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 circuits.

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 video display.

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:–

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 program.

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).