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






[Non-text portions of this message have been removed]

From experience I can tell you that the amount and color tone of
the visible light has little to do with the actual uv output. TI used to
sell tiny button uv leds that had almost no visible light output. However,
they didn't output much in the way of uv either. UV leds have come a long
way in the past couple years. Also, the MCD rating is pretty much useless
on most ebay leds. My experience has that they exaggerate the rating...
 When possible ask for and get the manufacturers datasheet.



On Tue, Feb 26, 2013 at 12:19 AM, James <bitsyboffin@...> wrote:

> **
>
>
> 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
>
> [Non-text portions of this message have been removed]
>
>  
>



-- 
If you ask me if it can be done. The answer is YES, it can always be done.
The correct questions however are... What will it cost, and how long will
it take?


[Non-text portions of this message have been removed]

> of uv either. UV leds have come a long way in the past couple years. Also,
the
> MCD rating is pretty much useless on most ebay leds. My experience has
that
> they exaggerate the rating...
>  When possible ask for and get the manufacturers datasheet.


Dunno about UV, but the mcd (milli-candela) number even when not exaggerated
is a bit of a lie.  A 'candle' rating is how much light a candle (wax,
flames and all that) puts out in your direction (lumens is total light).

So two LEDs may have the same chip (same power), but the one with the
smaller view angle (more focussed) will have a higher mcd as it appears to
be brighter.  Check the view angle on high mcd LEDs!  It's like measuring
sunlight intensity by using a magnifying glass.

Candle power is also based on how your eye perceives it, so unless you've
had the lens removed (cataracts etc) it's a bit useless for UV.  Can't see
it = 0 candle power.  Same for infra-red.

Tony

--- In Homebrew_PCBs@yahoogroups.com, James <bitsyboffin@...> wrote:
> 
> 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.
> 

Your film could be responding to both the UV and blue light.   Originally, photographic film would only record blue and UV because  silver halide is basically sensitive only to those wavelengths.  As people discovered what to add to the film emulsion the sensitivity was extended, first to green and then finally adding red.  So UV/Blue sensitive emulsion is the easiest and most straight forward to make.

I'm curious about the requirement for UV light boxes as opposed to blue light boxes.  Is this due to the sensitivity of the emulsions or is it a hold over from the days when UV fluorescent tubes were available but blue tubes were not.