Type 4 safety light curtains are specified for higher-risk machinery because they continuously self-test, contain redundant critical circuitry, and meet the maximum safety level defined by IEC 61496-1 and -2: SIL cl 3 and PL e per ISO 13849-1 [S1][S3]. Type 2 units, by contrast, only check for faults at start-up or re-start, do not require redundant circuitry, and are capped at SIL cl 1 / PL c [S1][S2].
The most cited number side-by-side is the effective aperture angle: Type 2 = 5°, Type 4 = 2.5° [S1][S2]. That single number drives how close the emitter and receiver can be mounted to reflective surfaces before an optical short circuit fools the receiver into seeing a clear field when a beam is actually blocked [S1]. Beam spacing is a separate axis: light curtains typically use 14–90 mm spacing for finger, hand, or body detection, while safety light grids use only 2–4 beams spaced 300–500 mm apart for body-only detection [S1][S3].
How IEC 61496 Defines "Type" for Photoelectric Safety Devices
When the safeguarding device is a light curtain or light grid, the relevant "Type" is set by IEC 61496-1 (general requirements) and IEC 61496-2 (AOPD-specific requirements), and it is independent of the SIL/PL classification, which is set by IEC 61508, IEC 62061, and ISO 13849-1 [S1][S3]. A Type 4 AOPD is the only category that simultaneously meets SIL cl 3, SIL 3, and PL e, which is why most safety authorities treat it as the default for new machinery guarding [S1].
Type 2 AOPDs are explicitly restricted to applications where a single fault cannot lead to an immediate loss of the safety function, a constraint that comes straight from the risk assessment philosophy of ISO 13849-1 and the type-testing rules of IEC 61496-1 [S1][S4]. Because of that, a Type 2 device is not a lower-grade Type 4; it is a different device class with a different fault model, not a "Type 4 lite" [S8].
Fault Detection, EAA, and Redundancy: Three Spec Axes That Drive the Choice
Three specification axes separate the two types in practice. First, fault detection: Type 2 runs a periodic self-test, so a fault that occurs between two test cycles can be missed for the duration of that interval, whereas Type 4 runs an active, continuous self-test that flags a fault within the same cycle [S2][S3]. Second, effective aperture angle (EAA): Type 2 = 5°, Type 4 = 2.5°; the wider EAA of Type 2 means the receiver can be tricked by specular reflection off shiny machine surfaces, an effect called an optical short circuit [S1][S2].
Third, circuit architecture: Type 2 does not require redundant critical components, while Type 4 must include duplicate critical components and circuits so the safety function survives a single component failure [S1][S2]. A direct, criteria-based comparison is laid out below.
Who Type 2 Is For, and Who It Is Not For
Type 2 is appropriate for lower-risk applications where the machinery can be stopped quickly, where a single fault cannot immediately lead to a hazardous situation, and where the operator's reach into the hazard is limited, for example small assembly benches, light-duty packaging conveyors, or low-tonnage manually fed presses [S1][S4][S5]. It is not appropriate for finger protection on power presses, for robotic cells where a person can be trapped, or for any application where the risk assessment under ISO 13849-1 lands above PL c [S1][S4].
Type 4 is the default for finger, hand, and body detection on hazardous machinery, including hydraulic and mechanical power presses, robotic work envelopes, palletisers, and high-speed converting lines [S1][S3][S4]. ifm, for one, only promotes light curtains and grids at Type 4 / SIL 3 / SIL cl 3 / PL e for these reasons, even though lower-cost Type 2 devices are widely available [S1]. The schmersalusa guidance reinforces that any AOPD must satisfy either Type 2 or Type 4 of EN 61496, and the choice is driven by the risk assessment, not by budget [S3].
Mounting, Safety Distance, and Stop-Time Discipline
Because a light curtain is a non-separating guard, the minimum safe distance from the hazard is calculated under EN ISO 13855 using the machine's measured stop time, the light curtain's response time, and a reach-over or reach-through factor based on the resolution [S3]. Resolution is the diameter of the smallest object the curtain can reliably detect and is set by the beam spacing: 14 mm for typical finger detection, up to 30, 40, or 50 mm for hand detection, and coarser for body detection [S1][S3].
