The backlight hardware buttons weren’t working on my HP EliteBook 8460w so I was using

sudo xrandr --output LVDS --brightness 1

and

sudo xrandr --output LVDS --brightness .5

That sort of worked but it only multiplied the gamma values. randr is a software solution – it doesn’t really change the backlight strength.

Here’s how I made my backlight buttons (the hardware buttons on the F9 and F10 keys) work in Debian Wheezy on my laptop.

1. Add contrib and non-free to the source list
2. Do

# aptitude update
# aptitude -r install linux-headers-amd64 fglrx-driver

3. Create an initial Xorg config file:

aticonfig --initial

3. Reboot.

Now I am one (mildly) illuminated guy \o/

Source: https://wiki.debian.org/ATIProprietary#AMD_Catalyst_12.6

/Edit
This breaks some compositing features on my desktop and the fan keeps on all the time. Also it broke Plymouth on my system but that is probably my own fault. For now I’ve removed the proprietary driver. Suggestions on the fan and compositing would be welcome.

/Edit 2
This works for me in Debian Jessie. Haven’t noticed any drawbacks yet.