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I'm trying to find a 940nm SMD infrared led with minimal red glow at night. The TSAL6400 gets close to it, but it's still not completely invisible and not SMD.

I've noticed IR leds have different wavelength curves in the lower range. TSAL6400 through hole with blue lens vs VSMY2943G SMD with clear lens.

I've tried covering them up with an infrared transmitting acrylic sheet but it's completely transparent to the red glow.

My guess it that the radiance on the lower end of the spectrum goes to close to the RED range. Although it's still infrared, the eye is still slightly sensitive to it.

How do I eliminate the red glow visible to the human eye from an IR led?

TSAL6400 VSMY2943G

VSMY2943G

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  • \$\begingroup\$ You have two problems: 1. Your eyes are really really sensitive when dark-adjusted. Detecting single photons is possible. 2. It's expensive to make an optical filter both highly wavelength-selective and cheap. So first you'll have to decide exactly how little visible light you want, depending on how dark you want it to be and still have this light be invisible. Then you might have to pay up for an expensive custom filter if you want to achieve that. \$\endgroup\$
    – The Photon
    Commented Oct 7, 2019 at 16:41
  • \$\begingroup\$ I thought the military uses 940nm was used precisely because it was completely invisible (as opposed to 850nm). You're saying you can still see the glow? \$\endgroup\$
    – DKNguyen
    Commented Oct 7, 2019 at 17:49
  • \$\begingroup\$ 5mm PD's use a black plastic lens for a "Daylight Blocking Filter" which is what you need. Same for old VCR's which used film window for Daylight blocking. Go look for these materials. \$\endgroup\$
    – D.A.S.
    Commented Oct 7, 2019 at 17:57
  • \$\begingroup\$ The @ThePhoton the goal is to have zero visible light. It's for a camera mounted over the bed, and any visible light is a distraction. Where could one get such a filter? \$\endgroup\$
    – pkuhar
    Commented Oct 7, 2019 at 20:24
  • \$\begingroup\$ @pkuhar, that's just not possible without also blocking all IR light. \$\endgroup\$
    – The Photon
    Commented Oct 7, 2019 at 20:26

2 Answers 2

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The human eye response stops at around 700nm to 750nm, you might be picking up a small amount of red because there are still a few photons in the red range produced and the eye is really sensitive to them in the dark. (its been theorized that the eye can be used as an individual photon detector.)

enter image description here
Source: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html

My suggestion would be to get a high pass optical filter at 850nm or 900nm and see if that solves your problem. And you can't stop all of the photons from getting through, only attenuate them. Maybe an IR laser, that transmitted at 850nm or 900nm might work also as the bandwidth is much smaller than an LED and will have less (almost no) 'leakage' into the 700-750nm range.

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    \$\begingroup\$ leefilters.com/index.php/camera-directory/camera-dir-list/… \$\endgroup\$
    – Scott Seidman
    Commented Oct 7, 2019 at 17:18
  • \$\begingroup\$ A laser wont work, since this is for night time camera illumination. \$\endgroup\$
    – pkuhar
    Commented Oct 7, 2019 at 20:26
  • \$\begingroup\$ @ScottSeidman I have something like this. But the glow is almost attenuated by it. It's funny, otherwise looks completely black. except for that glow. \$\endgroup\$
    – pkuhar
    Commented Oct 7, 2019 at 20:34
  • \$\begingroup\$ @pkuhar If this is for night time camera illumination, then why are you concerned with the glow? The camera won't be able to tell the difference \$\endgroup\$
    – Voltage Spike ♦
    Commented Oct 7, 2019 at 20:36
  • \$\begingroup\$ @VoltageSpike it's mounted over a bed, so could be visible by the user if he stares right at it. \$\endgroup\$
    – pkuhar
    Commented Oct 7, 2019 at 21:30
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It's almost certainly down to the filter.

It's hard to make a filter that has a very sharp cut-off between transmission and absorption, and make it cheap enough for use in consumer electronics. So the IR-blocking filter in your typical phone camera will still let a substantial amount of IR through, and the visible blocking filter on a typical IR remote control receiver will still let a fair amount of visible light through.

If you can afford a few 10's of dollars for this project, you could use a thin-film glass-substrate filter to get better selectivity.

Here's the response for the first one I found in a quick search. This is actually a "cold mirror" that reflects long wavelengths while transmitting short ones. You'd need to mount the light source and mirror so that the visible light is reflected away into some innocuous place to be absorbed by a dark surface.

enter image description here

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  • \$\begingroup\$ The cutoff is too low. I was testing with acrylite.co/extruded-black-9m020-gt.html with a slightly higher cutoff, and it has absolutely no affect on the glow. Do you know of any other ones? \$\endgroup\$
    – pkuhar
    Commented Oct 8, 2019 at 4:39
  • \$\begingroup\$ @pkuhar, we don't do specific product recommendations here. You can search and find for yourself if there's a product that fits your exact needs. The graph here is just to show how much more selective a (not too much) higher priced filter can be than a cheap plastic one. If you really have money to spend you could have a custom filter made . \$\endgroup\$
    – The Photon
    Commented Oct 8, 2019 at 4:42

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