Specifications
Video Display Technologies
873
How LCDs Work
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 fil-
ter, 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 electri-
cal 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 back-
ground, such as bright red, green, or blue dots. Although even a few of these can be distracting, man-
ufacturers 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 loca-
tions. 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 liq-
uid 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 indi-
vidual 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.
Chapter 15
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