Homebrew PCBs

Hi all,

does anybody know much about UV LEDs and the differences between them?

I have a batch of UV leds from ebay, and a batch of UV leds from another 
chinese wholesaler.

The ebay ones were sold as 395-400 nm, viewing angle 20-25 degrees, 
30mA, 3.2-3.8v
The other ones were sold as 395-400 nm, 3.2-3.4v, 20mA, viewing angle 30 
degrees.
The ebay ones stated 2000 mcd, but I wouldn't believe that (nor is it 
useful for UV to state an mcd which is about visible spectrum), the 
other ones claim 150-200 mcd which seems a reasonable claim for the 
visible output.

I had some time ago made an exposure box out of the ebay ones, and 
killed some over time (was pushing a bit much current through them I 
think, being they are tightly packed the heat was building up pretty 
good), so I replaced 1 complete column and a couple of other failing 
ones with some of the chinese wholesaler batch, in the slideshow here 
I've tried to capture the difference...

http://www.flickr.com/photos/93638530@N08/sets/72157632863117628/show/

The two batches in the off state are both water clear, 5mm, identical 
appearance, but as I hope you can see they differ REMARKABLY in their 
visible light in the on state - naturally visible light isn't too 
important here anyway, but such a big difference surprised me enough to 
think I'd been sold fakes.

The ebay ones look "like you would expect UV leds to look" they emit a 
little violet light, the other ones emit a quite strong light blue, 
cyan, sort of colour.

At an angle, the ebay ones in the on state are largely visibly 
transparent, the other ones are opaque with visible colour.

I did a quick test exposure (with dry film) and interestingly the 
wholesaler ones exposed MORE strongly, that is, the exposed areas were 
noticeably darker, so definitely they are putting out a decent amount of 
UV in at least a wavelength suitable to expose dry film, but I'm curious 
as to why there would be such a large difference visibly.

So, anybody able to share any insights into this phenomenon?

---
James Sleeman






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