I follow these instruction on Kubuntu 12.04. Perhaps you will serve:
But first I Install:
kgamma
kdegraphics
I gave then right clicked on the desktop, and then run order, and a
search box appears, put there "Kgamma" and appears "Gamma", I gave click
and a window which says: "Gamma control module KDE" and there are all
options to adjust the brightness of the monitor. I served to adjust the
brightness of an external monitor (I have a Dell Inspiron 1750 laptop)
brand VA1931wa-LED Viewsonic model and worked perfectly on Kubuntu 12.04
I published in:
kgamma on KDE 4.2.4