Last Updated: 2026-09-03 ·
Author: Ashton Kim, CEO, Fresnel Factory Inc. ·
Reading time: 7 min
Quick Answer:
Pet immunity in a PIR detector is a system-level optimization between the Fresnel lens and the sensor electronics.
- In our 42-point walk test, all pet detections occurred at 10 m or closer, with most occurring at 4 m and 8 m. No detections occurred at 12 m or 14 m.
- The remaining detections were concentrated toward the edges of the field of view, particularly around ±30° to ±45°, rather than along the optical axis.
- In this test case, the remaining pet detections were primarily related to the sensor sensitivity setting rather than a need to redesign the lens again.
- Compared with the previous lens of the same overall size and form, the new lens provided enough performance margin to operate the same detector system at 80% of the previous sensor sensitivity setting, while the PIR sensor, electronics, housing, and other system conditions remained unchanged.
- This is one practical benefit of higher optical performance: additional lens performance can be traded for lower sensor sensitivity, reducing the likelihood that pet signals will cause unwanted detections.
Why does this question come up at all?
“Pet immunity” is often written into a PIR detector specification as a single requirement: the detector should ignore animals below a certain size or weight while still detecting a walking person.
It is therefore tempting to treat pet immunity primarily as a Fresnel lens problem.
The lens is certainly important. Its zoning determines which parts of the monitored area are projected onto the PIR sensor, how the target crosses detection zones, and how much infrared energy reaches the sensing elements.
But the final detection decision is also affected by the sensitivity of the sensor module and its signal-processing electronics.
A higher-performing lens can increase the available signal from a human target, but it can also increase the signal generated by a nearby animal. The useful question is therefore not simply whether the lens produces a stronger signal.
The more useful engineering question is:
Does the lens provide enough optical performance margin to allow the detector sensitivity to be reduced while still meeting the required detection performance?
This distinction becomes especially important when a detector is close to final tuning and pet false triggers remain as one of the last open issues.
How do you measure pet immunity in the first place?
Pet immunity can be evaluated using a controlled walk test in which the target crosses predefined distances and angles within the detector’s field of view.
Our pet-immunity evaluation used the following geometry.
| Parameter | Value |
|---|---|
| Detector mounting height | 2.2 m |
| Pet target height | 0.6 m |
| Field-of-view sampling | −45° to +45°, 15° steps (7 points) |
| Distances sampled | 4, 6, 8, 10, 12, 14 m |
| Walk-test velocity | 0.5 m/s |
| Test basis | EN 50131-2-2 (Alarm systems — Intrusion and hold-up systems — Requirements for passive infrared detectors) |
| Total measurement points | 42 |
Pet-immunity walk-test geometry used in this evaluation.
Source: Fresnel Factory internal measurements, 2026-07.
Two aspects of this test are particularly useful for diagnosis.
The 0.6 m pet target height changes how the target intersects the detection zones compared with a standing human. A lower quadruped target presents a different thermal geometry to the optical system than a person at the same distance.
The 42-point grid is equally important. A simple pass/fail test tells you that an unwanted detection occurred. A distance-by-angle grid tells you where it occurred.
That spatial information helps determine whether the next engineering adjustment should focus on optical zoning, module sensitivity, or the test configuration itself.
For human detection-range evaluation, Fresnel Factory also operates a performance test system with a straight-line measurement distance of up to 24.5 m and holds IEC 63180 evaluation equipment for PIR detector performance measurement.
Where do pet false triggers actually occur?
The following results came from a recent pet-immune PIR lens development project for an intrusion alarm system manufacturer.
| Distance | Pet detections (of 7 FoV points) |
|---|---|
| 14 m | 0 |
| 12 m | 0 |
| 10 m | 1 |
| 8 m | 3 |
| 6 m | 0 |
| 4 m | 4 |
Pet detections by distance — 8 detections across 42 measurement points.
Source: Fresnel Factory internal measurements, 2026-07.
Three patterns are visible in the data.
Why were there no pet detections in the far field?
No pet detections occurred at 12 m or 14 m in this test configuration.
As target distance increases, the infrared signal received by the PIR system generally decreases. In this test, the pet target signals at the far-field measurement points did not reach the detector’s active detection level.
This does not mean that every pet-immune detector will automatically be immune to animals at long distances. The result depends on the lens, PIR sensor, electronics, mounting conditions, target, and sensitivity setting.
