Safety Laser Scanner Explained: How Area Scanners Work

A safety laser scanner is a non-contact, optoelectronic safety device that scans a flat, two-dimensional plane with a rotating laser beam and triggers a safety-rated output when any object — a person, a hand, a forklift — enters a defined protected zone. Unlike a simple presence sensor, a scanner creates a configurable map of the floor or work area, letting engineers draw virtual boundaries in software rather than weld physical guarding.

Safety laser scanners are found wherever hard guarding is impractical: collaborative robot cells, automated guided vehicles (AGVs), palletising stations, and access gates on large machinery. Understanding how they work at the controls level — not just the concept — is what separates a safe installation from a compliant-looking one.

What Is a Safety Laser Scanner?

A safety laser scanner is a SIL 2 / PLd-rated optoelectronic device that sweeps a pulsed infrared laser through a horizontal (or vertical) arc — typically 190° to 275° depending on model — and measures the distance to any object it strikes. The result is a real-time polar map of the surrounding area, sampled many times per second.

The scanner compares this live map against one or more pre-configured field sets. When the scanner detects an object inside the innermost, safety-rated zone, it de-energises its OSSD (Output Signal Switching Device) outputs, cutting the safe input to a downstream safety relay or safety PLC.

Key hardware attributes common to industrial safety laser scanners:

• Scanning angle: 190° to 275° — no single scanner covers a full 360° with a safety-rated field
• Range: Protective fields typically certified to 3 m–7 m; warning fields extend further
• Response time: Usually 60 ms–80 ms from intrusion to OSSD drop
• Safety rating: SIL 2 (IEC 62061) and/or PLd, Cat. 3 (ISO 13849) — always verify the manufacturer's certificate
• Minimum object resolution: Commonly 30 mm, 40 mm, or 70 mm depending on operating mode

Safety laser scanners are area-scanning devices, meaning they protect a zone rather than a line. That distinguishes them fundamentally from light curtains, which protect a plane.

How a Safety Laser Scanner Works

Time-of-Flight Measurement

The core measurement principle is time-of-flight (ToF). The scanner fires a short pulse of infrared laser light and measures the time interval between transmission and reception of the reflected pulse. Because the speed of light is constant, the round-trip time maps directly to distance:

Distance = (Speed of light × Time) ÷ 2

Modern scanners use highly accurate internal clocks and signal processing ASICs to resolve distances to millimetre-level precision, far exceeding what the eye can verify. Every measurement point is paired with the mirror's current angular position to produce a (distance, angle) coordinate — a polar data point.

Rotating Mirror

The laser does not physically rotate. Instead, a polygon mirror or an oscillating mirror assembly spins at high speed inside the scanner head, sweeping the laser beam through the full scanning arc. A full sweep across the scanning angle produces one complete scan cycle. The scanner repeats this cycle continuously — typically 25 to 50 times per second — giving the system a near-real-time picture of the area.

The motor driving the mirror is a safety-relevant component; internal diagnostics monitor mirror speed and overall system integrity. If an internal fault is detected, the OSSD outputs drop automatically without requiring any external command.

Building the 2D Area Map

Each scan cycle produces a ring of polar data points. The scanner's internal firmware converts these into a Cartesian map that it can compare against stored field shapes. Field shapes are programmed using the manufacturer's configuration software — typically running on a laptop connected via USB or Ethernet — and downloaded to the scanner's non-volatile memory.

The scanner does not send raw point-cloud data to the PLC during normal operation. It processes the comparison internally and communicates only the safety-relevant result: OSSD 1 and OSSD 2 energised (area clear) or de-energised (intrusion detected or fault).

Protective Field vs Warning Field

Every safety laser scanner installation uses at least two field zones, and understanding the difference is critical for both safety and productivity.

Field	Safety-rated?	Typical action	Output
Protective field	Yes — PLd/SIL 2	Immediate stop	OSSD de-energises
Warning field	No — monitoring only	Slow-down signal or alarm	Digital output (non-safety)

Protective Field

The protective field is the inner zone. It is safety-rated and certified. When any object larger than the scanner's minimum detectable object size enters this field, the OSSD pair drops within the device's stated response time. The machine receiving those OSSDs must stop within the stopping distance calculated during the risk assessment.

