It creates an invisible detection field; when an object — like a hand reaching in — breaks that field, the machine gets a stop signal. This one is in the F3SJ series, and the order code tells you it's a 25 mm resolution model, which means it catches fingers before they reach the pinch point.
Deployment Context
Out here in the grease, a safety light curtain like this one is what keeps a lube-tech's fingers out of a running press or a conveyor nip point. It mounts on the machine frame, not inside a panel, so the IP rating matters — this one is built for the factory floor, not a clean room.