Robot cell: light curtain vs scanner
Perimeter vs area protection. Use this engineering guide to choose between safety light curtains (perimeter/opening guarding) and safety laser scanners (area/zone guarding, potentially within a separately validated SSM application). Includes design rules, distance math inputs, and acceptance records for audits.
Use the safety light curtain range for opening-safeguard candidates and the industrial LiDAR range to distinguish safety scanners from non-safety sensing. DLD, SDLD, DLDS and JPTG are non-safety devices. The confirmed ST27 rating is Type 3 / PL d / SIL 2; its exact suitability still depends on the robot application and manual. The comparison of DQA light curtain versus non-safety DLD5 LiDAR separates protective opening detection from operational perception before considering coverage geometry.
Content updated: 2026-09-05.
1) Strengths at a glance
Light curtain — perimeter/opening
- Best for access points: doorways, material in/out chutes, teach gates.
- Actual detection capability and full machine stopping time determine the separation distance; no universal short-distance benefit is established.
- Check diagnostic, monitoring and reset provisions on the exact device and safety controller; do not infer them from the device category.
Safety laser scanner — area/zone
- Creates a 2D protective detection plane with model-specific field angle, range and detection capability.
- Where supported, field sets can be selected by validated safety-related mode inputs. A maintenance label is not permission to reduce protection.
- Potentially useful for irregular access areas, if required separation and complete detection coverage can be achieved without unprotected occlusions.
*SSM = Speed & Separation Monitoring. It requires the complete safety-rated separation assessment and robot speed/stop functions, not just a scanner warning output.
2) Quick decision table
| Design question | Prefer light curtain | Prefer laser scanner |
|---|---|---|
| Guarding geometry | A defined detection plane at a fixed opening; horizontal use needs its own assessment | A configured detection plane covering the required accessible area |
| After crossing | A vertical field alone does not detect a person remaining inside | Presence is detectable only where the approved plane remains unobstructed |
| Separation distance | Full stop time and actual approach/detection geometry govern | Full stop time plus scanner-specific measurement and geometry allowances govern |
| Material passage | Only supported and validated blanking/muting; no generic two-sensor recipe | Only supported field selection and safe transitions; no assumed automatic tracking |
| Project cost | Quote the complete guards, inputs, brackets and validation | Include occlusion controls, any additional devices and integration; no universal saving |
| Dust / spatter / environment | Verify exact device and approved covers; enclosure rating alone is insufficient | Verify contamination, reflectivity and operating limits; no assumed heater/filter capability |
If the required coverage is a volume rather than one detection plane, the 3D safety-camera versus light-curtain comparison examines that additional option, including occlusion and exact-device safety capability.
3) Application patterns to validate for robot cells
A. Perimeter fence + light curtains
- Fences on three sides; curtains at entries with the required presence, monitoring and restart provisions.
- Interlock doors (safety switches) tied into the same stop chain.
- Use supported blanking/muting only in a validated material-passage arrangement with residual access controlled and required indication.
B. Scanner-based open-side protection
- Determine the number and placement of scanners from required coverage; open-side protection is unsuitable where physical containment is needed.
- Use non-safety warning fields only for advisory or operational behavior; any safety-related slowdown must be a validated safety function.
- Assess crawl-under, stepping over, reaching over and occluded areas. One scanning plane does not automatically prevent every route.
C. Hybrid: scanner + light curtain
- Scanner covers long faces; curtain protects narrow load ports.
- Coordinate safety inputs, required monitoring and restart zones through the approved design; do not assume a shared feedback loop is compatible.
- Where field switching is supported, validate the correct selection and transition time for each operating mode.
4) Safety distance & timing inputs
Light curtain worksheet
Application / operating mode / approach geometry = ?
Actual detection capability and safeguarded height = ?
Worst-case total detection-to-hazard-stop time (s) = ?
Measurement start/end points; missing delays; no double-counting = ?
Applicable ISO 13855:2024 method and additional distances = ?
