Photoelectric Exposure Meter

an exposure meter in which the brightness or illumination of an object to be photographed is determined by means of a photoelectric optical detector, or photodetector, such as a photovoltaic cell or a photoconductive cell.

In a typical photoelectric exposure meter, the photodetector is connected to the circuit of a current indicator, usually a pointer-type d’Arsonval galvanometer; when a photoconductive cell is used, a power supply is also connected to the circuit. The brightness or illumination of the object to be photographed is estimated from the angular deflection of the galvanometer’s pointer, which is proportional to the current in the photodetector circuit, and consequently from the magnitude of the luminous flux incident on the light-sensitive area of the photodetector. The exposure parameters are determined by means of an exposure calculator, for which the galvanometer readings serve as the input.

A light-emitting diode (LED) connected across a bridge circuit may be used as the current indicator in a photoelectric exposure meter. To determine the exposure parameters with such an exposure meter, a rheostat slider coupled to an exposure calculator is turned until the bridge circuit reaches the equilibrium condition; the LED emission ceases when the bridge circuit is balanced.

Some photoelectric exposure meters are provided with a view-finder, so that the object whose brightness or illumination is being determined may be observed simultaneously with the meter readings.

A photoelectric exposure meter is a portable instrument in which all the main elements are mounted inside and outside a plastic housing. A special group of photoelectric exposure meters comprises those meters that are built into photographic cameras and motion-picture cameras. The mechanism of such exposure meters is coupled to the diaphragm control mechanism. However, in some camera models, the built-in exposure meter is an independent instrument, and the exposure calculator is mounted on the camera housing.