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-   -   How do I test for UV light in colored LEDs? (http://www.physicsforums.com/showthread.php?t=655835)

 R4za Nov30-12 05:06 AM

How do I test for UV light in colored LEDs?

I recently purchased two strings of RGB LED lights, the kind you can set to all sorts of colors by combining red, green and blue in various amounts.

Unfortunately, I strongly suspect that the blue and green led-pieces produce light that is contaminated with UV. Various UV-reactive things in my bedroom light up especially strongly under these colors; especially neon green and orange items. White UV reactive items just light up in the normal color of the lights. I don't want UV light in my room because it causes sharply accelerated skin aging.

However, I've never before had the chance to see my room lighted so brightly under so many different colors; I'm not sure what things are supposed to look like. I'd like to test the LEDS, to be sure.

Can anybody think of a (cheap) home experiment I could do, to definitively distinguish bright green or bright blue light from the same light mixed with UV?

 Pythagorean Nov30-12 05:36 AM

Re: How do I test for UV light in colored LEDs?

Maybe tonic water... Except that UV makes tonic water glow blue... So... Maybe not so helpful. Maybe though?

 256bits Nov30-12 07:43 AM

Re: How do I test for UV light in colored LEDs?

Try a US $20 dollar bill and look for the strip. Just make sure it is the correct year(s) of issue. Not sure what intensity and frequency is required for this to work. http://0.tqn.com/d/chemistry/1/0/A/X...blacklight.jpg Alos petroleum jelly spread on a surface will glow blue under a black light (UV)  RocketSci5KN Nov30-12 02:21 PM Re: How do I test for UV light in colored LEDs? I've never seen any R, G, or B LED datasheets that show any UV components whatsoever. They all look very narrow and Gaussian like in the spectrum with no higher frequency (UV) components. Dig up some datasheets on these and confirm for yourself. Likely, only the Blue LED's are worth looking at for this issue. For any fluorescent materials you may have, you also need to determine if the Blue color can activate them.  R4za Nov30-12 02:38 PM Re: How do I test for UV light in colored LEDs? Quote:  Quote by Pythagorean (Post 4179047) Maybe tonic water... Except that UV makes tonic water glow blue... So... Maybe not so helpful. Maybe though? I'll try it with the green ones on, thanks for the tip. Quote:  Quote by 256bits (Post 4179134) Try a US$20 dollar bill and look for the strip. Just make sure it is the correct year(s) of issue. Not sure what intensity and frequency is required for this to work. http://0.tqn.com/d/chemistry/1/0/A/X...blacklight.jpg Alos petroleum jelly spread on a surface will glow blue under a black light (UV)
I'm not in the US so no dollar bills available, but I'll try the petroleum jelly. Thanks. =)

Quote:
 Quote by RocketSci5KN (Post 4179532) I've never seen any R, G, or B LED datasheets that show any UV components whatsoever. They all look very narrow and Gaussian like in the spectrum with no higher frequency (UV) components. Dig up some datasheets on these and confirm for yourself. Likely, only the Blue LED's are worth looking at for this issue. For any fluorescent materials you may have, you also need to determine if the Blue color can activate them.
That's interesting info, thanks. Where would I start looking for datasheets on a string of leds? I live in a european, non-english speaking country and bought these locally, so the word alone probably won't get me anywhere.

 mfb Nov30-12 03:21 PM

Re: How do I test for UV light in colored LEDs?

I would expect that your total LED light output is negligible compared to the UV component of sunlight. Even if some fraction of the LED emission is UV (and I would be surprised by that), I don't think that it has any relevant effect.

Quote:
 Where would I start looking for datasheets on a string of leds?
At the producer of the LEDs or the string.

 RocketSci5KN Nov30-12 08:24 PM

Re: How do I test for UV light in colored LEDs?

Here's one of many LED vendors - check the datasheets.

http://www.kingbrightusa.com/Default.asp

 superdave Nov30-12 08:29 PM

Re: How do I test for UV light in colored LEDs?

Cheap plastic beads that change color in UV light.

