exposure time

the time interval t during which light-sensitive photographic material is exposed to the continuous action of light. If the radiation power (intensity of illumination on the emulsion layer) is varied during the time of exposure, a distinction is then made between the full exposure time t_f and the effective exposure time t_e, which is less than t_f. The effective exposure time is the interval of time during which the photographic layer would be exposed to the same amount of light as during the full exposure time if the radiation power were to remain constant and equal to its maximum value. If the variation of the exposure on the film layer is associated with the type of shutter used in cameras (for example, a diaphragm shutter whose blades are located in or near the pupil of the lens), the ratio of t_e to t_f is called the shutter efficiency. The shutter efficiency is higher with a longer exposure and a smaller lens aperture ratio. The product of the exposure time and the intensity of illumination L is called the exposure or the amount of illumination H = Lt. The same exposure can produce a slightly different photographic effect depending on the relationship between L and t; a similar photochemical phenomenon is called the phenomenon of noninterchangeability.