A pattern of x-rays corresponding to their differential absorption by the tissues leaves the body and strikes the film.
X-ray film is made of a flexible base coated on both sides with a gelatine emulsion containing crystals of silver halide, (90-99% silver bromide and 1-10% silver iodide). When energy of the proper wave length strikes these silver halide atoms, a chemical reaction occurs to form a latent image on the film. This latent image is invisible until the radiographic technician puts the film in a solution called developer. At this point, the silver halide crystals exposed to energy change to black metallic silver (Figure 5).
The problem with x-rays is that they have so much energy that most of them pass right through the film without affecting it. It has been estimated that only 1 to 2% of x-ray photons actually interact with the film—a very low number. To rectify this situation, devices have been developed called fluorescent intensifying screens. Two of these screens are sandwiched into a lightproof cassette on each side of the central film. When the x-ray photons leave the body, they strike the nearest intensifying screen. This is coated with a material called a phosphor that converts the x-ray energy to visible light energy by the process of fluorescence. The phosphor used until recently was calcium tungstate (CaWO4), which fluoresced in the blue-violet range. Film is particularly sensitive to light with these wavelengths and, thus, a better image is produced than if a phosphor fluorescing in a different wave length had been used. (The reason why red light can safely be used in the darkroom is that film is not sensitive to this wave length).
At present, rare earth phosphors are used much more commonly in intensifying screens than calcium tungstate, because their efficiency in converting x-rays to visible light is much higher.
Intensifying screens have allowed a large reduction in the amount of exposure needed to expose film. In fact, patients need only from 1/50 to l/100th as much radiation now. This, of course, makes radiography much safer.