 R4za Dec2-12 01:19 PM

Re: How do I test for UV light in colored LEDs?

Thanks again for the help, all of you. However, I'm getting conflicting and inconclusive results with these approaches.

Every SMD-5050 RGB LED datasheet I've found online lists focused wavelengths well out of the UV range. The manufacturer (and datasheets) specific to my own strips remains mysterious, however; I ordered them though Groupon, requiring no direct interaction between the supplier and myself, and the packaging doesn't list a company name. I've emailed Groupon about it, but not had a response yet.

That would indicate that LED strips like these don't generally produce UV light; however, every black-light-reactive item I've tried lights up intensely under these. Far moreso than under the low-powered actual UV lamp I've been using for comparison manages. If this fluorescence is caused by UV light, it is far from negligible.

Quote:
 Quote by RocketSci5KN For any fluorescent materials you may have, you also need to determine if the Blue color can activate them.
I can't find much of anything about blue light activated fluorescence online, except for some barely related stuff about animal proteins. Do you know if fluorescent reactions to blue light are possible/common in inorganic materials?

 greswd Dec2-12 10:12 PM

Re: How do I test for UV light in colored LEDs?

blue light can produce something that looks like a "fluorescent effect" on materials of certain colors, but there is actually no fluorescence at all.

 R4za Dec3-12 06:16 AM

Re: How do I test for UV light in colored LEDs?

Interesting. Does that give me anything I could use to tell the difference?

Anything I could reference for affected colors, maybe?

 NascentOxygen Dec3-12 06:36 AM

Re: How do I test for UV light in colored LEDs?

There was a roll of sticky tape whose edge would glow with a bluish halo when I carried it closer to an open window on a sunny day, I attributed that to UV fluorescence. I can't remember the brand, but it was the more costly vanishing/invisible sticky tape, quite colourless.

If it is summer where you are, string the lights near a window. Normal LEDs are reputed to emit practically no UV and so are not attractive to moths and insects, unlike fluoro-type globes which act like beacons to these Summer night nuisances.

 R4za Dec3-12 09:18 AM

Re: How do I test for UV light in colored LEDs?

That's a creative angle - thanks for the idea. S'too bad it's just nearing the end of fall here. I'll have to see if I can catch a straggler.

 R4za Dec4-12 09:50 AM

Re: How do I test for UV light in colored LEDs?

Well, I think I solved my problem. I took some of my reactive items to a store that sells colored LED strips, and found that theirs had the same effect. Combined with the various datasheets I've read that show fairly selective wavelengths to be the standard, I think that most likely all blue LED light causes the appearance of fluorescence in certain neon-colored materials.

Thanks for the help everybody! It was quite the puzzle.

 sophiecentaur Dec4-12 04:26 PM

Re: How do I test for UV light in colored LEDs?

It is unlikely that 'real' UV would be produced from a 'visible' LED because of the low supply voltage used. The energy of the photons produced will be limited by the electron energy - which will only be equal to or less than the supply voltage. You would need about 4eV to produce anything shorter than Violet light. What is the actual voltage across the diode itself?

The stripes on postage stamps and banknotes are fluorescent and a handy test.

 R4za Dec4-12 06:05 PM

Re: How do I test for UV light in colored LEDs?

That sounds like a useful approach. The packaging says "working voltage: 12 Vdc", and "Input Voltage: 100~240 Vac". Does that include the needed info?

I tried a bank note that's supposed to light up under UV, and it doesn't, but then it didn't under my 'real' blacklight either, leading me to think it might not respond to the whole spectrum of UV. Another explanation would be that the LEDs don't emit UV light and my old blacklight is just really crappy. =P

 sophiecentaur Dec4-12 06:11 PM

Re: How do I test for UV light in colored LEDs?

"Working voltage 12V" implies there's an internal series resistance in the package. So the info is no use, I think. You could chop one open carefully..... And use a meter.

Looks like all your money is fake too! Not your day is it? Lol.
Try stamps. UK stamps have uv strips.

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