Omron NT631 Operation Manuals page 218

Section 6-11
System Maintenance
Select "I/O Settings".
Select "Brightness Adjust".
Adjust the contrast with the "High", "Middle",
and "Low" touch switches in the left half of the
screen.
Press "Quit".
The contrast is set and the NT631/NT631C
returns to the "I/O SETTINGS MENU"
screen.
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