At 06:00 PM 1/13/03 +0000, you wrote:
>I think that using a UV LED as a plotter pen is a fantastic idea in theory
>and certainly a project that I would attempt myself if I could overcome a
>few problems I can foresee:

Agreed. abrilliant idea, and possible with a few mods.

> > ... The beamwidth is
> > 10 degrees for the narrow ones, which (if I've calculated correctly) would
> > give 0.4mm line width at 2.3mm ( (0.4/2) / tan(10/2) ) off the board
> > surface.

better yet, maybe one could grind off the lens of the led, and move the
emitter very close to the source of the photoresist. this increases light
intensity and permits for side to side movement as a determinant of trace
width.
>Your 'beam' would presumably need to be as narrow as the narrowest track
>you wish to expose, however I'm not sure that simple trigonometry is
>sufficient to model what will actually be happening in practice.  The
>beamwidth calculation makes the assumption that you are working with a
>point source, but would the 'light-giving element' in an LED be larger
>(possibly approaching 1mm) - therefore projecting a much larger
>image.  The other point worth mentioning is that the boundary between the
>light and dark parts of the beam probably isn't particularly well defined
>- I would suspect that the light output simply falls off at about 10
>degrees, rather than being a sharp edge at exactly 10 degrees.
>
>A huge obstacle for me in the past when exposing pre-sensitised PCBs
>through a printed film was light 'leaking' because the image on the film
>was not printed on the correct side of the film.  With the film flush with
>the surface of the board there was still enough light leakage through the
>thickness of the film (a few hundredths of millimetres?) for the image to
>be blurred.  I guess what I'm interested in finding out is if anyone has
>had any practical experience of focussing an LED's beam to close
>tolerances.  Maybe then I'll give this a go!
>
>Regards,
>Mark Brown
>
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