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Optical Proximity Sensor Range

Compare optical proximity sensor configurations using a relative detection factor derived from emitter power, detector responsivity, and target reflectivity. The output is dimensionless — use it to rank or compare configurations, not as an absolute distance.

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Formula

D_rel = √(P_e × R_d × (R_t/100)) / SF [dimensionless relative factor]

P_e— Emitter power (mW)
R_d— Detector responsivity (A/W)
R_t— Target reflectivity (0–100%) (%)
SF— Safety factor (≥1)

How It Works

Optical proximity sensors emit light (typically infrared at 850–950 nm) from an LED emitter and detect reflected or transmitted light with a photodiode or phototransistor detector. The received optical power follows an inverse-square law: P_received ∝ P_emitter × R_target / d², where d is the distance to the target and R_target is the reflectivity (0–1). For a diffuse reflective sensor, detection occurs when the received power exceeds the detector threshold: P_received > P_threshold. The detection range is therefore proportional to √(P_emitter × R_det × R_target), where R_det is the detector responsivity (A/W). Practical range estimation includes a safety factor (typically 1.5–2×) to account for lens dirt, LED aging (−30–50% over lifetime), and target surface variations. Background suppression sensors use triangulation to set a fixed detection window, immune to target colour/reflectivity variations.

Worked Example

Problem
An IR LED emits 15 mW. The detector has responsivity 0.65 A/W. Target is white paper (90% reflectivity). Safety factor = 2. Estimate detection range.
Solution
1. Normalised range = √(P_e × R_d × R_t) = √(15 × 0.65 × 0.9) = √(8.775) = 2.96 cm 2. Maximum detection range = 2.96 / 2 = 1.48 cm ≈ 1.5 cm 3. If target is black rubber (5% reflectivity): Normalised = √(15 × 0.65 × 0.05) = √(0.4875) = 0.70 cm Max range = 0.70 / 2 = 0.35 cm Result: White paper detected at ~1.5 cm; black rubber at ~0.35 cm — a 4× range reduction.

Practical Tips

  • ✓Use a pulsed (modulated) emitter with a synchronous detector to achieve 100–1000× better ambient light rejection compared to a DC-biased system.
  • ✓For precise distance measurement rather than simple presence detection, use a triangulation sensor (e.g., Sharp GP2Y0A02) or time-of-flight sensor (e.g., VL53L1X) instead of a simple reflective sensor.
  • ✓Mount the emitter and detector at a slight angle (5–15°) for reflective proximity to improve sensitivity and reduce direct cross-coupling at short range.

Common Mistakes

  • ✗Assuming maximum rated range is achievable with all targets — manufacturer range specs are for a standard white reflector (90% reflectivity); dark or absorbing targets reduce range significantly.
  • ✗Ignoring LED aging — IR LED radiant intensity drops 30–50% over the rated lifetime; the safety factor must account for end-of-life performance, not just initial output.
  • ✗Overlooking ambient light interference — bright incandescent or sunlight can saturate the detector, causing missed detections; choose sensors with optical filters and pulsed modulation to reject DC ambient light.

Frequently Asked Questions

Diffuse sensors have emitter and detector in the same housing and detect reflected light from the target — range depends on target reflectivity. Retro-reflective sensors detect light reflected from a corner-cube reflector — longer range and target-independent. Through-beam sensors place emitter and detector opposite each other; detection is by beam interruption — longest range and most reliable, but requires wiring to both sides.
Reflectivity varies widely: white matte surfaces 80–95%, grey plastic 40–60%, bare aluminium 65–75% (specular), black rubber 3–8%. Because range scales as √(reflectivity), a target that is 10× less reflective reduces detection range by √10 ≈ 3.2×. Always verify with the actual target material and colour.
Yes, but it requires modulated emitter and bandpass optical filter matched to the emitter wavelength (e.g., 850 nm, 940 nm). The synchronous detector rejects DC ambient light while passing the modulated signal. Check the manufacturer's ambient light immunity specification — industrial sensors are typically rated to 10,000 lux ambient.

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