What is a Fresnel Lens?
A Fresnel lens is a compact optical component that focuses infrared radiation onto the pyroelectric sensor element. It consists of concentric grooves that act as individual refractive surfaces, allowing a thin, lightweight lens to have a short focal length.
In PIR sensors, the white plastic dome you see is almost always a Fresnel lens. It plays a critical role in determining the sensor’s field of view and detection pattern.
How Fresnel Lenses Work in PIR Sensors
The lens divides the detection area into multiple zones (or “beams”). As a warm body moves through these zones, the sensor sees alternating infrared signals, creating the AC signal that triggers detection. The lens design determines:
- Detection range
- Detection angle
- Sensitivity pattern
- Number of detection zones
Common Fresnel Lens Types
1. Long Range (Curtain) Pattern
Characteristics: Narrow horizontal field of view (typically 5-20°), long detection distance (up to 15-20 meters). Creates a “curtain” of detection zones.
Applications: Hallways, corridors, perimeter security, entrance monitoring.
Example: Often used in outdoor security lights.
2. Wide Angle Pattern
Characteristics: Wide horizontal field (90-120°), medium detection range (5-10 meters). Multiple zones spread across the area.
Applications: Room occupancy detection, indoor lighting control, security systems.
Example: Most HC-SR501 modules come with a wide-angle Fresnel lens.
3. Pet Immune / Small Motion Rejection
Characteristics: Special lens design that creates “blind spots” near the ground, allowing pets to move without triggering the sensor while still detecting humans.
Applications: Pet-friendly security systems, indoor alarms.
Note: Works by focusing infrared from human height zones while ignoring lower zones.
4. Multi-Segment / High Density
Characteristics: Many small zones tightly packed, providing high sensitivity and ability to detect small movements. Range may be shorter.
Applications: Occupancy sensors for lighting control, presence detection.
Fresnel Lens Patterns Visualized
Imagine a top-down view:
- Long Range: Zones arranged in a narrow strip pointing outward.
- Wide Angle: Zones spread like a fan covering a wide area.
- Pet Immune: Zones above a certain height only.
How to Choose the Right Lens
Consider your application:
- What area needs coverage? Long hallway → long range. Room → wide angle.
- Do you have pets? Pet immune lens may reduce false alarms.
- What is the mounting height? Higher mounting may require different lens to avoid blind spots.
- Need to detect small movements? High-density lens.
Interchangeability
Many PIR modules allow you to swap the Fresnel lens. The lens usually snaps onto the sensor housing. However, ensure the lens is designed for the specific sensor’s pyroelectric element and package.
DIY: Modifying Fresnel Lenses
Some advanced users modify lenses by masking parts to change the detection pattern (e.g., covering sections to narrow field). Experiment cautiously as this can affect sensitivity.
Fresnel Lens Materials
Most Fresnel lenses are made from polyethylene or acrylic, which are transparent to infrared in the 8-14μm range. Avoid glass or standard plastic, as they block IR.
Conclusion
The Fresnel lens is an integral part of PIR sensor performance. Choosing the right lens pattern can make the difference between a reliable system and one plagued by false triggers or missed detections.