In the specific case of a photodetector, it measures the electrical output per optical input.
A photodetector's responsivity is usually expressed in units of amperes or volts per watt of incident radiant power.
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's properties, such as the bandgap of the material of which the photodetector is made.
[1] 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
For example, a neuroscientist may measure how neurons in the visual pathway respond to light.
The neural response measure depends on the part of the nervous system under study.
This article incorporates public domain material from Federal Standard 1037C.