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Glossary Terms

CdS — Cadmium Sulphide. In a photographic context, this refers to an electronic light meter using a CdS sensor, which varies its electrical resistance in response to the brightness of the scene. CdS meters became common in the 1960s, largely replacing Selenium cells, which required a much larger light-collecting area. While Selenium cells which generate their own electricity, CdS meters require a battery. CdS meters are sufficiently power-hungry that with most designs some form of switch is provided to turn off the meter circuit, to conserve battery life. (In a few cases, the photographer is simply expected to cap the lens; in darkness the current flow drops to a minimum.)