What Is Dirty Screen Effect And How To Solve It?
As a result of the current sharp drop in prices in LCDs (3), LEDs, (1) (2) we see that TVs and monitors are nowadays manufactured with visible issues (9) such as clouding, banding and dirty screen effect. These defects, specifically the dirty screen effect, are apparently viewed by manufacturers as within the product specifications framework (8) and patent of the product (14) because they define wide tolerance specification ranges (4) (5) in many different industries (12). Manufacturers consider that better product specifications, such as an improved LCD technology (10) (13), and the elimination of the dirty screen effect, improving CCFL technology (11) (15) or even replacing CCFL for better technologies (16) would turn devices more expensive and less competitive (6) (7).
The dirty screen effect is a defect in the depiction of the image in plasma and LED/LCD displays that pertains an absence of consistency regarding the projection of full-tone colors. In the dirty screen effect, the monitor predominantly outputs a bright white display, and the screen might appear to be virtually dirty, featuring patchy gray portions .
An overwhelming percentage of audiences will be unaware of this absence of homogeneity, that is nevertheless very evident under some rare occasions: in scenes featuring bright coloring or whiteness (such as scenes with snow, football, or the ocean), and instances during which the monitor is supposed to appear entirely black or gray and instead depict irregular, blotchy shapes accompanied by sometimes streaks or banding.
This dirty screen effect is related to bright pixels. This means that where dark areas used to be problematic and showed slightly lighter clouds, today it is light areas that show slightly darker clouds. So there, the light cannot pass through filters + LCD + filters completely unhindered.
The dirty screen defect is not as annoying as clouding, because there are only very few evenly bright areas in films and TV programs, and even then, this is only a really annoying problem when there is movement in the image, e.g. camera pans.
Since the dark clouds are formed by the panel, i.e. always stay in the same place on the picture, they do not pan with the picture. And then it just looks as if you have dust or some other veil of dirt on the screen. Hence, the name “Dirty Screen Effect”.
This means that many of these unsightly effects have one and the same cause: an imperfect interaction of the filter foils and the LCD layer.
Dirty Screen Effect Fix
You can reduce the effect by playing with the backlight and contrast but definitely increasing the brightness. Conversely, you reduce clouding when you reduce the lighting. Sounds easy, but that’s how it is. So how can you fix the dirty screen effect?
Usually, you can fix the dirty screen effect by loosening a bit the screws on the back side of the monitor because this relieves the back tension of the components which may be the cause of this defect.
So yes, you can often reduce DSE with the same effect of wiping or loosening the panel screws. If you have a stain due to a design error, e.g. a hot spot (heat stain) due to a hot component behind the panel, that is of course something else.
What Causes Dirty Screen Effect ?
The cause of the dirty screen effect (DSE) is the lack of uniformity in illumination as there are the gaps that are not reached by the CCFL (cold cathode fluorescent lamps)
Then they appear as dark lines or with direct LED as many dark spots and swim as “dirt” with camera pans because static. Only, that there should not be such spots, since the light is distributed very evenly via diffusers.
If you still have an old LCD with CCFL backlight, play a monochrome white image there, or just upload a blank image, and the dirty screen effect appears.
It is sometimes not so noticeable because it is more even because of the length of the CCFL tubes that extend over to the end of the screen. With LEDs, the lighting is more selective, which has also coined the term “spotting”, rarely used.
Cause In Light And Polarization Filters
So how has LCD technology influenced in causing this dirty screen effect? The LCDs should always deliver a black level that is as dark as possible. Although there is always a backlight that is often much brighter today than it used to be. Just think about how dark was the Pioneer Kuro long ago.
That means the LCDs have to cover ever brighter backlight more and more perfectly and thus get dark. A wide variety of LCD technologies are used to achieve this: TN, PVA, S-PVA, MVA, IPS, etc. etc. In principle, an LCD always consists of a polarization filter behind the LCD layer, then the LCD layer itself and another polarization filter in front of it.
The LCD layer rotates the light more or less in the direction of polarization, so that the second polarization filter either lets the light through more (= light pixel) or less (= dark pixel).
The various LCD technologies tell us how the whole thing is actually arranged. For example, you can align the polarization filters in front of and immediately afterwards, then the LCD must not turn the light if the pixel should be bright and must turn the light if the pixel should be dark.
Or you can, instead, arrange the polarization filters rotated to each other, then the light from the liquid crystal layer must be rotated if it should be bright or not rotated if it should be dark. It also plays a role how the thin film transistor (TFT) is constructed and installed.
The decisive factor is that all of these things have to work together perfectly in order to achieve the perfect black level: the filter films both in front of and behind the LCD have to be perfectly aligned, both in the direction and in relation to one another. And the LCD has to work perfectly. Which is not easy either.
The LCD layer consists of two glass plates with a space in between, in which the liquid with the crystals is introduced. If different pixels differ here, for example in the pressure on the liquid or in the distance between the glass plates, the crystals align themselves slightly differently, i.e. the light is rotated slightly differently.
Cause Related To Heat Effects
The LCD crystals also orient themselves differently depending on the temperature. So if there is a particularly warm spot because there is a hot component behind the panel, this can also affect the brightness somewhat.
That is why the manufacturer should make sure that you do not see a lighter or darker area because he is placing a very hot chip somewhere behind the panel. That would be a serial error, so to speak.
