Safety Light Curtain

What is a Safety Light Curtain

A safety light curtain consists of two opposing strips, a transmitter (sender) and a receiver, mounted on either side of an access opening. The transmitter emits a row of synchronized infrared beams, typically at a wavelength of 850 nm in the near-infrared band, and the receiver continuously confirms that every beam arrives. The set of parallel beams forms a flat detection field. The instant any beam is occluded by an opaque object of at least the rated detection capability, the receiver de-energizes its output signal switching devices (OSSDs), which command the machine control to enter a safe state. Because detection is non-contact and the field is invisible, the opening stays fully accessible for the operator while remaining guarded.

In safety terminology the curtain is one example of an electro-sensitive protective device, or ESPE, and the specific optical implementation is an active opto-electronic protective device, or AOPD. These terms matter because they tie the product to its governing standards: the general ESPE requirements in IEC 61496-1 and the AOPD-specific requirements in IEC 61496-2. A device that does not carry conformity to this pair is not a safety light curtain in the regulatory sense, regardless of how it is marketed; an ordinary photoelectric sensor or area scanner without redundant, self-tested architecture cannot substitute for one.

It is important to understand that the curtain alone does not make a machine safe. The protective function is a chain: the curtain detects intrusion, a safety relay or safety controller (or a safety PLC) processes the OSSD signals and any logic such as restart interlock, and final elements (contactors, valves) remove power or energy from the hazard. The overall reliability of this chain is what gets rated against a target, expressed either as a Performance Level PL a through PL e under ISO 13849-1, or a Safety Integrity Level SIL 1 through SIL 3 under IEC 61508 and IEC 62061. The light curtain contributes the input-subsystem portion of that budget through its published PFHd, the mean probability of a dangerous failure per hour.

Light curtains are deployed wherever an opening must stay open for production but a person could reach a hazard through it. Typical duties include point-of-operation guarding on mechanical and hydraulic presses, perimeter access to robot cells and palletizers, hand-protection on assembly and packaging machines, and entry or exit guarding on conveyor lines that feed automated equipment. They replace fixed fencing or movable gates where frequent operator access would otherwise slow the cycle, and they are often combined with two-hand controls, interlock switches, and emergency-stop devices in a layered guarding strategy. Regulatory drivers include the EU Machinery Regulation and, in the United States, OSHA 29 CFR 1910.212 and 1910.217 for power presses, alongside the ANSI B11 series.

The core engineering quantities that distinguish one curtain from another are its type (fault behavior), resolution (smallest detectable object), protective field height and operating range, and response time. These four, together with the machine stop time, feed directly into the mounting distance and therefore into whether the installation is actually safe. The remaining chapters decode each of them.

Type Classification under IEC 61496

IEC 61496-1 sorts ESPEs into types that describe how the device behaves when a fault occurs internally, not how small an object it detects. This is the single most misunderstood point in light-curtain selection. The two types commonly available as light curtains are Type 2 and Type 4. (Type 3 exists in the standard but is rare for curtains; it is more common in multi-beam and scanner products.) The table below summarizes the practical difference.

A Type 2 device runs a self-test at intervals. Between those tests, a single internal fault could in principle go unnoticed, which is why the standard caps its use at lower-risk applications. The current edition of IEC 61496-1 tightened systematic requirements so that, in practice, suppliers generally rate Type 2 curtains to SIL 1 / PL c rather than claiming PL d on electrical properties alone. Type 2 remains a valid, economical choice for warning fields, indirect access, or situations where the risk assessment genuinely lands at PL b or PL c.

A Type 4 device performs a continuous self-test and is designed so that a single fault does not lead to loss of the safety function and is detected before the next demand. It also carries stricter optical requirements: a narrower effective aperture angle (the standard limits the angle within which the receiver will accept light), which reduces the chance that a beam reflects off a nearby shiny surface and bypasses an obstruction. Type 4 is the only choice that reaches the highest ratings SIL 3, Category 4, and PL e, and it is the default for point-of-operation guarding on presses and for robot-cell access where a person can place their whole body in the danger zone.

