A photodetector is a device that converts an optical signal into a signal of another form. Most photodetectors convert optical signals into electrical signals, in the form of either current or voltage, that can be further processed or stored. All photodetectors are squarelaw detectors that respond to the power or intensity, rather than the field amplitude, of an optical signal. Based on the difference in the conversion mechanisms, there are two classes of photodetectors: photon detectors and thermal detectors. Photon detectors are quantum detectors based on the photoelectric effect, which converts a photon into an emitted electron or an electron–hole pair; a photon detector responds to the number of photons absorbed by the detector. Thermal detectors are based on the photothermal effect, which converts optical energy into heat; a thermal detector responds to the optical energy, rather than the number of photons, absorbed by the detector. Because of the difference in their fundamental mechanisms, there are a number of important differences in the general characteristics of these two classes of detectors.

The response of a photon detector is a function of optical wavelength with a longwavelength cutoff, whereas that of a thermal detector is wavelength independent. A photon detector can be much more responsive than a thermal detector in a particular spectral region, which typically falls somewhere within the range from the near ultraviolet to the near infrared. In comparison, a thermal detector normally covers a wide spectral range from the deep ultraviolet to the far infrared with a nearly constant response. Photon detectors can be made extremely sensitive.