IR filter made from a developed neg. film?

I'm working on a Infra-red laser beam perimeter detecter and need a infra-red pass filter has anyone heard of using developed black and white negitive film as a IR pass filter?

I've always heard it's best to use developed E6 color slide film. I have a 120-sized roll of it here and can send you a piece if you want (assuming I can still find it). It does let some visible light through, especially in the deep red. I had hoped to use it to "covert-ify" the IR emitters on my Sony Nightshot camcorder (which emit some visible red) but it doesn't work so well for that.

Something I found out quite accidentally I might add, is that in some black (apparently) glass astrays, such as those found in Vegas Hotels (yeah, I clepted one) and was just goofing around with my cam corder and my home made IR blinky, and I noticed that is is quite visible when obscured by the black ashtry. These were the ones with the LV Hilton logo on the side, and although apparently black at casual glance, if illuminated from behind with our ArKr white 1W laser that we use at work, the true color of very very dark amber becomes apparent. But to IR, even low power IR from a single IR LED as in the circuit that I whipped together with a 555 timer IC, it is nearly transparent. It is pretty much completely opaque to all other visible light including deep red so this should make an exceptional "covertification" device. I was completely shocked by this as I thought I was looking through X-Ray specs as the beam is clearly visible through my cheapie B/W video camera. Maybe some other apparent black glass will exhibit this phenomenon as well. I'll have to try some other in the lab when work resumes.


Ed.

Do you have a mass spectrometer, I wonder what the coloring agent is?

Hello to every body,
20 years ago I made IR system for detecting intruder. From one side IR LED, from the other phototransisitor (continuous wave). First I make without any filter but normal light decrease sensitivity of all system. I used piece of developed bw negative film made by some German company. It work.
I never measure any thing moore.
My friends were using this kind of filter also but depends on the manufacturer they get different result.
I have some very good small pieces of IR transparent plastic. For normal light is absolutely black. There are many version of this material for different cut-off freqency. If you need very small piece I can send to you.

Mastershake,
I forget if you are interested please send to me e-mail. I do not have acces to CPF every day.
This pieces which I have are 2 or 3 mm thick. It looks similar to acrylic but it is not normal acrylic.
Very similar product is used in SureFire IR filters.
I make few IR filters from this material. Normally this material is not cheap (from my source).

I don't mean to dis another guy's direct observations, but consider for a moment what we're talking about here.

We're not trying to filter out a different color of light here, that is to say there isn't some magic UV and some nasty old dark red we never really wanted and wish to get rid of mixed in, it's one color. And the 'spectral line' is fairly sharp, that is there's not a whole lot of difference between the 'reddest' IR light coming out and the 'not red at all' IR.

Bummer is that the specific color that comes from modern, high output LEDs (Gallium Aluminum Arsenide based) are around 880 nm, unlike 'the good old days' before the Aluminum when outputs were lower but at 950 nm. The rub is this is at the ragged edge of what the human eye detects. Meaning if it's bright enough (and the room dark enough) we could see it. The basic rub is neither our eyes, the LED nor the filter have an absolute edge to their performance around 850 or so nm. Ideally I guess you'd want LED chemistry (the exact center of emission varies a bit with slightly different ratios) as low in energy as possible a sharp edged filter carefully 'cut' to be just above that frequency, bright background, and old eyes.

It's clearly a compromise. Any filter that's effective WRT 'making it invisible' is gonna take away useful light at the same time.

I've got a couple of suggestions. First off, consider the less efficient GaAs devices, they emit at 950 nm or so and are quite a way out of the detection range of human eyes (no filter needed). As a filter material, consider the dark red plastic used on remote controls (where the 'hide the faint light' is also the goal), but expect losses. Forget B/W film, the dark part is suspended Silver Oxide grains (they mechanically block all light). If we need filters, the photography folks should be able to help out.

Doug Owen