Reducing the visible red glow on IR illuminators

Posted: 11/19/2008 9:27:46 PM EDT

LED's typically emit light in a relatively broad wavelength range.  

The peak wavelength output of an 850nm led is, obviously, 850nm.  But it also obviously emits some light well under the 780nm visible threshhold.  

This is the bright red glow you see on the face of your IR illuminator.  (Check this from a distance, not by holding the flashlight up to your eye and looking in it.)  

Anyway, the fix for hiding this red glow is to buy an IR-pass filter that has a cutoff of ~800nm.  IR-pass filters block all light below, and pass all light above, their respective wavelength cutoff.    

It would be nice if glass-lens IR-pass filters in various popular flashlight sizes such as 20, 22, or 26mm were available.  That would make it easy to install an IR-pass filter on all your IR flashlights:  just change the lens.

Currently, that option doesn't exist to my knowledge.  You can buy "slip-over" IR pass filters for flashlights, but it's not always clear what their cutoff is, and they are kind of pricey.  

Another option: get some IR-pass plastic-sheet, and cut out a 1 or 2 or 3 "lenses" of it to sandwich between your clear lens and reflector.

I first read about this on an old camera mailing list somewhere.  here's a link to a guy's comparison of under-exposed slide negative film to a Wratten professional IR pass filter.  

Bottom line - dark film negatives make good IR-pass filters.  

So I did some more reading, and evaluated 4 options for DIY IR-pass filter / IR-flashlight red-glow eliminator:

1 - Roscolux gels.  These are sheets of plastic film that cover stagelights to obtain the desired lighting effect for stage or film acts.  You can view spectral graphs online that show the wavelengths of light blocked or passed by the varius available gels.  The spectral graph for the #90 Dark Yellow Green looked perfect.  it blocked everything from 550 up to 750.  So i bought a sheet of that, plus a gel-sampler booklet.  The #90 was completely worthless at eliminating the red glow.  So was every other sheet in the sampler booklet.  Basically, Roscolux is a dead end. Cost for a sheet and sampler book delivered was $20.  

2 - Disassemble a 3.5" floppy diskette.  The magnetic media wheel inside is a sheet of dark plastic.  It is very effective at eliminating the red glow.  Unfortunately, it's also very effective at blocking all light, and cuts your IR illumination dramatically with just one layer of material.  Another dead end.  Cost = zero.  

3 - Slide film.  Have a roll of

un

exposed slide film developed.  This makes a maximum-darkness roll of negatives.  Cut out three disks of this material using the flashlight lens as a template.  Sandwich the disks between the lens and the reflector.  This will reduce the red glow to "barely noticeable", with very little effect on the IR illumination.  Four layers nearly eliminates the red glow, but also starts to reduce the IR illumination.  Cost - $14.

4 - Print film.  Have a roll of

over

exposed print film developed.  I bought a roll of "expired" 100 print film for $3 at Rite-aid.  I pulled it out of the cylinder, and then wound it back in.  The lady behind the counter thought it was funny, and she developed it for free (10 minutes).  I'm convinced it's a tiny bit better than the slide film.  Three layers blocks most of the red glow like the slide film does, but I have the sense that it's blocking even less of the IR illumination than the slide film.  I need to try 4 layers and see how that does.  Cost = $3.

Slide film is made to go in front of hot projector lamps, and therefore has a higher temperature threshhold than print film.  (before it starts to warp and melt.)

None of my IR flashlights get hot, so that's not a problem.  I imagine if you try this as a way to convert a "white" flashlight to IR, you're gonna develop some heat.  

Anyway, that's how I reduced the red glow on my IR illuminators.  Works great for either 5mm or surface-mount leds.

IR illuminators are typically made with 808 or 850nm leds. They can be built using arrays of standard T-1.75 5mm leds, or with surface-mount hi-power single-led emitters.LED's typically emit light in a relatively broad wavelength range.The peak wavelength output of an 850nm led is, obviously, 850nm. But it also obviously emits some light well under the 780nm visible threshhold.This is the bright red glow you see on the face of your IR illuminator. (Check this from a distance, not by holding the flashlight up to your eye and looking in it.)Anyway, the fix for hiding this red glow is to buy an IR-pass filter that has a cutoff of ~800nm. IR-pass filters block all light below, and pass all light above, their respective wavelength cutoff.It would be nice if glass-lens IR-pass filters in various popular flashlight sizes such as 20, 22, or 26mm were available. That would make it easy to install an IR-pass filter on all your IR flashlights: just change the lens.Currently, that option doesn't exist to my knowledge. You can buy "slip-over" IR pass filters for flashlights, but it's not always clear what their cutoff is, and they are kind of pricey.Another option: get some IR-pass plastic-sheet, and cut out a 1 or 2 or 3 "lenses" of it to sandwich between your clear lens and reflector.I first read about this on an old camera mailing list somewhere. here's a link to a guy's comparison of under-exposed slide negative film to a Wratten professional IR pass filter. Link Bottom line - dark film negatives make good IR-pass filters.So I did some more reading, and evaluated 4 options for DIY IR-pass filter / IR-flashlight red-glow eliminator:1 - Roscolux gels. These are sheets of plastic film that cover stagelights to obtain the desired lighting effect for stage or film acts. You can view spectral graphs online that show the wavelengths of light blocked or passed by the varius available gels. The spectral graph for the #90 Dark Yellow Green looked perfect. it blocked everything from 550 up to 750. So i bought a sheet of that, plus a gel-sampler booklet. The #90 was completely worthless at eliminating the red glow. So was every other sheet in the sampler booklet. Basically, Roscolux is a dead end. Cost for a sheet and sampler book delivered was $20.2 - Disassemble a 3.5" floppy diskette. The magnetic media wheel inside is a sheet of dark plastic. It is very effective at eliminating the red glow. Unfortunately, it's also very effective at blocking all light, and cuts your IR illumination dramatically with just one layer of material. Another dead end. Cost = zero.3 - Slide film. Have a roll ofexposed slide film developed. This makes a maximum-darkness roll of negatives. Cut out three disks of this material using the flashlight lens as a template. Sandwich the disks between the lens and the reflector. This will reduce the red glow to "barely noticeable", with very little effect on the IR illumination. Four layers nearly eliminates the red glow, but also starts to reduce the IR illumination. Cost - $14.4 - Print film. Have a roll ofexposed print film developed. I bought a roll of "expired" 100 print film for $3 at Rite-aid. I pulled it out of the cylinder, and then wound it back in. The lady behind the counter thought it was funny, and she developed it for free (10 minutes). I'm convinced it's a tiny bit better than the slide film. Three layers blocks most of the red glow like the slide film does, but I have the sense that it's blocking even less of the IR illumination than the slide film. I need to try 4 layers and see how that does. Cost = $3.Slide film is made to go in front of hot projector lamps, and therefore has a higher temperature threshhold than print film. (before it starts to warp and melt.)None of my IR flashlights get hot, so that's not a problem. I imagine if you try this as a way to convert a "white" flashlight to IR, you're gonna develop some heat.Anyway, that's how I reduced the red glow on my IR illuminators. Works great for either 5mm or surface-mount leds.