Homebrew_PCBs

Adam Seychell wrote:

> Its difficult finding a UV photodiode under AU$100.
> There are blue enhanced photodiodes go down to about 350nm with
> %10 sensitivity relative to peak sensitivity.
> Farnell have the BPW21R for AU$20 which I can live with.
> http://www.vishay.com/document/81519/81519.pdf
>
> Its the 365nm peak emitted from mercury vapor that is most
> important for photoresists. This is just within the passband
> range for standard glass, and the BPW21R photodiode. The inkjet
> transparency film with mylar base and its ink receptive coating
> no doubt has its own influence on the UV.
>
> NEC blacklight tubes (BL-B type) are widely available so thats
> not a problem, http://www.nelt.co.jp/nhe_hp/special/special.htm
>
> I might not be hugely important to get the camera taking images
> from a UV backlighted photomask. More important is knowing the
> average UV absorption from a solid black area of print. This
> absorption figure can be measured with a standard BL-B tube and a
> photodiode like the BPW21R. I'm guessing the relative absorption
> will be measured by taking intensity reading with solid black and
> another reading without print.
>
> A = log(Io/Ii)
>
> where Io= photodiode current without ink
> Ii= photodiode current with ink
> A = relative absorption
>
> I can measure and tabulate values of A for various inkjet
> transparencies / inkjet printers.

Beware that if a uv photodiode is "slightly" sensitive to IR, it
may still dominate the measurement because atleast one transparency
(epson ones) have low attenuation to IR. It can be tested easily
by biasing an IR photo-interruptor LED so that its detector is in the
linear region, then measuring the output change when some film is
inserted.