When detectors detect the physical signal for which they are designed, they generate an electrical signal. A similar function is performed by demodulators of RF receivers for radio or television, which generate audio or video signals, or which are used to trigger an alarm or other action.
Detectors are found in all sorts of technical applications. They are used for detecting differences in brightness and light pulses such as those used in infrared sensors or motion detectors. Others, like photodetectors, detect the light signals in optical fibers and still others detect the finger touch on multi-touch screens.