The mapping from machine risk to required type runs through the risk assessment. ISO 13849-1 produces a required Performance Level (PLr) from severity of injury, frequency of exposure, and possibility of avoidance; IEC 62061 produces a required SIL. If that target is PL d or PL e, a Type 4 curtain is mandatory because no Type 2 device can carry the input subsystem to that level. If the target is PL c or lower, Type 2 may be acceptable and cheaper. Selecting a Type 2 device for a PL e duty is a compliance failure that an auditor or a post-incident investigation will find immediately, so the type decision must be made from the risk assessment first, before any catalog is opened.

One further architectural note: Type 3 and Type 4 ESPEs are required to provide at least two OSSD outputs that both switch to OFF on detection, giving the dual-channel, cross-monitored output that downstream safety relays expect. A genuine Type 4 curtain therefore always exposes two safety outputs (commonly two PNP semiconductors, short-circuit protected and cross-circuit monitored), never a single channel. If a product offers only one safety output, it is not a Type 4 AOPD.

Sensing Technology and Detection Capability

The detection capability, often called resolution, is the diameter of the smallest opaque test rod that the curtain is guaranteed to detect at any point in the field. It is a function of both beam spacing and beam (lens) diameter, not beam spacing alone, because an object must block enough of a beam to register. Standard resolutions correspond to the body part being protected, and the choice drives both the mounting distance and the maximum range. The table below lists the common values and what each protects.

Finger protection at 14 mm is required where the operator's fingers can reach the hazard, for example at the point of operation on a press brake or a small-component assembly station. Finer detection has a cost: it requires more beams over the same height, which lengthens the scan cycle and can raise response time, and it usually reduces the maximum operating range. In return, it allows the curtain to mount much closer to the hazard, because the C intrusion term in the ISO 13855 distance formula is smallest at 14 mm (in fact zero, as Chapter 4 shows).

Hand protection at 30 mm is the workhorse value for general machine guarding where a hand, but not an isolated finger, can reach the danger zone. Limb or arm protection at 40 mm suits openings where access is at a greater distance and only an arm could intrude. Beyond 40 mm the device is no longer treated as a fine-resolution curtain; multi-light-beam safety devices use 2, 3, or 4 widely spaced beams (per ISO 13855 commonly 500 mm pitch for two beams, 400 mm for three, and 300 mm for four) to detect a whole body crossing a perimeter, which is the right tool for cell-entry guarding rather than reach-in protection.

Mechanically, the sender and receiver are usually housed in an extruded aluminum profile with a glass or polycarbonate front window, rated to IP65 and often IP67 against dust and washdown. Synchronization between sender and receiver is typically optical, so no separate sync wire is needed, and many families include an integrated alignment aid (a visible-red laser, Class 1) to speed setup over long ranges. The light source itself is invisible near-infrared, most commonly 850 nm, chosen because it is eye-safe at the operating power and is easy to filter against ambient light. Beam coding (assigning a modulation pattern to a pair) lets two curtains operate side by side without mutual interference, at the cost of added response time.

Two configuration features change the effective detection field. Blanking masks beams: fixed blanking permanently ignores beams blocked by a fixture or workpiece chute, while floating blanking lets a defined number of adjacent beams be interrupted anywhere without tripping, to allow material pass-through. Both alter effective resolution and therefore force a recalculation of the safety distance. Muting is different: it suspends the whole protective function for a supervised window so an authorized object (a pallet) can pass, using at least two independent muting sensors and a strict sequence and time limit per IEC/TS 62046. Muting must never be defeatable by a person, which is why single-sensor muting is prohibited.

Safety Distance and Standards

Selecting the right curtain is only half the job. Where you mount it decides whether the installation actually protects anyone. ISO 13855 establishes the minimum distance between the detection field and the nearest hazard so that the machine reaches a safe state before a person, reaching through the field at a defined speed, can touch the hazard. The governing equation for a normal approach is:

S = (K x T) + C

where S is the minimum distance in millimeters, K is the assumed approach speed, T is the total system stop time in seconds, and C is the intrusion allowance in millimeters. Each term hides engineering judgment, so each is explained below.

