# Puzzled over 365nm UV LED spectral content



## PhotonWrangler (Jul 9, 2011)

So I picked up a 365nm LED flashlight the other day and I'm a little puzzled.

This is listed as a 3W 365nm LED, and the spectral analysis that was performed on this light by Craig at the LED museum (thanks Craig!) shows little to no visible energy present in the output. So why do I see whitish light coming from it (in addition to a slight purplish glow) ? I don't think it's my corneas fluorescing because I see the same whitish content reflected from non-fluorescing surfaces.

I'm not complaining about the light at all, as it's outperforming all of my 395nm lights as expected. I'm just a little puzzled about the output of the LED vs the spectral response of the human eye, especially considering that I don't see this same whitish energy emanating from a conventional 365nm fluorescent black light (link goes to pdf from G-E site showing spectral distribution).

Anyone else notice this? Does the Nichia 365nm chip do the same thing? :thinking:

BTW I picked this up after dealing with a blown radiator and trying to use the UV tracing dye with several of my 395nm UV LED flashlights with little success. The radiator hass since been replaced but I need to have a usable UV light on hand for the next time I have a coolant leak.

Oh, and I'm now in the market for an orange peel reflector for this light. The beam shape is kind of weird and would benefit from a stippled reflector.

Update - 

I realized that the white plastic surrounding the chip die fluoresces and this is contributing to the visible light. I'm puzzled as to why they didn't make the plastic body black.


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## qwertyydude (Aug 1, 2011)

All 365nm uv leds emit some whitish/purplish light. I solved this problem in my led engine 365nm light by making a lens out of wood's glass. You can order woods glass filters online from photography supply stores like B&H. I just had a local stained glass place cut me a circle out of the glass and after, almost no visible light comes out, it's comparable now to the visible light output from fluorescent black light blue tubes. Getting rid of the visible light emission increases the contrast from the fluorescent substances from the background noticeably.


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## PhotonWrangler (Aug 1, 2011)

Thanks, Qwertyydude. I'm probably going to be fitting a Woods Glass filter on this light.


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## Yoda4561 (Aug 6, 2011)

I think part of it is a bit of fluorescing from impurities in the dome or the LED dome material itself. I have a chinese 5mm 375nm light, you can see a cone of hazy greenish light forward of the emitter inside the dome, and it appears to have a bit of white output because of this. The woods glass should take care of that, anything left is going to be the material fluorescing white.


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## calipsoii (Aug 6, 2011)

As everyone else has said, it's the stuff surrounding the emitter that's glowing. I use Nichia 375nm UV (their 5mm ones) regularly and even a little scratch in the 5mm epoxy is enough to cause the LED to emit a different color of light as the scratch fluoresces.


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## Norm (Dec 10, 2011)

I think you're making the mistake of thinking all the output of the LED is concentrated at 365nm.
Look at the spectragraphic displays in this link you'll see how the light peaks at 365nm but even getting up toward 400nm there is still some measurable output. 







Norm


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## PhotonWrangler (Dec 10, 2011)

Good point, Norm. What we need is a narrow bandwidth UV filter in front of the LED. Although I'm still puzzled over why the light appears a little whitish. It must be a combination of the longer wavelengths present and the surrounding materials fluorescing.

Earlier this year I was looking at a fluorescent based UV lamp that had virtually zero visible light output. It actually looked completely black at it's face, yet it was able to fluoresce nearby objects quite well. I guess I was hoping that a 365nm LED-based lamp would be able to approach this kind of performance without an external filter. Perhaps I've set my expectations a little too high.

So I need to pick up a piece of Woods Glass to fit onto the bezel of my flashlight.


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## FRITZHID (Dec 11, 2011)

well if anyone here knows of a dealer for cheap woods glass, i'm dieing to get my hands on a piece 4.5-5" square or round. please PM me if ya do! ty


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## PhotonWrangler (Dec 12, 2011)

Will do, Fritz.

I just checked the Day-Glo site - they _used_ to sell Woods Glass filters. I guess they've gotten out of that line of products and they're focusing mainly on their paints and pigments. Still looking around though...


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## alek (Dec 12, 2011)

On a semi-related note, I chimed in about my experience with a 365nm UV flashlight on this forum thread.

But I'm a bit of an unusual case as I can actually see Ultra-Violet! ;-)


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## bshanahan14rulz (Dec 14, 2011)

Or perhaps you could paint over the fluorescing offender with a paint that doesn't fluoresce and doesn't fog up or outgas. Or make a shield out of non-fluorescent non-conductive


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