I have the same problem as everyone else in these blogs. Basically the screen backlight gets bright when light colored images are on the screen and dark when dark colored images are on the screen. When watching movies the backlight is constantly getting brighter and darker. There is a common solution to this problem; all that needs to be done is disable the Vary-Bright setting under the graphics card settings. But like many people there is no Vary-Bright option. This problem only happens on battery. Please see the links below for more details on the problem and the ineffective solutions.
My laptop is
HP Pavilion g6t-2200 Select Edition Notebook PC
Windows 8 Pro 64
2nd generation Intel(R) Core(TM) i5-2450M Processor (2.5 GHz with Turbo Boost up to 3.1 GHz)
1GB AMD Radeon(TM) HD 7670M Discrete-Class Graphics
6GB DDR3 System Memory (2 Dimm)