How to Choose a Safety Light Curtain

What Does a Safety Light Curtain Do, and When Should You Use One?

A safety light curtain is an electro-sensitive protective device. Its job is to detect a person approaching a hazard early enough for the machine to stop before contact. It suits applications where operators interact with the machine every cycle — feeding a press, loading a molding machine, or passing parts into a robot cell — because there is no gate to open and close.

It is not the right tool everywhere. A light curtain cannot contain ejected parts, splashing coolant, or flying sparks; if those are present you still need a physical barrier, with the curtain guarding the access opening. And if the guarded opening is a door that people walk through occasionally rather than every cycle, a safety door interlock is often simpler and cheaper.

One more distinction matters at the start: safety light curtains are different from measuring light curtains and diffuse area sensors. DAIDISIKE's DD diffuse-reflection area grid, for example, is recommended by its own manual for object and area detection only — not for personnel protection. Always confirm the product family is intended for safety use before designing it into a guard.

Step 1: What Resolution (Beam Pitch) Does Your Hazard Require?

Resolution — set by the beam pitch, the center-to-center distance between adjacent beams — determines the smallest object the curtain reliably detects. Pick it based on what part of the body could reach the hazard through the opening.

Industry convention maps roughly like this: around 14 mm detects fingers, around 30 mm detects a hand, 40 mm suits general access-point detection, and 80–200 mm pitches are used as multi-beam barriers for whole-body access detection around perimeters, where nobody can reach the hazard point directly.

The DAIDISIKE DQC series covers this whole span in one family, with eight beam-pitch options: 10, 14, 20, 25, 30, 40, 80, and 200 mm. That matters practically: you can guard a punch-press feed opening at 10 or 14 mm and the fence opening behind the same machine at 80 or 200 mm without switching product families or wiring conventions. For applications that need a finer finger-protection grade, the DQT4 series offers 7.5 mm, 15 mm, and 30 mm resolution grades.

• 10–14 mm pitch — finger protection at the point of operation (press feed openings, small die areas)
• 20–30 mm pitch — hand protection (molding machine mold areas, assembly stations)
• 40 mm pitch — general access-point and arm detection
• 80–200 mm pitch — whole-body access detection at fence openings and conveyor entries, at the lowest cost per beam

Step 2: What Protective Height Covers the Opening?

Protective height is the vertical span the beams actually cover. It must cover the full zone through which a person can reach the hazard — measure the opening, not the machine.

On DQC-family products the arithmetic is transparent: protective height = (number of beams − 1) × beam pitch, and the model code spells it out as DQC + beam count + "/" + pitch + "-" + protective height. A DQC06/10-50 is six beams at 10 mm pitch giving a 50 mm protective height. Across the DQC range, protective heights run from 50 mm up to 3800 mm with up to 32 beams per unit.

If the opening needs more beams than a standard unit provides, move up a series rather than stacking units: the DQT series extends the same through-beam concept to 72 beams (e.g., DQT72/40-2840 with a 2840 mm protective height), and economy-line options such as the JER and MK series reach protective heights up to 7960 mm at 40 mm pitch with beam counts up to 200.

Step 3: How Far Apart Will the Emitter and Receiver Sit?

Operating range is the distance between the emitter and receiver bars. Standard-range curtains cover most single-machine openings: the DQC series works from 0.3 to 3 m as standard, extendable to 0.3–6 m on request. Measure the real mounting span, then add margin for alignment tolerance and future layout changes.

For wide spans — long conveyor openings, fence lines, large gantries — pick a long-range through-beam series from the start. In the DAIDISIKE catalog the DQA series is positioned exactly there: the long-range variant of the DQC, with the same eight pitch options and the same transparent model-code format, plus dedicated DQA-01/DQA-02 end-mounting brackets for long-span installation.

Also decide early whether a synchronization cable between emitter and receiver is acceptable. Optically synchronized series such as the JER and DQBT need no interconnecting sync cable, which simplifies wiring when the two bars sit on opposite sides of a wide or busy opening.

Step 4: How Close to the Hazard Can You Mount It?

