Cisco 8800 Series Manual page 256

Adjust Backlight Timer from the Phone Web Interface
Procedure
Step 1
Select Voice > User.
Step 2 In the Screen section, select a duration for the Back Light Timer parameter.
You can also configure this parameter in the configuration file (cfg.xml) by entering a string in this format:
<Back_Light_Timer ua="rw">30s</Back_Light_Timer>
The allowed values are 1m|5m|30m|Always On. The default value is 5m (5 minutes).
Step 3
In the Display Brightness field, enter an integer ranging from 4 to 15 for the desired brightness.
You can also configure this parameter in the configuration file (cfg.xml) by entering a string in this format:
<Display_Brightness ua="rw">15</Display_Brightness>
The allowed value is an integer ranging from 4 through 15. The bigger the value, the brighter the screen
display. The default value is 15.
Step 4
Click Submit All Changes.
Cisco IP Phone 8800 Series Multiplatform Phone Administration Guide for Release 11.3(1) and Later
236
Cisco IP Phone Configuration