I had same problem with my Sony Viao.. Found out about the command..
xrandr --output `LVDS_Number` --brightness `Zero_to_One`
LVDS_Number with your display number (possible LVDS-0 OR LVDS-1). And obviously without -> ` <-
Zero_to_One with any value between 0 to 1..It can be 0.1 for 10% brightness or 0.5 for 50% brightness.. And similarly without -> ` <-
Screen 0: minimum 8 x 8, current 1366 x 768, maximum 16384 x 16384
VGA-0 disconnected (normal left inverted right x axis y axis)
LVDS-0 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 344mm x 194mm
HDMI-0 disconnected (normal left inverted right x axis y axis)
You'll find your LVDS number in the output..
Which in my case is in third line, and is
So, to render the brightness to 50%.. We'd use the following command
$ xrandr --output LVDS-0 --brightness 0.5