>
> Many years ago, when I was doing my own photography using wet chemistry, I simply made a test exposure using an opaque card to expose the paper in steps. This is similar to the test wedge with graduated density but much handier and cheaper.
>
> Cover the entire area but a small portion to be exposed with the opaque card. Turn on the light source and then, at intervals, draw the card away so as to expose more of the surface. If you have some idea how much exposure might be required, you can expose the first area for the minimum expected time and space your additional exposure so that the last one is the correct distance "in time" from the first.
>
> Example - I suspect that the correct exposure for my 4x6 circuit board is between two and eight minutes. (120 to 480 sec). 360 sec difference between min and max, and I think I can accurately pull the card out the long way in 1/2 inch steps (12 total). Each step will be 30 sec long. I should be able to see (when development of the resist is complete) which time was best within a 30 second resolution. So I expose for 30 sec with 1/2" of board exposed, then pull the card back 1/2" every 30 sec thereafter, and turn the exposure lamp off 120 sec after the 11th step.
>
> If you have no idea what exposure is reasonable, go for 30 sec to 12.5 min in 60 sec increments or even 30 sec to 24.5 min in 120 sec increments (remember to set your stop watch to repeat and beep; it's easy to be distracted during longer exposure tests. :)
>
> Strip the resist and narrow the time window if necessary, or use the one that looks best to you.
>
> HTH,
> Donald.
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>
> ----- Original Message -----
> > From: "Rich Osman" <lists@...>
> > To: "Homebrew PCBs" <Homebrew_PCBs@yahoogroups.com>
> > Sent: Saturday, January 28, 2012 4:18:41 PM
> > Subject: Re: [Homebrew_PCBs] Re: UV light sources
> > Rich Osman; POB 93167; Southlake, TX 76092
> >
> >
> >
> > On Jan 28, 2012, at 10:54 AM, "Dave Sage" <davesage12@...>
> > wrote:
> >
> > > Lots of high tech ideas flying around here but if you really NEED to
> > > know
> > > why not just go on the website for the maker of the lamp and look it
> > > up OR
> > > ask them for the specs.
> > >
> > Mostly because I'm using unmarked used stuff AND because I want to get
> > a better idea how uniform illumination I'm getting.
> > > Regardless, you're going to have to make bracket
> > > exposures anyway to determine the best distance and time, so forget
> > > the
> > > theory and opt for the practical and "Get'er done".
> > >
> > Yeah, but I'd like to know I'm in the ballpark.
> > > Sage
> > >
> > >
> > >
> >
> >
> > [Non-text portions of this message have been removed]
> >
> >
> >
> > ------------------------------------
> >
> > Be sure to visit the group home and check for new Links, Files, and
> > Photos:
> > http://groups.yahoo.com/group/Homebrew_PCBsYahoo! Groups Links
> >
> >
> >
>