Photosensors or photodetectors appear in several varieties:
- Photoresistors or Light Dependant Resistors (LDR) which change resistance when illuminated
- Photovoltaic cells or solar cells which produce a voltage and supply an electric current when illuminated
- Photodiodes which can operate in photovoltaic mode or photoconductive mode
- Phototubes containing a photocathode which emits electrons when illuminated and in general behaves as a photoresistor.