Category: NIR FIR

Quick Answer

Achieving a reliable 120° horizontal detection area with a PIR sensor is difficult when the Fresnel lens must remain flat and flush with the product surface. In this lighting-control project, the central target was approximately 6 m, while the outer detection zones were expected to retain at least 50% of the central performance.

1. A nominal 120° lens FoV is not the same as a guaranteed 120° human-detection area.
2. The PIR sensor, lens, PCB position, focal distance, and enclosure must be designed as one optical system.
3. Increasing the sensor-to-lens distance from approximately 8 mm toward 9.5 mm can provide more optical design flexibility.
4. A lens width of approximately 40 mm may support a wide horizontal detection pattern, but the available vertical area must also be reviewed.
5. Adding a second PIR sensor does not automatically create a significantly wider FoV unless its position or installation angle is changed.
6. The Fresnel lens supplier should participate before the mechanical design and PCB position are frozen.

What Was the Customer Trying to Develop?

A global building-automation company approached Fresnel Factory during the early development stage of a new PIR-based lighting-control product. The device was intended to detect human movement across a wide area and use that information to control lighting automatically.

The customer had already selected a compact, low-profile SMD PIR sensor from Murata. The baseline concept used one PIR sensor mounted on a PCB parallel to the wall. The Fresnel lens also needed to remain flat and aligned with the exterior surface of the product.

The primary development targets were:

• Approximately 120° horizontal field of view
• Approximately 6 m detection range in the central zone
• At least 50% of the central performance in the outer zones
• A flat Fresnel lens flush with the product surface
• A clean exterior without excessive molding or shrink marks
• One PIR sensor as the baseline architecture
• Approximately 30° vertical FoV under review
• Target production by the end of 2027

The main question was not simply whether the selected sensor could detect a person at 6 m. The more difficult question was whether the system could create a reliable 120° horizontal detection area while remaining within the mechanical and cosmetic limitations of the product.

Why Is a 120° FoV Difficult With a Flat PIR Fresnel Lens?

A PIR Fresnel lens divides the monitored space into multiple optical zones and directs far-infrared energy from a moving person toward the pyroelectric sensing elements. The geometry of these zones depends on the sensor element arrangement, focal distance, lens dimensions, Fresnel segment geometry, and the position of the sensor relative to the lens.

Wide-angle detection becomes more difficult when the optical surface must remain completely flat. A curved or angled lens surface can provide more favorable geometry for collecting infrared energy from the far-left and far-right detection zones. A flat lens provides less freedom for redirecting these steep peripheral rays toward a small PIR sensing element.

To compensate for this limitation, the optical designer may need to use a larger lens area, increase the focal distance, adjust the sensor position, or use more aggressive Fresnel segment geometry. Each option affects other parts of the product.

• A larger lens requires more exterior and internal space.
• A longer focal distance increases the required housing depth.
• A tilted sensor may require a separate PCB or mechanical bracket.
• Aggressive segment geometry can make injection molding and cosmetic control more difficult.
• A wider horizontal FoV may reduce the optical area available for vertical coverage.

For this reason, a 120° requirement should not be treated as a lens specification alone. It is a system-level requirement involving the PIR sensor, optics, electronics, mechanical structure, and performance-test criteria.

How Should a 120° PIR Detection Requirement Be Defined?

PIR field of view can be defined differently by different sensor and lens suppliers. A nominal optical angle, a simulated detection zone, and a guaranteed human-detection area are not necessarily the same.

For this project, the 120° target was interpreted as an effective horizontal detection area. The central zone was expected to detect a person at approximately 6 m. The outer zones did not need to provide exactly the same distance, but they were expected to retain at least 50% of the central detection performance.

This is more useful than stating only “120° FoV,” because it defines how the product should perform across the monitored area.

A practical PIR requirement should specify:

• The guaranteed horizontal detection angle
• The guaranteed vertical detection angle
• The central detection distance
• The minimum peripheral detection distance or performance ratio
• The installation height
• The direction and speed of human movement
• The target size and clothing conditions
• The ambient and target temperature difference
• The PIR signal threshold and pass/fail criteria

Without a common test definition, one supplier may describe a lens as 110°, while another may describe a similar detection pattern as 120°. Fresnel Factory therefore recommends reviewing the actual detection map rather than comparing only the FoV number printed on a datasheet.

What Design Changes Were Evaluated During the Consultation?

1. Increasing the Sensor-to-Lens Distance

The preliminary mechanical drawing provided approximately 8 mm between the PIR element and the lens surface. Increasing this distance toward approximately 9.5 mm was discussed.

A longer focal distance can provide more design freedom when assigning the outer Fresnel zones. It does not guarantee 120° performance by itself, but it can make the target more feasible within a flat-lens structure.

2. Using the Available Horizontal Lens Area

The proposed lens width was approximately 40 mm. Fresnel Factory had experience with a similar wide-angle design using an optical surface of approximately 37 mm and a focal distance of about 9.5 mm.

This suggested that the available horizontal width could be sufficient for continued development, provided that the sensor position and focal distance were adjusted appropriately.

3. Increasing the Vertical Optical Area

The preliminary vertical lens area was only about 7 mm. If the customer retains an approximately 30° vertical FoV requirement, the vertical optical area may need to be increased.

A wide horizontal FoV and a wide vertical FoV compete for the limited lens surface. The final window dimensions should therefore be determined after both requirements are confirmed.

4. Evaluating Sensor Tilt

The baseline design positioned the PIR sensor parallel to the wall without horizontal or vertical tilt. A controlled sensor angle could improve coverage in a selected direction, but it would require mechanical or PCB changes.

Because the product was still in an early design phase, the customer agreed to review whether the internal structure could accommodate these changes.

