The emulsion will gradually darken if left exposed to light, but the process is too slow and incomplete to be of any practical use. Instead, very short exposure to the image formed by a camera lens is used to produce only a very slight chemical change, proportional to the amount of light absorbed by each crystal. This creates an invisible latent image in the emulsion, which can be chemically developed into a visible photograph. In addition to visible light, all films are sensitive to ultraviolet light, X-rays and gamma rays, and high-energy particles. Unmodified silver halide crystals are sensitive only to the blue part of the visible spectrum, producing unnatural-looking renditions of some coloured subjects. This problem was resolved with the discovery that certain dyes, called sensitizing dyes, when adsorbed onto the silver halide crystals, made them respond to other colours as well. First, orthochromatic (sensitive to blue and green) and panchromatic (sensitive to all visible colours) films were developed. The panchromatic film renders all colours in shades of grey approximately matching their subjective brightness. By similar techniques, special-purpose films can be made sensitive to the spectrum’s infrared (IR) region.
In the black-and-white photographic film, there is usually one layer of silver halide crystals. When the exposed silver halide grains are developed, the silver halide crystals are converted to metallic silver, which blocks light and appears as the black part of the film negative. Colour film has at least three sensitive layers, incorporating different combinations of sensitizing dyes. Typically, the blue-sensitive layer is on top, followed by a yellow filter layer to stop any blue light from affecting the below layers. Next comes a green-and-blue sensitive layer and a red-and-blue sensitive layer, which record the green and red images, respectively. The exposed silver halide crystals are converted to metallic silver during development, just as with black-and-white film. But in a colour film, the by-products of the development reaction simultaneously combine with chemicals known as colour couplers that are included either in the film itself or in the developer solution to form coloured dyes. Because the by-products are created directly proportional to the amount of exposure and development, the dye clouds formed are also in proportion to the exposure and development. Following the development, the silver is converted back to silver halide crystals in the bleach step. It is removed from the film during the process of fixing the image on the film with a solution of ammonium thiosulfate or sodium thiosulfate (hypo or fixer). Fixing leaves behind only the formed colour dyes, which combine to make up the coloured visible image. Later colour films, like Kodacolor II, have as many as 12 emulsion layers, with upwards of 20 different chemicals in each layer. Photographic film and film stock tend to be similar in composition and speed, but often not in other parameters such as frame size and length. Silver halide photographic paper is also similar to photographic film.