IR transmissive with visible color 2

[bblankst]

Hi. Usually, when I see an IR-transmissive window on a plastic part, it looks black or darkly tinted. My understanding is that this is usually the result of adding certain dyes which absorb light in the visual spectrum but allow some infrared wavelengths to pass through.

I was wondering if anyone knows anything about making an IR window which reflects some of the visual spectrum (so that it transmits IR but looks like opaque blue, for example). Is this possible?

 

[MikeHalloran]

It's possible to transmit IR through an opaque part of arbitrary color, if the IR is strong enough and the part is thin enough.

In fact, this is a major headache in using plastic 'flags' with IR interrupters. You crank the photo-gain down enough so the flag interrupts the beam, go into production, and the next lot of devices has stronger emitters or more sensitive receivers, and they see right through your flag all over again. This is why I use brass or other metal flags when possible.



Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

Why do you want the visible to be reflected rather than absorbed? The latter is far easier and will still give you the color you wanted.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[bblankst]

Re: MikeHalloran
Very intersting. Unfortunately, the IR emitters will be relatively low-power and the plastic has to be relatively sturdy as it is part of a case which will get banged around a lot. Thanks for the idea, though.

Re: Demon3
Thanks for responding. This may be a difference of semantics. I'm far from an expert on this, so please correct me if I'm misunderstanding. I was thinking that whatever light isn't absorbed (refracted, etc. I'm simplifying) gets reflected, right? A typical black-looking IR window absorbs most visible light wavelengths. To appear blue for example, the window would have to reflect the blue wavelengths back at the viewer, but continue to absorb other visible wavelengths. If this is correct, I believe we are saying the same thing in different ways. What I don't know is if there are dyes which will reflect enough blue light while absorbing other visible light and transmitting IR.

 

[IRstuff]

You are slightly confused. The typical "IR" window you refer to is technical NEAR infrared (NIR), i.e., from about 800 nm to 950 nm. The windows are high wavelength pass filters, i.e., they pass about 750 nm and above, and block visible wavelengths.

While what you're asking is hypothetically doable, most reflective materials are opaque from a transmission perspective, so while it might wind up looking blue, its NIR transmission would be quite pathetic.

TTFN

FAQ731-376

 

[bblankst]

Thanks, IRstuff. Just what I needed to know.

 

[patprimmer]

You should contact major dyestuff companies. This is a colouration rather than a plastics problem.

Some specialised colours will transmit some wavelengths and block or absorb others. Every single pigment type (and there are many thousands) has its own specific characteristic or transmission fingerprint.

BASF, Bayer and CIBA spring to mind as potential suppliers.

Regards
Pat
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