Definition: Device that responds very quickly and proportionally to light falling on it. * Used as light senors for measuring exposure in cameras and flash-guns; may also be found in specialist scanners. * It is a semi-conductor usually of the p-n or p-i-n type, operated at either below or above the breakdown voltage i.e. voltage at which there is a sudden decrease in the resistance of the device. The effect of light falling on the device is to create a current which can be measured and is largely proportional to the energy of incident light.

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