IBM RT PC Manual To Operations page 70

IBM 5151 Personal Computer Display
CONTRAST CONTROL
BRIGHTNESS CONTROL
The Brightness Control on the IBM 5151 Personal Computer Display
increases the brightness of the entire screen when it is turned clockwise, and
decreases the brightness when it is turned counterclockwise.
The Contrast Control increases the brightness of the data displayed on the
screen when turned clockwise, and decreases the brightness when turned
counterclockwise.
Section 3. Using the Displays 3-5