A stop-time analysis must take at least ten measurements of the machine's response time, factoring in the safety device, monitoring relay or safety controller, output contactors, and motor brakes, because the slowest event in that chain sets the minimum installation distance [S3]. A common field failure is to install a Type 2 device with its wider 5° EAA close to stainless steel guarding, where reflection creates a phantom clear path and defeats the safety function; the cure is either stepping up to a Type 4 with 2.5° EAA or moving the optics away from the reflective plane [S1][S2].
Light Curtain vs Light Grid: Picking Resolution That Matches the Hazard
Light curtains use many closely spaced beams (typically 14–90 mm apart) and can be specified for finger, hand, or body protection; safety light grids use only 2–4 beams spaced 300–500 mm apart and are limited to body detection [S1]. In practice, a finger-detection curtain at 14 mm resolution is the only option for power-press point-of-operation guarding where the operator's fingers can reach the tooling, while a 2-beam 500 mm grid is sufficient for perimeter access control around a robot cell where the hazard is whole-body entry [S1][S3].
Range scales vary by product: typical operating distances run from about 0.3 m (12 in) up to roughly 20 m (65 ft), and protection heights run from about 150 mm (6 in) to over 1.8 m (6 ft) per curtain, with cascadeable models available for taller hazards [S3]. The trade-off is resolution against range: a 14 mm finger-resolution curtain is usually limited to a shorter maximum range than a 30 mm hand-resolution curtain on the same product family, because narrower beams lose more energy per metre of air [S3].
Standards Stack Behind the "Type" Label
The safety claim on a light curtain is built on a stack of standards, not a single certificate. IEC 61508 is the umbrella for functional safety of electric, electronic, and programmable electronic devices, and defines SIL [S1]. ISO 13849-1 defines Performance Level (PL a through PL e) for machinery-specific safety functions, and IEC 62061 defines the SIL claim limit (SIL cl) that bridges the two regimes [S1].
IEC 61496-1 and IEC 61496-2 sit on top of that and define what "Type 2" and "Type 4" mean specifically for active optoelectronic protective devices such as light curtains and light grids [S1][S3]. EN ISO 13855 is then used at the installation stage to calculate the minimum safety distance, while a regional overlay such as EN 61496 in the EU or UL 61496 in North America makes the same IEC rules enforceable in the local regulatory framework [S3]. If a datasheet claims "Type 4 / SIL 3 / PL e," verify that it cites all three layers, not just the marketing headline. For background on the broader functional-safety vocabulary used on these datasheets, see the functional-safety overview in the encyclopedia.
Selection Checklist Before You Buy
Run the following checks before pinning a part number. (1) Confirm the risk-reduction target under ISO 13849-1; if the required PL is d or e, only Type 4 will satisfy it [S1][S4]. (2) Measure the machine's worst-case stop time with at least ten trials, including all safety components in the cut-off chain [S3]. (3) Compute the safety distance per EN ISO 13855 using the curtain's response time, the stop time, and the resolution-appropriate reach factor [S3].
(4) Audit the mounting geometry for reflective surfaces: if the optics sit within the 5° EAA cone of stainless or polished steel, choose Type 4 (2.5° EAA) or reposition the guard [S1][S2]. (5) Match the resolution to the body part at risk, 14 mm for finger, 30–40 mm for hand, 50+ mm or a 2–4 beam grid for body [S1][S3]. (6) Confirm the AOPD type, SIL cl, and PL appear together on the datasheet, with IEC 61496-1/-2, ISO 13849-1, and IEC 62061 all cited, not just one of them [S1][S3]. Skipping step 2 is the single most common reason safety curtains are installed at the wrong distance and then fail a plant audit. For more on the controls these curtains wire into, see the industrial test and measurement primer.
Watch the next revision cycle of IEC 61496-1 and -2 for any tightening of fault-model requirements that could reclassify borderline Type 2 installations, and track PL e and SIL cl 3 datasheet additions from the major AOPD vendors, which signal Type 4 becoming the de facto default for new machine builds even at the lower-cost end of the market [S1].
The underlying component specifications are covered under dry type transformer.
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