Blacklight, What color of led to use???

I wish to make some lights that act as blacklights. To make flouecent colors glow brightly. More specificaly, I work for a printer that produces postage stamps. One of the security features is what we call a "UV tag". This uv tag is invisible unless you view it under a blacklight. At key locations on the press and on some of the finishing equipment we have mounted blacklights in order for the workers to see if the uv tag is present. At times however it would be useful to be able to see this stuff in other parts of the plant. So it occured to me why not a flashlight with the correct colored led and voila. There in lies my problem. What color do I want? How easy is it to attain the desired color? Where might I purchase said leds? So many question. No doubt I'll have more as we go. /ubbthreads/images/graemlins/smile.gif

cheers,
zwf

Have you done a search for "UV LED FLASHLIGHTS" ?

Try it you may be surprised, at what you find. County Comm used to sell a little keychain one, so did ARC IIRC and Inova sells one. That's just off the top of my head....do the search....

From my own experiments I've found that the phosphors used in stamps will only respond to shortwave UV, and unfortunately there are no shortwave UV LEDs... not yet, anyway. It's possible that the phosphor/dye has changed since I experimented with those, so we might not be talking about the same thing...
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pw, have you tried the 385nm ones yet? (true 385s only recently becoming reasonable in price and widely available)

pw, how short are we talking here? Anything below ~350nm is not considered blacklight anymore, I believe, and naked 350nm LEDs are available (though not cheap).

The shortest wavelength of an LED in a product I know is the Nightcutter 5CSI with 375nm -- which typically means 377nm with a optical full width at half maximum of ~15nm. So you should get visible luminescence above 370nm or so.

Could you get a wavelength in nm from the fine print on one of the lamps you have on the line right now? That would seem to be the best info.

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vcal said:
pw, have you tried the 385nm ones yet? (true 385s only recently becoming reasonable in price and widely available)

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No, haven't tried them yet although I'm looking forward to getting a couple of them. Do you have a favorite vendor for them?

For less than $ 10, you can run a simple test. Buy a royal blue Lux 1 or III, aligator clip it to a 123 cell, and see if it will work. These run at 450 nm, which makes many materials light up.

If not, there are some shorter wavelength 5mm ones around.

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elgarak said:
pw, how short are we talking here? Anything below ~350nm is not considered blacklight anymore, I believe, and naked 350nm LEDs are available (though not cheap).


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The stuff I've been using up to now is primarily the 395nm variety, with a couple of 400nm units. I've seen the 350nm units in a news release, but even so, those are still within the neighborhood of an ordinary fluorescent blacklight tube, so if I really need that wavelength I'll just pull out one of my handheld 4w tube units rather than shelling out $$$ for an equivalent LED. No, I'm not a Luddite. /ubbthreads/images/graemlins/tongue.gif

The shortwave wavelength that I have in mind is around 265nm. This is the wavelength that can cause all sorts of minerals to fluoresce. It can also cause eye and skin damage, and in fact they use it as a germicidal lamp. My understanding is that it essentially tans the bacteria to death by breaking down the molecular bonds in their cells.
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265nm /ubbthreads/images/graemlins/eek.gif !? Ouch, too short. /ubbthreads/images/graemlins/cool.gif

But thinking of it, why should they use something like THIS on stamps?

Roithner Lasertechnik has 365nm 5mm LEDs if that'll do the job. The part number is RLT365-525, though I do not have additional information about this LED beyond what is on my UV LEDs page.
The UV LEDs web page on my website is right here if you're interested.

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elgarak said:
265nm /ubbthreads/images/graemlins/eek.gif !? Ouch, too short. /ubbthreads/images/graemlins/cool.gif

But thinking of it, why should they use something like THIS on stamps?

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So it'll be harder to detect and thus reproduce. It was probably also chosen for it's specific color and afterglow. As I recall the phosphorescence is extremely brief but bright.

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vcal said:
pw, have you tried the 385nm ones yet? (true 385s only recently becoming reasonable in price and widely available)

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Yes -- our 380nm and 390nm flashlights show up stamp phosphor well, if you are interested. And you can use it to find dog urine and cat vomit on your carpet too!

Hi.
...elgarak said:
265nm !? Ouch, too short.

But thinking of it, why should they use something like THIS on stamps?...

What about killing the germs in the saliva we put on when we lick it?

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xenopus said:
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vcal said:
pw, have you tried the 385nm ones yet? (true 385s only recently becoming reasonable in price and widely available)

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Yes -- our 380nm and 390nm flashlights show up stamp phosphor well, if you are interested. And you can use it to find dog urine and cat vomit on your carpet too!

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Is it true that even when run at recommended specs., that the (full brightness) life of these wavelength LEDs is less than 2500 hrs?

265nm, I just saw somewhere, germicidal replacement lamps. EBAY mabyee??? if these are small f-tubes, you could build smthn small to run it.
.02
Jeff

www.maxmax.com Lots of shortwave lights. 254mn

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xenopus said:
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vcal said:
pw, have you tried the 385nm ones yet? (true 385s only recently becoming reasonable in price and widely available)

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Yes -- our 380nm and 390nm flashlights show up stamp phosphor well, if you are interested. And you can use it to find dog urine and cat vomit on your carpet too!

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Hmm, based on this, I think we're talking about two different phosphors. I've seen the bluish fluorescence on stamps from longwave (blacklight) UV; I'm thinking of the other phosphors that they employed in order to automatically differentiate regular stamps from Air Mail stamps based on the color of the brief afterglow flash. Can't see that at all with longwave UV.

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vcal said:
Is it true that even when run at recommended specs., that the (full brightness) life of these wavelength LEDs is less than 2500 hrs?

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I believe the lifetime (to 50% intensity) of these LEDs operated at specifications is somewhere on the order of 200 hours.

Here is some text I had saved to my computer regarding UV LEDs:

According to another LED expert who has tested this, most UV and NUV LEDs in epoxy bodies last less than 200 hours to half-intensity. The kind in metal cases with glass windows last 1,000 to 2,000 hours to the half-intensity point, but they still have short device halflives.

Between the epoxy bodies degrading and the LEDs eating themselves at the atomic level (the UV radiation breaks atomic bonds in the GaN lattice), these LEDs have short halflives when compared with other, visible light LEDs.

Regarding the destructive effects of the UV or NUV radiation:
I think it's around 410nm or 420nm, with some degredation up to around 435nm. But I've never read a figure anywhere, so these figures are just guesses.

430nm whitish blue LEDs (such as Radio Shack # 276-311; GaN on a SiC substrate) appear to have fairly short halflives, but royal blue (444nm) LEDs don't, so I'm thinking this degradation begins around 435nm, and the shorter in wavelength you go, the shorter the "to half intensity" time is.

But remember, these are only guesses; I don't have or know of any documentation with which to back up these claims.

Thanks for that info Craig.
I have a collection of aviation instruments that I get to glow in the dark again. The original Ra226 has attacked the zinc sulfide scintillator -causing them not to glow in the dark anymore. So to simulate the original glow, I charge the dials with 2-5mA 400nm UV LEDs.

Looks g-r-e-a-t at night! /ubbthreads/images/graemlins/cool.gif