Comment 7 for bug 159255

to Андон Сикавица :
try this in a terminal window:

sudo su -

This will give you root clearance on the shell .
then :

echo -n VALUE > /proc/acpi/video/VGA/LCD/brightness

where argument VALUE represents any these: 12, 25, 37, 50, 62, 75, 87, or 100

for example a brightness level of 37 goes like this:
echo -n 37 > /proc/acpi/video/VGA/LCD/brightness

To quit the root terminal, type exit