5. Evaluating a Dual-Sensor Architecture

A second PIR sensor was considered as an optional way to increase coverage. However, placing two sensors on the same plane and at the same angle does not automatically double the field of view.

In the discussed geometry, the increase could be limited to approximately 10° unless the sensors were separated, tilted, or assigned different optical zones. A dual-sensor design must therefore be evaluated together with the PCB layout, signal-processing method, cost target, and enclosure space.

6. Using the Sensitivity of the Selected PIR Sensor

Based on Fresnel Factory’s previous design experience, the selected Murata PIR sensor was expected to provide strong signal sensitivity compared with several conventional alternatives.

This made the 6 m central detection target relatively manageable. The more significant design challenge remained the wide peripheral coverage created by the flat optical surface.

How Did the Engineering Direction Change After the Consultation?

Why Should the Fresnel Lens Supplier Join the Project Early?

In many PIR projects, the enclosure, PCB, and optical window are designed first, and the lens supplier is contacted only after the mechanical structure has been frozen. This sequence creates unnecessary design risk.

The following parameters are directly connected:

• PIR sensor element size and arrangement
• Sensor package height
• PCB position and installation angle
• Sensor-to-lens distance
• Lens width and height
• Horizontal and vertical detection zones
• Housing depth and exterior curvature
• Lens material and injection-molding conditions
• Cosmetic surface requirements

If the PCB position is fixed too close to the lens, the optical designer may not have enough focal distance to create the required peripheral zones. If the optical window is too narrow vertically, the target vertical FoV may not be achievable. If the enclosure must remain completely flat, a wider lens area or different sensor position may be necessary.

Early collaboration allows these trade-offs to be evaluated before tooling begins. In this project, the customer’s mechanical team could still review the additional 1.5 mm of sensor-to-lens distance, lens-window height, and possible sensor tilt before the internal structure was finalized.

What Information Should Engineers Prepare for a Custom PIR Lens Project?

Providing the following information allows an optical supplier to evaluate feasibility more accurately:

1. PIR sensor manufacturer and exact part number
2. Pyroelectric element dimensions and arrangement
3. Required horizontal and vertical FoV
4. Central and peripheral detection-distance targets
5. Product installation height and orientation
6. Expected human movement direction
7. Available sensor-to-lens distance
8. Maximum optical-window width and height
9. Flat, curved, tilted, or recessed exterior requirements
10. PCB position and allowable sensor angle
11. Cosmetic and injection-molding restrictions
12. Target development schedule and production volume

A 3D mechanical drawing is useful, but it should be considered a starting point rather than a fixed constraint during the first optical feasibility review.

How Can Fresnel Factory Support a New PIR Product?

Fresnel Factory supports PIR projects from initial feasibility review through optical design, tooling, manufacturing, and performance verification.

• Review of PIR sensor geometry and package dimensions
• Recommended sensor-to-lens distance
• Horizontal and vertical detection-zone design
• Custom Fresnel segment layout
• Mechanical integration guidance
• Evaluation of single- and dual-sensor architectures
• Optical simulation and design iteration
• Prototype tooling and injection molding
• PIR signal and detection-pattern testing
• Collaboration with customer mechanical and electronics teams

The objective is not simply to manufacture a lens that fits an existing opening. It is to define a sensor, lens, and mechanical arrangement that can be tested against a measurable detection requirement.

What Was the Main Engineering Lesson From This Project?

The primary challenge was not whether the selected PIR sensor could detect a person at 6 m. The greater challenge was achieving a reliable 120° horizontal detection area while keeping the lens flat, preserving a clean exterior, and fitting the optical system within the available mechanical space.

The practical solution was therefore not a lens-only modification. It required coordinated optimization of the lens area, focal distance, sensor position, vertical coverage, PCB structure, and enclosure design.

For hardware engineers and R&D managers beginning a PIR project, the most important step is to involve the optical supplier before the mechanical design is frozen. A few millimeters of additional focal distance or lens-window area during the concept stage can prevent major performance compromises and costly redesign later.

Frequently Asked Questions

Can a flat PIR Fresnel lens achieve a 120° field of view?

It may be possible, but feasibility depends on the PIR element size, focal distance, lens dimensions, required detection range, and test criteria. A flat surface makes peripheral-zone design more difficult than a curved or angled lens.

Does a 120° lens FoV guarantee 6 m detection across the entire angle?

No. A nominal lens FoV does not guarantee equal detection distance at every angle. The central and peripheral detection targets should be defined separately.

What sensor-to-lens distance is required for a wide-angle PIR design?

There is no universal distance. In this project, increasing the distance from approximately 8 mm toward 9.5 mm was considered to provide more optical design flexibility.

Does using two PIR sensors double the field of view?

No. If two sensors are installed on the same plane and at the same angle, the improvement may be limited. Their positions, angles, optical zones, and signal-processing method must be designed together.

Why can two suppliers state different FoV values for similar PIR lenses?

Suppliers may use different signal thresholds, walking paths, target conditions, and definitions of guaranteed versus non-guaranteed detection. Detection maps and test conditions should be compared instead of the angle alone.

Why does the vertical size of the PIR lens matter?

The vertical lens area determines how much optical space is available for vertical detection zones. A very narrow lens may restrict the achievable vertical FoV even when sufficient horizontal width is available.

When should a Fresnel lens supplier become involved?

The supplier should be involved before the PCB position, housing depth, optical-window dimensions, and sensor installation angle are finalized.

Start a PIR Optical Feasibility Review

To evaluate a new PIR project, prepare the sensor part number, target detection map, installation height, PCB position, lens-window dimensions, and preliminary 3D mechanical data.

Request a custom PIR lens feasibility review from Fresnel Factory

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