Required versus measured minimum separation distance = ?
Reach-over / under / around; presence behind field; reflections = ?
Approved configuration and validation record = ?Scanner field design notes
- Configure protective fields for the required safety function. A non-safety warning field is not an SSM protective boundary.
- Identify occlusions and low-reflectivity detection limits. Add appropriate guards or sensing if needed; ordinary mirrors are not an approved way to extend a scanner field.
- Validate switching inputs (modes) and teach-in seals/records.
5) Wiring baseline (both options)
FUNCTIONAL ROLES — not a terminal or pinout drawing
Approved protective-device outputs -> compatible safety inputs
Safety logic -> approved robot stop / safe motion and holding functions
Required final-element monitoring -> documented safety monitoring input
Reset -> documented safety restart interlock; no automatic hazardous start
Separate auxiliary status -> ordinary PLC / HMI diagnostics only
Do not parallel raw OSSDs into ordinary PLC inputs.
Exact OSSD / safety communication / feedback wiring follows both manuals.This is a role diagram, not a circuit or certified function block. Use the exact manuals; final elements may be approved robot safety interfaces rather than power contactors.
6) Commissioning & periodic tests
Commissioning
- Verify the documented safety interface and required monitoring with an approved safe fault-test procedure; do not hold contactors on an energized robot.
- Capture distance math, field screenshots, wiring photos.
- For scanners: verify field switching, blind-spot coverage, lamp/status IO.
Periodic tests at the required interval
- Use the manufacturer-specified test piece across the complete field and boundaries with hazardous motion prevented. Set test intervals from the manuals and risk assessment.
- Use the specified scanner test object and controlled procedure for coverage, occlusions and supported switching. Verify any claimed SSM function separately without exposing a person to moving hazards.
- Seal parameters; review change logs.
Test log (CSV copy)
Item,Test,Expected,Actual,Result,Notes
1,Required final-element monitoring,Approved safe fault test prevents unsafe restart,,,
2,Curtain separation and access,Full stopping chain and all access paths validated,,,
3,Scanner protective plane,Specified test piece and occlusion checks passed safely,,,
4,Claimed SSM function,Whole safety-rated separation and speed function validated,,,
5,Supported field switching,Correct safe field and transition for each mode,,,
Frequently asked questions
When should I choose a light curtain for a robot cell?
A light curtain can suit a defined access opening where every hazardous approach crosses the field at the required separation distance. It does not detect someone after they have passed through a vertical field, or contain ejected material. Assess presence behind the field, restart, bypass paths and any supported material-passage function separately.
When is a safety laser scanner a better fit?
A safety scanner can suit a validated detection plane over an accessible area, subject to its safety rating, range, detection capability, occlusions and environment. Multiple field sets are model-specific. A non-safety warning field may support operational slowdown but does not itself implement safety-rated speed and separation monitoring.
Do these devices replace the safety stop path?
No. The sensing device is one part of a complete safety function. Select and validate the required safety performance, compatible safety inputs, robot stopping functions, final-element monitoring and restart protection. Type 4 is not a rating for the entire stop path, and K1/K2 power contactors are not the only possible architecture.
Application sources: ISO 10218-2:2025, IEC 62046:2026 and ISO 13855:2024. The robot-cell standard guide separates robot and application responsibilities.
7) Common mistakes & quick fixes
| Mistake | Symptom | Fix |
|---|---|---|
| Ordinary PLC used for protective stop | Safety performance unvalidated | Use approved safety logic and stop functions; separate auxiliary diagnostics |
| Scanner occlusions | A person can occupy an undetected area | Redesign coverage or residual guards and revalidate; do not add arbitrary mirrors or raise a plane blindly |
| Warning field called SSM | Operational slowdown mistaken for a safety function | Validate the complete safety-rated separation and robot speed function |
| Combined OSSD channels | Channel independence or fault detection compromised | Restore the documented compatible input circuit; wiring and fusing follow the exact manuals |