The Photo Eye That Saw the Background (No BGS)
We used a photo eye (article 108) to detect a part (on a conveyor, article 121). The part was on a conveyor belt (a dark part). Behind the part (a meter away) was a metal wall (bright). The photo eye (a standard diffuse, article 108) saw the wall (the wall reflected more than the part). The sensor didn’t trigger (it saw the background). The problem: no background suppression (the sensor saw the bright wall, not the dark part). We switched to a background-suppression (BGS) sensor. The BGS sensor only sees the part (within its sensing range). It ignores the wall (beyond the range). The part was detected. The mistake was using a standard diffuse (it saw the bright background).
Photoelectric background suppression (BGS) sees the part (ignores the background). This article covers the selection.
The Photo Eye Types (Revisit, Article 108)
- Through-beam: Emitter + receiver (across). Best for a long range, a transparent part.
- Retro-reflective: Emitter + reflector. For a reflective part.
- Diffuse (proximity): Emitter + receiver (same housing). Sees the part (within range). But it sees the background (a bright wall).
- Background suppression (BGS): A diffuse that ignores the background (only sees within a set range). Uses two receivers (triangulation).
Step 1: The Background (Bright?)
Is there a bright background (a metal wall, a light surface) behind the part? A standard diffuse (article 108) sees the bright background (it triggers on the wall, not the part). Use a BGS sensor (it only sees within the set range, ignoring the wall beyond).
The BGS rule: For a dark part (with a bright background), use a BGS sensor. The sensor that saw the wall was a standard diffuse. Use BGS (set the range to the part’s distance, ignore the wall). The part is detected (not the background).
Step 2: The Range (Set It)
The BGS sensor has a set range (e.g., 100 mm). It sees the part (at 100 mm). It ignores the wall (at 200 mm, beyond the range). Set the range (to the part’s distance). The BGS uses triangulation (two receivers, it can tell the distance).
Step 3: The Contrast (Part Color)
A BGS sensor works regardless of the part’s color (it sees the distance, not the reflectance). A dark part (low reflectance) and a bright background (high reflectance) — the BGS sees the part (by distance). A standard diffuse (by reflectance) sees the bright background (wrong).
Step 4: The Forbidden Zone (Blind)
A BGS sensor has a “forbidden zone” (a range too close, where it can’t see). Mount the sensor beyond the forbidden zone (e.g., 30 mm away from the part). Don’t mount it too close (it’s blind).
| Sensor | Sees |
|---|---|
| Through-beam (article 108) | Long range (transparent) |
| Retro-reflective | Reflective part |
| Diffuse (standard) | Part + background (bright) |
| BGS (background suppression) | Part only (set range) |
A BGS Checklist
- Is there a bright background? (Wall?)
- Is the part dark? (Low reflectance?)
- Is a standard diffuse seeing the background? (Wrong?)
- Use a BGS? (Set range?)
- Is the range set? (Part distance?)
- Is the forbidden zone avoided? (Mounted?)
- Does it see the part? (Not the wall?)
- Is it consistent? (Repeats?)
- Is the lens clean? (Article 146?)
- Is the light OK? (Article 193?)
The Bottom Line
Photoelectric background suppression (BGS) sees the part (ignores the wall). The sensor that saw the wall was a standard diffuse. Use a BGS (set the range). For a dark part on a bright background, BGS. The sensor that saw the part (not the wall) wasn’t the most sensitive — it had BGS.
