Safety Light Curtain

A safety light curtain consists of two columns: an emitter containing a vertical array of infrared LEDs and a receiver containing a matching array of photodetectors. Beams scan across the protected area sequentially — typically thousands of times per second. If any beam is interrupted, both output signal switching devices (OSSDs) drop to 0 V within one scan cycle.

The OSSD (Output Signal Switching Device) pair provides two monitored semiconductor safety outputs to a compatible safety relay or safety PLC. The controller evaluates their switching and discrepancy behavior and moves the safety function to its defined state when a blockage or detected fault occurs. The performance level or SIL belongs to the complete input–logic–output safety function, not to “two wires” by themselves; it depends on the selected devices, architecture, reliability data, diagnostics, common-cause measures, and validation.

Resolution is the diameter of the smallest object the curtain will reliably detect. Finger-resolution curtains detect objects ≥ 14 mm in diameter (a finger). Hand-resolution curtains detect objects ≥ 30 mm. Body-resolution curtains (≥ 70 mm) guard larger openings where only whole-body intrusion matters.

Muting — temporarily suspending the curtain — allows material (pallets, parts) to pass through while keeping it active against personnel. Muting requires two independent muting sensors whose activation sequence and timing are verified by the safety controller.

Light curtains require competent design and commissioning. Their resolution, response time, mounting position, reach-over/reach-under protection, reset behavior, and machine stopping time must all be validated. Connect them through the specified safety-rated control path, not ordinary PLC I/O.