Protective field shapes are drawn in the configuration software. Common shapes are a semicircle in front of a machine, a rectangle around a robot cell, or a custom polygon that accounts for fixed obstacles like machine legs or conveyor frames. Fixed obstacles inside the field must be masked (taught as permanent contours) — otherwise every scan cycle would see them as intrusions.

Warning Field

The warning field is an outer zone that does not carry a safety certification. It is typically configured larger than the protective field — extending two to three times further out — and its output is a standard digital I/O line, not an OSSD. Common uses:

• Trigger a slow-down signal to a drive, reducing conveyor or robot speed before the protective field is reached
• Activate a flashing light or audible horn to warn pedestrians
• Signal the machine PLC to pause non-safety operations (e.g., inhibit a new pallet cycle)

Relying on the warning field alone for stopping a hazardous motion is a design error. It is not safety-rated. The warning field buys time; the protective field enforces the stop.

Configurable and Switchable Field Sets

Real-world machines rarely have a single operating mode. A robot cell may run at high speed when the gate is closed and slow speed during a teach session. A press may have a maintenance mode with different access requirements. Switchable field sets handle this.

A field set is a pairing of one protective field and one or more warning fields that the scanner activates simultaneously. Most scanners support between 4 and 16 field sets in memory. The active field set is selected by a combination of digital inputs to the scanner — typically two or four input pins forming a binary or Gray-code selection.

Field-Set Selection From the Safety PLC

The safety PLC or safety relay drives the scanner's field-select inputs based on machine state. A typical arrangement:

1. Mode 1 — Normal production: Large protective field, wide warning field. OSSD connected to main drive safety function.
2. Mode 2 — Slow/setup: Smaller protective field matching the reduced stopping distance at low speed. OSSD still connected to drive; drive already speed-limited by a separate safety function.
3. Mode 3 — Muted passage: Protective field temporarily shaped around a narrow material entry corridor; warning field still active across the full arc.

The inputs selecting the field set are safety-relevant in context — if someone defeats the mode selection to force a smaller protective field during high-speed operation, the risk assessment is violated. Engineers should route field-select signals through the safety controller rather than a standard PLC output, and document the selection logic in the safety function description.

Switching between field sets does not require a manual reset unless the safety function demands one. The OSSD will re-energise automatically once the selected field is clear — or hold a latch state if the safety controller requires a deliberate restart acknowledgement.

Safety Scanner vs Light Curtain — Which to Use?

Both devices protect personnel from hazardous zones. The right choice depends on the geometry of the access point and the operational requirements. This question comes up often enough that it is worth a direct comparison. For a deeper look at the light curtain side, see light curtain explained.

Factor	Safety Laser Scanner	Light Curtain
Protection geometry	2D area (floor plan)	1D plane (vertical or horizontal slice)
Access direction	Any direction within scan arc	Perpendicular to curtain plane only
Material passage	Configurable muting or field shaping	Requires external muting sensors
Coverage range	Up to 7 m+ for protective field	Up to 20 m beam height
Finger/hand detection	Depends on resolution mode (min ~30 mm)	Down to 14 mm finger detection
AGV / mobile use	Yes — standard for AGV bumper and side scan	Rarely practical on mobile platforms
Cost	Higher initial cost	Lower cost for simple linear access
IP rating	Typically IP65	IP65 or better available

Use a safety laser scanner when:

• The hazard can be approached from multiple directions across a floor area
• Material must pass through the guarded zone on a conveyor or by forklift
• The installation is on a mobile platform (AGV, AMR)
• Fixed guarding or a light curtain would block legitimate access paths

Use a light curtain when:

• Access is through a defined vertical opening (a press gate, a machine access door)
• You need high resolution for finger and hand detection at the point of operation
• Cost is a primary constraint and the geometry permits a planar guard
• The standard machine guarding analysis points to a fixed-opening access point

Neither device is universally superior. The risk assessment — not the sales catalogue — determines which technology fits.

