Type 2 vs Type 4 Safety Light Curtains
Two safety light curtains can share the same housing, the same beam pitch, the same protected height and the same range — and still belong to different worlds. The difference is invisible in a product photo: it is how the device treats its own internal faults. That is what the Type rating in IEC 61496 classifies, and it is the single most misunderstood line on any light-curtain datasheet.
The short version up front: a Type 4 device must keep its protective function even when a single internal fault occurs, which it achieves with dual output channels and continuous self-monitoring. A Type 2 device is allowed to rely on periodic self-tests instead — meaning a fault that appears between two tests can go undetected until the next test cycle runs.
This guide explains where those classes come from, how they map to Performance Levels, when a Type 2 curtain is a legitimate engineering choice rather than a false economy, and how to verify what a supplier — including us — actually claims.
Where "Type" Comes From: IEC 61496 in Plain Terms
IEC 61496 is the standard that governs electro-sensitive protective equipment — ESPE for short, the family that includes safety light curtains. Part 1 defines the general requirements and part 2 covers active opto-electronic devices specifically. The standard's job is not to rate how well a curtain sees; optical performance like resolution and range is specified separately. Its job is to rate how the device behaves as a safety component: what happens inside it when a component drifts, an output stage fails, or interference corrupts a signal.
The Type number is therefore a fault-behaviour class, full stop. It is not a quality grade, not a precision grade, and not a durability grade. A Type 2 curtain from a serious factory can be beautifully built; a Type 4 curtain is not sharper-eyed. What the higher class buys you is a guarantee about failure: the protective function survives it, or the device forces a safe stop the instant it is detected.
The Real Difference: What Happens When a Fault Occurs
A Type 4 device runs two output signal switching device channels — dual OSSDs — and cross-checks them continuously, alongside periodic internal pulse tests that exercise the signal path many times per second. If channel A sticks on while channel B releases, the disagreement itself is detected and the device drops to the safe state immediately. Our DQT4 family describes exactly this construction on its series page: dual independent OSSD channels with continuous self-diagnosis, and the DQO series carries the same dual-output discipline.
A Type 2 device is permitted a lighter architecture: a test signal exercises the protective function at intervals, and between those intervals the device is trusted. If a dangerous fault occurs mid-interval, the curtain may keep signalling "clear" until the next test catches the problem. The standard accepts this because the class is intended for lower-risk duty — but the buyer must understand that the acceptance window is real, not theoretical.
Type Maps to Performance Level: 2 → PL c, 4 → PL e
The bridge between the sensor standard and your machine's paperwork is ISO 13849-1. A risk assessment on the machine produces a required Performance Level — PLr — and the protective devices you specify must be capable of meeting it. In practice the mapping is simple: a Type 2 ESPE is the right building block for safety functions up to PL c, typically realised in a Category 2 architecture; a Type 4 ESPE serves safety functions up to PL e in a Category 4 architecture, where dual channels and continuous monitoring are structural requirements.
Reading that backwards is the practical skill: if your risk graph lands on PL c, paying for Type 4 is legitimate conservatism but not an obligation. If it lands on PL d or PL e, a Type 2 curtain is not a budget option — it is a non-compliant one, whatever its other virtues.
Resolution Has Nothing to Do With Type
The most common confusion we see in inquiries is treating Type as a fineness grade — assuming a finger-protection curtain must be Type 4 and a body-detection curtain must be Type 2. The two axes are fully independent. A 14 mm finger-resolution curtain built as Type 2 exists; a 200 mm perimeter curtain built as Type 4 exists. Resolution is chosen from the hazard geometry and mounting distance; Type is chosen from the fault-tolerance the risk level demands.
Choosing the pitch itself — 7.5 mm finger grade up to 200 mm perimeter beams, protected heights, mounting distance arithmetic — is a separate decision path with its own guide on this site, and the two decisions should be made in that order: resolution from geometry first, Type from risk second.
When a Type 2 Light Curtain Is Enough
Honest answer: more often than the safety-equipment industry likes to admit. Duty points where the assessment routinely lands at PL c include buffer and storage zones a person enters occasionally rather than every cycle; slow machinery whose hazardous motion can be seen and avoided; stations where the worst credible injury is reversible; and secondary openings already backed by other measures. In those places a Type 2 device is not a compromise — it is the proportionate engineering answer, and the saved budget frequently buys a second protected opening that would otherwise have stayed bare.
The discipline is documentation: the choice is defensible exactly when the risk assessment that produced PL c is on file. A Type 2 curtain specified by assessment is good engineering; the same curtain specified by price list alone is a liability wearing a CE mark.
When Type 4 Is the Only Defensible Choice
Reverse every condition above and you have drawn the Type 4 map: point-of-operation guarding on power presses, press brakes and shears; any opening a hand enters on every machine cycle; hazards fast enough that avoidance is fantasy; injuries that do not heal. Here the acceptance window of a periodic-test device is not an abstraction — a mid-interval fault coincides with a hand in the opening as a matter of statistics, and the assessment output will read PL d or PL e accordingly.
Press and press-brake duty deserves its own note: beyond demanding Type 4-class fault behaviour, hand-fed bending changes the guarding geometry itself, which is why our press and press brake protection category pairs curtains with ram-tracking laser systems rather than treating every machine as a flat opening.
Why Most of the Market — Including Us — Builds to Type 4 Architecture
Walk our own safety light curtain category and a pattern is obvious: the DCE, DQA, DQC, DQE, DQO, DQR, DQZ and DQSA series are all designed to the Type 4 architecture of IEC 61496 — dual outputs, continuous self-monitoring — while the JER and MK economy lines state their design levels in their own words on their own pages. The commercial logic is plain. A curtain family is designed once and sold for a decade; the incremental cost of the Type 4 architecture is modest at design time, while the cost of a customer discovering mid-audit that their point-of-operation guard is the wrong class is unbounded.
So the market converged: build the platform to the demanding class, let the risk assessment decide where lighter duty is acceptable, and keep economy lines honestly labelled for the duties they fit. That is the shape of our range, and of most credible ranges.
How to Verify a Claimed Type Rating
"Type 4" printed in a listing is a claim, not a property. Verification is asking for documents, and a serious supplier answers without friction.
- Ask which edition of IEC 61496-1/-2 the design was evaluated against, and whether the wording is "certified to" or "designed to" — both are meaningful, but they are different statements and an honest supplier distinguishes them unprompted.
- Ask for the type-examination or test documentation matched to the exact model code you are ordering — not a certificate for a sibling series.
- Ask for the functional-safety data your machine file needs: PFHd, mission time, and the response time at your beam count, which feeds the mounting-distance calculation.
- Check the output structure physically: a device claiming Type 4 duty should present two OSSD channels to your safety relay, not a single switched line.
- Be suspicious of Type 4 claims attached to prices that undercut the class's own architecture — dual monitored channels cost real money to build.
Our own paperwork policy is stated on every series page: design-basis wording on the web page, certificates and functional-safety documents supplied with the order, and data gaps flagged rather than filled with plausible numbers. For the electrical side of verification — checking OSSD behaviour at the terminals before first power-up — the OSSD wiring and verification walkthroughs on fsddsk.com in the DAIDISIKE engineering library cover the test sequence in wiring-level detail.
Send us your risk assessment's required PLr and the opening dimensions via the inquiry form — the reply is a model code and the documentation list, not a brochure.