Monday, November 10, 2008

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 ...


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michal pfeil said...

Just reading a past post from your blog and with all this technical knowhow that you have I am wondering why you are not using Plasmas?. They have far superior color and blacks than any LCD.

eric said...

Michal said:

Just reading a past post from your blog and with all this technical knowhow that you have I am wondering why you are not using Plasmas?. They have far superior color and blacks than any LCD.

I do have a Hitachi plasma flat panel, about 5 yrs. old. The technology has improved a lot, but this model left much to be desired, as I mentioned in some of the earliest posts in the blog:

(1) Its reds are biased towards orange, and no amount of tweaking with the various advanced color adjustments seem to fix that.

(2) It tends to give a greenish cast to black & white images.

(3) It can't get close to a true black, only a dark charcoal color where black ought to be.

Newer plasmas do have a much-improved black level and truer color renditions.

But the same is true of LCDs. I don't have one, but an LCD panel that has LED backlighting can give really deep blacks, I hear. The colors on the two LCD flat-panel HDTVs I do have -- a Sony and a Samsung -- are spot on. LCDs' former problem with image lag seems a thing of the past, especially now that the pixel refresh rates are up to 240 Hz or more in some models.

My main gripe with the two LCD panels I have is uneven backlighting that shows up as impure blacks or grays. I imagine LED-backlit models might conquer that issue.

Anonymous said...

I have found on my LEDTV, which I have had exchanged to a newer model, only displays this phenomena on low res. I play low res off a hard drive and it looks terrible with a large uneven dull line about 20cm from left to right. however, when playing high resolution Blueray or movie files 720i+p plus it looks superb.

I'll give it a few weeks and see what happens

eric said...

Anonymous said...

I have found on my LEDTV, which I have had exchanged to a newer model, only displays this phenomena on low res. I play low res off a hard drive and it looks terrible with a large uneven dull line about 20cm from left to right. however, when playing high resolution Blueray or movie files 720i+p plus it looks superb.

I'll give it a few weeks and see what happens


Your "large uneven dull line about 20cm from left to right" sounds similar to my complaint.

However, even though I don't have any experience with LED models, I would have thought them immune to uneven backlighting!

As I understand it, the backlight on LED TVs is controlled on a pixel-by-pixel basis. That's called "local dimming." But some models of LED TV apparently don't use local dimming. "Full array" LED TVs don't have different brightness levels, pixel-by-pixel. "White edge" LEDs produce light only at the edges of the screen and then use a special diffusion panel to spread the light (supposedly) evenly behind the screen.

I wish you would post another comment saying what specific model TV you have ...

Anonymous said...

I just brought home a Sansui 46" led tv with a very pronounced uneven back-lighting problem. The right half of the screen is MUCH lighter than the left side. On a black screen it it almost black and white in difference from side to side. I will probably return it tomorrow, it's just too big a problem.