WHY PIR LENSES MUST BE "NON-IMAGING": UNDERSTANDING THE REAL OPTICS BEHIND MOTION DETECTION
In this article, we’ll dive into the core principles of PIR detection, explain how materials affect detection performance, and reveal why false alarms are often symptoms of optical misalignment or material mismatches—not just electronic noise.
Introduction: The Power Behind Invisible Sensing
Passive Infrared (PIR) sensors are a cornerstone of motion detection in smart lighting, security systems, and automated appliances. But what most engineers or product managers rarely pause to ask is:
What makes a PIR sensor “see” motion?
At the heart of this lies a clever combination of pyroelectric detection and non-imaging optical design, particularly a plastic Fresnel lens optimized not for image formation, but for field coverage and signal enhancement.
In this article, we’ll dive into the core principles of PIR detection, explain how materials affect detection performance, and reveal why false alarms are often symptoms of optical misalignment or material mismatches—not just electronic noise.
1. The Real Principle: Pyroelectric Sensing + Optical Funneling
PIR sensors don’t form images. Instead, they detect changes in infrared (IR) energy—especially from warm bodies—within their field of view (FOV). Here's how they work:
Pyroelectric detectors convert IR radiation fluctuations into electrical signals.
These detectors are typically very small, with narrow receptive fields.
Enter the Fresnel lens: A structured optical component that focuses and segments the incoming IR light into multiple discrete zones.
This segmentation allows the PIR sensor to pick up movement as a subject passes from one zone to another, triggering a distinct change in received IR energy.
In essence, the Fresnel lens acts as a spatial amplifier: not capturing a clear image, but helping the sensor detect when and where IR energy changes.
2. Why Non-Imaging Optics Are Necessary
In traditional optics, lenses are used to form sharp images. But in PIR sensors, this would be a problem.
PIR sensors don't need image resolution—they need wide field coverage and dynamic IR signal changes.
Imaging lenses would introduce unnecessary focal depth, increasing the complexity and narrowing the useful field.
Non-imaging Fresnel lenses, on the other hand, are designed to split the field into multiple IR beams, enhancing motion sensitivity.
As described in the recent work by Vo Quang Sang et al., non-coaxial Fresnel designs are increasingly used to avoid excessive central light concentration, leading to more uniform sensitivity across the sensor surfacessrn-5093890.
3. Material Makes the Difference: Not All Plastics Are Equal
Choosing the right material for the Fresnel lens is critical. A poor choice can result in:
Insufficient transmission at target IR wavelengths (usually 8–14 μm).
Surface degradation over time (especially in outdoor or humid environments).
Increased absorption that causes lens heating, distorting sensitivity.
Key material traits to consider:
| Property | Desired Value |
|---|---|
| IR transmittance | >85% at 10 μm |
| Refractive index | ~1.5–1.6 (e.g., PMMA, HDPE) |
| Molding capability | High (to allow micro-structuring) |
| Thermal stability | ≥100°C (for outdoor use) |
Materials like HDPE (High-Density Polyethylene) and PMMA (Polymethyl Methacrylate) are common, but their actual IR performance depends heavily on additives, surface finish, and coating. Some advanced PIR lenses now use multi-layer or black additives to reduce stray IR and suppress ghost reflections.
4. The Truth About False Alarms: It’s Not Just Noise
False alarms in PIR-based systems often get blamed on ambient temperature swings or electrical noise. But the optical system—especially the lens geometry and material—plays a huge role:
Common causes:
Symmetrical Fresnel lens patterns causing central overfocus (hot spots).
Thermal lensing due to sunlight heating the plastic lens and warping its performance.
Ghost zones from internal reflections in multi-layer lenses or poorly coated surfaces.
Improper zone segmentation, resulting in poor signal contrast for subtle motion.
In practice, some installations show high false trigger rates not because of bad sensors, but due to misaligned lenses, wrong focal distances, or simply using a general-purpose PIR lens in a highly specific scenario (e.g., narrow hallways or outdoor bushy areas).
The choice of lens and its optical profile must match the detection algorithm, mounting height, and environmental noise profile.
Aubor Perspective: Optical Root Cause Solving
At Aubor, we’re often brought in when a client’s PIR system is “failing silently” or producing too many false positives. In our experience, 7 out of 10 such issues stem from optical design mismatches, not electronics.
Our approach includes:
IR spectrum testing of customer materials (with FTIR spectrometers).
Zemax simulations of FOV coverage, IR energy zones, and ray-tracing under different angles.
Material matching: Selecting polymers with high transmittance, then pairing with anti-fog or anti-reflective coatings.
Iterative lens shaping: Adjusting groove pitch, pattern asymmetry, and zone density based on real-world motion testing.
We believe in designing PIR lenses not just as components, but as integrated optical systems, each tailored to match detection expectations with environmental constraints.
About Aubor Optical
Aubor Optical is a precision optics company specializing in polymer-based IR lenses for sensing applications. We combine:
In-house tooling with CNC and diamond turning for microstructured Fresnel geometries;
Full-stack lens design from optical simulation to injection molding and surface finishing;
Custom solutions for PIR, ToF, LiDAR, and beyond.
Our proprietary workflow—from design to test to mass production—enables rapid turnaround for clients across security, smart home, and robotics sectors.
At Aubor, we don’t just mold lenses—we shape how the future perceives the world.