Mobile Use on AGVs and AMRs

Automated guided vehicles present a guarding challenge that fixed scanners cannot solve: the hazard moves with the machine. Safety laser scanners mounted to AGV and AMR platforms are the dominant solution to this problem, and their use on mobile platforms introduces considerations that do not apply to fixed installations.

Typical AGV Scanner Layout

Most AGVs carry at least two scanners — one at the front and one at the rear — oriented horizontally at approximately bumper height (150–300 mm from the floor). Each scanner covers a forward arc of roughly 190°. Together they provide near-360° coverage without requiring a single 360° device.

Larger or faster vehicles may add side-facing scanners for lateral protection during turns, or additional scanners aimed at floor level to detect low obstacles.

Dynamic Field Sets by Vehicle Speed

A fundamental requirement for AGV scanner safety is that the protective field must be sized for the stopping distance at the current speed. A vehicle travelling at 1.5 m/s needs a longer front protective field than one travelling at 0.3 m/s during a tight manoeuvre.

The AGV controller — often a safety-rated motion controller — selects the appropriate scanner field set based on the vehicle's actual speed signal. A typical configuration:

• Speed 0–0.3 m/s: Short protective field (e.g., 300 mm in front of bumper), used in dock and narrow-aisle manoeuvring
• Speed 0.3–0.8 m/s: Medium field
• Speed 0.8–1.5 m/s: Long protective field sized to ensure the vehicle stops before the scanner's face reaches the detected object

The stopping distance calculation must account for the scanner's response time, the safety controller's reaction time, the drive's deceleration capability, and a safety margin — typically per ISO 13855.

Warning Field for Slow-Down

On AGVs, the warning field triggers a speed reduction before the protective field is reached. A person entering the warning zone causes the AGV to slow from travel speed to creep speed. If they continue into the protective field, the drive cuts. This two-stage approach reduces unnecessary emergency stops and improves throughput while maintaining the safety function.

For a broader overview of AGV system architecture and navigation principles, see AGV explained.

Mounting and Resolution

Mounting Height and Scan Plane

Safety laser scanners are almost always mounted to scan a horizontal plane at a defined height above the floor. IEC/TS 62998-1 and ISO 13855 give guidance on mounting height relative to the floor and the minimum detectable object size (MDOS) required to detect a lower leg reliably.

For floor-level area guarding, a typical mounting height is 200–300 mm. At this height, the horizontal scan plane reliably intercepts the lower leg of a standing adult. Crouching, crawling, or lying personnel may not be detected — which is why a presence-sensing mat or a pre-entry restart interlock is often required alongside the scanner for zones where people might be standing inside when the guard resets.

Minimum Object Resolution

Resolution — the minimum object size the scanner will reliably detect — is not a fixed number. It depends on:

• The operating range (further away, the beam diameter is wider)
• The scan angle increment (angular step between samples)
• The manufacturer's specified minimum detectable object size (MDOS) for the certified field size

Configuring a protective field at a range beyond the scanner's certified maximum for the selected resolution invalidates the safety function. Always stay within the manufacturer's certified operating envelope, documented in the device's safety manual.

Environmental Considerations

Scanners use infrared light. Reflective surfaces, dust, steam, and smoke all affect performance. Most scanners implement contamination monitoring — if the front window's signal attenuation exceeds a threshold, the device signals a warning before dropping below reliable detection. However, environments with heavy airborne contamination may require alternative guarding strategies or environmental controls.

Wiring: OSSD Outputs to Safety Relay or Safety PLC

The OSSD (Output Signal Switching Device) is the electrical heart of the scanner's safety output. It consists of two independent PNP transistor outputs — OSSD 1 and OSSD 2 — that must both be wired into the safety-rated input of a downstream safety relay or safety PLC input module.

