How-it-Works: LCD Display Flat-Panel Technology

Liquid Crystal Displays

What is LCD display technology, and how do LCD panels work? Discover more in this short easy-to-follow illustrate description of the basic operational principles behind flat-panel LCD technology.

Sony KDL40S2000 40" Bravia Flat Panel LCD HDTV

Introduction to Liquid Crystal Displays

LCDs - short for Liquid Crystal Display devices - have become an important part of our everyday life. Their use range from wristwatches, calculators, and mobile phones, to more demanding high-resolution applications in test instrumentation, computer monitors and high definition LCD Televisions.

The use of LCD panels is growing at an incredible rate. Suffice to say that in the world of HDTV, LCD is already the dominant non-CRT HD display technology; sales of LCD HDTV sets are expected to exceed 4million units by end 2006 in the US alone.

LCD panels are common because they offer some real advantages over other display technologies.

These flat-panel displays are slimmer - LCD Televisions hardly exceeds 3.5 inches in dept, and lighter than an equivalent screen size cathode ray tube (CRT) TV or plasma television.

Further more, they draw much less power than alternative flat-panel displays; typically, LCD panels use only about 60% of the power requirements normally associated with plasma displays.

In addition, LCD display panels do not emit harmful electromagnetic waves. To top-up this whole pro list, LCD panels have a half lifetime of around 60,000hrs - meaning that the image brightness of an LCD video panel will fall to half its original value after approximately 60,000 hours of use!

Mind you, liquid crystal displays do not represent the perfect display technology; LCD displays have their drawbacks as well. Restricted viewing angle, limited contrast ratio, and display response time may be issues of concern especially with LCD panels from 2^nd and 3^rd tier display manufactures.

Price is also an issue. LCD TV sets are substantially more expensive than their CRT or plasma counterpart, even though prices have fallen considerably, and are expected to continue to fall by a further 40 per cent within the next three years.

In this article, we look at the underlying technology that makes LCD display panels work in order to represent numbers, words, and high-resolution images.

We explain the complexities involved in the physical setup of an LCD display and how this impacts the production of LCD displays. In the process, we discuss the issue of 'bad' pixels. We also explain why manufacturers never guarantee that LCD panels are 100% free of bad, stuck or dead pixels.

LCD Display Technology: Basic Operational Principles:

LCD displays consists primarily of two sheets of polarized glass plates with some liquid crystal solution trapped between them. The type of liquid crystals used in LCD panels have got very specific properties that enable them to serve as effective 'shutters' that close or open to block or otherwise, the passage of light - in a predicable manner - depending on the application of an electric current.

This current through the liquid crystals is controlled by a voltage applied between the glass plates through the use of transparent electrodes that form a grid - with rows on one side of the panel and columns on the other - representing the picture elements or pixels.

But what are Liquid Crystals?

Though the three most common states of matter are solid, liquid, and gaseous, yet some substances can exist in a totally odd state that is a sort of liquid and a sort of solid at the same time.

Equally odd is the behavior of their molecules when substances are in this state, since these tend to maintain their orientation, like the molecules in a solid, but at the same time, they also move around to different positions, like the molecules in a liquid.

This means that liquid crystals are neither a solid nor a liquid, even though from a behavior perspective, these are closer to a liquid state than a solid.

Use of Liquid Crystals in LCD Display Panels

There is a variety of liquid crystals - each with different properties. The liquid crystals used in LCD panels are referred to as Nematic Phase liquid crystals. These have their molecules arranged in a definite pattern.

Basic Operational Principle of an LCD Display Panel (Click on image to enlarge)

One type of nematic liquid crystal, called twisted nematic (TN), has its molecular structure naturally twisted.

The orientation of the molecules in the nematic phase is based on the 'director'; this can be anything from a magnetic field - say resulting from the application of an electric current due to an applied voltage across the glass plates holding the liquid crystal solution, to a surface that has microscopic grooves in it.

In the later, the molecules at the various layers of the liquid crystal will gradually align themselves till the molecules at the layer adjacent to the surface will be exactly in line with the direction of the microscopic grooves on the surface.

Microscopic grooves in LCD display panels are applied on the surface of the glass plate that does not have the polarizing film on it, to help align the molecular structure of the liquid crystals as these approach the glass surface in line with the polarization filters on either side of the LCD panel. 

Now, the polarization filters on either side of an LCD display are set at 90 degrees to each other (ref. to above diagram). This means that the crystal lineup will go through a 90 degrees twist from one panel surface to the other.  When a light shines on the glass surface of the first polarization filter, the molecules in each layer of the liquid crystal solution will guide the light they receive to the next layer. In the process, they will also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through.

When an electric current is passed through these liquid crystals, they will untwist to varying degrees, depending on the current's voltage. This untwisting effect will change the polarization of the light passing through the LCD panel. As the polarization changes in response to the applied voltage across the glass plates, more or less light is able to pass through the polarized filter on the face of the LCD display. 

Backlit versus Reflective

Unlike CRT or plasma displays, LCD displays require an external light source to display the picture. Most inexpensive LCD displays make use of a reflective process to reflect ambient light over to display the information. However, computer and LCD TV displays are lit with an external light source, which typically takes the form of built-in micro fluorescent tubes - often a few millimeters in diameter - above, besides, and sometimes behind the LCD. A white diffusion panel is used behind the LCD to redirect and scatter the light evenly to ensure a uniform display.

