Update the description
It's not just for "new generation" cards: due to new kernel improvements this method now works all the way back to the original GM 915 card from 2005!
% cat /sys/class/backlight/intel_backlight/brightness
1734375
% cat /sys/class/backlight/intel_backlight/max_brightness
3538125
(And yes... those 3538125 levels, they all work, even overriding the upper and lower limits imposed by the BIOS/EC!)