Almost all such devices work using a physical principle called the “photoelectric effect”. For certain carefully engineered materials, when light hits it, it creates a small electrical current. For imaging or sensing light, these currents are very small and detected by very sensitive electronics. For generating electricity from the Sun, these currents can get very large, large enough to run the electrical needs of a boat or a house. :: More Information on the photoelectric effect ::

The most common material used for this purpose is silicon. This is the same material of which most modern electronics (including computer chips) are made. ::  More Information on silicon ::

Silicon can be manufactured so that when visible light hits it, some electrons are “energized” from their normal energies in the material, and can hop out of its resting place and move around. These electrons are what make up the current. Thus, whenever a unit of light called a “photon” hits the material, it can make a “photoelectron” in the material, which can be detected as an electrical current.

For simple sensors, we can attach two wires to the silicon, and detect these electrons as a current. More complicated arrangements are needed when we are interested in more that one sensing element.

Bottom image courtesy of Process Specialties Inc.