Responsivity

Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, responsivity measures the electrical output per optical input.

The responsivity of a photodetector is usually expressed in units of either amperes or volts per watt of incident radiant power. For a system that responds linearly to its input, there is a unique responsivity. For nonlinear systems, the responsivity is the local slope. Many common photodetectors respond linearly as a function of the incident power.

Responsivity is a function of the wavelength of the incident radiation and of the sensor properties, such as the bandgap of the material of which the photodetector is made. One simple expression for the responsivity R of a photodetector in which an optical signal is converted into an electric current (known as a photocurrent) is

where is the quantum efficiency (the conversion efficiency of photons to electrons) of the detector for a given wavelength, is the electron charge, is the frequency of the optical signal, and is Planck's constant. This expression is also given in terms of , the wavelength of the optical signal, and has units of amperes per watt (A/W).