The Light Curtain That Didn’t Catch the Finger
We installed a light curtain (article 33) to guard a press. The light curtain had 30 mm resolution (it detected objects over 30 mm). On the floor, the operator reached in (a hand, about 80 mm). The curtain stopped the press (good). But on a later check, a finger (about 14 mm) entered the curtain. The press didn’t stop (the finger was smaller than the 30 mm resolution). The problem: the curtain’s resolution (the smallest object it detects) was 30 mm. A finger (14 mm) passed through undetected. We switched to a 14 mm resolution curtain (detects fingers). The press stopped. The mistake was choosing a body-resolution curtain (30 mm) for a point-of-operation (where fingers reach in). The resolution must match the body part that needs protection.
Safety light curtain resolution is the smallest object it detects. This article covers the selection.
The Resolution Classes
A light curtain’s resolution (the beam spacing) determines the smallest object it detects. Common classes:
- 30 mm (body): Detects a hand (80 mm) or arm. Used to guard a body (a person reaching in). Not for fingers. Used for a hazardous area (a robot, a conveyor).
- 14 mm (finger): Detects a finger (14 mm). Used at point-of-operation (a press, where fingers reach in). The finger class.
- 7 mm (small part): Detects a small object (a pencil). Used for very fine protection (rare). Overkill for most.
Step 1: Match the Resolution to the Hazard
The resolution is chosen by what body part needs protection:
- Whole body (a person walks in): Use 30 mm (body). The curtain detects the body.
- Hand/arm (the operator reaches a hand in): Use 30 mm (hand is over 30 mm).
- Fingers (point-of-operation): Use 14 mm (finger class). The finger (14 mm) must be detected.
The light curtain that didn’t catch the finger had 30 mm (body) resolution on a press (point-of-operation). Use 14 mm (finger) for point-of-operation.
The resolution rule: At point-of-operation (a press, where fingers reach in), use 14 mm (finger) resolution. For a body guard (a person walking into a robot cell), use 30 mm (body). The curtain that missed the finger was 30 mm on a press. The resolution must match the body part that needs protection.
Step 2: Safety Distance (Article 74)
The light curtain is mounted at a safety distance (from the hazard). The distance is based on the curtain’s response time (and the resolution). A finger-resolution curtain (14 mm) has a shorter safety distance (the finger reaches the hazard faster). A body curtain (30 mm) has a longer distance (the body takes longer to reach). Calculate the safety distance per the standard (ISO 13855). Don’t mount the curtain too close (the finger reaches the hazard before the stop).
Step 3: Protective Height
The curtain’s protective height (the length of the beam array) must cover the hazard. For a press (the operator reaches over the table), the curtain covers the table height (from the floor to above the table). For a robot cell, the curtain covers the full opening. Don’t leave a gap (under or over the curtain) — the operator reaches around.
Step 4: Muting (Article 33)
For a conveyor (parts pass through the curtain), use muting (article 33) — the curtain ignores the parts (but stops for a person). The muting sensors detect the parts (they’re known). The resolution (14 mm) is for the person (fingers), not the parts.
| Resolution | Detects | Best For |
|---|---|---|
| 30 mm (body) | Hand, arm, body | Robot cell, body guard |
| 14 mm (finger) | Finger | Press, point-of-operation |
| 7 mm (small part) | Pencil, small object | Very fine (rare) |
A Light Curtain Checklist
- What is the hazard? (Point-of-operation? Body?)
- For fingers (press): 14 mm resolution?
- For body (robot): 30 mm?
- Is the protective height enough? (Covers the opening?)
- Is the safety distance right? (ISO 13855?)
- Is muting needed? (Conveyor? Article 33?)
- Is the curtain dual-channel? (Article 98?)
- Is the safety relay monitoring? (Article 127?)
- Does it stop on a finger? (Test?)
- Is the reset monitored? (Article 150?)
The Bottom Line
Safety light curtain resolution matches the resolution to the body part. The curtain that missed the finger had 30 mm (body) on a press. For point-of-operation (fingers), use 14 mm. For body (robot cell), use 30 mm. Calculate the safety distance. The curtain that protected the fingers wasn’t the largest array — it was 14 mm resolution.
