CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [320]
That’s how light really acts. Well, I guess we could take the analogy one step further by saying the person has an infinite number of arms, each holding a jump rope shooting out in every direction to show the three- dimensionality of light waves, but (a) I can’t draw that and (b) one jump rope will suffice to explain LCD panels. The varying speeds create wavelengths, from very short to very long. When light comes into your eyes at many different wavelengths, you see white light. If the light came in only one wavelength, you would see only that color. Light flowing through a polarized filter (like sunglasses) is like putting a picket fence between you and the people shaking the ropes. You see all of the wavelengths, but only the waves of similar orientation. You would still see all of the colors, just fewer of them because you only see the waves of the same orientation, making the image darker. That’s why many sunglasses use polarizing filters.
Now, what would happen if you added another picket fence, but put the slats in a horizontal direction? This would effectively cancel out all of the waves. This is what happens when two polarizing filters are combined at a 90-degree angle—no light passes through.
Now, what would happen if you added a third fence between the two fences with the slats at a 45-degree angle? Well, it would sort of “twist” some of the shakes in the rope so that the waves could then get through. The same thing is true with the polarizing filters. The third filter twists some of the light so that it gets through. If you’re really feeling scientific, go to any teacher’s supply store and pick up three polarizing filters for about (US)$3 each and try it. It works.
Liquid crystals take advantage of the property of polarization. Liquid crystals are composed of a specially formulated liquid full of long, thin crystals that always want to orient themselves in the same direction, as shown in Figure 19-11. This substance acts exactly like a liquid polarized filter. If you poured a thin film of this stuff between two sheets of glass, you’d get a darn good pair of sunglasses.
Imagine cutting extremely fine grooves on one side of one of those sheets of glass. When you place this liquid in contact with a finely grooved surface, the molecules naturally line up with the grooves in the surface (see Figure 19-12).
Figure 19-11 Waves of similar orientation
Figure 19-12 Liquid crystal molecules tend to line up together.
If you place another finely grooved surface, with the grooves at a 90-degree orientation to the other surface, opposite of the first one, the molecules in contact with that side will attempt to line up with it. The molecules in between, in trying to line up with both sides, will immediately line up in a nice twist (see Figure 19-13). If two perpendicular polarizing filters are then placed on either side of the liquid crystal, the liquid crystal will twist the light and enable it to pass (see Figure 19-14).
If you expose the liquid crystal to an electrical potential, however, the crystals will change their orientation to match the direction of the electrical field. The twist goes away and no light passes through (see Figure 19-15).
A color LCD screen is composed of a large number of tiny liquid crystal molecules (called sub-pixels) arranged in rows and columns between polarizing filters. A translucent sheet above the sub-pixels is colored red, green, or blue. Each tiny distinct group of three sub-pixels—one red, one green, and one blue—form a physical pixel, as shown in Figure 19-16.
Figure 19-13 Liquid crystal molecules twisting
Figure 19-14 No charge, enabling light to pass
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NOTE LCD pixels are very different from the pixels in a CRT. A CRT pixel’s size changes depending on the resolution. The