What the test does show is that, for this particular configuration, the far-field positions were not responsible for the remaining false detections.
Why were the near and mid fields more important?
All eight detections occurred at 10 m or closer, and seven of the eight occurred at 4 m or 8 m.
This is consistent with the fact that a nearby thermal target generally generates a stronger signal at the detector than the same target at a much longer distance.
For pet-immunity development, this means that simply confirming long-distance immunity is not enough. Near- and mid-field positions require particular attention.
Why did the remaining detections concentrate near the FoV edges?
At 4 m, the residual pet detections occurred around ±30° and ±45° rather than at the 0° optical axis.
This does not necessarily mean that the optical signal is simply “weaker” or “stronger” at the edge. Fresnel lens zone geometry changes across the field of view, and a low-height target can intersect those zones differently at different angles.
The practical result is more important: in this test, the remaining pet-immunity challenge was concentrated toward the outer field of view rather than uniformly distributed across the coverage area.
For comparison, the previous-generation lens tested on the same 42-point grid generated pet detections at 15 of 42 points, compared with 8 of 42 points for the new lens.
A caution when comparing the two lenses:
the two optical designs do not necessarily divide their near-, mid-, and far-field zones in exactly the same way. The reliable comparison is the number and location of triggered measurement points, rather than assuming that a named optical zone in one lens corresponds directly to the same named zone in the other.
How can a better PIR lens allow lower sensor sensitivity?
This project provides a useful example because the new lens was not simply made larger.
The new lens had the same overall size and the same external form as the previous lens. The PIR sensor, electronic circuit, housing, and other detector conditions were also kept unchanged.
The primary changed component was the Fresnel lens.
| System condition | Previous lens | New lens |
|---|---|---|
| Overall lens size | Same | Same |
| Overall lens form | Same | Same |
| PIR sensor | Same | Same |
| Electronics | Same | Same |
| Housing / mechanical conditions | Same | Same |
| Relative sensor sensitivity setting | 100% reference | 80% of previous setting |
Under the same PIR module conditions, the new lens also achieved a detection distance more than 10% greater than the previous lens in comparative testing.
More importantly from a system-design perspective, the improved lens performance created enough margin that the detector could subsequently be operated with the sensor sensitivity setting reduced to 80% of the level used with the previous lens.
This is a more useful way to think about PIR lens performance than simply saying that a lens “detects farther.”
A higher-performing PIR lens creates optical margin that the system designer can trade for lower sensor sensitivity.
Lower sensor sensitivity makes it more difficult for unwanted pet signals to reach the detector’s decision level. At the same time, improved optical performance provides additional margin on the human-detection side of the system.
In other words, the Fresnel lens and the electronic sensitivity setting should not be treated as two unrelated design problems.
Pet immunity is a system-level optimization.
Does reducing sensor sensitivity mean the lens no longer matters?
No. In fact, this test demonstrates the opposite.
If the optical system has little performance margin, reducing sensor sensitivity may also reduce the detector’s useful human-detection range.
A higher-performing lens creates more flexibility. The system designer can use that extra performance in several ways:
- increase maximum human-detection distance,
- improve signal margin at the required distance,
- reduce sensor sensitivity to suppress unwanted pet signals, or
- balance sensitivity and coverage according to the final detector specification.
In this project, reducing the sensor sensitivity to 80% of the previous setting was possible because the new lens provided better optical performance than the previous lens under otherwise identical system conditions.
The lens therefore did not replace electronic tuning. It created the margin that made more aggressive electronic tuning possible.
Lens or electronics? A quick decision table
| Observed symptom | What to investigate | Likely engineering action |
|---|---|---|
| Pet detections concentrated at short distances while human range has margin | Module sensitivity setting | Reduce sensor sensitivity while confirming required human-detection performance |
| Pet detections concentrated at specific FoV angles | Lens zone geometry and target intersection | Review optical zoning together with sensitivity settings |
| Pet detections appear broadly across most distances and angles | Detector configuration and sensitivity | Verify the DUT configuration and electronic settings before redesigning the lens |
| Human-detection range falls below specification after sensitivity reduction | Available optical margin | Improve lens efficiency or revise zone design |
| Human range is sufficient on-axis but coverage gaps remain near ±45° | FoV-edge zone layout | Review Fresnel zone placement at the FoV extremes |
What does this mean for PIR detector development?
