LCD Flat Panels: Uneven Backlighting

I have two 1080p flat-panel LCD HDTVs, a new Samsung LN52A650 (52") and a two-year-old Sony Sony KDL-40XBR2 (40"). They both exhibit a problem with uneven backlighting, the Samsung to a very minor degree, the Sony to a more noticeable (but still hard to spot) extent. Research on the web indicates that this and related problems — "cloudy" backlighting, a "flashlight" effect, "leaky" backlighting, "mura" (a Japanese word) — are not uncommon on large 1080p LCD flat-panel HDTVs.

Click here to see an official thread about the clouding/flashlighting problem with the Samsung LNxxA650 model HDTVs. Click here for a similar thread about Sony LCDs.

Here are some pictures showing what the problem can look like when an artificially solid black/dark image (or a "regular" image with a very dark background) is on the screen:


To test your TV: some TVs will give you a solid black screen if tuned to an unused input; or, you can use an appropriate image from a DVD, such as the end credits from many movies.

The number of individual TVs about which forum posters have complained concerning this type of problem is so large, it has to be said that the uneven backlighting problem is not confined to just a few unfortunate set owners. This sort of problem is showing up on a great many high-priced LCD flat panels from Sony and Samsung, and perhaps from other manufacturers as well. For Sony and Samsung, there seem to be similar problems with successive generations of LCD flat-panel HDTVs; for instance, the current Sony KDL-nnXBR4 and XBR5 models seem to be affected, and not just the older XBR2 and XBR3 models. Multiple Samsung models over the last few years are also known to be prone to the problem.

Many buyers have replaced their original LCD sets by exchanging them at the store where they bought them or by taking advantage of the manufacturer's warranty, and found the replacement TVs too often have the same problem, if to a greater or lesser extent.

There is little reliable information on what causes the problem, or what solves the problem.

There is some unscientific, anecdotal evidence that the problem stems from mechanical or heat-induced stresses on the LCD panel itself. (This is the theory I personally subscribe to.) An LCD panel works by using electrical signals to tell molecules in individual pixel-sized locations in the panel to block light from a large fluorescent backlight from passing through the panel and reaching your eyes. Ignoring details concerning how this is done separately for the red, green, and blue components of the color TV picture, the basic idea is that as certain light-blocking molecules (the "liquid crystals") twist and untwist, light from the backlight passes through the panel unobstructed or is blocked to one degree or another. The three-dimensional geometric alignment of all the pixel-sized cells containing the twisty light-blocking molecules has to be absolutely uniform. Otherwise, you get "uneven backlighting" and see "clouds."

If there are uneven mechanical or thermal stresses on the LCD panel, perhaps coming from physical distortions transmitted from the frame of the TV assembly, the panel can warp, dimple, twist, crumple, etc., to a very minute degree. Now the twisty light-blocking molecules aren't squarely in front of the spots of light which they are intended to control. The effect is similar to what happens when you view an LCD flat panel from off-axis and see increased overall brightness and reduced overall contrast. The difference here is that only certain spots on the screen are, in effect, being viewed off-axis, owing to the warping, dimpling, and distorting coming from uneven mechanical or thermal stresses that have been applied to the physical LCD panel.

At least, that is the theory. Giving it some support is the fact that some people have reported being able to reduce the severity of the problem by slightly loosening some or all of the screws holding the TV assembly together, after carefully laying the TV face down on a soft cloth covering a perfectly flat and level floor. (This is a procedure that can cause major problems if botched, and it may void your warranty, so don't try it lightly.) Many who have done this have suggested leaving the TV in this face-down position, powered on so that it stays at normal operating temperature, for hours or days before re-tightening the screws to a lesser degree than they were originally tightened.

Other people have reported that their uneven backlighting problems have gone away or diminished on their own after several weeks or months of TV use, presumably because the cumulative effect of cycles of heating/cooling the TV over and over again has eliminated or minimized the mechanical stresses the TV assembly as a whole puts on the LCD panel it houses. Yet other people report that the problem either appears or disappears (depending on the individual TV) each time the set warms up, after not having been used for a while.

All of these reports tend to confirm that mechanical/thermal stresses on the LCD panel are the prime culprit.


There have been, in addition, reports that the cloudiness/unevenness problems tend to vanish if you simply turn down the LCD backlight — not contrast or brightness, but "backlight" per se — as you can do from the onscreen menu systems of these TVs. Sony, moreover, has apparently issued a firmware upgrade for some of its XBR models that will apparently do this automatically when the scene on the screen is dark. (I have ordered the upgrade but have not tried it yet.)