A light curtain only protects if the machine can stop before a hand reaches the hazard. That is what the minimum safety distance formula in ISO 13855 captures conceptually: S = K × T + C, where K is the approach speed of the body (2000 mm/s for hand approach at close range), T is the total stopping time — curtain response time plus machine stopping time — and C is a penetration allowance that grows as resolution gets coarser.

Two practical consequences follow. First, a faster curtain can be mounted closer to the hazard: the DQC series' response time of 15 ms or less keeps the curtain's contribution to T small, which helps on compact machines where every millimeter of mounting depth counts. Second, coarser resolution pushes the curtain farther away, because the C term increases — another reason not to over-economize on pitch at the point of operation.

Treat the formula as a design obligation, not a datasheet line: you must measure your machine's actual stopping time and do the calculation per ISO 13855 for your installation. Beam pitch, response time, and mounting distance are one linked decision, not three separate ones.

How the Curtain Gets Into the Stop Circuit

A curtain that changes state but cannot remove power from the hazard performs no safety function at all. This is the single most common serious finding when our engineers audit machines that already have guarding fitted, and it is worth being explicit about what "wired in" actually means.

The outputs must reach the final switching elements — the contactors, valve or drive enable — through a device that can detect faults in that path. On a curtain with transistor OSSD outputs that means a safety relay or a safety PLC. On a series supplied with its own controller, such as the DQS press guard, the controller already carries the dual relay outputs that go into the press circuit, which removes the most common excuse for leaving the connection half-finished.

Two further connections decide whether the installation is honest. External device monitoring (EDM) feeds the contactor auxiliary contacts back to the safety device so that a welded contact is detected instead of silently disabling every future stop command. And the reset must be deliberate: a curtain that lets the machine restart the instant the beams clear will restart it while the operator is still inside the guarded zone.

Get the polarity right before anything is powered. PNP sourcing outputs suit the sourcing input cards common on European and North American controls; NPN sinking outputs suit the sinking inputs common on Japanese equipment. The wiring diagram below shows the NPN case — for PNP the load moves to the 0 V side, and the two are not interchangeable on the same input card.

Step 5: Which Output Type and Power Supply Do You Need?

Match the curtain's output to your control system before ordering. DAIDISIKE safety light curtains use transistor outputs with NPN or PNP signal selection available across most series — PNP (sourcing) for the sourcing-type PLC inputs common on European and American equipment, NPN (sinking) for the sinking-type inputs common on Japanese equipment. Confirm N.O. or N.C. logic and the exact wiring variant with the factory when you finalize the order.

The output signal typically lands on a safety relay or safety controller, which performs the monitored switching of the machine's contactors. DAIDISIKE's DQSRN and DA31 safety relays are the matching in-house option for building out that circuit.

On power: DQC-family curtains run natively on DC12/24V. If you are retrofitting an older machine that only has AC available, an external controller option accepts AC110–220V, so you can add guarding without pulling a new low-voltage supply. Wiring is available in 4-core, 5-core, or 6-core configurations depending on series and signal options.

Step 6: Which Housing and Environment Rating Fit Your Machine?

Standard housings suit general workshop conditions: the DQC series is rated IP65, which handles the dust and oil mist of typical stamping and molding shops. For washdown or genuinely wet environments, step up to the DQR waterproof series — IP65 as standard, with the DQRF variant adding a waterproof enclosure to reach IP68.

Space constraints drive a second set of choices. The DQB side-emitting series has a 15 × 30 mm cross-section with beams exiting the narrow side; the DQZ front-emitting series measures 17.2 × 30 mm; and the MK mini economy series has a 25 × 23 mm profile. These slim housings fit inside machine frames and between guides where a full-size bar will not.

Two further special cases: if the hazard extends right to the end of the opening, the DQO no-blind-zone series places its first and last beams flush with the housing ends, eliminating the end dead zone of standard curtains. And for automation-line object detection alongside personnel guarding, the DCE automation series covers 10/20/30/40 mm pitches up to a 2840 mm protective height.

Muting and Blanking — and Why They Are Not the Same Thing

Sooner or later something has to pass through the detection plane that is not a person: a pallet on a conveyor, a fixture, a workpiece taller than the opening. Two mechanisms exist for that, they are routinely confused, and confusing them is how a curtain quietly stops protecting anyone.

