http://superuser.com/questions/127835/how-to-disab le-the-light-sensor-on-a-vaio-laptop

Look under the Vaio Control Center » Display » Automatic Brightness Settings and uncheck the "Adjust brightness automatically" option.

If that doesn't work, go to the graphics properties control center: Right click on the Desktop home screen and select "Graphics Properties" > select the "Power" tab > select "On Battery" > and under "Display Power Saving Technology" click OFF