Photoelectric sensor

The circuitry for power input, amplification, and output switching is located elsewhere, typically in a control panel.

They have no electrical circuitry and no moving parts, and can safely pipe light into and out of hostile environments.

In this mode, an object is detected when the light beam is blocked from getting to the receiver from the transmitter.

A proximity-sensing (diffused) arrangement is one in which the transmitted radiation must reflect off the object in order to reach the receiver.

The emitter sends out a beam of light (most often a pulsed infrared, visible red, or laser) that diffuses in all directions, filling a detection area.

The target then enters the area and deflects part of the beam back to the receiver.

Conceptual through-beam system to detect unauthorized access to a secure door. If the beam is interrupted, the detector triggers an alarm.
Certain types of smoke detector use a photoelectric sensor to warn of smouldering fires.