Industrial presses close with enough force to stamp sheet steel. Palletisers swing loads a person couldn't lift. Both are routinely operated by people gtric standing right next to them, with no cage in the way.
What's between the operator and the machine is usually a curtain of invisible light, and the engineering behind it is more interesting than it looks.
The basic idea, and why the basic idea isn't enough
A safety light curtain is two aluminium posts facing each other. One holds a column of infrared emitters, the other a matching column of receivers. Each emitter fires at its opposite number in sequence, many times a second. Break any beam and the outputs open, and the machine stops.
Simple enough. The complications are what make it a safety device rather than a light switch.
The emitters fire in a timed sequence and each receiver only accepts light arriving in its own slot, so ambient light and reflections from a neighbouring curtain don't produce a false clear. The device continuously tests itself, checking that each channel can actually detect an interruption. It has two independent output channels, monitored against each other, so a single internal fault is caught rather than hidden.
And critically, its outputs are wired so that a failure looks like an obstruction. Lose power, break a wire, cook a transistor — the machine stops. Never the other way.
Resolution decides what it can see
The spacing between beams determines the smallest object that's guaranteed to break at least one of them. This is quoted as the curtain's resolution, and it's the number that decides what the device is for.
Around 14 mm resolution will reliably detect a finger. Around 30 mm detects a hand. Anything above roughly 40 mm detects an arm, a leg or a body, but a hand can pass between the beams without being seen.
You can't just fit the finest resolution everywhere. Finer resolution means more beams over the same height, which means more electronics, higher cost, and more sensitivity to alignment and contamination. The choice comes out of a risk assessment: what part of a person can reach the hazard, and how fast.
The bit that surprises people: the distance is calculated
A light curtain protects nothing if it's mounted where somebody can cross it and reach the hazard before the machine has stopped.
So the mounting distance is calculated, not chosen. The standard approach multiplies an assumed approach speed — typically 2 metres per second for a hand — by the total time between the beam breaking and the machine actually coming to rest. That total includes the curtain's response time, the safety relay or controller, the valve or contactor, and the mechanical run-down of the machine itself.
Then a further allowance is added for how far a hand can penetrate a curtain of that resolution before it's detected. Coarser resolution means a bigger allowance, so a curtain that only sees hands has to be mounted further back than one that sees fingers.
The arithmetic gives a minimum distance, often surprisingly large. A machine taking 300 milliseconds to stop needs the better part of a metre before you add anything else. People who assume the curtain goes right at the opening are usually wrong by a factor of several.
Where it goes wrong in practice
Reflective surfaces nearby. A polished guard or a stainless panel can bounce a beam around an obstruction, so the curtain sees light that didn't travel the direct path. Standards require a minimum clearance from reflective surfaces for exactly this reason.
Mounting it as a convenience. Curtains get chosen because they're faster to walk through than a gate. That's a legitimate benefit, but the device has to be selected against the assessed risk, not against how annoying the gate was.
Nobody checking it. These devices are tested periodically, by deliberately interrupting them with a test rod of the rated diameter, at several points across the field. It takes a minute and it's the only thing that proves the device still does what it says.
The technology is decades old and thoroughly solved. Almost every incident comes from the installation, not the curtain — the distance, the reflection, the muting logic somebody added to keep production moving. Reference material and selection data from manufacturers such as gtric covers the device specifications; the mounting geometry is on whoever installs it.
