IBM RT PC Manual To Operations page 76

IBM 6153 Advanced Monochrome Graphics Display
Brightness and Raster Test Control
The Brightness and Raster Test Control increases the intensity of the entire
screen when it is turned clockwise and decreases the intensity when it is
turned counterclockwise.
When the Brightness and Raster Test Control is turned fully clockwise, a
raster is displayed regardless of any input to the monitor. A raster is a
predetermined pattern of lines that provides uniform coverage of a display
space. The raster position is used to test the display for proper operation.
For normal operation, the Brightness and Raster Test Control should not be
turned fully clockwise.
Section 3. Using the Displays
3-11