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ATARI Light Pen Operator's Manual
APPENDIX A - BASICS OF LIQHT PEN OPERATION
The ATARI Personal Computer System produces 60 "frames" (pictures)
each second
on the television screen.
Each of these 60 individual
still pictures is produced by a beam of electrons sweeping across a
light-sensitive phosphor on the inner surface of the screen. When this
beam hits the surface/ that section of the surface which is being
swept lights up. (Only when the beam is there does it light/ and the
light dies down very quickly once the beam passes. ) Refer to Figure
A. 1 for a simplified sketch of this picture-producing activity.
Figure A. 1
Television Screen Simulation
Only a very few lines are shown here for discussion purposes. (There
are approximately 262 lines per screen scan in each 1/60 second.) The
solid lines represent the time when the trace beam is visible
(actively producing light on the screen). The dotted lines represent
the retrace period when the beam is not visible/ while it is returning
to the left edge to begin a new scan line or to the upper-left corner
to begin the scanning of the whole screen again.
The ^eam begins at the upper-left corner and goes left to right/
retraces/ moves down a little bit (1/262 of the total height of
the screen) repeats that process until it gets to the lower right
corner/ then retraces to the upper-left corner and starts all
over again.
Different colors are produced at different locations on the screen
because the electron beam is being changed by the computer hardware.
As it sweeps/ the computer controls its hue and luminance (color and
brightness) by reading a selected group (graphics block) of memory
locations and determining which color is to be displayed at each
specific physical location on the surface of the screen.
Basics of Light Pen Operation - 45
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