[mindslight]

Well then, I must have been unlucky with the brand I attempted to use as a flag for an opto interrupter - it definitely passed enough IR for the thing to not trip.

And yeah, red lasers are quite common. But that doesn't rule out the possibility of a reader using IR - my experience with the opto left me paranoid on the matter.

[sjruckle]

I mean, a flag that moves in front of an ir sensor might not work if the light can leak between the flag and the sensor housing. Also, not all sensor housings are sealed from behind. Some of them are just a tiny folded-metal can that have gaps at the edges. It really depends on the configuration of your setup.

Unless the tape has holes, though, you can consider it opaque. Actually, because it absorbs and blocks ir, it's often used as a target for infrared temperature measurement of difficult-to-measure objects.

[mindslight]

This was many years ago, so I don't remember the exact details besides it being one of those basic black plastic housing straight-through ones. I had been surprised because the quite-black electrical tape turned out to be seemingly transmissive. I probably solved the problem with something equally hacky, like sandwiching a piece of plain paper between the folded over tape, or perhaps a bit of metal.

If it blocked say 90% of IR, it would congruent with both of our points. I'll have to investigate further - it would make a much simpler ID mask.

(hm. Just did an experiment with two IR remotes and it did appear to block for the most part. One of the remotes worked really close up, but I can't easily rule out leakage or some other type of coupling.)