# I have an LED flashlight question about IR Filters?



## jeena1 (Mar 23, 2009)

Hello,

If LED flashlights do not emit any infrared light unlike the fluorescent flashlights, how can an IR Filter cancel out the non-infrared portion of the light spectrum on an LED flashlight if there isn't any infrared light?

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## asdalton (Mar 23, 2009)

There aren't any fluorescent flashlights really, although there are a few lanterns that use fluorescent tubes. Do you mean incandescent?

You're right that white LEDs emit very little infrared light. Infrared filters will not produce an infrared beam when used with LEDs; the result will instead be *no* beam.

The flexibility to use infrared filters is one of the reasons why incandescent lights still have their place.


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## Marduke (Mar 23, 2009)

jeena1 said:


> Hello,
> 
> If LED flashlights do not emit any infrared light unlike the fluorescent flashlights, how can an IR Filter cancel out the non-infrared portion of the light spectrum on an LED flashlight if there isn't any infrared light?



Filters block any wavelength they are not designed to allow through. An IR filter allows IR through, and blocks everything else. If the light produced to start with is "everything else", then everything is blocked.


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## DevL (Mar 24, 2009)

asdalton said:


> There aren't any fluorescent flashlights really, although there are a few lanterns that use fluorescent tubes. Do you mean incandescent?
> 
> You're right that white LEDs emit very little infrared light. Infrared filters will not produce an infrared beam when used with LEDs; the result will instead be *no* beam.
> 
> The flexibility to use infrared filters is one of the reasons why incandescent lights still have their place.



Umm no, thats why you use a LED visible and IR LED combination... not some silly filter. A filter makes no sense at all... it gives poor performance on both ends vs the dual LED giving good performance on both ends. Also filtered lights will glow red and are more visable in the normal light spectrum.


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## asdalton (Mar 24, 2009)

What do you mean, "Umm no"? Everything that I wrote was true.

A white/IR dual-LED setup has the disadvantage of being pretty much unobtainium right now.


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## Marduke (Mar 24, 2009)

asdalton said:


> What do you mean, "Umm no"? Everything that I wrote was true.
> 
> A white/IR dual-LED setup has the disadvantage of being pretty much unobtainium right now.



Not really. All the model lights that have them are still available. It's also not particularly hard to swap out the LED in many lights to be IR instead.

Filtering incan for IR is still horribly inefficient.


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## Chrontius (Mar 25, 2009)

He's thinking of a white/IR combination _in the same light,_ like the Surefire V2L Vampire. Incandescent gives you the option of switching from one to the other _in one light_, which is important for things like weapon lights where you really only get one (handgun) or pay a penalty in complexity (two switches, different locations). One or the other is easy, but high-powered IR LEDs are difficult to find and hard to work with. Also, the Luxeon ones seem to be three 1-watt dies that produce a crappy beam that demands lots of texture and give a floody beam.

Edit: filtering for IR is actually fairly efficient - a xenon lamp produces mostly IR light, and the visible light is absorbed by the filter, and then eventually re-radiated as very long-wavelength IR when it gets warm.


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## Marduke (Mar 25, 2009)

Chrontius said:


> He's thinking of a white/IR combination _in the same light,_ like the Surefire V2L Vampire. Incandescent gives you the option of switching from one to the other _in one light_, which is important for things like weapon lights where you really only get one (handgun) or pay a penalty in complexity (two switches, different locations). One or the other is easy, but high-powered IR LEDs are difficult to find and hard to work with. Also, the Luxeon ones seem to be three 1-watt dies that produce a crappy beam that demands lots of texture and give a floody beam.



I was talking about lights that can do both. There are a number of available lights that have both, and any light which has white and red can be swapped to have white and IR.




Chrontius said:


> Edit: filtering for IR is actually fairly efficient - a xenon lamp produces mostly IR light, and the visible light is absorbed by the filter, and then eventually re-radiated as very long-wavelength IR when it gets warm.



Filtering an inefficient light source to produce even less light. Yeah, efficient....


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## asdalton (Mar 25, 2009)

Incandescent is only "inefficient" in the visible range. Most of the energy consumed by the filament is radiated away in the IR spectrum.


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## Marduke (Mar 25, 2009)

asdalton said:


> Incandescent is only "inefficient" in the visible range. Most of the energy consumed by the filament is radiated away in the IR spectrum.




Crunch the numbers on energy output vs energy consumed. That's what I mean by "inefficient".


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## asdalton (Mar 25, 2009)

energy output = energy consumed 

The question relevant to this problem is: What fraction of the energy is radiated into the beam, versus conducted away through the filament legs, reflector, etc.?


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## asdalton (Mar 25, 2009)

I should add that one way to definitively test this is through calorimetry, and I may have the equipment needed for this.


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## Marduke (Mar 25, 2009)

asdalton said:


> energy output = energy consumed
> 
> The question relevant to this problem is: What fraction of the energy is radiated into the beam, versus conducted away through the filament legs, reflector, etc.?



Okay, rephrase:

USEFUL energy output divided by total energy consumed =! 1

Hence your efficiency.

Yes, filtering incan for IR is better than filtering LED, but it's still vastly outgunned by using a IR LED to start with, which requires no filtering.


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## pOkiE (Mar 26, 2009)

Surefire has a new light and replacement heads called the Vampire series which has both visible and IR options together in the one package. I dont know when they actually plan to release it but I am quite excited.


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