Photosensors or photodetectors are sensors of light or other
electromagnetic energy. There are several varieties:
Optical detectors, which are mostly quantum devices in which an
individual photon produces a discrete effect.

photoresistor


A photoresistor or Light Dependent Resistor or CdS Cell is a resistor
whose resistance decreases with increasing incident light intensity.
It can also be referred to as a photoconductor.




http://electronicstopics.blogspot.co...oresistor.html