It stands to reason that reducing the backlight intensity would camouflage the problem, which is basically that too much light is leaking through in certain spots, in dark scenes. Lower the amount of light that the backlight produces, and there is consequently less light to leak through.

As a sheer guess, it may be that keeping the backlight low also generates less heat, reducing thermal stresses on the LCD panel.


So far, there appears to be little hard evidence that these problems derive from poorly manufactured LCD panels per se. I had the panel inside my Sony replaced under warranty, and the new one turned out to be exactly like the original, insofar as this problem was concerned. It looks as if the installation of the new panel inside the TV assembly created the problem all over again. This is more evidence that the problem has to do with mechanical/thermal stresses coming from the TV assembly as a whole.

Some posters to the threads mentioned above have tried to assign blame for the problem to certain ranges of the manufacturers' series of serial numbers, or to certain specific months of manufacture, or to what country the TV was assembled in. I have seen little hard evidence that any of these factors is important.

Nor does there seem to be any correlation between this problem and that of "dead" or "stuck" individual LCD pixels — which are defects in the manufacture of the LCD panels themselves.

It also needs to be noted that this problem is typically hard to spot. Unless you are looking at just the right kind of image on the LCD screen and the "cloudiness" just happens to catch your eye — or possibly you have made a point of trying to find it — you may in fact have the problem and never become aware of it. Some people, once they have discovered it, pretty much dismiss it from their minds as relatively unimportant. Others become semi-obsessed with it. I have personally found that it is better to be in the former group than the latter ...

My New Samsung LN52A650 TV

The LN52A650 is one of Samsung's 2008-model TVs — this one is a 1080p 52" LCD flat panel — that have been improved in several ways over last year's models. I just broke down and got an LN52A650 for my living room, to go where I used to have a rear-projection Samsung 61" DLP HDTV.

I also have a five-year-old 32" non-1080p Hitachi plasma HDTV in my basement and a two-year-old 40" 1080p Sony LCD flat panel in my bedroom. See the posts in my Sony KDL-40XBR2 series for more on the latter. The Sony is an excellent TV — though it has some minor problems with imperfect brightness uniformity across the screen, seemingly due to uneven backlighting — but the new Samsung is even better overall and has no such uniformity issues.

The main reason it beats the Sony: the Samsung has black levels I'd truly describe as "inky." The Sony just can't match them.

The Samsung advertises a dynamic contrast ratio of 50,000:1. (The Sony's is a comparatively paltry 7,000: 1.) That means the brightest white the Samsung can produce is 50,000 times brighter than the inkiest black. Actual video pictures don't need that much contrast, needless to say, but the figure is still meaningful — provided, that is, that the stunningly high contrast ratio is achieved (at least in part) by lowering the level at which black is displayed, not by just upping the level of white. And that's exactly what Samsung has done with the LN52A650.

The 50,000:1 ratio, by the way, isn't the best Samsung has to offer. The best would be the 1,000,000:1 dynamic contrast ratio (!) possessed by a handful of its other models (none of which comes in the 52" size I wanted). That superior ratio is a direct result of backlighting the LCD screen using an array of tiny, individually controllable LEDs, not the uniform fluorescent backlight of the LN52A650. LED backlighting provides a yet further step up in contrast ratio/black level, but at a super-premium price.


You can watch the LN52A650 in a well-lit room and still get a dazzling picture with all those inky blacks, I have found. And not only are the blacks convincing, the portions of the picture that are near-black have excellent definition (fine "shadow detail"). Some HDTVs like my Hitachi tend to "swallow" shadow detail to fool the eye into thinking blacks are being displayed darker than they really are.

Color renditions are superb on the LN52A650, as are the smoothness and naturalness of the brightness gradations between black and white. And I have seen no color banding due to suboptimal internal handling of digital signals — my Hitachi exhibits a lot of banding, my Sony only a little bit.

In fact, the LN52A650 is the first HDTV I've had that gives me a marvelous picture right out of the box, without a lot of fussing and tweaking. I find the "Standard" preset looks just fine to my eyes, though I do turn off "Dynamic Contrast" as providing too much contrast for my taste. There is also, separately, a "Dynamic" preset — too jazzed up to suit me — and a "Movie" preset that I find a little too restrained for daily use.

