Homebrew PCBs

Adam Seychell wrote:
> Ross McKenzie wrote:
>> Twelve months ago I bought 50 UV leds from an ebay seller in Homg Kong
>> for US$1.99 plusd US$11 postage. One was dead on arrival but the others
>> were perfect. My UV light box with 7 strings each of 7 leds in series
>> spaced at 20 mm works perfectly for me. But you do need a diffuser to
>> get an even light (no hot spots). Believe it or not, I used two sheets
>> of baking paper. Lovely even glow. Ten minute exposure times at 60 mm
>> above the leds. I mount my sensitied board and the photoartwork inside a
>> slide in photoframe with a single sheet of glass in front. Very tight
>> package. No undercutting.
>>
> 
> Thanks Ross. It looks like ebay is the best to get LEDs for this 
> project. Diffusers only undermine light undercut. When I'm pushing the 
> limits out of what my inkjet printer can produce, I need minimum 
> undercut. I've tried banks of fluorescent tubes in the past with dismal 
> results. I'm using large HID mercury lamp for the time being.
> 
> I had the idea of mounting the LEDs in an array of 5mm diameter holes 
> drilled through a 18mm (3/4") thick MDF particle board. This will 
> guarantee a maximum +-8 degree from each LED. As Markus Zingg, suggested 
> I'd need to experiment a bit until I find optimum layout.
> 
> I know that all LEDs aren't made equal. From experimenting with normal 
> colour LEDs I notice some manufactures have fairly poor uniformity or 
> patterns of rings when shined on a white surface.

You could measure the intensity spread by using a photodiode behind
a pinhole in alfoil.