PIR field of view

Larry Molter

The pir sensor's field of view is too wide for my application. I'm using the sensor to trigger an infrared camera but the sensor fires before the subject is in the field of the camera. The camera is fixed focus and has no zoom. So... can I put tape or someting around the lens of the pir to restrict its view?

John R.

I'm a bit rusty on my optics, but the long wavelength of the IR "light" may make the tape less than useful. You may have more luck with a tube around the sensor, and adjusting the length.

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John R.
Click here to see my Nomad Build Log

SRLM

Note that if you put a tube around the PIR (or dramatically restrict it's view in some other way) then the sensitivity of the module can significantly decrease.

Chris Savage

That is correct. A piece of PVC tubing can be used to obstruct the view of the sensor, reducing it's field of view.

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Chris Savage

Parallax Engineering
50 72 6F 6A 65 63 74 20 53 69 74 65
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ElectricAye

Chris Savage (Parallax) said...
That is correct. A piece of PVC tubing can be used to obstruct the view of the sensor, reducing it's field of view.

You might want to watch out for reflections taking place inside the tube, though. If the tube is short and the inside of the tube is smooth, the tube might not restrict the field of view as much as you'd expect because light entering at somewhat of an angle (and not head-on) can bounce off the smooth walls. Even if you paint the inside of the tube black, if the paint is smooth and somewhat glossy, reflections can still be a problem. If you want to get fancy, there's something called "flocking paper" that will kill those reflections. Or you can make little baffles, etc. or even rough up the tube walls.

my .2 cents worth.

John R.

If you use flocking, or paint, or anything like that to alter the inside of the tube, remember that you are dealing with infrared light. That is, black isn't necessarily "black", and if you use flocking, and things are stationary for a period of time, you might defeat the purpose, as even of reflections are stopped, you may heat up the flocking enough so that it starts to emit its own IR radiation. This could even happen with "white light" hitting a black surface (painted or flocked).

I also seem to recall that long wavelengths behave a lot differently around obstacles than shorter ones. It's a function of the ration of the wavelength to the obstacle size and shape. I believe that as the ratio of the wavelength to obstacle grows, the obstacle has less effect, but I could have that backwards. I also don’t know if we’re talking about enough of a difference between visible light and IR to make a difference. I’m only mentioning it in case it helps trigger someone else’s thoughts, and/or if things just are not working like it seems they should, maybe this points in a direction to research further.

Given the resources most of us have, trial and error is probably the fastest/easiest way to work this out.

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John R.
Click here to see my Nomad Build Log

Chris Savage

White PVC has been used without issue because the PIR sensor is not sensitive to visible light that reflects...it responds to rapid changes in the IR (heat) band. Several halloween and movie prop builders have used PVC without reporting any reflection issues so it shouldn't be a problem. Take care.

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Chris Savage

Parallax Engineering
50 72 6F 6A 65 63 74 20 53 69 74 65
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John R.

That sounds like our actor friend in action!

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John R.
Click here to see my Nomad Build Log

hover1

Something a little cheaper than PVC. A paper towel tube. The PIR sensor fits nicely into it. I just spray paint inside and outside black and hide it in the tree branches of some of our children's interactive installations. Toilet paper tube just as usefull.

Note: Paint does not do anything to reflectivity, just to hide the assembley. And this was done for indoor only. Outdoors, go PVC.

Jim