Light curtains are also called light screens, optical guards, and presence sensing devices. Light curtains emit a "curtain" of infrared light beams in front of the hazardous area being protected. A stop signal is sent to the machine being guarded when any of the beams are blocked. Light curtains guard areas many meters wide, and can be diverted around corners using mirrors. Application areas include perimeter guarding for industrial robots and machinery, and point of access guarding for automated machine assemblies. A photoelectric transmitter projects an array of synchronized, parallel infrared light beams to a receiver unit. When an opaque object interrupts one or more beams, the light curtain controller sends a stop signal to the guarded machine. The transmitter unit contains light emitting diodes (LEDs) that emit pulses of invisible infrared light when energized by the light curtains timing and logic circuitry. Light pulses are both sequenced (one LED after another) and modulated (pulsed at a specific frequency). The receiver unit contains phototransistors and supporting circuitry to detect only the specific pulse and the frequency designated for it.