It now became possible to boot a live DVD of the latest development
release of ubuntu (64 bit) on my HP ZBook. I installed this system of
the machine's HDD. I have applied all available updates via the
UpdateManager.

The brightenss problem remains.

Afterwards I tested also the latest mainline kernel, i.e. file
linux-image-3.13.1-031301-generic_3.13.1-031301.201401291035_amd64.deb
(Of course, I rebooted the machine after having the kernel installed.)
Even with the newest kernel it remains impossible to controll brightness using 
the tool in the Control Centre.
The other "official" way to control brightness should be the  Fn F9   and  Fn 
F10  key combinations. I can see that pressing _either_ of these key pairs has 
the effect of immediate maximisation of brightness. No way to decrease it.

I can still controll brightness (fortunatelly) using the workaround proposed by 
ebutler (see above). In its essence it is done by variation of the number 
written in file
/sys/class/backlight/intel_backlight/brightness
I my opinion this bug should be easy to fix bacause it looks to be only the 
issue of the desktop environment.

BTW, even if I set brightness to an acceptably low value for me, it
remains at this value only untill the screensaver is started. Pressing a
key (to cancel the screensaver) turns the brighteness to its maximum.

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https://bugs.launchpad.net/bugs/1257766

Title:
  can not reduce brightness on HP ZBook 15

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