On Jul 17, 19:17, Tony Duell wrote:
> [Lightpen over dark screen]
> If that's the article I am thinking of, they made a mistake and suggested
> the device you needed was the plain phototransistor. The next month they
> publised a correction that said you needed the phototransistor + schmitt
> trigger Sweet-Spot.
Yes, but the schmitt-triggered one has a different problem -- it's too
slow! It's a long time ago, and the articles and my notes are buried near
the bottom of a heap of other stuff, but I seem to remember that the
frequency response of the schmitt device was quite low. I didn't use it in
the end; I built a custom amplifier with the front end in the pen tube.
> > brightness was set so that black was *just* not visible, a suitably
>
> Yes, but as you imply, that's not truely a dark screen :-)...
>
> > adjusted lightpen on a short-persistence monitor could detect dark
pixels.
> > It was, however, *much* easier with lit pixels.
>
> And the adjustments (monitor brightness, light pen sensitivity/threshold)
> are _very_ touchy!
Indeed. If you're not careful, the detector that can "see" the spot on a
"dark" screen gets dazzled by a lit pixel, or fooled if you tilt the pen so
ambient light leaks past the tip. I gave it up.
--
Pete Peter Turnbull
Network Manager
University of York
Received on Tue Jul 17 2001 - 18:29:42 BST