Why is cadmium sulfide used in visible-light sensors and exposure meters?

Cadmium sulfide is used in visible-light sensors and exposure meters because it is a strongly photoconductive semiconductor.1

In these devices, the material functions as a variable resistance element. When exposed to light, photons with sufficient energy—specifically 2.42 electron volts for cadmium sulfide—excite electrons from the valence band to the conduction band, across the band gap. This process increases the material's electrical conductivity, causing its electrical resistance to change in response to variations in light intensity.12

By connecting the cadmium sulfide cell to a battery-powered circuit, the resulting change in current can be measured and calibrated to accurately indicate light levels, making it highly effective for photographic exposure meters. This mechanism is distinct from the photoelectric effect in that it relies on photoconductivity rather than the direct emission of electrons from a surface.21