HP L1535, L1735, L1908wi, s9500 CRT, v930 CRT Reference Guide

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Understanding pixel

 

defects in LCD

 

monitors

 

Bob Myers

 

Displays Business Unit

 

Last revised: July 1, 2009

 

Introduction.........................................................................................................

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Executive summary .............................................................................................

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Understanding TFT-LCD technology ......................................................................

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How LCDs work ..............................................................................................

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Why TFTs? ......................................................................................................

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Understanding pixel defects.................................................................................

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How pixel defects occur...................................................................................

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How to spot a sub-pixel defect ........................................................................

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HP pixel-defect standards....................................................................................

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HP specifications .............................................................................................

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International standards (ISO-9241)..................................................................

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Defect type definitions.....................................................................................

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The HP advantage...............................................................................................

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HP quality and reliability.................................................................................

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Designed with the environment in mind ...........................................................

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HP service and support..................................................................................

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For more information ........................................................................................

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Introduction

Liquid crystal display (LCD) technology was first invented decades ago and has been improving ever since—to the point that today’s high-quality flat panel displays deliver crisp, clear visual quality at a reasonable price. Even so, some LCD monitors may harbor tiny defects due to the extreme complexity of the manufacturing process. To deal with these inevitable minor flaws, HP has developed a set of policies and detection methods to help ensure that each customer receives the highest quality product available.

Executive summary

Flat panel LCD technology is a complex subject. To help you understand how pixel and sub-pixel defects occur, and what HP does about them, this white paper explains:

What are sub-pixels and how do they work? A detailed look will show that millions of tiny sub-pixels cover the typical flat panel screen, producing the mixture of color and detail that forms the sharp, vibrant images flat panel users have come to expect.

How do pixel and sub-pixel defects occur? The HP specification does not allow for any full or complete pixel defects. It does, however, allow for some minimal sub-pixel defects. This is because the current state-of-the-art in manufacturing processes still may produce a few sub-pixel defects per screen. These defects can be extremely hard to see unless they are viewed under special conditions, or unless they happen to be clustered in groups. Nevertheless, special practices and policies have been devised to reject any complete pixel defects and minimize sub-pixel defects.

What is HP doing about it? HP has conducted a detailed study of its standards for sub-pixel defect specifications, and as a result, has adopted a more stringent unified standard for all models, which is discussed in greater detail later in this paper.

Why is this important to me? Doing business with HP gives you the advantage of dealing with a company that strives to consistently deliver a higher standard of quality to its customers. In this case, no full or complete pixel defects, and fewer sub-pixel defects than most competitors. This means better quality for the customer and ultimately greater satisfaction for the end user because the user is viewing a cleaner image without the distraction of pixel defects.

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Understanding TFT-LCD technology

Thin-film transistor (TFT) technology refers to a type of liquid crystal display (LCD), also known as an active-matrix LCD (AMLCD), used in all HP LCD monitors as well as HP iPAQ devices and HP notebook displays. To understand how pixel defects occur, it helps to understand the technology behind this type of LCD.

How LCDs work

A liquid crystal is exactly what it sounds like: a fluid substance which also exhibits some properties—such as an ordered arrangement of its molecules—similar to a solid crystal. In 1963, an RCA researcher discovered that liquid crystals can be used to control light, by switching a voltage across the material on and off. Before long, the technology was being applied to everything from calculators to computer monitors.

Because of their long, rod-like shape and electrical properties, liquid crystal molecules tend to line up in parallel rows in their natural state. When used in computer displays, this material is enclosed between two pieces of glass (in a flat panel display) and an image is generated by controlling the alignment of the LC molecules electrically, causing them to effectively act as “light valves”—letting light either pass through the panel or be blocked, depending on the voltage applied across the material. Color filters are placed on one side of the glass panels, with three different colors (red, green, and blue) for each pixel. Combining red, green, and blue light in different amounts, controlled by the voltage applied across the LC material at each of these individual areas (called subpixels) lets each pixel appear as any color and any brightness, and the combination of all the pixels on the panel creates the complete image.

Fluorescent lights (very similar to the fluorescent tubes used in standard office lighting, but much smaller) or light-emitting diodes (LEDs) provide the “backlighting” for the LCD display—the light that will pass through the panel from the rear, and so create the image. The light from the backlight unit passes through a translucent plastic diffuser layer, which spreads the light evenly across the screen.

Why TFTs?

In the simplest LCD technology, the voltage applied across the LC material is delivered by transparent row and column electrodes, lines of conductive material which cross at 90 degrees (one set on the “top” glass, and the other on the bottom). The intersection of rows and columns define the pixels and subpixels of the display, and applying a voltage to a given row and column switches the pixel at that particular intersection. If the panel is driven such that the pixels are switched, in order across the display, very rapidly, the appearance of a complete image is produced. Unfortunately, when the drive voltage is removed from a given pixel and we move on to the next, that first pixel starts to switch back to its “off” state. This limits the contrast, resolution, and the response time that can be achieved with such a simple, passive-matrix drive system.

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HP L1535, L1735, L1908wi, s9500 CRT, v930 CRT Reference Guide

The advent of thin-film transistor (TFT) technology allowed transistors to be placed at each picture element or sub-pixel. These can switch very quickly, and then hold the state of the sub-pixel while the panel drivers take care of the other rows and columns of the display. This results in a great improvement in the contrast and response time possible with LCD technology, and permits the manufacture of largesize, high-resolution displays which rival any other display technology in performance.

Figure 1. How thin-film transistors are placed in the LCD array.

Electrodes across

LC material

Understanding pixel defects

Active-matrix TFT-LCDs require at least one transistor to be created at each subpixel on the panel. This makes the average AMLCD an enormously complex device. For example, producing one of today’s high-resolution WUXGA displays. with a 1920 x 1200 pixel native format requires embedding nearly seven million transistors in the screen (1920 x 1200 x 3 = 6.91 million). This is more than double the number of transistors found in the original Intel® Pentium® processor.

How pixel defects occur

Damage to any one of the millions of transistors within the LCD panel may leave a sub-pixel permanently on or off, creating a tiny dark spot or bright spot on the display. This is fairly common, even for small TFT displays on handheld computers.

Minute specks of dust on the panel, slight errors in the panel processing, and other problems encountered during manufacturing of the TFT array on the glass substrate cause these defects. When we look at the total number of pixels and subpixels on a 1920 x 1200 display, we see that the failure of one sub-pixel out of the 6.91 million is a very low failure rate indeed—only about 14 millionths of one percent (0.000014%). For lower-resolution SXGA displays, a single sub-pixel defect still represents a failure rate of only 25 millionths of one percent.

To look at it another way, having 10 sub-pixel defects on a 1280 x 1024 color panel means that the panel is still 99.9999% defect free!

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