The sensitivity of a photographic medium (e.g., a radiograph) to light or radiation. The film speed is determined by: the size of its silver halide crystals; the thickness of the emulsion; and the presence of radiosensitive dyes.
In radiography, the term “speed” denotes the comparative degree of darkening produced on a film, taking into consideration the characteristics of the film or screen, in relation to a specific amount of radiation. It is compared to an exposure made with a different film or screen, as applicable. The terms “speed” and “sensitivity” may be used interchangeably in this context.