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Introduction: Why "Distance" Is Not a Straightforward Concept in PIR Sensing

In product catalogs and datasheets, you’ll often see PIR sensors described as having a "detection range of 10 meters" or even more. But what does that really mean? Can every PIR lens see clearly at that distance? And does "10 meters" apply in all directions, angles, and environmental conditions?

The truth is more complex. A PIR lens does not operate like a camera lens. Instead, it functions as a passive energy concentrator, and the effective detection distance depends on multiple factors: lens geometry, material choice, object temperature, angle of movement, and even the ambient thermal background.

This article breaks down what actually determines the sensing distance of a PIR system and why a one-size-fits-all answer is misleading. For engineers and integrators working on motion detection applications, understanding these principles can help avoid overpromising—and underperforming—designs.

Section One: The Five Misconceptions About PIR Distance

Many false assumptions circulate in the sensor industry. Let’s clarify five of the most common:

1. "Maximum distance = guaranteed detection"
   The listed value often represents an ideal-case scenario under controlled conditions. In real-world environments, detection effectiveness declines sharply beyond the center of the lens field.

2. "The sensing range is symmetrical"
   Most Fresnel lenses are not isotropic. Their field of view is segmented by multiple facets, each with a different angle. The horizontal and vertical ranges are often very different.

3. "Detection stops abruptly beyond a fixed range"
   In reality, PIR signals degrade gradually. Movement at the edge of the field may still generate a weak signal, but not strong enough to exceed the comparator threshold, leading to a missed detection.

4. "All PIR lenses of the same size behave the same"
   Lens design, focal length, and microstructure profile dramatically affect detection performance. A 15 mm lens with a narrow pitch will behave very differently from a dome-shaped 15 mm lens with broad grooves.

5. "Mounting height doesn’t matter"
   On the contrary, mounting height changes the geometry of the detection zones and alters the effective distance on the floor plane. A 10 m detection angle at 2.5 m height might only give 5 m usable distance at ankle level.

Section Two: The Real Physics of PIR Distance

The Fresnel lens focuses infrared radiation from the surrounding environment onto the surface of a pyroelectric detector. The following factors directly influence the effective distance:

• Focal length and aperture
  A longer focal length increases sensitivity to far-field movement but reduces coverage width. Shorter focal lengths do the opposite.

• Zone geometry
  A typical curtain lens creates multiple narrow vertical zones. A dome lens, by contrast, creates wide-angle circular zones. The spacing between these zones determines how easily a moving subject crosses them, generating signal changes.

• Subject temperature and speed
  A warm object moving rapidly across zones will trigger a stronger signal. A slow, small, or cooler object at the same distance might go undetected.

• Environmental background
  A hot day or warm wall surface can reduce the signal-to-noise ratio of the infrared change detected, effectively shortening the useful range.

According to field tests, typical PIR lenses used in commercial-grade motion sensors have a reliable detection range of about 5 to 8 meters under most indoor conditions, even if datasheets claim 10 meters or more.

Section Three: Comparing "Curtain" and "Dome" Lens Behavior

Two common PIR lens types dominate the market: the narrow-view curtain lens and the wide-view dome lens.

• Curtain lenses
  Optimized for corridors, fences, or perimeters, curtain lenses focus on linear motion. They can achieve 10 to 12 meter distances in controlled setups but have a very narrow horizontal field. Minor vertical misalignment often causes blind zones.

• Dome lenses
  Used in ceiling-mounted occupancy detectors, dome lenses offer full 360-degree horizontal coverage but limited vertical depth. Their effective range is often 3 to 6 meters, ideal for rooms and offices.

At Aubor, we simulate both designs using Zemax software and adjust parameters like pitch depth, angle spacing, and segment count to meet specific detection profiles.

Section Four: Real Customer Case — When 10 Meters Was Too Much

A European client came to us in 2024 with a request for a PIR lens that could “reliably detect a person walking at 10 meters distance.” After reviewing their use case, we found three major problems:

1. Their system was ceiling-mounted at 2.4 meters, and the sensor angle aimed downwards, reducing the effective detection zone on the floor to only 6 meters.

2. Their target subjects moved slowly and occasionally paused—this produced weak PIR signals.

3. Their original lens design used symmetrical concentric grooves, which created a central hot spot but neglected side zones.

Our solution was to redesign the lens with a non-coaxial groove pattern, optimizing field uniformity and introducing asymmetric angular zones. The resulting lens passed field testing with over 90 percent detection accuracy at 7 meters and 70 percent at 10 meters.

The key insight: instead of chasing unrealistic maximums, we focused on usable uniformity.

Section Five: Aubor’s Lens Design Recommendations

For customers who need to understand and define the real working range of their PIR system, we propose this practical approach:

• Use ray tracing tools to simulate zone activation at multiple distances and object sizes.

• Account for mounting angle, height, and wall reflection in optical modeling.

• Calibrate the Fresnel pitch and groove depth to fit your desired detection speed and zone width.

• Choose materials with verified IR transmission above 85 percent at 10 microns.

• Combine PIR data with digital filtering and directional logic to reduce false negatives at longer distances.

Every lens should be treated not as a commodity part, but as a tuned system element within your overall product.

About Aubor Optical

Aubor Optical specializes in the research, design, and manufacturing of custom infrared Fresnel lenses for motion detection and smart sensing. Our factory supports injection molding, diamond turning, and optical coating processes, enabling end-to-end control from prototype to volume production.

We work closely with clients to build lenses that match exact FOV, IR wavelength, and detection profile requirements, serving industries from security to robotics to smart building automation.

With over a decade of experience in lens simulation, materials testing, and production scaling, Aubor is committed to helping partners turn motion into meaning—through precision optics.