July 11-12:BILLINGS, MONTANA: Annual show; Billings Gem and Mineral Club; Sat. 10-5, Sun. 10-4; Free admission!
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PostPosted: Mon Dec 10, 2007 4:15 am 
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I am in Silicon Valley and there are a number of local tech shops that might stock something like that. Let me know if you want to keep my eyes peeled.


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PostPosted: Mon Dec 10, 2007 10:27 am 
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Brian if you look on fleabay, ledwholesalers have flashlights that supposedly peak at 385. Not expensive.

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Posted by: "Dr "
Sun Dec 9, 2007 4:39 pm (PST)
I would counsel against using white LEDs in gem assessment and
photography. If you look up white LED spectra in Wikipedia, you will
find that these devices have no UV emission, but they do have a strong
sharp peak at 465nm (blue) and a broad but lower intensity band due to
fluorescence of a cerium YAG phosphor in the yellow/red region. Now if
you have a gem that has a strong absorption in the blue-green at
around 500nm, the LED will be very weak there, and the gem will appear
far less pink (due to the absorption of green) than it should. Other
gems may appear to have the same colour as in daylight: it all depends
on the structure of the absorption spectrum of the gem and that of the
light.
Until LEDs are made with a really flat unstructured spectrum, tungsten
halogen bulbs run at a high voltage or with a blue corrector filter
will provide a far more realistic appearance for gem colour.
The human eye is easily satisfied , taking as white a source with
a very strange peaky spectrum. WHen you start to look at gems, the
inadequacies of this 'white' become apparent. Color temperature, a
measure of the overall bias of the spectrum towards blue, is not a
good guide when one is dealing with gems with distinct band absorptions.


The ongoing discussion on LEDs on the faceter's list brought this post from
a member who has credentials at the Nobel prize level and whose area of expertise is the design of advanced microscopy systems for molecular biology research. IMO the last word on the subject. He knows everything.
He recently published a method called. "Brewster Conoscopy A novel method for determining high refractive indices in gemstones."
http://homepage.ntlworld.com/w.amos2/Brad%20Amos's%20Website/Brewster.pdf
You can read that here if you want.


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PostPosted: Mon Dec 10, 2007 1:15 pm 
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I'll have a look around for the 385 nm. Not that that particular UV wavelength would be useful for making a "flat visible spectrum" phosphor-based LED. Any LED line has such a broad peak, with full-width-half-maximums of between 25-50 nm, so you'd still see a whole lot of excess light in the 400-425 nm range.

Even so, that wavelength would make a great addition to my experiment. I will have to locate some of these LEDs before this time next year.


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PostPosted: Mon Dec 10, 2007 4:24 pm 
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I have a bunch of these, including their UV and sodium yellow, but I don't know if they have what you're looking for, Brian. You might want to check here if you haven't already: www.photonlight.com :D

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PostPosted: Mon Dec 10, 2007 6:37 pm 
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Thanks guys, but I've got a year before I'll be needing them. As they say... why do today what you can put off til tomorrow?

Checking the Nichia and Fox Group websites, I see they make 365 nm - 350 nm LEDs with basically the whole line contained below 400 nm. Those guys are amazing.


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PostPosted: Wed Dec 12, 2007 12:30 am 
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Darn it, you guys!

I have many different color LEDs from different manufacturers, but for some time I've wanted a complete spectral suite made by a single manufacturer. For months now, I've been able to suppress that desire. But along comes this thread... So now I'm spending way too much time surfing manufacturer websites and the ledmuseum website. Today I sent contact e-mails to four manufacturers.

I can't even collect spectra from those Fox Group UV LEDs. Who knows what I'm going to do with 'em? But I've got to have 'em. :twisted:

Thanks a lot you guys for resurrecting my LED addiction.

Ah well, I guess it could be worse... even the most expensive LEDs are pretty cheap. Not at all in the same league as a gem addiction.


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PostPosted: Wed Dec 12, 2007 12:50 am 
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Glad to see the debbil can get into you too. :lol:


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