Screen Burn (Yosemite)
Something like this might help


  Create an all-white screen in a graphics application such as
  AppleWorks or Photoshop, and save it as a JPEG file.
  
  Use this as the image displayed by the screen saver.
  
  Turn the display brightness down (but not off) to preserve backlight
  bulb life.


Leave the white screen saver on for a while.

or more simple, use the build in "Flurry" screen saver and let it run overnight.