Safety light curtain

A safety light curtain is a row of infrared beams that stops a machine the moment a hand or body enters the danger zone. Unlike a guard door, the operator can load parts freely, so light curtains are common on presses, robot cells and packaging machines.

How it works

An emitter column sends many parallel infrared beams to a receiver column. The beams are pulsed in sequence and checked continuously. If any beam is interrupted, or the device detects an internal fault, both of its safety outputs switch off.

It looks like a through-beam photo-eye, but it is a certified safety device (Type 2 or Type 4 per IEC 61496) with self-monitoring. A normal photo-eye must never be used to protect people.

Resolution

OSSD outputs

The receiver has two OSSD (output signal switching device) outputs: PNP outputs that are ON when the field is clear. They are pulse-tested to detect short circuits and wired to a safety relay or safety PLC, which drives the contactors. Both OSSDs must be used.

Safety distance

The light curtain must be far enough from the hazard that the machine stops before a hand can reach it. ISO 13855 gives:

S = K × T + C

• S: minimum distance (mm)
• K: approach speed, 2000 mm/s (or 1600 mm/s for larger distances)
• T: total stopping time of light curtain + safety relay + machine (s)
• C: extra distance depending on resolution, C = 8 × (d − 14) mm for d ≤ 40 mm

Example: 30 mm resolution, total stop time 0.2 s: S = 2000 × 0.2 + 8 × 16 = 528 mm.

Muting and blanking

• Muting: temporarily suspends the curtain while a pallet passes, using extra muting sensors in a defined sequence.
• Blanking: permanently ignores certain beams where a fixed part of the machine passes through.

Both reduce protection and must be configured carefully as part of a risk assessment.

See also emergency stop circuits. In the PLC Sim Lab Playground the safety example lets you break the light curtain and watch the machine stop.