask that improves colour accuracy; printing reverses it again. Slide film produces positive transparencies directly. Colour processing became so common that some black-and-white films were designed to be developed in colour chemicals.
Source: Photographic film
