Light Barrier (Safety Light Curtain): Working Principle, Types

photoelectric_ safety barrier / _safety light barrier
Updated: 2025-09-08 · DAIDISIKE
In industrial machine guarding, “light barrier” usually refers to a safety light curtain—an array of infrared beams forming a protective field. This article gives a practical, standards-oriented explanation you can use on real machines: definition, how it works, Type 2 vs Type 4, safety distance, and a selection workflow for presses and robot cells.

Safety light curtain working principle illustration
Safety light curtain (light barrier) — principle overview
On this page
Working Principle of Light Barrier (Safety Light Curtain)
Types of Light Barrier (Safety Light Curtain)
Guide to Machine Protection with Light Barriers
FAQ
Working Principle of Light Barrier (Safety Light Curtain)
A safety light curtain uses modulated infrared photoelectric detection. When any beam in the protective field is interrupted, the system switches its safety outputs, stopping hazardous motion via a safety relay or a safety PLC.

Signal flow: emit IR → receive → detect interruption → switch OSSD → machine stop
Working principle of safety light curtains – step 1
Working principle of safety light curtains – step 2
Working principle of safety light curtains – step 3
Main components
Emitter: An array of IR diodes fires beams in a timed sequence. The light is modulated (tens of kHz) so ambient light is rejected.
Receiver: A matched array of photodiodes/phototransistors reads each beam in order and reports any loss.
Control/Safety Output Module: Processes receiver data, runs internal diagnostics, and drives OSSD outputs to the machine’s safety circuits.
Key technologies for reliability
Modulation & optical filtering: Frequency-coded pulses and electronic filtering reduce false trips from lamps/sunlight.
Redundant architecture & self-tests (Type 4): Dual-channel electronics (often dual MCUs) cross-monitor critical paths. Any internal fault forces a safe state.
OSSD safety outputs: Modern curtains provide two independent PNP sourcing outputs. Safe/clear state = high; interruption or fault = outputs switch off (low). Cross-short and wire-break detection is built-in. Always follow the product datasheet if NPN variants are used.
Installation & maintenance essentials
Precise alignment & rigid mounting (use the built-in alignment indicators).
Calculate safety distance so the machine fully stops before access is possible.
Daily functional test: Before each shift, block with a test rod and confirm stop.
Keep optics clean: Oil and dust reduce margin; clean windows regularly.
Wire to safety circuits only: Route OSSD directly to a safety relay/PLC, not general I/O.
DAIDISIKE notes. DQC/DQE/DQT/DQA/DQO/DQR/DQL/DQM/DQS/DQB/DQZ series use wired synchronization for robust signal timing. Use shielded cables; in noisy environments bond the shield per manual (0V/PE as specified) to avoid ground loops. MK/JER series support optical sync for short opposed distances (≤ ~6 m) to simplify cabling.
Types of Light Barrier (Safety Light Curtain)
Safety light curtains can be compared from two angles: installation form (how beams traverse the area) and safety performance (Type 2 vs Type 4 design targets based on ISO 13849 principles).

Feature Through-beam (opposed units) Single-side with safety mirrors / retro-reflective solutions Type 2 Type 4
Working principle Dedicated emitter ↔ receiver across the opening One active side; beam path redirected via safety mirrors / certified reflectors Basic fault checks at start/restart Continuous diagnostics & dual-channel architecture
Range & accuracy Long range, high stability, small-object detection Medium range; care with mirror contamination/alignment For lower-risk applications For higher-risk zones
Installation Needs mounting on two sides Single-side mounting; good for tight spaces or multi-side perimeters PL up to ~c / SIL 1 (typical) Targets PL e / SIL 3 (Cat.4 designs)
Cost Higher (two active units) Lower hardware; mirrors need upkeep Lower Higher
Typical uses Presses, shears, large machine openings Confined cells, perimeter guarding with L/U/□ beam paths Choose based on formal risk assessment (PLr)
Key performance parameters
Resolution (beam spacing): 14 mm (finger), 30 mm (hand), 40–200 mm (body/area).
Protection height & operating range.
Response time: < 10 ms.
Environmental ratings: IP, vibration, oil mist, EMC.
Mounting solution 1
Mounting solution 2
Mounting solution 3
Mounting solution 4
Guide to Machine Protection with Light Barriers (Safety Light Curtains)
Resolution vs protection objective
Protection objective Recommended resolution Machine examples Notes
Finger 10–14 mm Small presses, assembly jigs, press brakes 14 mm is the global mainstream for finger protection; 10 mm for higher protection.
Hand 20–30 mm Power presses, feed areas of machine tools 30 mm widely used; prevents palm/wrist access.
Body / area 40–200 mm Packaging lines, logistics cells, access gates For area isolation or entry detection.
Special cells Customized Robot welding, spray booths, test rigs Usually multi-side guarding with mirrors + muting/blanking.
Resolution vs protection objective illustration
Safety distance quick guide (ISO 13855):
Measure total stop time T (light curtain + logic + machine).
Use approach speed K from the standard (e.g., 2,000 mm/s for hand/arm).
Compute S = K × T + C (C = reach-over constant by resolution).
Mount the light curtain at distance ≥ S from the hazard.
Verify with a test rod and record results in the maintenance log.
Get a light barrier selection sheet Type 4Press brake
FAQ
Is “light barrier” the same as “safety light curtain”?
In industrial safety, they are used interchangeably. Check the Type rating and confirm the design meets the required Performance Level (PLr) for your machine.

Type 2 vs Type 4 — how should I choose?
Decide by risk assessment (PLr). Type 4 has dual-channel diagnostics aimed at PL e/SIL 3 architectures for higher-risk zones (e.g., presses, robot cells). Type 2 suits lower-risk tasks.

What is an OSSD safety output and why are there two channels?
OSSD are self-monitoring safety outputs. Two independent channels let the safety relay/PLC detect shorts, breaks, and faults. If any issue occurs, both channels switch off to force a safe stop.

Should I use PNP or NPN outputs?
Use what your safety relay/PLC input expects. PNP (sourcing) is the global mainstream for OSSDs. If using NPN, ensure the entire safety circuit is designed for sinking logic and maintains diagnostic coverage.

Do I need a safety relay or can I wire to a normal PLC?
Use a certified safety relay or a safety PLC. General PLC inputs are not safety-rated and cannot guarantee the required Performance Level, even if they read the signals.

14 mm vs 30 mm — which resolution should I pick?
14 mm targets finger protection; 30 mm targets hand/palm protection. Choose by the smallest body part that must be prevented from entering the hazard and the machine opening size.

Video demonstration of safety light curtains
Video demonstration of safety light curtains

Video demonstration of DQL detection series
Video demonstration of DQL detection series

3D animation demonstration of light curtains
3D animation demonstration of light curtains

DAIDISIKE DQT Type 4 safety-rated light curtain
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Author: DAIDISIKE Engineering Team · Last reviewed: 2025-09-08 · Contact: About & Team
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