[krisoft]

> One thing that we were surprised about was that it could see clear through latex balloons.

This was something which surprised me too. Not specifically latex, but how many manufactured materials appear to be transparent in IR.

The explanation I heard is that it is because dye manufacturers usually want to make their product keep their colour long even when exposed to direct sunlight. And the easiest way to achieve that is by selecting materials which are transparent in IR. If they weren't the absorbed IR energy would break them down faster.

So people usually don't care about the IR opaqueness of their pigments/dyes but they care about colour fastness. And that is what selects for IR transparency.

[alnwlsn]

I was once involved in a project which projected Aruco markers in IR. So you get a slide projector, swap the bulb for an LED, have some slides made at the photo place, and you're done, right?

No - slide projector slides are IR transparent on purpose, so that they don't absorb heat from the bulb.