Abstract

Conventional silver halide-based photography is discussed. The key subprocesses in the photographic industry are examined. The manufacture of photographic films and papers begins with the production of silver halide microcrystals which are grown via the controlled mixing of silver nitrate and alkali halides in an aqueous environment, a process generally referred to as emulsion precipitation. To increase the light sensitivity of the resulting microcrystals, two sensitization processes are employed: chemical sensitization and spectral sensitization. Chemical sensitization usually involves the addition of trace levels of gold- and sulfur-containing compounds to the surface of the microcrystalline silver halide grains. These sensitizing chemicals increase the sensitivity of the crystals to photons absorbed during exposure. Because silver halides do not significantly absorb photons with wavelengths greater than 500 nanometers, spectral sensitizing dyes are adsorbed to the surfaces of the microcrystals. Dyes are selected to provided sensitivity to different wavelengths depending on the photographic application. After the sensitization process is complete, the silver halide crystals can be coated on film or paper bases. Gelatin is used as the supporting vehicle for the sensitized microcrystals. Exposure generates mobile conduction band electrons and valence band holes and ultimately yields small nuclei which contain only a few atoms of metallic silver. These silver nuclei are referred to as latent-image centers and provide a site for subsequent catalytic amplification during development. High amplification gains during development, image sharpness, low granularity, and archival permanence of developed photographic images are some of the reasons why conventional photography holds a preeminent position in the image-formation industry.