This worked for my Lenovo T530 with Nvidia/Intel...
Note. My BIOS graphics settings were changed from Nvidia Optimus to Discrete Graphics (this was so I can drive a 4K external monitor from the Display Port). Also no need to have the file /etc/X11/xorg.conf on your system. The following assumes you are running the Nvidia driver and have access to the NVIDIA X Server Settings app in Unity.
First, bring up the text editor and I'll show you how to add the right contents to this file for your graphics card.
gksu gedit /usr/share/X11/xorg.conf.d/20-nvidia.conf
If your file is empty as mine was then open up the app "NVIDIA X Server Settings" from Unity.
Then go to the "X Server Display Configuration" section.
At the bottom of the screen click the button "Save to X Configuration". We don't want to actually save anything here, but click on the "Show Preview" button.
Scroll through until you see Section "Device"
and the following EndSection
. Highlight and copy the entire section. Mine looked like this:
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "NVS 5400M"
EndSection
Paste this into the /usr/share/X11/xorg.conf.d/20-nvidia.conf file opened before.
Now paste the following line just before the EndSection
Option "RegistryDwords" "EnableBrightnessControl=1"
It should look like this:
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "NVS 5400M"
Option "RegistryDwords" "EnableBrightnessControl=1"
EndSection
Now save and close /usr/share/X11/xorg.conf.d/20-nvidia.conf and reboot. My brightness control now works. Got the idea from this related post.