by D. Parkinson
What is a light pen?
A light pen is essentially a photo-detector held in a pen-shaped carrier that is
used in conjunction with a video display. When the light pen is held against the
screen of a video display the photo-detector produces an output whenever it is over
an illuminated part of the screen. A conventional video display uses a raster-scan,
where the apparantly steady display is in fact being re-written at 50 frames per
second. Every time the section of screen under the photo-detector is re-written a
pulse is output from the light pen. If this pulse is routed to the display
generation circuits of the system the position of the light pen on the screen can
be determined from the current values held in the video display generation
In order for the system to work the screen used should not have a long- persistence
phosphor, the photo-detector should have a fast response, and the light pen should
be adequately shielded against unwanted pick-up from the deflection coils of the
It is basically a poor man’s Bit-Pad, costing in the 10s of £s rather than the
£1000s. (A Bit-Pad is a tablet and stylus combination, where the computer can
sense the position of the stylus on the tablet. These can offer large “draw” areas
with a resolution of 0.004" or better).
What is a light pen for?
Good question, I offer two stock answers:–
a) It’s use is limited only by your imagination. (i.e. I haven’t a clue –
or b) Menu driven programs.
With a menu driven program the user is presented with a “Menu” of options on the
screen. To select a given option all he has to do is to place the light pen against
the appropriate option on the screen and press the button. This generally will
bring up another sub-menu, and the process can be repeated until the final goal is
reached. An example of such a program would be an information retrieval program for
the inexperienced user (or inept typist!).
Such an approach can also be used to help the casual user run a complex analysis
Light Pens and Nascom/Gemini owners
There are a variety of commercial light pens available, and they are not beyond the
capability of the home constructor. The Gemini IVC will support a light pen
directly, but those intending to add one to a Nascom will need to exercise some
ingenuity as there is no in-built support. (Possibly cross connect the outputs of
the counters that address the screen ram to the inputs of a PIO???). I have only
used the Arfon light pen with the Gemini IVC, but it should be possible to
interface one of the other commercial pens to it. (Necessary now as, alas, Arfon is
no longer with us).