Sensitivity of Film Photographic Emulsion
Photographic emulsion is the emulsion coating applied to film base, composed of silver halide layered particles suspended in a colloidal medium. The silver halide particles are light sensitive and with the help of initiating agents an exposure of light will initiate an electrochemical reaction that reduces the silver halide particles to metallic silver, then darkening the film base on which it is coated. It is the sensitivity of the silver halide crystals to incoming light, which determines the films sensitivity or speed.
ISO speed, which is a measure of the speed of the film, allows a photographer to determine the correct exposure for a given situation. However, ISO speed does not tell a photographer the actual sensitometric response of the film to light. For example, the ISO rating of a certain film could be 400ASA, which means that if a photographer were to expose it for two seconds, a third of the available film density (residual light) would be achieved. The actual sensitometric response of this particular film could indicate that if it were been exposed for twice as long it would have achieved twice the density.
Film speed is an indicator of the light sensitivity of the emulsion, and is determined by the size, shape and spatial arrangement of the silver halide crystals. Larger silver halide crystals are more sensitive than smaller crystal and will achieve a given photographic density with fewer photon captures. The speed of the emulsion is also adversely affected by the presence of initiating and development inhibiting agents (DIA), which must be present and activated to complete the development process. These DIA agents significantly reduce the speed of the film.
When film is exposed to light it loses some of its sensitivity to further exposure as the chemical structure of the crystals changes due to light energy captured by the crystals. The more energy that has been captured, the more irreversible the photochemical reaction becomes and the crystal will yield a lesser amount of energy with each successive energy capture. This process takes place more quickly with higher speed film, as more energy is required for the formation of image silver, leaving the photographic formulae with less energy to drive chemical reaction in the DIA agents, resulting in slower speeds or shorter development processes.
For many years, film emulsion speed was limited by the size and shape of the silver halide crystals that could be manufactured; however, with the introduction of tabular grain emulsions, this speed limitation was largely overcome. Tabular grains have a larger surface area in relation to their volume, allowing them to capture more photons than conventional crystalline grains, and in turn achieve greater speeds.
In addition to crystal geometry, the sensitometry of film emulsion is also dependent on the activity of the developing agent that is used during the film processing step. As the activity of the developer increases, so does the speed of the film. However, the most accurate measure of a film’s speed is the Dmin or “Minimum Density”, which is determined by exposing the film to light and then measuring the amount of density (as measured by a densitometer) that has been achieved.
In conclusion, film speed is a measure of the light sensitivity of the film emulsion, which is determined by the size, shape, and spatial arrangement of the silver halide crystals, as well as the activity of the developing agent. The Dmin or “Minimum Density” is the most accurate way to measure a film’s speed.