(Edit: After a couple of days of using the "Standard" preset, I switched to "Movie." The latter gives a less dazzling picture, but one that ultimately seems more realistic. I have found that I like to up the "Gamma" setting from 0 to +3 in "Movie" mode, which brings out yet more shadow detail. Other than that, I have left the "Movie" preset as it comes out of the box.)

Each preset, including "Movie," can have a raft of video (and audio) parameters individually adjusted, if need be, independently for each video input. Or, if you decide you have made things worse and not better, it's easy to reset any preset to its original settings.

There are also separate, non-adjustable "entertainment mode" presets that I haven't tried yet. (Edit: I've tried them now, if briefly, and found them nothing special.)


You can read CNet's full review of the LN52A650 here. All in all, CNet rates the LN52A650 "excellent," giving it four stars out of five.

CNet marks this generally excellent set down for the reddish tint of its bezel, which I find so hard to see as to be a non-factor. They correctly note that (as is the glossy bezel) the front of the screen is shiny, not matte-finished, allowing reflections of brightly lit objects in the room to bounce off the screen or bezel and into the viewer's eye. In my room, that doesn't happen to be much of a problem. (The shiny face of the screen is part of Samsung's strategy to give you deeper blacks and steeper contrast ratios.)

CNet also dislikes the "awkward click wheel remote"; I agree. The wheel spins under your finger and accomplishes what up-down-right-left clicks of the same wheel also accomplish: navigating on-screen menus, lowering or raising volume, etc. But it's unnecessary, clumsy, hard to get used to ... and can't be turned off.

Finally, CNet says there are "some [visible] artifacts when de-judder modes are engaged"; I haven't noticed those.


Another nice thing about the LN52A650 is its combination of a 120-Hz refresh rate with a 4-millisecond response time. Each pixel is refreshed twice as often as with the more customary 60-Hz rate for LCD TVs, and it takes a swift 4 ms for the refreshed pixel to stabilize. My Sony's response time is double that: 8 ms.

The difference in response time and refresh rate can be detected by the eye. Imagine a motionless closeup of a face in repose. Since nothing is in motion, you can see every last nuance of facial detail. Now, say the face starts to move as the camera pans. On my Sony, the details of the facial rendition soften due to "motion blur." Then when the camera stops panning, the details turn crisp again.

This can lead to eye fatigue, since the softening of detail tricks the eye into thinking it has to refocus. Then, when the detail turns crisp again, the eye says, "Whoops! Let's go back to that earlier focus." On the Samsung, the fast response time and the rapid refresh rate eliminate motion blur and eye fatigue.


Also, the colors on the Samsung just seem right — as they do on the Sony, but not on the Hitachi plasma. On the Hitachi, the reds seem orange, and a black-and-white picture can take on a green tinge. (On a color TV, B&W is really the sum of red, green, and blue signals. If the TV messes up the computation, a "colorless" picture can wind up with a tint.)

To be fair, plasmas and LCD flat panels have come a long way since the Hitachi was made, and it would be a mistake to downrate plasmas based on what is now a Stone Age model.

The Samsung produces sound through its internal speakers that my not-so-wonderful ears can make good sense of, in terms of comprehending dialogue. Neither the Sony nor the Hitachi render dialogue as well. Music on the Samsung sounds great as well.

The December 2008 issue of Consumer Reports gives the Samsung LN52A650 the highest rating of all the LCD and plasma HDTVs of various sizes that it tested (though no LED-backlit LCDs were among them).


The Samsung LN52A650 is priced in the mid-to-upper tier of 52" LCDs. You can pay a lot less for an LCD with the same screen size (or you can pay more if you want more bells and whistles).

There are, I'd say, three tiers of HDTV buyers. The top tier will spare no expense to get the absolute best, and will undoubtedly prefer an LED-backlit LCD, when one becomes available in their preferred screen size, or one of the top plasmas, which sometimes produce even inkier blacks than the LN52A650. Better still, for some buyers, are the front-projection TVs that fill huge screens in home theaters with to-die-for images.

The bottom tier of HDTV buyers want the lowest price, or close to it, on any given screen size. They are willing to sacrifice performance for economy — and who can blame them, since even the cheapest flat panel today gives a picture far superior to anything that was available just a few years ago?

Then there's the middle tier, in which I proudly place myself. I and those like me will pay extra for noticeably better performance and features ... but we don't absolutely have to have the state of the art in a TV set.

In which tier would you place yourself? If you, too, are in the middle tier, and if you want a 52" flat panel HDTV, you would do well to look into the Samsung LN52A650.