Since their first use about 150 years ago, silver halides have remained the primary photosensing materials in an image capturing process known as photography. Silver halides have retained their pre-eminent role despite numerous attempts to replace them with other materials. Even in the present era of electronics, conventional photography remains the most popular format for recording still images. When compared with electronic imaging, silver halide photography retains an edge in resolution, latitude, and convenience.

The current superiority and popularity of conventional photography stem from an unusual confluence of properties found in the silver halides. The ensuing articles will discuss the properties that make silver halides particularly well suited for photography. The article by L.M. Slifkin discusses the improbability of any one material having the characteristics of the silver halides. F.C. Brown describes the special electronic properties, and P.W.M. Jacobs covers ionic characteristics. C.R.A. Catlow gives us some insight into the nature of the silver halides based on their interatomic potentials. This introductory article presents an overview of the photographic process.

The theory of the photographic process has its roots in the Gurney-Mott model of latent image formation. Since it was first proposed over 50 years ago, this model has formed the basis of mechanistic thinking about photography.

The modern photographic emulsion is an extremely sensitive detector of light quanta. In fact, high speed black-and-white film (which consists mainly of microcrystalline grains of AgBr or AgBrI suspended in gelatin on an inert base) ranks with the photomultiplier tube as capable of detecting a very few individual light quanta. Exposure times can vary from minute fractions of a second to hours. Unlike the multiplier tube, the photographic emulsion also responds well to relatively high light flux, that is, film can have wide exposure latitude. In many respects such as sensitivity, latitude, resolution, etc., film sets the pace as new multiplier tubes and charge-coupled devices are developed.

Of course, the high sensitivity of film comes about because light absorbed within an emulsion grain causes a latent image to be formed (usually on the surface), which then renders the entire grain developable. The tabular grains of a modern high speed emulsion are largely triangular or hexagonal single-crystal platelets approximately 10 ü across and 0.1 ü thick. Such a grain will contain about 109 silver atoms, and it can be sensitized by a few molecules of gold sulfide or other additives adsorbed on its surface.

In the concentration speck theory of the latent image, light is absorbed throughout the grain, but this energy is transported by the motion of electrons to a sensitivity center usually on the surface where the latent image is formed.

Modeling methods are now established as a major technique in defect physics and chemistry. Indeed, much of the contemporary understanding of complex defect processes derives from applying these techniques in conjunction with experimental methods.

The silver halides possess unusual defect properties and pose unique challenges to simulation studies. This article aims to outline the general features of defect simulations, and to review the status of their application to the defect properties of silver halides.

Technical aspects of the field have been detailed elsewhere, so this account is brief. An important feature of recent studies of the silver halides concerns developments in interatomic potentials, which will be considered, followed by a presentation and discussion of the calculated defect parameters.

The basis of the simulation techniques is the specification of an interatomic potential model for the system, i.e., an analytical or possibly a numerical description of the energy of the system as a function of atomic coordinates. For polar materials, the model must include the following terms: Coulomb and short-range energies, and ionic polarization.

To evaluate these terms, charges must be assigned to all atoms. In most studies reported to date, the fully ionic model has been used, i.e., integral charges have been assigned. The procedure has been followed in modeling the silver halides. Care must be taken in summing r-1 terms. Simple truncation procedures give unreliable results.

Silver chloride and silver bromide, like sodium chloride and sodium bromide, are ionic crystals with the rock-salt structure, but despite the similarity in the crystal radii of Ag+ (1.29 Å) and Na+ (1.16 Å) the properties of these silver halides are very different from those of the corresponding sodium salts. Much of this difference is due to the filled 4d shell in the electron configuration of Ag+, which is responsible for the higher polarizability of Ag+ (2.37 − 2.44 Å3) than Na+ (0.41 Å3 and for the tendency of Ag+ to undergo quadrupolar deformation.

Associated with the high polarizability of Ag+ are the large van der Waals cation-cation and cation-anion interactions in AgCl and AgBr and the consequent high cohesive energies of these silver salts (9.26 eV and 9.12 eV, respectively). Bear in mind, however, that the large van der Waals coefficients that result from empirical fitting of two-body model crystal potentials to crystal properties may be partly due to the model attempting to simulate features of the real crystal which the model does not really include, such as many-body interactions. That such many-body interactions are important in AgCl and AgBr is evident from the large violations of the Cauchy relation between the elastic constants, C12 = C44, which ought to hold when each ion is at a center of cubic symmetry and the inter-ionic forces are derivable from two-body potentials.

The very high sensitivity of silver halide photographic emulsions is the result of a surprisingly high quantum efficiency in the formation of the latent image, combined with a large amplification of the stored optical signal upon photographic “development.” The efficient formation of the latent image can be traced to the effects of an unusual set of physical properties of silver bromide and silver chloride, involving the electron energy band structure, the dynamics of photoelectrons, the nature and mobilities of the ionic point defects, and the existence of a sub-surface electric field.

Despite the current availability of a wide variety of optical recording systems, the standard silver halide photographic emulsion continues to offer a unique combination of sensitivity, resolution, tone quality, convenience, and economy. This article will outline the physical processes that operate in forming the image. It will emphasize the unusual set of properties of silver bromide and silver chloride that are involved, and that impart to the process its extraordinary efficiency. Only black-and-white images will be considered, but the same basic imaging process is involved in color photography as well.

This discussion is, of necessity, brief and often qualitative, but more complete treatments of the scientific aspects of the photographic process can be found in the other articles in this issue of the MRS BULLETIN, as well as in the bibliography.