K (approach speed) is taken as 2000 mm/s for hand and arm movement toward the hazard. ISO 13855 permits reducing K to 1600 mm/s only after a first calculation using 2000 mm/s yields S greater than 500 mm, in which case the slower whole-body approach becomes the governing case. Using 1600 mm/s prematurely understates the required distance and is a common error.

T (total stop time) is the sum of every delay from beam interruption to the hazard halting: the curtain's own response time, the safety relay or safety controller reaction time, and the machine run-down time of the final elements. The machine stop time should be measured on the actual equipment, ideally with a stop-time measurement instrument, and re-verified periodically because brakes and valves degrade with wear. Forgetting to add the device response time to the machine stop time is the most frequent sizing mistake.

C (intrusion allowance) accounts for how far a body part can penetrate the field before a beam is broken. For a vertical curtain with resolution d of 40 mm or finer, C = 8 x (d minus 14) mm. This makes the arithmetic concrete: a 14 mm curtain gives C = 0, a 30 mm curtain gives C = 8 x 16 = 128 mm, and a 40 mm curtain gives C = 8 x 26 = 208 mm. For coarser detection above 40 mm, including multi-beam guards, C is a fixed 850 mm. The table below works a representative example end to end.

The example shows why resolution and mounting interact. With the same 100 ms stop time, the 14 mm curtain can sit 200 mm from the hazard, while the 30 mm curtain must sit 328 mm back. If the cell layout cannot accommodate the larger distance, the answer is finer resolution or a faster stop, not fudging the numbers. Note that ISO 13855 adds further cases the formula above does not cover: horizontal (floor-level) fields use a different C based on field height, and any gap under or around the curtain must itself satisfy reach-over and reach-under geometry from ISO 13857.

The standards landscape around the calculation is worth fixing in one place. IEC 61496-1 and IEC 61496-2 define the device. ISO 13855 defines positioning. ISO 13849-1 (PL and Category) or IEC 62061 / IEC 61508 (SIL) rate the safety function. IEC/TS 62046 covers application, muting, and blanking. ISO 13857 governs reach distances and gaps. In North America, UL 61496 is the device standard, ANSI B11.19 and ANSI/RIA R15.06 cover application and robots, and OSHA 29 CFR 1910 sets the legal floor. A complete submittal cites the device certificate and the calculation, not just a part number.

Key Specification Parameters

Datasheets for Type 4 curtains list dozens of entries, but a manageable set actually drives selection: resolution, protective field height, operating range, response time, safety rating (type, SIL, Category, PL) with PFHd, output type, supply voltage, enclosure rating, and operating temperature. The comparison below pins three verified mainstream Type 4 families against these axes, then the parameters are decoded individually.

Response time is the interval from beam interruption to the OSSDs reaching the OFF state, and it feeds straight into the ISO 13855 stop time. It grows with field height (more beams to scan), with beam coding, and with cascading. The SICK deTec4 Set 14/1950, for instance, publishes 21 ms uncoded but 48 ms when code 1 or code 2 beam coding is enabled, more than doubling its contribution to the safety distance. Always size the mounting distance with the response time of the exact configuration you will deploy, including coding and any guest segments.

Resolution and protective field height were covered in Chapters 3 and 4; on the datasheet they appear as fixed catalog options. Note that protective field height is the guarded length, not the device length, and that some families offer the same height in several resolutions, so confirm the exact ordering code matches both the height and the resolution you sized.

Operating range is the maximum sender-to-receiver distance at which detection is guaranteed; it shrinks for finer resolutions because each lens captures less light over distance. Verified examples include up to 20 m for the SICK deTec4 at 14 mm and 0.3 to 30 m for the Omron F3SG-SR at coarser resolutions, dropping toward 15 m for its finest 14 mm option. Long-range duties also demand careful alignment, which is why an integrated laser alignment aid is a practical selection criterion.

