In A New Bedroom HDTV for Me, Part III I described further the "mura" problem of my new Sony Bravia KDL-40XBR2 1080p LCD HDTV, originally mentioned in A New Bedroom HDTV for Me, Part II.
Since I discovered the problem, I've been hanging out in the Official Sony 46" XBR LCD Uneven Backlight/Cloudy Thread. It represents the problems of hundreds of owners of flawed 46" and 40" XBR LCDs who have responded to a poll. The poll asks about Sonys with "clouds" of extra brightness on portions of the screen.
At present writing, 283 responders have said, "YES, clouds can been seen when dark colors are displayed or when switching inputs." Only 133 say, "NO, my screen has a perfect, fully even, backlight."
Any problem with the picture uniformity of an LCD panel can be called a mura defect. Picture uniformity is what happens when the panel can display full-field test patterns of various shades of gray, ranging from reference black to peak white, with no blemishes or signs of unevenness. Mura is different from dead or stuck pixels, problems which can also beset LCD panels.
Most mura problems involve variations in brightness, not color, but sometimes it is the case that color is not uniform over the entire screen as well. The variations, whether of brightness or color, are often subtle. A brightness variation, for instance, will typically be of such low contrast that one simply cannot see it with ordinary program material. That's when full-field test patterns come in handy.
A poor man's test pattern for the kind of "cloudy backlighting" mura most posters to the thread complain of happens when the Sony is tuned to an unused video input. That turns the screen black, and any clouds can readily be seen. Another way to get the same result is to cue up program material with a black background, such as the end credits of a movie.
To the right is a photo of one poster's problem "clouds." It comes from this album which contains several other shots posted to the thread by various Sony customers.
Notice that the "clouds" problem, which is called "cluster mura," looks different than the "vertical-band mura" my set has (see image at right). My problem shows up on light-gray fields. The "clouds" show up on dark fields.
I have also seen reports of horizontal banding and of light "bleeding" at the very edges of the screen.