Photodetectors are devices, as the name suggest, which detect or respond to photo or light energy. Photodetectors consist of a list of distinct devices that are used through unique principles to detect light.

Some common and popular types of photodetectors are photodiodes, photoresistors, phototransistors and photomultipliers. Several of these types of detectors a semiconductor type of device—although semiconductor photodetectors are not the only type. A photomultiplier is based on vacuum tubes, for instance.

Photodetectors can be used to detect continuous or constant light. Other types of photodetectors can be used for detecting pulsed light, as opposed to continuous light. This later photodetector is often used to assist in counting photons as they are detected by this type of pulsed light photodetector.

Each photodetector, because of its unique characteristics, will respond differently to light. Photoresistors, for instance, will change their resistance according the light intensity incident on the device. Photovoltaic cells, also known as solar cells, will produce a voltage and drive an electrical current when exposed to light.

Photodiodes, when reversed biased, will allow current to flow, by mean of a p-n junction. There are several different types of photodetectors, each with their response and reaction to the presence of light. Each of these reactions can be incorporated into different systems to have a photo-dependent event take place as one considers different design alternatives.