Common Clear to IR material?

Acrylic won't work with far infrared. Those acrylic filters pass from

I did some testing today. The unit was set under the stairsteps at the outdoor entrance to my business. I then looked like a nut walking up and down the steps and listening for the alarm. It work... almost. It triggered over 80% of the time maybe 90%. But it didn't work 100%. So, I took off the plastic lid cover I had installed for weather protection. The unit then work flawlessly. Oops, I had a false alarm, nope there were two sparrows that tripped it. :-) I can probably make it work without the weatherproofing. I have not removed the cover from the original unit and it now has additional covering. I may need to make a weep hole in case any water does blow in. Some back story, I'm in a Marina with plenty of car and foot traffic. I needed to limit the area to prevent false alarms. This thing would trigger on a car 40ft away. One half of my possible mounting locations are eliminated because I can't point it towards where I work for obvious reasons. The opposite direction has auto and foot traffic. So I have a small operating area. But what I did today makes me think it wiil work good.

I'll try a layer of black plastic bag when I get it all going.

Thanks all, Mikek

Um, isn't standard exposed photographic film largely infrared transparent?

It is to near IR but not to LW IR, i.e. 2 to 12 nm.

BTW, you can check the NIR with some digital cameras. Use your tv remote as a source and insert the material to see the loss.