Safety rating and PFHd together define how much of your SIL or PL budget the curtain consumes. A genuine Type 4 device states all four descriptors (Type 4, SIL 3, Category 4, PL e) and a PFHd, for example 15.3 x 10^-9 per hour for a single SICK deTec4. Cascading raises PFHd: SICK publishes 30.5 x 10^-9 for a host plus one guest and 45.6 x 10^-9 for two guests, so the architecture must be checked against the target, not just the bare device value. Mission time (TM), commonly 20 years under ISO 13849-1, is the basis for these probability figures and sets the replacement horizon.

Output, supply, enclosure, and temperature round out integration. Expect two PNP OSSDs, short-circuit protected and cross-circuit monitored, typically rated to 500 mA each, on a 24 V DC supply (commonly 19.2 to 28.8 V). Enclosure ratings of IP65 and IP67 suit dusty and washdown environments; for the deTec4 the ambient operating range runs -30 to +55 degrees C, wide enough for cold stores and outdoor canopies. Confirm vibration and shock ratings (for example 5 to 150 Hz per EN 60068-2-6) match presses and other high-vibration hosts.

Selection Decision Factors

To convert the preceding chapters into a defensible model choice, follow the decision sequence below. The order matters: most failures come not from one wrong number but from deciding range or brand before the risk assessment has fixed the required type. These nine steps double as an RFQ template.

1. Run the risk assessment first: derive the required PL (ISO 13849-1) or SIL (IEC 62061) from severity, exposure, and avoidability. This fixes whether you need a Type 4 (PL d / PL e) or may use Type 2 (up to PL c). Never start from a catalog.
2. Fix the resolution to the body part at risk: 14 mm for fingers, 30 mm for hands, 40 mm for arms, or multi-beam for whole-body perimeter access. This choice sets the C term in the safety distance.
3. Calculate the ISO 13855 safety distance: S = (K x T) + C, using measured machine stop time plus the chosen device's response time. Confirm the cell layout can physically accommodate S; if not, choose finer resolution or a faster stop.
4. Size protective field height and range: the guarded height must cover every reach path, and the sender-to-receiver range must exceed the opening width with margin. Check that the finest resolution you need is available at that height and range.
5. Select the safety output architecture: two OSSDs into a safety relay, safety controller, or safety PLC, with restart interlock and external device monitoring (EDM) if the duty requires manual reset and contactor feedback.
6. Specify application functions: muting (with at least two sensors per IEC/TS 62046) for authorized material pass-through, fixed or floating blanking for fixtures, cascading for L-shaped or U-shaped openings, and beam coding where adjacent curtains could interfere. Recalculate distance whenever blanking or cascading changes resolution or response time.
7. Match environment ratings: enclosure IP65 or IP67 for washdown and dust, operating temperature span for the site, and vibration and shock ratings for the host machine. Add deviating-field or weld-spark-resistant front screens for foundries and welding cells.
8. Confirm certification and conformity: CE type-examination plus declaration of conformity to IEC 61496-1/-2, UL 61496 for North America, and any sector requirements (press guarding under OSHA 1910.217, robots under ANSI/RIA R15.06). File the certificate with the calculation.
9. Total cost of ownership: purchase price plus mounting and alignment labor, plus the periodic stop-time re-verification and functional test the standard requires. A curtain with an integrated alignment aid, NFC or IO-Link diagnostics, and local spare-part stock lowers lifetime cost even at a higher list price.

One dimension that buyers routinely underweight is serviceability over the machine's life: local spare-part inventory, field alignment and validation support, diagnostic outputs that pinpoint a blocked or misaligned beam, and firmware or configuration tooling that survives a decade of production. SICK, Omron, Banner, Pilz, Rockwell Allen-Bradley, Schmersal, Keyence, and Leuze all maintain regional support and certified spares, which is why they dominate shortlists for critical lines. The cheapest compliant curtain that cannot be re-aligned or re-validated quickly after a nuisance trip can cost more in downtime within a single year than the price gap to a serviceable brand.