LCD Display Systems - Passive vs Active Matrix Displays

There are two main types of LCD displays - passive matrix and active matrix.

Passive Matrix: These are the type of LCD display panels that rely on the display persistence to maintain the state of each display element (pixel) between refresh scans. To a certain extent, the resolution of such displays is limited by the ratio between the time to set a pixel and the time it takes to fade.

To operate, passive-matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. The grid is made up of conductive transparent material - usually indium-tin oxide - over two glass layers (called substrates) housing the liquid crystal solution, with one substrate taking the columns, and the other the rows. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The point of intersection of the row and column represents the designated pixel on the LCD panel to which a voltage is applied to untwist the liquid crystals at that pixel to control the passage of light.

A display can have more than one pixel 'on' at any one time because of the response time of the liquid crystal material. Pixels have a short turn-on time during which the liquid crystal molecules will untwist to control the passage of light. Once the voltage between the respective electrodes addressing a pixel is removed, the pixel behaves similar to a discharging capacitor, slowly turning off as charge dissipates and the molecules return to their twisted orientation.

Because of this response time, a display can scan across the matrix of pixels, turning on the appropriate ones to form an image. As long as the time to scan the entire matrix is shorter than the turn-off time, a multiple pixel image can be displayed.

Passive matrix LCD displays are simple to manufacture, and therefore cheap, but they have a slow response time - in the order of a few hundred milliseconds - and a relatively imprecise voltage control. These characteristics render images that are somewhat fuzzy and lacking in contrast. Passive matrix LCD displays are therefore unsuitable for most of today's high speed, high resolution video applications.
 

Active Matrix: Active-matrix LCD display panels depend on thin film transistors (TFT) to maintain the state of each pixel between scans while improving response times.

TFTs are micro-switching transistors (and associated capacitors) that are arranged in a matrix on a glass substrate to control each picture element (or pixel). Switching on one of the TFTs will activate the associated pixel.

The use of an active switching device embedded onto the display panel itself to control each picture element helps reduce cross-talk between adjacent pixels while drastically improving the display response.

By carefully adjusting the amount of voltage applied in very small increments, it is possible to create a gray-scale effect. While most of today's LCD displays support 256 levels of brightness per pixel, yet some high-end LCD panels used in HDTV LCD televisions support up to 1024 different levels of brightness. This results in improved gray scale performance and therefore improved picture detail in those areas of the image that are primarily all dark or all bright.

Color

For an LCD display to show color, each individual pixel is divided into three sub-pixels with red, green and blue (RGB) color filters to create a color pixel. This is somewhat similar to the way CRT and Plasma use different phosphors to glow red, green, or blue to create color.

With a combination of red, blue and green sub-pixels of various intensities, a pixel can be made to appear any number of different colors. The number of colors that can be made by mixing red, green and blue sub pixels depends on the number of distinct gray scales (intensities) that can be achieved by the display.

If each red, green and blue sub-pixel can display 256 different intensities of their respective color, then each pixel can produce a possible palette of 16.8 million (256x256x256) colors.

Manufacturing Process Challenges and 'Bad' Pixels

The issue of 'bad' pixels is surely a hot topic with LCD Display panels. Why?

Color TFT LCD TV displays require as many controlling transistors as the number of sub-color pixels forming the display.

This means that the manufacturing process associated with the production of color LCD display panels involves also the production of an enormous number of thin film transistors etched onto the glass substrate to control each and every sub-pixel. Simple mathematics shows that a typical wide screen panel with a screen resolution of 1366 x 768 pixels would require over 3.1 million transistors!

Any faulty transistor during the manufacturing process cannot be replaced - leading to what are know as 'bad pixels' - mainly visible only during static displays. A bad pixel can show up as a black spot if it remains always off, as a white spot of light if it is permanently always on, or as a colored spot of light if it is a damaged sub-pixel.

For these reasons, 'bad' pixels are at times also referred to as 'dead' or 'stuck' pixels.

If the number of 'bad' pixels is above normal, the whole LCD display panel will have to be rejected. In the process, some 30 to 40 per cent of all manufactured LCD TV panels have to be rejected because of bad pixels. This relatively low yield is the primary reason behind the high cost of LCD panels, as the price for 'good' panels will have to make  up for the manufacturing costs of all rejected screens.

The large number of TFT's on LCD display panels also means that even brand new LCD panels may contain a few bad pixels. It is unfortunate here that most large manufactures of electronic gear do not have clear policies when it comes to replacing LCD display panels with bad pixels - rather, they consider the presence of a few 'bad' pixels not as a sign of some malfunction, but rather, as something inherent within the production process itself.  It is as if you are buying a brand new car, but then it is OK to have a few dents on its sparkling paintwork!

Luckily, some manufactures are realizing that what they may consider as an inherent aspect of the LCD display panel manufacturing process, may eventually turn out to be of great concern to end customers. For this reason, we are starting to experience a shift by top flat-panel display manufacturers, towards a 'zero bad pixel' policy; Samsung and Viewsonic are among the first to have moved in this direction.

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