The Photo Eye That Saw the Background: Background Suppression, Not the Sensor
A photoelectric sensor detecting a box on a conveyor. It triggered randomly. The box was 200 mm away. The background (a steel wall) was 500 mm away. The sensor was a through-beam type. It saw the wall. This is about photoelectric sensor types, not about the sensor being defective.
The photoelectric sensor types
| Type | How it works | Use for |
|---|---|---|
| Through-beam | Emitter and receiver separate. Object breaks the beam. | Long range, reliable detection (but needs alignment) |
| Retro-reflective | Emitter + receiver in one unit, reflector opposite. Object breaks the beam. | Medium range, easier alignment |
| Diffuse (proximity) | Emitter + receiver in one unit. Object reflects light back. | Short range, simple (but sees background) |
| Background suppression (BGS) | Like diffuse, but ignores objects beyond a set distance. | Detection of objects near the sensor, ignoring background |
| Laser distance | Measures distance to the target. Triggers at a set range. | Precise distance detection |
What we found
The sensor was a diffuse (proximity) type. It detected any reflection. The box at 200 mm reflected light. The wall at 500 mm also reflected light. The sensor couldn’t distinguish. It triggered on the box, then on the wall, then on the box again. Random triggers.
What we did
1. Switched to a BGS sensor. We replaced the diffuse sensor with a background-suppression sensor. The BGS triggers at 200 mm (the box) and ignores anything beyond 300 mm (the wall). The wall doesn’t cause a trigger. The sensor only sees the box.
2. Set the sensing distance. The BGS sensor has a potentiometer to set the cutoff distance. We set it to 250 mm. The box at 200 mm is detected. The wall at 500 mm is ignored.
3. Considered through-beam. If the application is a box on a conveyor, a through-beam is the most reliable. It triggers when the beam is broken. No background issue. But it needs alignment (emitter and receiver on opposite sides of the conveyor). We used BGS because it was easier to install (one side only).
The photo eye rule: If a diffuse sensor triggers on the background, switch to background suppression (BGS). BGS ignores objects beyond a set distance. For conveyor detection, through-beam is the most reliable. The sensor that saw the background wasn’t defective — it was the wrong type for the application.
