Introduction
Sometimes you want to hide a PIR sensor behind a plastic cover for aesthetics or protection. But many plastics block infrared, rendering the sensor useless.
Why Most Plastics Block IR
Common plastics like acrylic (Plexiglas), polycarbonate (Lexan), and ABS are opaque to infrared in the 8-14µm range. They may look clear to visible light, but they absorb thermal IR.
What Materials Work?
Materials that transmit IR:
- Polyethylene (PE): Thin film transmits IR well.
- Polypropylene (PP): Similar to polyethylene.
- Silicon: IR-transparent, expensive and fragile.
- Germanium: Excellent IR transmission, very expensive.
Testing Your Cover Material
- Place the sensor facing a warm object (like your hand).
- Measure the output signal.
- Place the candidate material between sensor and hand.
- If signal drops significantly, the material is not suitable.
Solutions
- Use IR-transparent material (thin polyethylene film).
- Leave an opening for the sensor.
- Use a sensor with built-in IR-transparent cover.
- Recess the sensor behind an opening, not a cover.
Conclusion
Don’t assume that clear plastic is IR-transparent. Test your cover material or use an open design.