A common development mistake is to evaluate pet immunity only after the lens design has been completed, or to treat every remaining pet trigger as evidence that the lens must be redesigned.
A better approach is to measure both human detection and pet detection on a distance-by-angle grid and then evaluate the available system margin.
The development sequence can be summarized as follows:
- Measure the human-detection coverage with the intended lens and PIR module.
- Measure pet-target detections at the same distances and FoV angles.
- Identify whether unwanted detections are concentrated by distance, angle, or both.
- Determine how much human-detection margin the optical system provides.
- Reduce sensor sensitivity where sufficient optical margin exists.
- Re-test both human coverage and pet immunity after the adjustment.
- Redesign optical zoning only when the remaining problem is clearly associated with the lens coverage pattern.
This process makes it easier to separate an optical limitation from a sensitivity-tuning problem and can prevent unnecessary lens or mold changes late in development.
Where does this approach show up in real motion-sensing programs?
Fresnel Factory has supplied PIR optics for consumer and professional motion-sensing applications, including the Samsung SmartThings Motion Detector and Blink doorbell and outdoor camera products.
Fresnel Factory is also an optical partner for STMicroelectronics TMOS sensor applications and supplies HDPE lens modules for the FLIR Boson 320.
These applications differ significantly in sensor technology, detection range, field of view, mechanical constraints, and signal-processing requirements.
The common engineering principle is that the optics determine what infrared information becomes available to the sensor, while the electronics determine how that information is interpreted.
For PIR detector development in particular, good results usually come from optimizing both together rather than maximizing either one independently.
FAQ
Does a pet-immune Fresnel lens alone make a PIR detector pet-immune?
No. The Fresnel lens determines the optical zones and how infrared energy reaches the PIR sensor, but the final detection behavior also depends on sensor sensitivity and signal-processing settings. Pet immunity should therefore be treated as a system-level requirement.
Why do pet false triggers often occur at shorter distances?
A nearby animal generally produces a stronger signal at the detector than the same animal at a longer distance. In our 42-point test, all pet detections occurred at 10 m or closer, and seven of eight detections occurred at 4 m or 8 m.
Why can pet detections concentrate near the edges of the field of view?
Fresnel zone geometry changes across the field of view. A low-height pet target may therefore intersect optical zones differently near the FoV edges than along the optical axis. In our test, the 4 m residual detections were concentrated around ±30° and ±45°.
Can a better PIR lens allow the sensor sensitivity to be reduced?
Yes. In this development case, the new lens had the same overall size and form as the previous lens, while the PIR sensor, electronics, housing, and other system conditions remained unchanged. The improved lens performance allowed the sensor sensitivity setting to be reduced to 80% of the previous setting.
Does lowering PIR sensor sensitivity reduce detection range?
It can. Lower sensitivity generally reduces the available signal margin. This is why optical performance matters: a lens with additional detection margin gives the system designer more room to reduce sensitivity without immediately requiring a larger lens or a complete optical redesign.
What target height does Fresnel Factory use for pet-immunity testing?
In this test configuration, the detector was mounted at 2.2 m and the pet target height was 0.6 m. The test grid covered six distances and seven FoV angles for a total of 42 measurement points.
What walk speed was used in the pet-immunity test?
The test was conducted at 0.5 m/s on the basis of EN 50131-2-2. The applicable certification and test requirements should still be confirmed for each detector program.
How can I tell whether a pet-immunity problem is caused by the lens or the electronics?
Measure detections by both distance and FoV angle. If the problem is concentrated at specific optical regions, the Fresnel zone design should be reviewed. If the detector has sufficient human-detection margin but nearby pet signals still trigger it, the sensor sensitivity setting should also be investigated before changing the lens.
Next steps
If you are developing or tuning a pet-immune PIR detector, measure the detection pattern before changing the optics.
A distance-by-angle map can show whether the remaining unwanted detections are related primarily to optical zoning, available signal margin, or sensor sensitivity.
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About the author
Ashton Kim, CEO, Fresnel Factory Inc. Fresnel Factory designs and manufactures Fresnel optics using hot-press, injection molding, casting, roll-to-roll, and hot-embossing processes and participates in international sensor performance standards development through IEC TC47 and ISO TC22 SC32 WG6.
Related reading
- How Is PIR Detection Range Actually Measured? Inside IEC 63180 (planned)
- TMOS vs Traditional PIR: When to Switch (planned)
- 5 Fresnel Lens Manufacturing Methods Compared (planned)