Epson GT-6500 EN - Manual (Page 47)

Image Setting Commands
Image setting commands are commands that tell the scanner what
kinds of image processing to perform on the image data. Brightness
settings and gamma correction (adjusting for input and output
brightness) settings are examples.
1. Set brightness (ESC L)
This command sets the scanning brightness. If the standard brightness
setting results in images that are too dark or light, use this command
to adjust the brightness. You can select from among seven brightness
levels; settings in between these seven levels are not possible.
The Set brightness (ESC L) command is ineffective when a gamma
table defined with the ESC z command is selected by the ESC Z (Set
gamma correction) command.
Command Usage 2-9