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CdS — Cadmium Sulphide: in photographic terms, this refers to an electronic light meter using a CdS light-dependent resistor as a sensor. These became common in the early 1970s, largely replacing the less sensitive Selenium meters. Unlike Selenium, which was powered by light, these meters required a battery, and were sufficiently power-hungry that the meter needed to be switched on for as short a time as possible. Thus CdS meters need some sort of switch, where Selenium meters were just left to run all the time.