Introduction
We’ve covered why standard glass blocks PIR detection. But what if you absolutely must detect through a window? This article covers advanced, sometimes experimental, solutions.
Why Standard Glass Blocks IR (Recap)
Ordinary soda-lime glass is opaque to 8-14µm infrared due to molecular absorption. It may transmit visible light but blocks thermal IR completely.
Advanced Solution 1: IR-Transparent Window Materials
If you can replace a small pane of glass with an IR-transparent material, detection becomes possible. Options include:
Polyethylene Film
Thin (0.1-0.5mm) high-density polyethylene (HDPE) film transmits IR well. It’s flexible, cheap, but not durable for long-term outdoor use. Can be stretched over a frame.
Zinc Selenide (ZnSe)
Excellent IR transmission from visible to beyond 14µm. Used in laser optics. Extremely expensive and fragile. Not practical for most applications.
Germanium
Good IR transmission, but expensive and brittle. Requires anti-reflection coating.
Calcium Fluoride (CaF2)
Transmits from UV to mid-IR, but hygroscopic (absorbs moisture).
Special IR Plastics
Some manufacturers produce IR-transparent plastics for sensor windows. These are proprietary and may be available as custom components.
Advanced Solution 2: Active IR Illumination
Instead of relying on passive body heat, use an active IR source and look for reflections. This is essentially an active IR sensor, not PIR.
A pulsed IR LED (850-940nm) and a photodiode can detect reflections from people, even through glass. The LED must be bright enough to penetrate the glass and return a signal. This is how many automatic door sensors work.
Advanced Solution 3: Millimeter-Wave Radar
mmWave radar (24GHz, 60GHz, 77GHz) penetrates glass easily and detects motion and presence. It’s a completely different technology but solves the through-glass problem elegantly. Modules like the HLK-LD2410 are now under $10.
Advanced Solution 4: Dual-Window Differential Measurement
This experimental technique uses two sensors: one with a view through the glass, one with a view of the glass itself. By subtracting the signals, you might cancel out the glass’s thermal signature. This is complex and not commercially common.
Practical Recommendation
For most users, the best solution is to avoid putting the sensor behind glass. Mount it outside in a weatherproof enclosure. If that’s impossible, consider switching to mmWave radar.
Conclusion
While standard glass remains a barrier, advanced materials and alternative technologies offer solutions for through-window detection.