Possible to focus PIR "beam" ?

[Cliff Karlsson]

I would like to monitor a narrow area about 10-20cm from a distance of around 50cm. Is it possible to remove/modify the PIR lens so that it only detects movement in that small area?

If I paint the whole lens black/white and only leave a smal hole would that be enough? or have I misunderstood how the PIR works.

[Sparkman]

@Cliff-Karlsson That should work, but I would try using tape first so that it is easy to readjust. Once you have it adjusted the way you want, then replace the tape with paint.

[kimot]

But remember, that PIR sensor detects changes in temperature in focused area.
So you can use it only to monitor peoples, animals etc.
What about using ultrasonic range sensor, like cheap HC-SR04?

[m26872]

I have a CVD ZnSe (Chemical Vapor Deposited Zinc Selenide) lens I'm planning to experiment with in this kind of use case. It should work in theory. It bought it for my IR camera, but I've understood it's commonly used in laser cutters.