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A light meter is an instrument for measuring light intensity, illumination, or luminance. Light meters used in photography to determine proper camera settings are called exposure meters. Incident-light exposure meters measure the illumination reaching the scene to be photographed, and reflected-light meters measure the luminance of the scene.
The first light meters were actinometers (devised by J.B.F. Soleil in 1840). With these instruments, camera exposure was monitored by measuring the time it took for a photosensitive paper to darken to a standard tint. Extinction light meters (C. F. Albinus, 1844) have been used to observe the scene being photographed through a wedge of graduated density and to note the step through which the scene details remained just visible. Exposure photometers (J. F. Taylor, 1866) have been used to compare a target illuminated by a standard light source with another target illuminated by the scene.
Most light meters now in use are photoelectric instruments with a photovoltaic or photoconductive cell as the light-sensitive element. The photovoltaic cell, typically a selenium cell, generates an electric current when light falls on the cell. This current is measured by a microammeter calibrated to indicate proper camera settings. The photoconductive cell typically is a cadmium sulfide (CdS) cell. A battery provides a current through the cell, and this current increases as the light intensity on the CdS cell increases. The current is measured by a microammeter, as in photovoltaic meters. The CdS light meter has largely replaced the selenium-cell meter because it can be made much smaller and much more sensitive.
Camera Exposure Meter Systems
The photoelectric light meter, originally a separate instrument; now is usually built into the camera and linked to the camera controls. Links to the lens diaphragm, the exposure-time adjustment of the shutter, or both, provide semiautomatic or automatic operation. In modern single-lens-reflex still cameras and in movie cameras, exposure meters frequently measure the luminance of the image produced by the lens on a viewing screen or at some intermediate point in the lens' optical path. Such through-the-lens measurement can more accurately relate the required exposure to the average luminance of the image produced by the lens.
In automatic camera light-meter systems, the photoelectric cell current indirectly (via a mechanical sensing system) or directly adjusts the lens diaphragm or the shutter setting. In the simplest system, widely used in inexpensive movie cameras, the microammeter is linked with a pair of diaphragm blades. As the microammeter responds to an increase (or decrease) in the cell current, the diaphragm blades close (or open) the lens aperture. In more advanced systems the CdS cell is part of a bridge circuit that controls a servomotor, which in turn adjusts the diaphragm. In some still cameras, the cell is part of a capacitor-transistor circuit. During film exposure, this circuit keeps the shutter open for a time depending on the light received by the cell. Such systems typically provide exposure times ranging from 10 seconds to 1/500 second.