Introduction
Ceiling-mounted PIR sensors are common in offices and commercial spaces. However, they often have a dead zone directly below the sensor – you can stand right under it and not be detected.
Why the Dead Zone Exists
The Fresnel lens creates detection zones that project outward at an angle. Directly below the sensor, there are no zones. The size of the dead zone depends on mounting height and lens design.
Calculating the Dead Zone
For a sensor with vertical field of view θ_v, mounted at height H, the radius of the dead zone is approximately:
R_dead = H × tan(90° – θ_v/2)
Example: H=3m, θ_v=90°, R_dead = 3 × tan(45°) = 3m. So a 3m diameter circle directly below is blind.
Symptoms
- People standing directly under sensor are not detected
- Detection works at distance but fails close to sensor
Solutions
Use a Sensor with 360° Coverage and Proper Lens
Ceiling-mount sensors designed for the purpose often have lenses that create zones directly below.
Tilt the Sensor (If Possible)
Some ceiling-mount sensors can be tilted slightly to cover the area below.
Use Multiple Sensors
Overlapping coverage from multiple sensors can eliminate dead zones.
Add a Secondary Sensor
A small, downward-facing PIR can cover the area directly below the main sensor.
Lower the Mounting Height
Lower mounting height reduces the dead zone radius.
Conclusion
Dead zones under ceiling-mounted sensors are normal but manageable with proper lens selection and supplemental coverage.