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PICList Thread
'First PIC project...[OT]'
2000\01\29@081906 by Robert A. LaBudde

<x-flowed>At 08:33 PM 1/28/00 -0500, Sean wrote:
>You are correct about glass ATTENUATING UV, but I don't think that a think
>sheet of glass blocks it ENTIRELY. Also, I think the UV wavelength used for
>PCB exposure is considerably longer than that used for EPROM erasure, so it
>might go through glass better than the short wavelengh UV.

No, glass and plastic roll off in frequencies transmitted below 300 nm.
(Visualize them as bandpass filters, similar to the electrical ones.)

>It seems to me that what is probably happening is that the glass cuts the
>power considerably, but since the other Tim :-) has a full 60 watts of
>power (of which a considerable fraction is probably longwave UV, because we
>are talking flourescent bulbs), enough gets through to expose the film.

What's happening is that thin glass might pass 5% of the power at 250 nm,
so you have to boost your output 20x more than necessary.

You can develop with a lot less power sunlight if you avoid the glass or
plastic, or reduce the thickness (absorption is proportional to thickness).

It would be nice if someone could dredge up the spectra for lime glass,
quartz, acrylic and polycarbonate: then we could figure out the optimal
material and thickness for both strength and transmission above 250 nm.

Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail:
Least Cost Formulations, Ltd.                   URL:
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"


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