These are repairable, they use the same lg panel found in the 2011 27 inch Imac. This repair is not for the average repair person. As any apple repair person knows, (official or not!) the 2011 Imac had serious backlight issues. Repair Information is sparse and difficult to find, not to mention the many (So bloody many!) people who refuse to believe you. The repair is simple: Thunderbolt display or Imac 2011 27 inch. Check all functions before hand, rule out backlight driver, psu, cables etc. Take out lcd panel, disassemble lcd panel, only as far as you need to to get at the backlight strip. remove backlight strip, resolder 4 or 5 pin plug on backlight strip even if it looks clean. This is a factory defect. I am a computer technician in Australia who has to do these and other repairs that apple won't admit are issues fairly often.