1. A device that generates electrical energy from light energy, usually as a voltage or current.
2. An instrument which converts light into electrical energy or uses it to regulate a flow of current, often incorporated into automatic control systems for doors and lighting.
3. An electronic device having an electrical output that varies in response to incident radiation; especially, to visible light.
4. A small cathode-ray tube having a fluorescent pattern whose size varies with the voltage applied to the grid.

It is used in radio receivers to indicate accuracy of tuning and as a modulation indicator in some tape recorders.

5. An electric eye can operate a mechanism so as to open a door when its invisible beam is interrupted by the approach of a person and includes a photoelectric cell which is used as an automatic controlling appliance.

It is also used in motion pictures, television, and many other industries.