The Sensor That Triggered on the Background
We used a diffuse (proximity-mode) photoelectric sensor to detect boxes on a conveyor. The sensor worked on light-colored boxes. On a dark box (the box was black), the sensor didn’t trigger (it reflected too little light). We switched the sensitivity up. Then the sensor triggered on the background (the belt behind the box) — the sensor saw the belt (which was gray). The problem: a diffuse sensor (article 108) detects based on the reflected light. A dark box reflects less (no trigger). A light belt reflects more (false trigger). We switched to a through-beam sensor (article 108) — the box breaks the beam (regardless of color). The trigger was consistent. The mistake was using a diffuse sensor for varying-color parts.
Photoelectric sensing mode matches the mode to the application. This article covers the choices.
The Three Modes
Through-Beam (Emitter + Receiver)
An emitter (one side) and a receiver (other side). The part breaks the beam. Long range (up to 10 m). Reliable (regardless of color). But needs two sides (emitter and receiver). Best for: parts on a conveyor (any color), large parts.
Retroreflective (Emitter/Receiver + Reflector)
A single sensor (emitter and receiver in one housing) and a reflector (across the way). The light goes to the reflector and back. The part breaks the return beam. Medium range (up to 5 m). Needs one side (the sensor) and a reflector. Best for: mid-range, single-side mounting.
Diffuse (Proximity Mode)
A single sensor (emitter and receiver). The light hits the target, reflects back to the sensor. The sensor detects the reflected light. Short range (up to 1 m). But depends on the target’s reflectivity (a dark target reflects less). Best for: consistent-color targets, close range.
| Mode | Range | Color Sensitivity | Best For |
|---|---|---|---|
| Through-beam | Up to 10 m | No (breaks beam) | Any color, conveyor |
| Retroreflective | Up to 5 m | Low (return beam) | Single-side, mid-range |
| Diffuse | Up to 1 m | High (reflects off target) | Consistent color, close |
Step 1: Match the Mode to the Target
For a target of varying color (dark, light, shiny), use through-beam (the color doesn’t matter — the beam is broken). The sensor that triggered on the background used diffuse (color-sensitive). Switch to through-beam.
For a consistent-color target (a specific plastic), diffuse is fine (cheaper, single-side). For a shiny (reflective) target, use retroreflective with a polarizer (the shiny surface reflects like a reflector).
The sensing mode rule: For varying-color parts, use through-beam (color doesn’t matter). The sensor that triggered on the background used diffuse (color-sensitive). Through-beam breaks the beam (reliable). For a consistent target, diffuse (cheaper). For shiny targets, retroreflective with polarizer.
Step 2: Background Suppression (Diffuse)
A diffuse sensor with background suppression (BGS) ignores the background (it only detects objects within a set range). For a box on a belt (the box is 100 mm away, the belt is 200 mm away), a BGS sensor detects the box (within 150 mm) but not the belt (beyond 150 mm). Use a BGS diffuse for close targets with a variable background.
Step 3: Range
The range (distance) determines the mode. For a long range (over 2 m), through-beam. For mid-range (0.5–2 m), retroreflective. For close (under 0.5 m), diffuse. Don’t force a diffuse sensor to reach 2 m (it can’t — the light is too weak).
Step 4: The Target Size
A small target (a 5 mm hole) needs a focused beam (a through-beam with a small spot). A diffuse sensor’s beam is wide (it might miss a small target). For small targets, use a through-beam (or a laser sensor, article 134).
A Sensing Mode Checklist
- What is the target? (Varying color? Consistent?)
- For varying color: through-beam?
- What is the range? (m?)
- Is it single-side mounting? (Retro or diffuse?)
- Is the background variable? (BGS diffuse?)
- Is the target shiny? (Polarizer?)
- Is the target small? (Laser? Article 134?)
- Does the sensor trigger on the background? (Diffuse problem?)
- Is the lens clean? (Article 147?)
- Is the stability LED good? (Article 147?)
The Bottom Line
Photoelectric sensing mode matches the mode to the target. The sensor that triggered on the background used diffuse (color-sensitive). For varying colors, use through-beam (breaks the beam). For consistent targets, diffuse (cheaper). Use BGS for variable backgrounds. The sensor that detected every part wasn’t the most sensitive — it was the right mode.
