Moving objects can be detected with touching them by means of light barriers. Light is emitted in the visible, ultraviolet, or infrared spectrum. Depending on the application, primarily in automation technology or logistics, a distinction is made among various designs. In hybrid light barriers and one-way light barriers, transmitters and receivers a mounted next to each other or in a single housing. They are especially well suited to industrial applications such as trigger-point monitoring and position determination. In retro-reflective light barriers, too, transmitter and receiver are in a single housing, but a parallel to one another. The light is reflected back with additional reflectors. The quality criteria of optical systems are range, switching interval, and measurement accuracy.