IBM RT PC Manual To Operations page 77

IBM 6154 Advanced Color Graphics Display
Power-On
Brightness Indicator
The single control on the Advanced Color Graphics Display is used to
power on the display and for brightness control. The Brightness Control
increases the brightness of the entire screen when it is turned clockwise.,
and decreases the brightness when it is turned counterclockwise. The
display power is turned off, when the Power OnlOff and Brightness
Control is turned fully counterclockwise.
The Power-On Indicator shows when power is available at the display.
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IBM RT PC Guide To Operations