During Runtime (while communicating with the device/PLC), you can adjust the display unit screen brightness and contrast with the Brightness/Contrast Control Bar.
You cannot adjust the brightness/contrast when the display unit is starting up.
The GP-4100 series does not allow display of the Brightness/Contrast Control Bar while displaying an error message or system menu.
On the GP-4100 series, when the alarm banner is showing and the Brightness/Contrast Control Bar is activated, the alarm banner becomes hidden. The alarm banner will re-appear when the Brightness/Contract Control Bar is closed.
Brightness/Contrast Control Procedure
Touch the upper right then bottom left corners, or the upper left and bottom right corners of the panel (within a 40 pixel area) in 0.5 seconds.
When using Win GP, touch the upper right then bottom left corners, or the upper left and bottom right corners of the panel (within a 40 pixel area) in 5 seconds.
When using the GP-4100 Series, touch the upper right then bottom left corners, or the upper left and bottom right corners of the panel (within a 20 pixel area) in 0.5 seconds.
You can adjust the level of brightness intensity by touching [+] / [-] on the Brightness/Contrast Control Bar.
You can also adjust the contrast for non-TFT color LCD model units. 
By touching the icon indicated on the left, the Contrast Control Bar appears, allowing you to adjust the contrast.
Touch [X] on the Brightness/Contrast Control Bar to end adjustment.
Brightness/Contrast Control Bar display setting
Changes the setting in Offline mode when the Brightness/Contrast Control Bar does not display on the screen.
For details about the setting screen, refer to the setting guide.
M.16.2.3 Main Unit - Display Settings
Go to Offline mode and touch [Display Unit] on the item changeover switch.
Touch [Display Settings].
Select [Enable] on the [Show Brightness / Contrast Control Bar]
Touch [Exit] to exit offline mode.
When using GP-4100 series, touch [Top] to return to the initial screen. Touch [Exit] and in the confirmation screen, touch [Save changes and exit].