Also found in: Medical.
a time-delay relay that automatically shuts off the lamp of an enlarger, reproducing unit, or similar device after a specified time interval—the exposure time—which is measured from the beginning of exposure of the photosensitive layer of a photographic material.
Depending on the method used to regulate the exposure time, phototimers are classified as mechanical (clock-driven), pneumatic, electromechanical, and electronic. Electronic phototimers, in which the exposure time is determined by the charging time of a capacitor, are the most advanced types. The exposure time may be varied over a specific range by using a switch to change the parameters of the circuit to which the capacitor is connected (for example, by means of additional resistors) or to change the capacitance of the capacitor itself. The exposure times of various phototimers range from tenths of a second to several tens of seconds. [27–1774–1 ]