Blanking permanently ignores specific beams so that a stationary object — a chute, a bracket, a fixture — can occupy the field continuously. Fixed blanking holds one defined zone; floating blanking lets a small object move within the field. In both cases the blanked region no longer detects anything, including a hand, so it must be physically guarded if a body part could occupy it.

Muting temporarily suspends the whole protective function during a part of the cycle that is genuinely non-hazardous, then restores it automatically. A correctly designed mute is driven by at least two independent sensors in a defined sequence and has a time limit, so that a single failed sensor or a stalled pallet cannot leave the guard suspended indefinitely.

The failure pattern is the same for both: a blanking zone sized to swallow the path a hand actually takes, or a mute that never ends. If you specify either, document the reason in the risk assessment and make the blanked or muted region unreachable by other means.

Commissioning Checks Before the Machine Runs

These are the checks our engineers run at handover. None of them needs specialist equipment beyond a stop-time analyser and a test piece cut to the curtain’s stated detection capability.

• Interrupt each beam in turn with a test piece of the rated diameter, at the top, middle and bottom of the protective height — not just at a convenient height.
• Break a beam while the machine is running and confirm the hazardous motion actually stops, rather than an indicator merely changing colour.
• Attempt to reach the hazard over, under, around and behind the detection plane. If you can get there without breaking a beam, the geometry is wrong regardless of the device.
• Measure the real mounting distance and compare it against the calculation for the resolution actually installed — not the resolution originally specified.
• Confirm the machine stopping time by measurement and file the result with a date, so the next brake service has a baseline.
• Simulate a contactor failure if the safety device supports the test, and confirm EDM prevents a restart.
• Verify the reset requires a deliberate action from a position where the whole guarded zone is visible.
• Check the ambient conditions the curtain will actually meet: reflective surfaces close to the beam path, another curtain nearby that could interfere, and any welding or inverter noise on the same panel.

Re-run the distance and stopping-time checks annually, and after any work on the brake, clutch, valves or safety circuit. Stopping time is the one input that degrades invisibly, so a distance that was correct at commissioning can quietly become inadequate without anything looking different.

What Actually Makes a Curtain Fail in Service

Curtains rarely fail electrically. They fail because of what surrounds them, and the patterns repeat across every plant we visit.

Alignment drift is the commonest. Long spans, machine vibration and thermal movement all walk the beams off axis until the curtain trips intermittently — which then invites somebody to widen a blanking zone rather than realign the bars. Optical interference is the second: a reflective surface within a metre or so of the beam path can carry light around an obstruction, and two curtains facing each other on adjacent machines can trigger each other unless the series supports interference-free coding.

Environment does the rest. Coolant film and metal dust on the front lens reduce received energy until the curtain reports a fault, which is why an IP65 or IP67 housing and a routine wipe-down matter more on a machining centre than any headline specification. And temperature swings outside the rated range shift response times in ways the safety distance calculation never accounted for.

None of these is exotic, and all of them are visible during a five-minute walk along the machine. The reason they persist is that a curtain that trips too often is treated as a nuisance rather than as a symptom.

Which DAIDISIKE Series Should You Start With?

If you are unsure, start from the DQC general-purpose series and move outward only when a constraint forces you to. The decision usually resolves in one pass:

• General machine guarding, 0.3–3 m span — DQC (eight pitches, 50–3800 mm heights)
• Long spans or perimeter lines — DQA (long-range through-beam), DQT for more than 32 beams
• Finer resolution grades (7.5/15/30 mm) — DQT4
• Wet or washdown areas — DQR / DQRF (up to IP68)
• Tight mounting spaces — DQB (side-emitting, 15 × 30 mm), DQZ (front-emitting, 17.2 × 30 mm), MK mini (25 × 23 mm)
• No end blind zone — DQO (flush end beams, 30 × 28 mm cross-section)
• Budget projects — JER economy series (optical sync, no interconnect cable, IP65)
• Pair any of the above with DQSRN or DA31 safety relays for the monitored safety circuit

One more pairing worth knowing before you finalise the layout: openings guarded by a curtain often sit next to crush edges and standing zones that a curtain cannot cover. Safety edges close off moving-edge pinch lines, and safety mats hold a floor zone - the three devices are specified together on most well-guarded cells.

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