What is a light curtain?
A light curtain (or safety light curtain) is a non-contact, electro-sensitive protective device that guards an opening with a grid of infrared beams and stops a hazardous machine motion as soon as a hand, arm or person interrupts the protective field. In the standards it is called an AOPD (active opto-electronic protective device), one form of ESPE under IEC 61496. Light curtains are used where a fixed guard or door is impractical because operators need frequent access, such as at robot cells, presses, packaging lines and palletisers. They are part of machine safety and are only effective when type, resolution, distance and wiring are all correct.
🧠 How does a light curtain work?
A light curtain consists of a transmitter and a receiver mounted opposite each other. The transmitter fires pulsed infrared beams in rapid sequence, one after the other; the receiver checks that each beam arrives at exactly the expected moment. This makes the system immune to ambient light and to a second transmitter nearby.
When a single beam is interrupted, the receiver switches off its two safety outputs:
- OSSD outputs (Output Signal Switching Device) - two redundant 24 V PNP outputs. In the clear state they briefly pulse low with test pulses, staggered per channel, so the light curtain itself detects a short to 24 V, to 0 V or between the two channels.
- Safety logic - the OSSDs feed a safety relay or a safety PLC, which removes motor power or performs a controlled stop according to the stop categories of IEC 60204-1. A safety PLC must filter the test pulses, otherwise it reads them as an interruption.
- EDM (External Device Monitoring) - through a normally closed contact on the contactors, the light curtain checks that the contactors have really dropped out and refuses to restart if a contact has welded.
- Start/restart interlock - after an interruption, the machine restarts only after a deliberate press of a reset button located outside the hazardous area with a view of the zone.
The response time of the light curtain itself is usually between roughly 5 and 20 ms on modern devices. What counts for safety, however, is the total time to standstill, including relays, the control system and the stopping distance of the machine.
📐 Which types and resolutions are there?
IEC 61496 classifies electro-sensitive protective equipment into types. The type defines how well the device detects faults and withstands disturbances such as reflections, and therefore which Performance Level or SIL you can reach with it.
| Type | Fault detection and optics | Maximum achievable | Typical use |
|---|---|---|---|
| Type 2 | Periodic testing; aperture angle ±5° | PL c / SIL 1 | Low risks, e.g. small conveyors |
| Type 3 | Also described for light curtains since 2020 | PL d | Medium risks |
| Type 4 | Faults detected immediately; aperture angle ±2.5° (at 3 m) | PL e / SIL 3 | Presses, robot cells, cutting machines |
The narrower aperture angle of Type 4 reduces the chance that a beam reflects around a hand via a shiny surface. Besides the type, you choose the resolution (detection capability): the smallest diameter that is guaranteed to be detected.
| Resolution | Detects | Application |
|---|---|---|
| 14 mm | Finger | Manual loading close to the hazard |
| 20–40 mm | Hand | Operating presses and packaging machines |
| 50–90 mm | Arm or leg | Access protection at a greater distance |
| 2–4 beams (multi-beam light grid) | Whole body | Access to robot cells and palletisers |
You also choose the protection height (the vertical coverage of the field) and the range between transmitter and receiver, typically a few metres up to around 12 m for light curtains, depending on manufacturer and resolution. On multi-beam light grids the beams sit at fixed heights above the floor: 400 and 900 mm with two beams (only if the risk assessment allows it), 300, 700 and 1100 mm with three, and 300, 600, 900 and 1200 mm with four beams.
📏 How do you calculate the safety distance?
A light curtain does not stop a machine instantly. It must therefore be mounted far enough from the hazard point that a hand breaking the field reaches the hazard only after the motion has stopped. ISO 13855 provides the calculation rules. The classic formula from the 2010 edition is:
S = K × T + C
- S - minimum distance between the protective field and the hazard point (mm)
- K - approach speed: 2000 mm/s for hand and arm, 1600 mm/s for walking
- T - total time: light curtain response time + relay/controller response time + machine stopping time (s)
- C - additional distance for intrusion into the field; for a resolution d up to 40 mm, C = 8 × (d − 14)
Worked example for a vertical light curtain with 30 mm resolution and a measured total stopping time of 0.3 s: S = 2000 × 0.3 + 8 × (30 − 14) = 728 mm. Because this exceeds 500 mm, you may recalculate with K = 1600: S = 1600 × 0.3 + 128 = 608 mm, with a lower limit of 500 mm. A 14 mm light curtain with a 0.2 s stopping time gives 400 mm. For resolutions above 40 mm and for multi-beam grids, C = 850 mm and K = 1600 mm/s, because an arm can reach between the beams. The distance may never be less than 100 mm.
The ISO 13855:2024 edition revised the calculation to S = K × T + DDS + Z, with a separate intrusion distance (DDS) and additional allowances (Z). It also takes account of a hazard point that keeps moving after the field is interrupted. For new designs, check which edition you follow and record it in the technical file.
🔧 What are muting and blanking?
On production lines, material sometimes has to pass through the protective field, for example pallets leaving a palletiser. Two functions exist for this:
- Muting - temporarily and automatically bypassing the light curtain while material passes. Two or four muting sensors (usually photoelectric or reflective sensors, sometimes proximity sensors) recognise the material by sequence, timing and size. Muting sensors must become active together within a short time window (often around 4 s) and the muting time is limited. A person must never be able to trigger muting; application and positioning are covered in IEC 62046.
