UNDERSTANDING PIR SENSING: FROM PHYSICS TO FALSE ALARMS
Passive Infrared (PIR) sensors are among the most widely used motion detection technologies in the world. From automatic lighting and smart thermostats to alarm systems and occupancy monitors, these small devices offer low-cost, low-power solutions to human presence detection.
Introduction: The Simplicity and Subtlety of PIR Motion Detection
Passive Infrared (PIR) sensors are among the most widely used motion detection technologies in the world. From automatic lighting and smart thermostats to alarm systems and occupancy monitors, these small devices offer low-cost, low-power solutions to human presence detection.
But while the principle of PIR detection appears simple, the performance of a PIR system—its range, reliability, and error rate—depends heavily on the optical lens, the chosen materials, and the surrounding environment. In this article, we examine how PIR sensors actually work, how materials affect signal transmission, and why false alarms occur even in well-designed systems.
How PIR Sensors Work: Thermal Contrast and Zone Interruption
PIR sensors operate on the pyroelectric effect, detecting changes in infrared radiation within a designated field of view. All objects emit infrared energy, especially warm bodies like humans. The core components of a PIR system include:
A pyroelectric detector, typically a dual-element structure that responds to differential heat changes.
A Fresnel lens, which focuses and segments infrared radiation into discrete sensing zones.
An amplifier and comparator circuit, which converts the voltage output into a binary detection signal.
When a person walks across multiple IR zones, alternating thermal exposure causes voltage fluctuations between the two detector elements. These fluctuations are interpreted as motion.
Importantly, the sensor does not capture images. It registers only the rate of infrared change across its detection surface. This is why the lens structure and placement are critical.
Material Impact: Why Transmission Efficiency Matters
The Fresnel lens in a PIR system is not just a mechanical accessory—it directly influences how much infrared energy reaches the sensor. Most PIR lenses are made from thermoplastics such as:
HDPE (High-Density Polyethylene): Common, low-cost, with acceptable IR transmittance.
PMMA (Polymethyl Methacrylate): Easier to mold precisely, better for complex zone structures.
TPX or COC: Higher cost, but offer excellent mid-IR transparency and stability.
Key optical performance metrics include:
Infrared transmittance in the 8–14 micron range, which should ideally exceed 85 percent.
Refractive index around 1.5–1.6 for effective light bending in lens grooves.
Stability under temperature and humidity, especially in outdoor or industrial settings.
Low-grade plastics, poor molding, or incorrect additives can lead to lens fogging, yellowing, or microcracking—each of which diminishes IR transmission and increases system error rates.
False Alarms: When Optics and Environment Mislead the Sensor
False positives in PIR systems are not always caused by electronics. In many cases, the root causes are optical or environmental:
Thermal reflections: Infrared energy bouncing off glass or metallic surfaces can enter unintended zones.
Lens misalignment: If the Fresnel pattern does not match the detector geometry, zone overlap and signal ambiguity occur.
Sunlight interference: Direct or reflected sunlight can create strong background IR radiation, overwhelming the sensor’s baseline.
Material mismatch: Using a lens material with poor IR transmittance shifts the signal threshold, making minor disturbances look significant.
Poor lens design: Overly symmetrical or shallow lens grooves can create hot zones in the center and blind spots at the edge.
Moreover, the installation context matters. Sensors placed near HVAC vents, incandescent lights, or large thermal masses (like windows or radiators) may generate spurious signals due to thermal convection or radiant noise.
Detection vs. Disturbance: The Fine Line
A well-functioning PIR system should respond only to human-scale movement within its designated zones. But if the optical signal reaching the pyroelectric sensor is distorted, the system cannot distinguish between a person and a passing heat current.
To minimize false alarms and missed detections, PIR system designers must take a holistic view of:
Lens optical performance
Material spectral properties
Mechanical alignment
Environmental variability
At Aubor, we routinely see over-specification in detection range without accounting for material limitations, or under-designed optics that fail to create usable thermal contrast zones. These design shortcuts often result in higher support costs and customer dissatisfaction downstream.
About Aubor Optical
Aubor Optical is an advanced manufacturer of precision polymer optics, specializing in infrared Fresnel lenses for sensing applications. Our expertise covers:
Custom lens design based on specific detection fields and signal response requirements
High-transmittance material selection and IR band testing
Tooling and molding for stable, high-resolution Fresnel structures
Simulation-driven optimization using Zemax and physical prototyping
We support clients across security, smart home, healthcare, and robotics industries who require reliable, application-matched motion detection optics.
For developers seeking consistent PIR performance, Aubor offers more than components—we offer confidence, built into every lens.
