If you have the same problem like me – your notebook automatically adjusts the backlight brightness depending on the brightness of the shown image, this could be your solution.
Inside the Catalyst Control Centre of your graphic chip there is an option named Vari-Bright. You’ll find it there: Energy Savings>PowerPlay>Vari-Bright. Turn it off and the annoying behaviour (a lot of fun during browsing -.-) should be gone.
Hint for owners of Intel graphics: http://adrynalyne.blogspot.de/2012/10/fix-brightness-control-for-windows-8.html