This is an old question, but an answer may help out others. I ran into an issue upon a fresh installation of
i3wm on my laptop where, for whatever reason, my
XF86MonBrightnessUp/Down keys weren't being registered (I checked with
xev). What I ended up doing is creating
acpi actions and events which corresponded to the keys being pressed.
The following are the actions/events I defined in
echo $(($(cat $bl_device)-1)) | sudo tee $bl_device
echo $(($(cat $bl_device)+1)) | sudo tee $bl_device
event=video/brightnessdown BRTDN 00000087 00000000
event=video/brightnessup BRTUP 00000086 00000000
You can verify your brightnessup/down acpi event codes by using
acpi_listen in your terminal and then pressing the relevant key combination (e.g., for me, it's Fn + Down Arrow for brightness down).
Finally, don't forget to restart
sudo /etc/init.d/acpid reload
Note: Your backlight device may be defined in a different location than
/sys/class/backlight/acpi_video0 - that's just where mine happened to be. Do some poking around.