LCD screen quality control

There in ISO quality standard for measuring defects in LCD display panels so that manufacturers, repairers and customers have a clear definition of what a good LCD panel is compared to a faulty LCD panel.

ISO 13406-2 is an ISO standard, based on flat panel. It is best known to end consumers for defining a series of flat-panel display "classes" with different numbers of permitted defects.

As part of an ISO standard, the classes are guidelines, and not mandatory. Where implemented, the interpretation of the standard by the panel or end product manufacturer and effects in terms of labeling of products, what class of panel is used, etc. can vary. Most flat-panel makers use this standard as the excuse (it is not clear the legal base for it) for not accepting flat-panels returned that have defects. Many customers argue that it's not very honest in the makers' part to sell a product that most people wouldn't accept if they knew it had these defects.

During use it can be hard to spot these defects but you can test for them by testing each pixel state ie;

Full black screen	Full white screen	Full red screen	Full green screen
Full blue screen	Full magenta	Full cyan	Full yellow

You are looking for a tiny dot which is not behaving as expected; a small black dot in your red screen for example signifies that the TFT (thin Film Transistor) for that dot (pixel) in the red chanel is a dead pixel. A red dot in the black screen signifies that pixel is a stuck pixel.

Windows uses a lot of white back grounds so it is more common for users to notice a defect during something like a Microsoft Word document which has a white "page". As white is a mix of all 3 channels (red, green & blue) if one is faulty you won't get white.

In practice Class I panels are limited and highly priced, used in price premium products and markets. It's difficult to buy a "standard" laptop or screen product with a guaranteed panel condition. If this is claimed by the manufacturer there is usually a price premium. In general, the higher the class of defects, the cheaper the display panel. Manufacturing techniques for LCD screens are constantly improving so it is expected that quality panels are/will improve in quality as time goes by but whether manufacturers will be willing to accept returns based on a these pixel defects alone will probably be de LCD pixel array cided by market forces.

Pixel Fault Classes

The standard lists four pixel fault classes. Three types of defective pixels are described:

type 1 = a hot pixel (always on, being colour white)
type 2 = a dead pixel (always off, meaning black)
type 3 = a stuck pixel (one or more sub-pixels (red, blue or green) are always on or always off)

The table below shows the maximum number of allowed defects (per type) per 1 million pixels.

Class	Defect type 1	Defect type 2	Defect type 3	Cluster with more than one type 1 or 2	Cluster of type 3
I	0	0	0	0	0
II	2	2	5	0	2
III	5	15	50	0	5
IV	50	150	500	5	50

Pixel fault classes - maximum number of faults per million pixels. A good laptop screen resolution is 1024x768,

thats 2.35 million pixels.

As of 2007, most manufacturers specify their products as Pixel Fault Class II.

Hot pixel

A permanently lit white pixel is called a glowing pixel. Hot pixels are usually best seen against a dark background.

Dead pixel

A dead pixel is a defective pixel that remains unlit. Dead pixels are usually best seen against a white background.

Stuck pixel

A stuck pixel will usually be most visible against a black background, where it will appear red, green or blue, sometimes cyan, magenta or yellow. Aan LCD monitor is composed of three subpixels (one red, one green, and one blue) which produce the visible color of the pixel by their relative brightness. A stuck pixel results from a manufacturing defect, which leaves one or more of these sub-pixels permanently turned on or off.

Stuck versus dead pixels

Stuck pixels are often incorrectly referred to as dead pixels, which have a similar appearance. In a dead pixel, all three sub-pixels are permanently off, producing a permanently black pixel. Dead pixels can result from similar manufacturing anomalies as stuck pixels, but may also occur from a non-functioning TFT resulting in complete lack of power to the pixel. Dead pixels are much less likely to correct themselves over time.

Stuck pixels, unlike dead pixels, have been reported by LCD screen owners to disappear, and there are several popular methods purported to fix them, such as gently rubbing the screen (in an attempt to reset the pixel), cycling the color value of the stuck pixel rapidly (in other words, flashing bright colors on the screen,) or simply tolerating the stuck pixel until it disappears (which can take anywhere from a day to years). While these methods can work on some stuck pixels others cannot be fixed by the above methods. Also some stuck pixels will reappear after being fixed if the screen is left off for several hours. We do not adopt any of these "repair" approaches as their reliability and success rate are un-acceptable.

Manufacturer policy

In LCD manufacture, it is common for a display to be manufactured that has a number of sub-pixel defects (each pixel is composed of three primary-colored sub-pixels). The number of faulty pixels tolerated before a screen is rejected is dependent on the class that the manufacturer has given the display (although officially described by the ISO standard, not all manufacturers interpret this the same way, or follow it at all).

Some manufacturers have a zero-tolerance policy with regard to LCD screens, rejecting all units found to have any number of (sub-)pixel defects. Displays meeting this standard are deemed Class I. Other manufacturers reject displays according to the number of total defects, the number of defects in a given group (e.g. 1 dead pixel or 3 stuck sub-pixels in a 5x5 pixel area), or other criteria.

In some cases, the manufacturer sends all screens to sale, and then replaces the screen if the customer reports the unit as faulty and the defective pixels meet their minimum requirements for return. Some screens come with a leaflet stating how many dead pixels they are allowed to have before you can send them back to the manufacturer. Dead pixels may tend to occur in clusters; in most cases displays with such a problem can be sent back to the manufacturer.

Some of this information came from Wikipedia