Radiography in Modern Industry - Kodak

Chapter 18: The Photographic Latent ImageAs shown in Figures 67 and 68, a photographic emulsion consists of a myriad of tiny crystals ofsilver halide--usually the bromide with a small quantity of iodide--dispersed in gelatin and coatedon a support. The crystals--or photographic grains--respond as individual units to the successiveactions of radiation and the photographic developer.The photographic latent image may be defined as that radiation-induced change in a grain orcrystal that renders the grain readily susceptible to the chemical action of a developer.To discuss the latent image in the confines of this site requires that only the basic concept beoutlined. A discussion of the historical development of the subject and a consideration of most ofthe experimental evidence supporting these theories must be omitted because of lack of space.It is interesting to note that throughout the greater part of the history of photography, the nature ofthe latent image was unknown or in considerable doubt. The first public announcement ofDaguerre's process was made in 1839, but it was not until 1938 that a reasonably satisfactoryand coherent theory of the formation of the photographic latent image was proposed. That theoryhas been undergoing refinement and modification ever since.Some of the investigational difficulties arose because the formation of the latent image is a verysubtle change in the silver halide grain. It involves the absorption of only one or a few photons ofradiation and can therefore affect only a few atoms, out of some 10 9 or 10 10 atoms in a typicalphotographic grain. The latent image cannot be detected by direct physical or analytical chemicalmeans.However, even during the time that the mechanism of formation of the latent image was a subjectfor speculation, a good deal was known about its physical nature. It was known, for example, thatthe latent image was localized at certain discrete sites on the silver halide grain. If a photographicemulsion is exposed to light, developed briefly, fixed, and then examined under a microscope(See Figure 126), it can be seen that development (the reduction of silver halide to metallic silver)has begun at only one or a few places on the crystal. Since small amounts of silver sulfide on thesurface of the grain were known to be necessary for a photographic material to have a highsensitivity, it seemed likely that the spots at which the latent image was localized were localconcentrations of silver sulfide.