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

Understanding pixel
defects in LCD
monitors

Bob Myers
Displays Business Unit
Last revised: July 1, 2009

Introduction......................................................................................................... 2

Executive summary ............................................................................................. 2

Understanding TFT-LCD technology ...................................................................... 3

How LCDs work .............................................................................................. 3

Why TFTs? ...................................................................................................... 3

Understanding pixel defects ................................................................................. 4

How pixel defects occur................................................................................... 4

How to spot a sub-pixel defect ........................................................................ 5

HP pixel-defect standards .................................................................................... 5

HP specifications ............................................................................................. 5

International standards (ISO-9241) .................................................................. 6

Defect type definitions ..................................................................................... 8

The HP advantage ............................................................................................... 9

HP quality and reliability ................................................................................. 9

Designed with the environment in mind ........................................................... 9

HP service and support .................................................................................. 10

For more information ........................................................................................ 11

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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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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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