- Blanking - disabling specific beams. With fixed blanking, a fixed machine part protrudes through the field; with floating blanking, an object of limited size may move within it. Blanking increases the effective resolution, so you must calculate the safety distance with that larger value.
Some presses also use PSDI (presence sensing device initiation), where clearing the light curtain starts the next cycle. This is permitted only under strict conditions set out in the press standards.
🏭 Where are light curtains used?
| Application | Why a light curtain | Point of attention |
|---|---|---|
| Press brake and mechanical press | Fast manual access to the tooling | Type 4, finger resolution, accurate stopping time measurement |
| Robot cell with loading station (Robotics) | Operator places parts without opening a door | Combined with interlocked doors for maintenance |
| Palletiser and stretch wrapper | Pallets must enter and leave the cell | Four-sensor muting |
| Packaging and cutting machine | Product changeovers and infeed faults | Resolution and restart interlock |
| CNC machine with loading zone | Loading without a physical guard | Distance and reaching over the field |
A maintenance door is not a light curtain application: it needs a guard with an interlocking device according to ISO 14119. For cobots, AGVs and AMRs, safety laser scanners are more common; these are described as AOPDDR in IEC 61496-3.
🛡️ Which standards and laws apply?
| Standard or law | Role for light curtains |
|---|---|
| IEC 61496 | Product standard: part 1 general requirements, part 2 light curtains (AOPD), part 3 laser scanners (AOPDDR) |
| ISO 13855 | Positioning and minimum safety distance |
| ISO 13849-1 / IEC 62061 | Design and validation of the complete safety function (PL or SIL) |
| ISO 12100 | Risk assessment from which the choice of a light curtain follows |
| IEC 62046 | Selection, positioning, muting, commissioning and periodic testing |
| Machinery Directive | Devices that detect the presence of persons are listed in Annex IV (high-risk products) |
| Machinery Regulation | Replaces the Machinery Directive on 20 January 2027 |
A Type 4 light curtain does not automatically deliver PL e. The final result depends on the whole chain: light curtain, logic and output stage together. In modern plants, that chain increasingly runs over a safety network protocol such as PROFIsafe or CIP Safety.
🔐 How do you ensure a safe installation in practice?
- Start with the risk assessment - determine which body part can reach the hazard and which PL or SIL is required.
- Measure the stopping time - use a stopping time analyser in the worst case (maximum speed and load) instead of relying on a catalogue value alone.
- Calculate and secure the distance - record S and prevent anyone from reaching around, under, over or behind the field. A person who can stand behind the field without being detected needs additional detection.
- Wire dual-channel - OSSDs to a safety relay or safety PLC, with EDM and restart interlock.
- Validate and test - at handover (SAT), check the whole field with a test rod of the right diameter, and repeat this daily or every shift. Record periodic inspections.
- Prevent tampering - taping over, mirrors or bypassing often happen under production pressure. Signal muting and override states and restrict parameter changes with passwords.
In the Netherlands, the Working Conditions Decree (Arbobesluit) requires that safeguards cannot easily be bypassed and that work equipment remains safe, building on the Arbowet. Employers therefore include light curtains in the RI&E and the maintenance plan; in an accident investigation, the Netherlands Labour Authority may also examine the distance and the test regime. Any change to the machine or its control system calls for a new check of stopping time and distance.
❓ Frequently asked questions
What is the difference between a light curtain and a light grid?
A light curtain has many closely spaced beams and detects fingers or hands. A light grid has two to four beams at fixed heights and only detects a whole body walking into an opening. A light grid is therefore suited to access protection at a greater distance, not to keeping hands away from the hazard point.
Which type of light curtain do I need?
The required light curtain type follows from the risk assessment and the required Performance Level. PL e requires a Type 4 light curtain; for PL d a Type 3 device formally suffices, but most designers choose Type 4 then as well. Type 2 is limited to PL c and SIL 1. Presses and robot cells almost always use Type 4.
How far from the hazard must a light curtain be mounted?
You calculate the minimum distance of a light curtain with ISO 13855 from the approach speed, the total stopping time and the resolution. A common formula is S = K × T + C, with K = 2000 mm/s for a hand. With a stopping time of 0.2 s and 14 mm resolution, the result is 400 mm.
May a light curtain restart a machine?
After an interruption, a light curtain may in principle not restart a machine by itself; a deliberate reset from outside the danger zone is required. Only on certain presses is PSDI permitted, where clearing the field starts the next stroke. Strict requirements then apply to resolution, distance and the design of the press.
How often must a light curtain be tested?
A light curtain should be tested at every commissioning, after every change and periodically with a test rod of the specified resolution. Many manufacturers recommend a daily or per-shift function test by the operator and a more extensive inspection by a competent person. The results belong in the safety file.
Can a light curtain be hacked or tampered with?
Most tampering is physical: taping over, mirrors or bypassing the OSSD wiring. With light curtains configured over a network or through a safety PLC, muting, blanking or parameter settings can also be changed. Restrict that access with passwords and roles and log changes, as the Machinery Regulation requires from 2027.
📌 In summary
A light curtain is a non-contact safety device that stops a machine motion as soon as a body part interrupts the infrared field, and it is only safe when type, resolution, safety distance and wiring are correct. Choose the type according to IEC 61496, calculate the distance with ISO 13855, design the complete safety function according to ISO 13849-1 and test the light curtain routinely as part of safe industrial automation and CE marking.