Latent Photographic Image

an invisible change that occurs in light-sensitive material upon the action of optical radiation and is capable of conversion by photographic treatment into a visible image. For this conversion, known as development, in conventional photography, the capacity of the latent images in photographic emulsions to catalyze reduction reactions of silver halides (AgHal, Hal ≡ Br, Cl, I, most frequently Br) to Ag is used. In electrostatic photography, the capacity of latent images to electrostatically attract pigment particles is used.

Only the principal property of the latent image, the property whereby the latent image serves as cause and precursor of the visible image, was given in the above definition. This definition applies to the most varied processes for formation of latent images. Examples of these processes are the photochemical changes in light-sensitive salt crystals, the cross-linking of molecules in light-sensitive polymers, and the changes under the effect of light in the distribution of surface charge of polarized or charged dielectrics or in the space charge in semiconductors.

The latent image is a record of objects or other optical information (spectra, interference fringes). In principle, a subsequent visual observation of the record is not necessary; the recorded information may be obtained directly from the latent image by, for example, holographic techniques or electron beams. However, in any such method of retrieving information, the latent image gives a signal much weaker than its developed counterpart, and the level of the signal is insufficiently higher than the minimum discernible. As a consequence, the signal’s resistance to various disturbances is low. In addition, latent images are not always sufficiently stable over time, and prolonged storage can result in spontaneous development.

In the most common photographic process, the latent image is formed in the emulsion layer of silver halide in gelatin by small groups of Ag atoms, known as sensitivity specks, located at discrete points on the surface or in the volume of AgHal mi-crocrystals. These groups of silver atoms, which are not yet bound in the crystal lattice, are generated in the following way. Upon exposure, an internal photoelectric effect, involving the liberation of halide ion electrons, occurs in the semiconductor AgHal microcrystals. In addition, a certain number of free, mobile Ag⁺ ions, which have been expelled from their positions as a result of thermal fluctuations of the lattice, are already present in AgHal crystals. The free electrons and silver ions attract one another electrostatically and recombine to generate neutral Ag atoms. This process is localized at sites on the surface of the mi-crocrystals where there are lattice defects, primarily defects from the impurities (in particular Ag₂S) formed in the preparation of the emulsion. The formation of latent image specks at each defect site involves the multiple repetition of two elementary steps: the capture of a photoelectron from the volume of the microcrystal (electronic step) and the electrostatic attraction of the mobile Ag⁺ ion to the electron (ionic step). When the illuminance of the emulsion layer is low, the first step proceeds slowly (the release of electrons is infrequent), and the neutral Ag atom formed may undergo ionization before the next photoelectron is released. Thus, the probability of forming a latent image speck, which must consist of not one but several silver atoms, is reduced, which in turn serves to reduce the light sensitivity with prolonged exposure time.

In photographic development (the visualization of latent images), the exposed AgHal microcrystals are reduced to metallic Ag. One of the components of the developer (developing agent) is adsorbed on the microcrystals and transfers electrons to the microcrystals, thus undergoing oxidation. This electron transfer is possible only in the presence of latent image specks, which must be in contact with molecules of the developing agent; that is, the specks must be on the surface of the microcrystals. In the absence of latent image specks, the reduction reaction does not proceed; the specks therefore act as catalysts in this reaction. Each time a latent image speck becomes charged, acquiring an electron, this charge is neutralized by the closest Ag⁺ ion, and the conversion of AgHal into Ag continues until the microcrystal has been completely reduced. Thus, development in the case of silver halide emulsions enormously increases the amount of the product of the primary photochemical process.

The quantum efficiency of the formation of latent images in AgHal microcrystals, that is, the ratio of the number of neutral silver atoms formed to the number of absorbed quanta, is close to one. Thus, an AgHal microcrystal must absorb from 10 to 100 quanta, on the average, for the formation of a latent image speck, which usually contains from several atoms to several tens of atoms. After reduction (development), an Ag microcrystal contains from 10⁸ to 10¹⁰ Ag atoms, which coresponds to an amplification factor of up to 10⁹ (relative to the number of absorbed quanta). Amplification of the latent images also occurs in other photographic processes, but not to such a large extent. Thus, as of 1976, the ordinary photographic process on AgHal emulsion layers remains unsurpassed in sensitivity, although in certain respects, for example, image characteristics, it is inferior to a number of other proposed processes.