Changes on the backlight do not have effect. Only possibility to change the backlight is to boot OS X, change the backlight there and boot back into Ubuntu.
The bug is also discribed here: https://forum.ubuntuusers.de/topic/helligkeit-anpassen/ (german Website)
I followed this guide to produce the bug-report: https://wiki.ubuntu.com/Kernel/Debugging/Backlight
On this website the problem I have is also discribed: "Backlight control does not work, but there are entries in /sys/class/backlight. This may require the ACPI backlight driver to not take control so that a vendor specific driver will have a chance to register (see below)."
Kind regards
Jojo
ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: linux-image-3.19.0-51-generic 3.19.0-51.58~14.04.1
ProcVersionSignature: Ubuntu 3.19.0-51.58~14.04.1-generic 3.19.8-ckt13
Uname: Linux 3.19.0-51-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Mar 7 16:09:37 2016
InstallationDate: Installed on 2016-02-22 (14 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
SourcePackage: linux-lts-vivid
UpgradeStatus: No upgrade log present (probably fresh install)
There where also files from ssdt5.dat up to ssdt9.dat generated. If they are of use, I can add them, too.