A Type 4 safety light curtain holds its sensing field continuously by combining redundant self-monitored optics, a tight 2.5 degree effective aperture angle, and constant OSSD cross-checking against the standards IEC 61496-1 and -2, achieving SIL cl 3 and PL e [S1].
The protective plane is defined by the emitter-to-receiver range (commonly 12 inches to over 65 feet) and the protection height (around 6 inches to over 6 feet), with beams spaced 14-90 mm apart to match finger, hand, or body detection resolution [S5][S6].
Optical geometry: from one beam to a continuous plane
A safety light curtain starts as a through-beam photoelectric pair, then becomes a "curtain" by stacking many parallel infrared beams between one emitter stick and one receiver stick [S1][S2]. The spacing of those beams, called the resolution, sets what the field can detect: 14 mm for finger protection, around 30 mm for hand protection, and coarser spacings for body or perimeter guarding [S1][S5]. The emitter typically uses infrared LEDs or laser diodes pulsed in sequence, while the receiver uses an aligned array of photodiodes or phototransistors that only accept pulses from the matching transmitter [S2].
Range and protection height together define the actual plane being monitored; in current product lines, the emitter-receiver gap can run from roughly 12 inches up to about 65 feet, and the protection height (the distance from the first to the last beam) commonly spans 6 inches to over 6 feet, with the physical housing a bit longer than that active span [S5]. Multiple parallel infrared beams fill that plane to form a continuous safety detection zone, and the interruption of any single beam is enough to drop the safety outputs [S8].
Type 4 vs Type 2: what makes the field "continuously reliable"
Type 4 devices are built so the field is trustworthy every scan cycle, not just at start-up. Per IEC 61496-1/-2, Type 4 requires continuous self-monitoring, redundant critical circuitry with a back-up path, a tighter 2.5 degree effective aperture angle (versus 5 degrees for Type 2) to cut the optical short-circuit risk near reflective surfaces, and the ability to reach SIL cl 3 and PL e [S1][S3].
Type 2 only checks for faults during start-up or re-start and leaves gaps between internal tests, so a fault can sit undetected until the next periodic check; that is acceptable for lower-risk cells but not for higher-risk points of operation [S1][S3]. In a Type 4 curtain, the Output Signal Switching Devices (OSSDs), typically solid-state transistors, drop out the moment any fault is detected, and the machine's EDM (External Device Monitoring) loop "checks back" on contactors and MPCEs with a diagnostics coverage that the literature places at 99.9% [S2][S4]. The practical result is that the sensing field is treated as a single safety function, with the beam array, OSSD outputs, and external contactors all cross-checked on every cycle.
Beam coding, blanking, and muting: keeping the field continuous in real plants
Three operating features decide whether the curtain can stay armed while real production objects move through it. Beam coding lets adjacent curtains run on different pulse patterns so two systems in the same optical neighborhood do not false-trigger each other [S4]. Floating blanking allows a configured number of beams to be broken anywhere in the field without tripping the OSSDs, which is useful when small fixtures or cables legitimately sit in the plane [S1].
Permanent blanking locks out specific fixed beam positions, used when a support table or bracket permanently occupies part of the field [S1][S4]. Muting goes further, temporarily suspending the safety function only when a muting sensor confirms a defined object (such as a car body on a conveyor) is passing, while still tripping on a person [S4]. Together with recommence interlocks in automatic or manual reset modes, these features let a Type 4 system hold a continuous protective plane through normal material flow rather than forcing the line to stop every time a fixture enters the field [S4][S5].
Standards, stopping time, and the minimum safe distance
The Type 4 architecture is only as good as its installation math. The relevant standard stack is IEC 61496-1 and IEC 61496-2 for the AOPD itself, EN ISO 13855 for the safety distance from the hazard, ISO 13849-1 for Performance Level, and IEC 62061 / IEC 61508 for SIL cl 3 claims, with the curtain treated as an ESPE (electro-sensitive protective equipment) or AOPD (active opto-electronic protective device) [S1][S2][S5].
Because the curtain is a non-separating guard, the safety distance calculation under EN ISO 13855 must use the machine's measured stopping time, which comes from a stop-time analysis (minimum 10 measurements) covering the safety device response, monitoring device, output triggers, motors, and any other element in the stop path [S5]. If the dangerous movement cannot be stopped in time, the field has failed its job even though every beam is intact, so the stop-time and minimum-distance calculation is part of "maintaining a continuous sensing field" in the engineering sense, not just a paperwork step [S5].
Comparison at a glance: Type 2 vs Type 4 on the four decision criteria
On fault detection, Type 2 runs periodic internal tests with gaps, while Type 4 cross-checks every cycle and drops the OSSD outputs on any detected fault [S1][S3]. On optical robustness, Type 2 has a 5 degree EAA, and Type 4 has a 2.5 degree EAA, so Type 4 is specified closer to shiny surfaces where an optical short circuit (an unsafe alternate reflection path) is a real risk [S1].
On achievable safety level, Type 2 reaches SIL cl 1 and PL c, while Type 4 reaches SIL cl 3 and PL e under IEC 61496, the highest level the standard defines for AOPDs [S1]. On cost, Type 2 is lower, but several Type 4 lines have closed that gap to the point where the decision is driven by risk level, not by budget [S1]. For a deeper Type 2 vs Type 4 risk and spec map, see the related selection reference on Type 2 vs Type 4 safety light curtain selection.
Where Type 4 continuous fields earn their keep, and where they do not fit
Type 4 is the right pick for finger- or hand-resolution points of operation on presses, robotic cells, and palletisers where any undetected intrusion could cause serious injury, and where the risk assessment demands PL e [S1][S3][S5]. It is also the right pick when the curtain must sit close to reflective guarding, when EDM on downstream contactors is required, or when the cell must keep running with floating blanking, fixed blanking, or muting during material transfer [S1][S4].
It is the wrong pick for a low-risk perimeter around a slow-moving automated stacker that only needs body detection, where a Type 2 light grid, or even a Category 3 device, can deliver the needed risk reduction at lower cost [S1][S3]. It is also the wrong pick if the dangerous movement cannot be reliably stopped, because no amount of self-checking optics will fix an installation whose safety distance under EN ISO 13855 is too short [S5]. On the broader machine-safety layer, these curtains sit next to other protective devices described in the fire-safety and safety light curtain encyclopedia entries.
What to watch in 2026: solid-state OSSDs, muting logic, and connectivity
The 2026 trend lines for Type 4 curtains are solid-state OSSD diagnostics with richer fault logs, deeper muting and blanking sequencing tied to safety controllers, and more connectivity into plant-wide safety networks for diagnostics rather than for the safety function itself [S2]. Vendors continue to push beam coding and tighter EAA optics so curtains can be mounted closer to stainless guarding in food, pharma, and washdown cells without optical short circuits, and so multiple curtains can share one work envelope without cross-talk [S1][S4].
Two trackable signals for the next planning cycle: confirmation that the OSSD diagnostic coverage figure used in vendor literature still maps to ISO 13849-1 and ANSI B11.26 diagnostic coverage rules, and any update to EN ISO 13855's safety-distance formulas as AOPD response times keep shrinking. For broader process-control context on the scan and cycle times these safety devices plug into, see the engineering reference on PLC scan time vs task cycle time.
The underlying component specifications are covered under dry type transformer.