Light energy is transformed into an electrical signal by the light sensor, a passive device. Because they transform light energy (photons) into an electrical signal, light sensors are more frequently referred to as photoelectric devices or photosensors (electrons).
These and other light sensors may be divided into two groups. The first category produces electricity when illuminated and is classified as either photovoltaic or photo-emissive. Photovoltaic generators convert solar radiation into direct-current electricity, while photo-emissive generators release free electrons from light-sensitive materials like cesium when sufficiently energetic photons strike them. A photoresistor, whose resistance reduces with increased incident light intensity, or a photoconductor, which changes its electrical characteristics in some way, are included in the second category (which varies electrical resistance when subjected to light).
Devices in these two broad categories can be further subdivided into these classifications:
- Photoconductive cells
- Photo-junction devices
- Photovoltaic cells
