PIR and Fresnel lens setup

[Jon Wobbly]

Hi and thanks for adding me to the site.

I need some advice on a Fresnel lens and the distance from the PIR as used in a human movement sensor.

At the moment I have a curved plastic Fresnel lens with a focal point of 29mm and the PIR is in the unit is 21mm from the lens.
I have been told that this should not affect the detection and the unit should work fine, is this correct for this type of system or should I be looking at a more precise lens?

Many thanks

 

[dknguyen]

The image will just be blurry but you only have "two" pixels and you're not taking a picture. It just won't be as sensitive as it could be.

 

[Jon Wobbly]

Thanks for your reply dknguyen that is useful to know.
On the same theme, the lens I have has an 80° wide view , I am considering replacing that with a 110° wide view lens of the same size to get a better coverage, does increasing the angle of view have any impact on the sensitivity?
Thanks

 

[dknguyen]

A wider field of view captures more light but over a wider area. A narrower field of view captures less light because it's over a narrower area. So "sensitivity" could have more than one meaning in this case (detecting something moving slowly, something with low body heat, or something from farther away). The wider FOV will work over a wider FOV, but the narrower field of view will be able to detect things from farther away within it's narrower FOV.

 

[Jon Wobbly]

Thanks for the info. As the PIR is being used in a domestic house where the distance is not so important, being able to see a wider FOV in a room would be more beneficial for my type of application.
Thanks again for your input.

 

[dknguyen]

narrower will also pick up things moving more slowly than wide FOV.