Light-Operated Proximity/Presence Sensors
A number of different light sensors are available/on the market.
Devices such as photoconductive cells, photovoltaic celts, photodiodes, and phototransistors are used in various applications.
Wavelength sensitivities can run from the far infrared to the far ultraviolet or near X-ray regions of the electromagnetic spectrum.
These sensors provide an output voltage, output current, or change of resistance in response to an impinging Light.