 Photographic film is a strip or sheet of transparent plastic film base coated on one side with a gel of emulsion containing microscopically small light-sensitive silver halide crystal. The sizes and other characteristics of the crystal determine the sensitivity, contrast and resolution of the film. The emulsion will gradually darken if left exposed to light, but the process is to slow and incomplete to be of any practical use. Instead, a 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, x-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 colored 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 colors as well. First orthochromatic sensitive to blue and green and finally panchromatic sensitive to all visible colors films were developed. Panchromatic film renders all colors in shades of gray approximately matching their subjective brightness. By similar techniques, special-purpose films can be made sensitive to the infrared IR region of the spectrum. In 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. Color 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 filtered layer to stop any remaining blue light from affecting the layers below. Next comes the green and blue sensitive layer, and a red and blue sensitive layer, which record the green and red images respectively. During development, the exposed silver halide crystals are converted to metallic silver, just as with black and white film. But in the color film, the byproducts of the development reactions simultaneously combined with chemicals known as color cutlers that are included either in the film itself or in the developer solution to form colored dyes. Because the byproducts are created in direct proportion to the amount of exposure and development, the dye clouds forms are also in proportion to the exposure and development. Following the development, the silver is converted back to silver halide crystal 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 phylosulfate or sodium phylosulfate hypofixor. Fixing leaves behind only the formed colored eyes, which combine to make up the colored visible image. Later color films, like Coca Cola II, have as many as 12 emulsion layers, with upwards of 20 different chemicals in each layer.