Light pens?

[Jim Leonard]

Peter C. Wallace wrote:
>> Long persistence phospor doesn't preclude the use of a light pen, it
>> just introduces some limitations on the usage model.  The DEC Type 30
>> and 340 displays as used on the PDP-1 through PDP-10 use a P7 phosphor
>> which has very high persistence, yet light pens were commonly used with
>> them.
> 
> You can cheat with P7 and other double layer phosphors by using a blue 
> filter as they have a medium or short persistance blue phosphor and a 
> very long persistance yellow or orange phosphor.

Okay, now my age and lack of experience is going to show:  Can someone explain 
to me exactly *how* a light pen works?  What is the feedback mechanism if 
drawing on, say, a completely black screen?  How does the computer get 
coordinates for where the pen is touching?
-- 
Jim Leonard (trixter at oldskool.org)                    http://www.oldskool.org/
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