Photographic Sensitivity

the ability of photographic materials to form images after exposure to light and subsequent development; also, a value quantitatively representing this ability that is used in photography for selecting correct exposure conditions.

In silver halide gelatin films, which are the most widely used photographic films, the nature and level of sensitivity depend on several factors. One factor is the type of light absorption in silver halide crystal lattices and in the layer of sensitizing dye adsorbed by the silver halide. A second factor is the photoelectric effect that occurs in silver halide lattices and determines the photochemical efficiency of light absorption. A third factor affecting the nature and level of sensitivity is the presence of freely mobile interstitial silver ions in the lattice, which can act as nuclei for the formation of a latent photographic image. A fourth factor is the presence of sensitivity specks, or impurity centers (Ag₂S, Ag), on the surface of microcrystals in the photographic emulsion. These centers are formed during the preparation of the emulsion when silver halides react with active components of the gelatin. Latent image centers are created by the action of light on or near these sensitivity centers. A fifth factor affecting photographic sensitivity is the extent of selectivity of photographic development. Silver halides alone are sensitive to light with wavelengths below 500 nanometers (nm), which corresponds to the blue-violet region of the visible spectrum. The halides respond only slightly to wavelengths in the yellow, green, red, and infrared regions. The sensitivity of the halide itself is termed intrinsic sensitivity. Sensitivity to light with wavelengths exceeding 500 nm is attained by adding special dyes to the photographic emulsion and is referred to as extended sensitivity. The spectral sensitivity of virtually all present-day photographic materials has been broadened by similar methods.

Sensitivity is represented quantitatively by the value S, the inverse of the exposure time H. Exposure creates the desired photographic effect on photographic materials after development or other chemical processing. The desired effect is most often a defined optical density of blackening D. Thus, S = k/H, where the value of H has been chosen with D held constant.