Learn About... LCDs!

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Have you ever wondered what lights up the numbers on your watch? Or pondered how the pictures form on your computer as you watch movies or play games? These are just a few everyday examples when you might be using items containing LCDs (liquid crystal displays). LCDs are common because they offer advantages over other display technologies previously used. They are thinner and lighter and draw much less power than cathode ray tubes (CRTs), which were used in the first television sets manufactured by Telefunken in Germany in 1934.

But just what are these things called liquid crystals? The name "liquid crystal" sounds like a contradiction. We think of a crystal as a solid material like a diamond, usually hard as rock, and a liquid as a free-flowing material such as water. How could there be materials that exist in a state in between such different forms?

Most of us have learned that there are three common states of matter: solids, liquids and gases. Solids act the way they do because the molecules they consist of remain in the same orientation and position with respect to one another. The molecules in liquids do the opposite: their orientation and position are always changing. As the name might suggest, liquid crystals (LCs) are substances that can exist in an odd state that is sort of like a liquid and sort of like a solid. In a liquid crystal, molecules tend to maintain their orientation, like the molecules in a solid, but also change positions freely, like the molecules in a liquid. This means that liquid crystals are neither a solid nor a liquid, but rather a state in between the two.

LCs are closer to liquids than solids as far as characteristics are concerned. It generally takes more heating to turn a solid into a liquid crystal than it does to turn that liquid crystal into a liquid.

Nematic Phase Liquid Crystals

Can you think of more than one solid substance? How about a liquid? You can probably name many of each, and liquid crystals are no different—there are many types of LCs. Liquid crystals can be in different phases based on the molecules that make them up as well as the temperature of those molecules. The nematic liquid crystal phase is characterized by molecules that have no positional order but tend to point in the same direction, like in the picture on the right. Liquid crystals that are in the nematic phase are the ones used in LCDs.

Photo credit: Dr. Oleg Lavrentovich, Liquid Crystal Institute

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An important property of LCs is that they are affected by electric current. A special kind of nematic LC, called a twisted nematic (TN), is naturally twisted, as depicted on the left. When a current is applied to this type of LC, it will untwist a certain amount depending on the current's voltage. LCDs use these liquid crystals, since they react predictably to electric current, and the degree of the twist can control how light passes through the material.

Creating an LCD

There is more to building an LCD than simply creating a sheet of liquid crystals. The combination of four facts makes LCDs possible:

          • Light can be polarized.
          • Liquid crystals can transmit and change polarized light.
          • The structure of liquid crystals can be changed by electric current.
          • There are transparent substances that can conduct electricity.

An LCD is a device that uses these four facts in a surprising way.

An LCD starts with two pieces of polarized glass. During the manufacturing process of this glass, a special chemical is applied to polarize it. The chemical is laminated in a vertical pattern, which reorganizes light. This pattern blocks the light that is horizontal to it, similar to how a window blind works. See how light is blocked in the animated image of a polarizing lens to the right.

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A special material is then applied to make microscopic grooves in the surface of the side of the glass that does not have the polarizing film on it. These grooves are aligned with the polarizing film. A coating of nematic liquid crystals is then added to one of the filters. The grooves on the filter cause the first layer of molecules of the LCs to align with the filter's orientation. The second piece of polarized glass is then added with the polarizing film at a right angle to the first piece. Each layer of twisted nematic molecules between the two filters will gradually twist until the uppermost layer is at a 90-degree angle to the bottom, matching the orientation of the filter that is closest to it, as in Fig. (a) on the left.

As light strikes the first filter, it is polarized, or oriented in a particular direction. The molecules in each layer then guide the light they receive to the next layer. As the light passes through the LC layers, the molecules also change the light's orientation to match their own. When the light reaches the far side of the liquid crystal substance, it is in the same orientation 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.

Remember, if we apply electric charge to liquid crystal molecules, they untwist. When the LCs straighten out, they change the angle of the light passing through them so that it no longer matches the angle of the top polarizing filter. Consequently, no light can pass through that area of the LCD, which makes that area darker than the surrounding areas, as in Fig. (b) below.

A basic LCD diagram is shown to the right. It has a mirror (A) in back, which makes it reflective. A piece of glass (B) with a polarizing film is placed on the bottom side, and a common electrode plane (C) made of indium-tin oxide is placed on top in order to receive electricity. A common electrode plane covers the entire area of the LCD to pass electrical current along. Above that is the layer of liquid crystal substance (D). Then comes one more piece of glass (E) with an electrode in the shape of the rectangle on the bottom and, on top, another polarizing film (F), at a right angle to the first one.

To power the LCD, the electrode is hooked up to a power supply, such as a battery. When there is no current, light entering through the front of the LCD will pass through the LCs and polarized glass to the mirror and bounce right back out. But when the battery supplies current to the electrodes, the liquid crystals between the common-plane electrode and the electrode shaped like a rectangle untwist and block the light in that region from passing through. This untwisting and subsequent blockage of light makes the LCD show the rectangle as a black area.

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Backlit vs. Reflective

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The LCD above requires an external light source. This is because LC materials emit no light of their own. Smaller LCDs are often reflective, which means to display anything they must reflect light from external light sources. Look at an LCD watch: the numbers appear where small electrodes charge the LCs, making them untwist so that light cannot pass through the polarized film.

Most computer displays are lit with built-in fluorescent tubes above, beside, and sometimes behind the LCD. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform display. On its way through filters, liquid crystal layers, and electrode layers, a lot of this light is lost—often more than half!

LCD History

Today, LCDs are commonplace, but they didn't appear out of thin air. It took a long time to get from the discovery of liquid crystals to the multitude of LCD applications we use now. LCs were first discovered in 1888, by Austrian botanist Friedrich Reinitzer. He observed that when he melted a curious cholesterol-like substance (cholesteryl benzoate), it first became a cloudy liquid before clearing up as its temperature rose. Upon cooling, the liquid turned blue before finally crystallizing. Eighty years passed before RCA made the first experimental LCD in 1968. Since then, LCD manufacturers have steadily developed many variations and improvements on the technology, taking the LCD to amazing levels of technical complexity.

Color LCDs

An LCD that can show colors must have three subpixels with red, green, and blue color filters to create each color pixel.

Careful control of the voltage applied can then change the intensity of each subpixel to generate over 256 shades. On the right, you can see large blocks of color or images under the subpixel display used to make them. Combining subpixels like this gives the LCD a possible 16.8 million colors (256 shades of red x 256 shades of green x 256 shades of blue), to choose from!

LCD technology is constantly evolving. These are just the basics of what makes it work. Different liquid crystals are being used to create new LCD materials. It can be very complicated, but all these new technologies depend on a special phase of material which can bend and unbend light and polarization films which can block light out.

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References

Castellano, Joseph A (2005). Liquid Gold: The Story of Liquid Crystal Displays and the Creation of an Industry. World Scientific Publishing. ISBN 978-981-238-956-5.
Kawamoto, Hiroshi (2002). "The History of Liquid-Crystal Displays" (PDF). Proceedings of the IEEE. 90 (4): 460–500. doi:10.1109/JPROC.2002.1002521.
Jonathan W. Steed & Jerry L. Atwood (2009). Supramolecular Chemistry (2nd ed.). John Wiley and Sons. p. 844. ISBN 978-0-470-51234-0.
Gray, George W.; Kelly, Stephen M. (1999). "Liquid crystals for twisted nematic display devices". Journal of Materials Chemistry. 9(9): 2037–2050. doi:10.1039/a902682g.
Jeff Tyson "How LCDs Work" 17 July 2000. HowStuffWorks.com. 23 July 2018.