The Definition of Light Curtain (Safety Light Curtain)

Definition of Light Curtain

A light curtain is a presence-sensing safeguarding device that uses a row of infrared beams between an emitter and a receiver to detect when a hand, arm or body enters a hazard zone and to signal the machine control to stop hazardous motion.

Light curtains let operators load and unload parts without opening a physical guard on every cycle. That makes them common where a fixed barrier would slow production or block access. When the beams are interrupted, the curtain sends a stop signal before the person can reach the moving parts, as long as the device is mounted at the correct safety distance and the machine can stop reliably.

You will find light curtains on press brakes, power presses, robot cells, palletizers and packaging lines. They are one tool within a broader machine guarding program, usually protecting the point of operation or the entry to a cell. Maintenance technicians, machine operators, controls engineers and safety managers all need to understand how they work and where they fall short.

How a Light Curtain Works

Core Components

• Emitter: a column of infrared LEDs that sends pulsed beams across the opening.
• Receiver: a matching column that detects each beam. If one or more beams are blocked, its outputs switch off.
• Safety outputs and controller: redundant, monitored outputs that tell the machine control to stop hazardous motion and prevent a restart until the zone is clear.
• Resolution (object sensitivity): the smallest object the curtain will reliably detect. Finger resolution is typically around 14 mm, while hand and body detection use larger spacing.

Common Features

• Muting: temporarily suspends the safety function during a non-hazardous part of the cycle or to let material pass, such as a pallet leaving a cell. Muting must be designed so a person cannot pass through undetected.
• Blanking: ignores specific beams so a fixture or workpiece can sit in the field. Fixed blanking ignores set beams. Floating blanking allows an object to move within the field. Both reduce effective resolution and increase the required safety distance.
• External device monitoring (EDM): checks that the relays or contactors controlled by the curtain actually change state, so a welded contact is caught.
• Brake monitor: on presses, watches stopping performance and blocks the next stroke if stopping time grows beyond the set limit.

Limitations

• No protection from ejected parts: beams do not stop flying chips, broken tooling or parts thrown from the machine.
• Depends on reliable stopping: the curtain only signals a stop. A machine with a worn brake or a full revolution clutch cannot respond in time.
• Can be defeated: people can reach over, under or around a poorly placed curtain. Muting and blanking set up wrong are common forms of bypassing safety devices.

Regulations and Standards

OSHA 29 CFR 1910.217

For mechanical power presses, 29 CFR 1910.217(c)(3)(iii) sets rules for presence-sensing point of operation devices. The device must protect the operator by preventing or stopping normal stroking of the press if the operator's hands are inadvertently within the point of operation. It may not be used on machines using full revolution clutches. It may not be used as a tripping means to initiate slide motion, except when used under the presence-sensing device initiation (PSDI) rules in 1910.217(h). The safety distance is Ds = 63 inches per second multiplied by Ts, where Ts is the stopping time of the press measured at about 90 degrees of crankshaft rotation. Guards must protect all areas of entry to the point of operation that the curtain does not cover. See mechanical power press safety for the wider rule.

ANSI B11.19

ANSI B11.19 gives a more complete safety distance formula: Ds = K x (Ts + Tc + Tr + Tbm) + Dpf. K is the hand speed constant of 63 inches per second. Ts is the machine stopping time, Tc is the control system response time, Tr is the curtain's response time and Tbm is the additional time allowed by the brake monitor. Dpf is the depth penetration factor, which depends on the curtain's object sensitivity. Coarser resolution means a larger Dpf.

ISO 13855 and IEC 61496

ISO 13855 covers positioning of safeguards based on approach speed. It uses K = 2,000 mm/s for close mounting and 1,600 mm/s at larger distances. IEC 61496-1 and IEC 61496-2 define the device itself. Type 4 curtains are used for high-risk applications. Type 2 curtains are limited to lower risk. The required performance level under ISO 13849 for the stop function helps decide which type fits.

Good Practices for Light Curtains

• Measure stopping time: test the actual stopping time with a stop-time meter and calculate the safety distance from real data, not the catalog.
• Retest after changes: repeat stop-time tests after brake work, control changes, die changes that add weight, or relocation of the curtain.
• Check with the test rod: at the start of each shift, pass the correct test rod through the whole field and confirm the machine stops.
• Close the gaps: add hard guards so no one can reach over, under or around the curtain or stand between it and the hazard undetected.
• Control blanking and muting: restrict configuration to trained, authorized people and recalculate safety distance whenever blanking is used.
• Use the right type: match the device type and resolution to the risk and the body part being protected.
• Train operators: explain what the curtain does and does not protect against, and how to report faults.

Example of a Light Curtain

A contract metal stamping shop mounted a light curtain on a part revolution press after a die change. During a routine audit, a supervisor noticed operators could reach under the bottom beam to clear scrap. A stop-time test also showed the press now took longer to stop because the new die was heavier. The curtain sat several inches closer than the recalculated safety distance allowed.

The shop moved the curtain back to the correct distance and added a fixed guard below the lowest beam. It installed a brake monitor set to the new stopping time and made stop-time testing part of every die change. Operators now run a test rod check at each shift start and record it on the press checklist.

Frequently Asked Questions: Light Curtain

What is the difference between a Type 2 and Type 4 light curtain?

Both are defined in IEC 61496. Type 4 curtains have stricter fault detection and a tighter optical design, so they suit high-risk machines such as presses. Type 2 curtains are limited to lower-risk applications.

How do you calculate light curtain safety distance?

Under OSHA 29 CFR 1910.217, Ds = 63 inches per second multiplied by the stopping time. ANSI B11.19 adds control and device response times, brake monitor allowance and a depth penetration factor based on resolution. ISO 13855 uses 2,000 mm/s or 1,600 mm/s depending on distance.

Can a light curtain be used on a full revolution press?

No. OSHA does not allow presence-sensing devices on presses with full revolution clutches because those presses cannot stop mid-stroke. Use guards or other devices that keep hands out entirely.

How often should a light curtain be tested?

Check it with the manufacturer's test rod at least at the start of each shift and after any adjustment. Measure stopping time at regular intervals and after maintenance or changes that could affect stopping.