Video Hardware, Part 1 - How LCDs Work
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In an LCD, a polarizing filter creates two separate light waves. The polarizing filter allows light waves that are aligned only with the filter to pass through. After passing through the polarizing filter, the remaining light waves are all aligned in the same direction. By aligning a second polarizing filter at a right angle to the first, all those waves are blocked. By changing the angle of the second polarizing filter, the amount of light allowed to pass can be changed. It is the role of the liquid crystal cell to change the angle of polarization and control the amount of light that passes. The liquid crystals are rod-shaped molecules that flow like a liquid. They enable light to pass straight through, but an electrical charge alters their orientations and the orientation of light passing through them. Although monochrome LCDs do not have color filters, they can have multiple cells per pixel for controlling shades of gray.
In a color LCD, an additional filter has three cells for each pixel—one each for displaying red, green, and blue—with a corresponding transistor for each cell. The red, green, and blue cells, which make up a pixel, are sometimes referred to as subpixels. The ability to control each cell individually has enabled Microsoft to develop a new method of improving LCD text quality. Beginning with Windows XP, you can enable a feature called ClearType through the Display properties sheet. However, individual cells can also fail.
Dead Pixels
A so-called dead pixel is one in which the red, green, or blue cell is stuck on or off. Failures in the on state are more common. In particular, those that fail when on are very noticeable on a dark background, such as bright red, green, or blue dots. Although even a few of these can be distracting, manufacturers vary in their warranty policies regarding how many dead pixels are required before you can get a replacement display. Some vendors look at both the total number of dead pixels and their locations. Fortunately, improvements in manufacturing quality make it less and less likely that you will see a screen with dead pixels either on your desktop or in your notebook computer display.
Although there is no normal way to repair bad pixels, there might be a simple fix that can help. I have actually repaired bad pixels by gently tapping on the screen at the pixel location. This seems to work in many cases, especially in cases in which the pixel is always illuminated instead of dead (dark). Because I find a constantly lit pixel to be more irritating than one that is constantly dark, this fix has saved me a lot of aggravation.
Active-Matrix Displays
Most active-matrix displays use a thin film transistor (TFT) array. TFT is a method for packaging from one (monochrome) to three (RGB color) transistors per pixel within a flexible material that is the same size and shape as the display. Therefore, the transistors for each pixel lie directly behind the liquid crystal cells they control.
Two TFT manufacturing processes account for most of the active-matrix displays on the market today: hydrogenated amorphous silicon (a-Si) and low-temperature polysilicon (p-Si). These processes differ primarily in their costs. At first, most TFT displays were manufactured using the a-Si process because it required lower temperatures (less than 400°C) than the p-Si process of the time. Now, lower-temperature p-Si manufacturing processes are making this method an economically viable alternative to a-Si.
To improve horizontal viewing angles in the latest LCDs, some vendors have modified the classic TFT design. For example, Hitachi's in-plane switching (IPS) design—also known as STFT—aligns the individual cells of the LCD parallel to the glass, running the electric current through the sides of the cells and spinning the pixels to provide more even distribution of the image to the entire panel area. Hitachi's Super-IPS technology also rearranges the liquid crystal molecules into a zig-zag pattern, rather than the typical row-column arrangement, to reduce color shift and improve color uniformity. The similar multidomain vertical alignment (MVA) technology developed by Fujitsu divides the screen into different regions and changes the angle of the regions.
Both Super-IPS and MVA provide a wider viewing angle than traditional TFT displays. Other companies have different names for the same technology—for example, Sharp calls it Ultra High Aperture (UHA). Manufacturers often like to think up their own buzzwords for the same technology because it makes their products seem different. Because larger LCDs (17'' and wider) are large enough to cause shifts in viewing angle even for an individual user, these advanced technologies are being used primarily on larger and more expensive panels and have been licensed to other display vendors.
 | This chapter is from Upgrading and Repairing PCs, 16th edition,by Scott Mueller. (Que Books, 2004, ISBN: 0789731738). Check it out at your favorite bookstore today. Buy this book now! |
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