Photoresistors, which are sometimes also referred to as Light Dependent Resistors (LOR), are based on the internal photo effect , are made mostly from semiconductor material such as Si, Ge and CdS (in both the intrinsic and doped form) and have a logarithmic transfer characteristic, as shown in Figure 1 for a typical CdS photoresistor.
Because of the relatively long life time of the optically induced charge carriers (i.e. the time taken for an electron in the conduction band to recombine with a hole in the valence band) and the dependence of this process on previous conditions (i.e. a large hysterisis), photoresistors are used mainly in low-frequency control applications where their slow response time and relatively high hysteresis are either acceptable or an actual advantage.
Photoresistors therefore find limited applications in the robotics field where they are used mainly in components detecton on conveyor systems or as part of an optical safety ‘curtain’ around industrial robots.