Important: Usually Mitsubishi requires one of their authorized servicers to check the TV first, before they will offer a concession. It is best to have an estimate of the repair from the technician before you call Mitsubishi.
Unfortunately, this is a defective light engine. I have changed many of these simply because they were covered in dust.
If you were to consider the fan on a desktop computer and the amount of dust collected in the vents, this is what the light in your TV is passing through on its way to displaying the picture. The fan draws the dust into the light engine. It is a common flaw with LCD projection TVs.
I am fairly certain Mitsubishi would offer to supply the part for your set. They will take your old light engine, disassemble it, clean it, and replace any damaged filters and panels, and then resell it. Yours is certainly not the first (or the last) light engine to be covered in dust.