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I need to block the 'faint glow', which is to say red light noise, of an 850nm-LED IR floodlight.

Since green absorbs red, and acrylic transmits IR, natural approach seems to be to use a sheet of green transparent acrylic in front of the light. That correct?

(I'm aware there are 940nm-IR floodlights out there. For some reason, they are hardly available in Germany though.)

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  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Commented Oct 14 at 20:48
  • $\begingroup$ Is red light blocked? If yes, to what extent? $\endgroup$
    – Peterש
    Commented Oct 14 at 21:01
  • $\begingroup$ My question essentially is a question of principle. There are no further details yet. Sometimes questions are non-detailed, more about basic ideas, yet important. $\endgroup$
    – Peterש
    Commented Oct 14 at 21:16
  • $\begingroup$ For example, mathematics needed thousands of years to arrive at the principle of an algebraic field in its current form, including its intricacies. $\endgroup$
    – Peterש
    Commented Oct 14 at 21:21
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    $\begingroup$ The chemical engineers who chose (or invented) the dye (or dyes) that make the plastic look green to human eyes probably were not at all interested in whether or not those same dyes absorbed infrared light. The absorption spectrum of organic dyes can be quite complex, having many peaks and valleys. $\endgroup$
    – Solomon Slow
    Commented Oct 14 at 22:41

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It looks like your approach is correct. The only real questions are whether the dye used also blocks IR, and whether the dye is effective enough at blocking your red glow. My guess is that it won't block IR and that it will effectively block the red glow. Faraday would have determined the answer empirically. Here's a link to a study using a blue dye in different amounts. Note the dip in the visible spectrum centered on red, and the ~90% transmission above 850nm. https://scijournals.onlinelibrary.wiley.com/doi/abs/10.1002/pi.6082

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