But the LEDs of the backlight themselves are often significantly warmer than the rest of the area. If these are attached to the panel on the left and right, for example, the picture may also be a bit darker or lighter, especially on the left and right edges, simply due to the warmth of the LEDs. This is often referred to as the “dirty edge”.
All of these considerations were previously based on an optimal, even backlight. Now, however, the backlight is usually generated by LEDs. It is possible that the LEDs do not all shine with the same brightness. For this purpose, the light from the LEDs is distributed as evenly as possible behind the image surface via light guides and diffusers.
These light guides may also not be perfectly aligned. Especially if the LEDs are only located on the edge (edge LED), the way to the center of the image is quite long and the chance for errors due to manufacturing tolerances in the quality and alignment of the light guides increases. This means that in addition to the effects of the light processing in the actual LCD part, there is also the possibility that the backlight itself reveals slight irregularities.
Incorrect Handling During Storage And Transportation
It is a very minor cause, but I think that a part of damage to the image is caused by the handling of the package by the suppliers, warehouses, dealers and also by the end customer himself.
Complete monitoring of the device after it has left production, i.e. during transport to the dealer and to the end customer (e.g. lying in the car, or cramped, falling or thrown boxes at the shipping company), handling at the dealer (e.g. stacked boxes in the warehouse, driving against them with a forklift) and last but not least also at the end customer (handing when setting up during the suspension on the wall, etc.) is practically impossible.
Is Dirty Screen Effect Normal ?
The dirty screen effect is not normal and it is a defect. However, manufacturers would inform to you that this effect is within the product specifications and will regard it as normal.
Dirty Screen Effect In New Monitors
With new TVs, the evidence is that this still occurs and really it is a bit difficult to explain, except that there are certain irregularities in the control of the pixels, so that these pixels simply shine unevenly brightly on their own.
It is difficult to manufacture batches of LEDs of the same brightness, even with LCDs with LED backlight. Therefore, it is also possible with these self-illuminating devices that the pixels do not shine 100% equally bright by default, simple due to tolerances in manufacturing, e.g. in the row and column drivers or the transistors in the pixels. Manufacturers have a wide ranges of tolerances in their specifications and consider within their framework, what a specialized user would see as a defect.
With older devices, however, this can also be a matter of so-called burn-in effects. With plasma and OLED (generally with all self-luminous objects), the pixels age with the amount of light that they have to emit. A pixel that had to emit a lot of light is then no longer quite as bright as a pixel that previously had to emit less light.
For example, one could imagine that the pixels that are in the black 4: 3 bars on the left and right or in the CinemaScope bars above and below remain a little brighter because they only had to show the black bar for many hours, i.e. in the In contrast to the pixels in the center of the picture, they did not have to emit any light during this time.
In principle, this also applies to other image content. So if, for example, someone had displayed a computer desktop very often over the years,
In the extreme case, such an effect would probably really be visible, for example if a pure white picture is displayed. Then you would see slight differences in brightness, you could see a silhouette of the frequently shown image, so to speak. It is therefore said that the image has burned itself into the screen.
However, since such monochrome surfaces are rarely found in practice except in test images, such burners will probably first draw attention to themselves as a DSE when panning cameras or similar movements.
Dirty Screen Effect Test
To create a dirty screen test effect, you can use any image editor (e.g. Windows Paint, or GIMP or Photoshop), and create a new image with a suitable size, e.g. 1920×1080 pixels. Then you usually already have a white picture. Just make one yourself and do not buy any test.
Most TVs can now display such images directly for testing (e.g. from a USB stick).
You can also use your search engine and search for a white monochrome image.
Does Dirty Screen Effect Go Away ?
The dirty screen effect does not go away because it has relation with gaps in the cold cathode fluorescent lamps’ depiction patterns, and other related causes, so it is not a defect that is eliminated after some time utilizing the affected device.
All these dirty effects with unequal illumination come from the fact that you always want to be better. Where previously an LCD still had a static contrast of 500: 1, none of that was an issue. But there was also the black level: then the black level got better (2000: 1, 3000: 1), but the clouding effects got more drastic.
Then we worked against clouding, but the dirt effects increased in light images. In addition, the screen diagonals are getting bigger and thinner at the same time, so that it is getting more and more difficult to get the backlight evenly.
If you consider how complicated it all has become, what we are offered today is perhaps actually not that bad. On the other hand, effects like DSE interfere. Especially football, where there are a lot of camera pans over a halfway uniform light green area, reveals the DSE quite clearly. And that bothers a lot of people.
However, these effects are apparently viewed by manufacturers as tolerable and therefore not much is being done about them at the moment.
In other words, with the current sharp drop in the price of LCDs, one apparently has to live with these negative phenomena from the manufacturer’s point of view, because more precise production, better measurement and even better LCD technology would be significantly more expensive.
As end customers, we just have to consider whether we can put up with it. If we accept these deficiencies in the image as systemic, then not much will change.
But if we throw these TVs and monitors with picture problems at the feet of the retailers by the dozen and insist on replacing them, then at some point it will be so expensive for the manufacturers to handle all the negative reviews, customer returns and product recalls, that they might think about how to master this DSE, dirty screen effect problem, and reformulate their bill of materials, production plans and product specifications.
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I am Chris, a musician. I work as an audio engineer in mastering and arranging bridges in existing songs and the arrangement and orchestration of chorus. In Firing Squad I test gear provided by local distributors during a couple of days and write a review. I also write about AV topics, amplifiers, speakers and headphones.