The Infrared Spectrum and Human Body
All objects with a temperature above absolute zero emit infrared radiation. The wavelength of peak emission depends on the object’s temperature, described by Wien’s displacement law:
λ_max = b / T (where b ≈ 2898 μm·K)
For the human body at approximately 300K (27°C/80°F), the peak emission wavelength is around 9.7 μm. Most of the emitted energy falls within the 8-14 μm atmospheric window.
Why 8-14μm?
PIR sensors are specifically designed to be sensitive in the 8-14 μm range for several reasons:
- Human Body Emission: This range captures the peak of human thermal radiation, maximizing signal-to-noise ratio.
- Atmospheric Transmission: Water vapor and CO2 absorb IR strongly in other bands, but the 8-14 μm region has relatively low absorption, allowing detection over reasonable distances.
- Background Rejection: Many common sources of infrared (sunlight, incandescent lights) have different spectral characteristics, so limiting the bandwidth helps reduce false triggers.
Physics of Pyroelectric Detection
Pyroelectric materials generate a temporary voltage when subjected to changing temperature. In a PIR sensor, two pyroelectric elements are arranged in a differential configuration:
- Both elements receive background IR equally → no signal.
- A warm body moves across the field, affecting one element more than the other → differential signal.
This common-mode rejection reduces sensitivity to uniform temperature changes (e.g., ambient warming).
Filtering: The Silicon Window
Most PIR sensors have a silicon window coated with a filter that only transmits IR in the 8-14 μm band. This filter:
- Blocks visible light and near-IR (sunlight)
- Rejects shorter wavelengths from artificial lighting
- Passes the human body radiation band
Practical Implications
Why PIR Sensors Don’t Work Through Glass
Ordinary glass is opaque to infrared in the 8-14 μm range. This is why PIR sensors cannot detect motion behind a window.
False Trigger Sources
While the filter helps, some sources still emit in this band:
- Heat vents (warm air currents)
- Direct sunlight (contains some 8-14 μm component)
- Incandescent bulbs (hot filament emits broadband IR)
- Warm electronics
Comparison with Other Motion Sensors
| Sensor Type | Detection Principle | Spectral Range |
|---|---|---|
| PIR | Passive IR (heat) | 8-14 μm |
| Microwave | Doppler radar | RF (5.8 GHz, 24 GHz) |
| Ultrasonic | Sound reflection | 40 kHz |
| Thermopile | IR intensity | Broadband IR |
Design Considerations
When designing with PIR sensors, remember:
- The sensor is optimized for human detection; other warm animals may also trigger it.
- Cold-blooded animals (reptiles) emit less IR and may not be detected.
- Very hot objects (e.g., a car engine) can trigger from farther away.
- Temperature fluctuations (e.g., heater turning on) can cause false triggers.
Conclusion
The 8-14 μm sensitivity band is a carefully chosen compromise that maximizes human detection while minimizing environmental interference. Understanding this helps in proper sensor placement and application selection.