OSSD Signal Behaviour

When the protective field is clear and the scanner is healthy:

• Both OSSD outputs are HIGH (supply voltage, typically 24 V DC)
• The outputs periodically pulse LOW for a few microseconds as a test pulse — this is normal and confirms the output transistors are functional

When an intrusion is detected or a device fault occurs:

• Both OSSD outputs switch to LOW (0 V)
• The downstream safety relay or input module recognises this as a de-energise command and opens the safety circuit

Cross-Channel Monitoring

Because safety functions require redundancy, the two OSSD channels must connect to separate input channels on the safety relay or safety PLC. The safety controller monitors that both channels switch simultaneously — a discrepancy between OSSD 1 and OSSD 2 indicates a fault (wiring short, output failure) and forces a lockout.

Never wire OSSD 1 and OSSD 2 to the same input terminal, and never use a single-channel safety relay that reads only one OSSD line. Both steps violate the dual-channel architecture that achieves PLd / Cat. 3.

Typical Wiring Topology

Scanner OSSD 1 ──► Safety Relay / Safety PLC Input Channel A
Scanner OSSD 2 ──► Safety Relay / Safety PLC Input Channel B
Scanner Field-Select IN1 ◄── Safety PLC Safety Output (mode selection)
Scanner Field-Select IN2 ◄── Safety PLC Safety Output (mode selection)
Scanner Restart IN ◄── Reset pushbutton (monitored)
Scanner 24 V / 0 V ◄── Safety-rated 24 V DC PSU

The restart input requires a deliberate signal — typically from a monitored reset pushbutton located outside the hazard zone with line-of-sight to the cell interior — before the OSSD outputs will re-energise after a protective field interruption. This prevents automatic restart after a person clears the field.

Integration Into the Safety Function

The scanner's OSSD pair is one element in a larger safety function. The safety controller must:

1. Monitor both OSSD channels simultaneously
2. Cross-reference the scanner's status with the active field-set selection to confirm the correct mode is active
3. Command the hazardous drive to a safe state (STO — Safe Torque Off, or a controlled stop) within the required reaction time
4. Require a deliberate restart acknowledgement before restoring hazardous motion

Defining slow-down vs stop zones — where the warning field output triggers a drive speed reduction and the protective field triggers STO — should be documented in the safety function specification and validated during commissioning with timed stopping-distance measurements.

---

Frequently Asked Questions

How does a safety laser scanner work?

A safety laser scanner fires short infrared laser pulses and measures the time each pulse takes to return after reflecting off a surface — a technique called time-of-flight. A spinning mirror sweeps the beam through a wide arc (typically 190° to 275°), building a polar map of the surrounding area many times per second. The scanner compares this live map against pre-configured field shapes stored in its memory. If any object appears inside the safety-rated protective field, the scanner de-energises its two OSSD outputs, signalling the downstream safety relay or safety PLC to stop the hazardous machine.

What is the difference between a protective field and a warning field?

The protective field is the inner, safety-rated zone. When breached, it causes the scanner's OSSD outputs to drop — triggering an immediate, certified safety stop. The warning field is a larger outer zone that is not safety-rated. It produces a standard digital output used to slow a machine, activate a warning light, or alert an operator before the protective field is reached. The warning field improves productivity by reducing unnecessary emergency stops, but it cannot substitute for the protective field as a safety function.

Safety scanner vs light curtain — which should I use?

Use a safety laser scanner when the hazard zone can be approached from multiple directions across a floor area, when material must pass through the zone, or when the guarding must be mounted on a mobile platform like an AGV. Use a light curtain when access to the hazard is through a defined vertical opening and you need high resolution (down to 14 mm) for point-of-operation guarding. The risk assessment geometry determines the correct choice — not cost alone.

How are safety laser scanners used on AGVs?

On AGVs and AMRs, safety laser scanners are typically mounted at bumper height on the front and rear of the vehicle, scanning horizontal planes to detect obstacles in the travel path. The AGV controller selects different scanner field sets based on the vehicle's current speed — larger protective fields at higher speeds to account for longer stopping distances. An outer warning field triggers a speed reduction before the protective field is reached. If an object enters the protective field, the OSSD outputs drop and the drive cuts power. This dynamic field-switching approach is required by ISO 3691-4 for industrial trucks with automated functions.