Further more, a recent study published by iSuppli Corp., a global leader in technology value chain research and consultancy, indicates that despite the present global economic uncertainties, worldwide shipments of LCD TVs are expected to continue to grow - though at a slower rate - reaching 112.6 million units for 2009.
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. In particular, viewing angle and display response time may be issues of concern especially with LCD panels from 2nd and 3rd tier display manufactures. And price may also be an issue especially as one moves towards LCD HDTV sets in excess of 50-inch screen size.
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 consist 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. This blocking takes place in a perpendicular manner to the passage of light 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.
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. The least expensive 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 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 in LCD Display Panels
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.
Source: http://www.practical-home-theater-guide.com/lcd-display.html
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