echo -n 50 > /proc/acpi/video/VGA/LCD/brightnessd.
Correct. Or at least for now. /proc is depreciated, and it's moved over to /sys. But we are using both for now. The paths to the brightness file vary. Depending on the system. echoing a random value to /proc/acpi/*/*/brightness will not work. The exact accepted value needs to be used. The user can view the brightness file in a text editor to find the accepted values.
The brightness file located in /sys has been evolving with each kernel release it seems. Echoing a value to it fails with the latest kernel we have available. Now the brightness values are not in percents any longer. It is controlled by levels. This machine has 7 levels of brightness. So to make the display change to full brightness I have to echo a 7.
You may not have a brightness file. It may not be supported on your machine. I "think" most intel video cards and maybe "some" others support this. The same goes for adjusting your brightness via xbacklight. Make sure the video kernel module is loaded. This is needed.
If trying xbacklight fails, also try this in a terminal:
xrandr --output LVDS --set BACKLIGHT_CONTROL native
Don't know why, but sometimes I need to do this, depending on what kernel I am using. Then try to set your brightness to half:
xbacklight -d 0:0 -set 50
If you can change your brightness with echoing to a brightness file, or using xbacklight, life is good. Even if your FN+Key buttons don't send a acpi event, or aren't visible to the kernel, we can always map other keys or key combinations to get the job done.
I've written several versions of "brightness" changing scripts that might get you started if you can manually change your brightness. One of them is somewhat "smart" and will read your available brightness levels, what level is currently used, and bump up or down according to what key is pressed. I'm also working on a HAL powered one that does all the thinking and should work for all hardware that supports brightess, but I got bored with it, and didn't finish.