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I am using camera traps a with passive infrared sensor for my research and last week I decided to test their detection zones. I expected to find detection zone looking like this:

enter image description here

with a certain angle and range in which the sensor would operate, but instead I kept finding that the camera would only detect within a more 'rounded' zone:

enter image description here

Is this normal for passive infrared sensors? Or is it a consistent measurement error? Someone mentioned directivity to me but I have not been able to understand how that applies here. I hope this is the right place to ask, otherwise please point me in the right direction.

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  • \$\begingroup\$ don't know if this helps but take a look at this datasheet for a PIR sensor element. futurlec.com/PIR_D203S.shtml If you look on page 5 of 6 you can see that it uses multiple element pairs to get the horizontal view pattern. That pattern doesn't look uniform like your first drawing either. there's also a fresnel lens in front of it and the angle that makes with the detector could also have something to do with the pattern you're seeing. \$\endgroup\$ – Some Hardware Guy Jun 22 '15 at 2:49
  • \$\begingroup\$ The detection pattern is almost entirely due to the lens (facet design), not the PIR element so this is an optics question. \$\endgroup\$ – Martin Jun 22 '15 at 6:31
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PIR sensors work by focusing several narrow "zones" within the field of view onto a pyroelectric sensor. When a warm body moves into and out of these zones, it is the amplitude of the AC signal produced by the sensor that triggers a detection.

The effective sensitivity of the detector across its field of view depends on many factors in addition to an object's distance from the sensor, such as the size and temperature of the moving body, its speed and direction, etc.

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