PHOTOGRAPHIC SENSITIZING FOR THE INFRARED
Practically all materials used in photography consist of a suspension of minute crystals of a halide of silver in a colloidal medium which is usually gelatin. The halides employed are the chloride, bromide, and iodide of silver. The chloride finds its application primarily in papers for making photographic prints by contact printing, in lantern-slide plates, and in some materials giving high contrast and used by photoengravers instead of wet plates. The bromide, although used also in enlarging papers, films for positive transparencies, and some lantern-slide plates, is the basis of all the films and plates employed for making negatives in the camera. In the majority of these, particularly where high sensitivity to light is desirable, the silver bromide contains a small percentage of silver iodide. The iodide alone, used originally in the daguerreotype process, is now used only in photoengravers' wet plates, suspended in collodion.
The photographic process is a photochemical reaction. There is a basic law of photochemistry, known as the Grotthus-Draper law, from which it follows that only the radiation which is absorbed by a system can bring about a photochemical change in it. That is, it is only the light absorbed by the silver halide in a photographic plate or film which can bring about the chemical change which makes the material darken in a developer. The radiation absorbed by the silver halides is confined to the invisible ultraviolet and to the shorter wavelengths of the visible spectrum adjacent to it. The extent of the sensitivity in the visible part of the spectrum differs from one silver halide to another. In the case of silver chloride and iodide, it extends through the violet and part of the blue. That of the bromide includes the whole of the violet and blue and part of the green. By giving very long exposures it is possible to extend these limits