Light Pens ... - test-drb@ccmp.vtda.org

16 Jul 2001

On Jul 16, 18:39, Tony Duell wrote:
...
   So here's
a trick question ..  what happens when a person holds
 a lightpen to a dark section of the screen.  Is the lightpen 
 Nothing happens (literally). The light pen never sees any light, so it
 can't send a signal to the video card, so the CRT controller can never
 report the position.
  completely blind unless come pixel is lit up?

 Yes. 
That's not necessarily *quite* true.  I built a lightpen for my Beeb years
and years ago, based on an article in a magazine.  It used one of the RS
"Sweet Spot" devices and at first I had some trouble making it work.  The
following month the magazine published a followup indicating that not all
Sweet Spot devices worked, and suggesting alternatives, but by then I'd
figured out that there was a speed-of-response problem and also a
sensitivity issue and had played with other detectors.  I found that if the
brightness was set so that black was *just* not visible, a suitably
adjusted lightpen on a short-persistence monitor could detect dark pixels.
 It was, however, *much* easier with lit pixels.
--
Pete                                            Peter Turnbull
                                                Network Manager
                                                University of York