After a couple of months I bought the Cinema Display 27 second hand I started noticing the same streak / stain on the screen. I decided to open it up and tried different things like cleaning the LCD assembly, air blow the fan, or change the kapton tape in the back of the LCD asssembly but only to find out that none of this was the problem, as no dust or condensation were found (i cleaned it like 3 or 4 times with isopropyl and a microfiber cloth that came with my LG TV, and the process was a pain in the ass).
Then I noticed that this mark was very noticeable after a couple of hours of use when the display was hotter because it gets get hot, but it gets even hotter when it’s been used intensively, like watching a movie or playing a game, so I totally discarded the dust / humidity option apple has been giving.
The way I somehow managed to work around this problem was:
- Charge my computer with the Macbook charger, not the display, as it used to get hotter where the power supply its located. I remembered that during the first couple of months I didn’t have this problem (or at least I didn’t notice) because I was waiting for the magsafe to magsafe 2 adapter to arrive, so the problem basically started when I started charging my macbook with the integrated cable in the display.
- Disconnected every USB peripherals in the back. I had it crowded, like the 3 usb ports were used, my iPhone, a little usb fan and numeric keypad. I know it might seem like its not too much, but every watt counts to avoid the display getting hotter.
- Put a fan directed to the vents of the display in the right side. It is a mini fan, with AC, not USB powered. It keeps it cool by constantly blowing air inside the display.
At the moment the stain / streak is there, but is definitely less noticeable and the display rarely gets hotter again, just regular hot / warm. I know that this also might be a problem of the fan, I’m waiting for a replacement, but right now they’re not really easy to get where I live, so I guess I’ll have to wait and try that.
Another thing I read is that maybe some of the LCD diffusers might be slightly burnt or damaged, but I don’t know if its even possibly to repair.
I hope it works for you.