How LCDs Work

Transflective LCD
Transflective LCDs are a mixture of the reflective and transmissive types, with the rear polarizer having partial reflectivity. They are combined with a backlight for use in all types of lighting conditions. The backlight can be left off where there is sufficient outside lighting, conserving power. In darker environments, the backlight is turned on to provide a bright display. Transflective LCDs will not "wash out" when operated in direct sunlight.

Transflector bonded to the rear polarizer reflects light from front as well as enabling lights to pass through the back. Used with backlight off in bright light and with it on in low light to reduce power consumption.

Transmissive LCD
Transmissive LCDs have a transparent rear polarizer and do not reflect ambient light. They require a backlight to be visible. They work best in low light conditions with the backlight on continuously.

Without reflector or transflector bonded to the rear polarizer.
Backlight required. Most common is transmissive negative image.

Positive/Negative
Another feature of the viewing mode is whether the LCD is a positive or negative image. The standard image is positive, which means a light background with a dark character or dot. This works best in reflective or transflective mode. A negative image is usually combined with a transmissive mode. This provides a dark background with a light character. A backlight must be used to provide good illumination. In most graphic applications, the transmissive negative mode is inverted. This combination provides a light background with dark characters.

Viewing Mode	Display Description	Application Comments	Direct Sunlight	Office Light	Subdued Light	Very Low Light
Reflective Positive Image	Dark segments on light background	Not backlit. Provides best head-on contrast and environmental stability.	Excellent	Very Good	Average	Poor
Transflective Positive Image	Dark segments on grey background	Can be viewed by reflected ambient light or with backlighting.	Excellent (No backlight)	Good (No backlight)	Good (Backlit)	Very Good (Backlit)
Transflective Negative Image	Light grey segments on dark background	Needs high ambient light or backlighting. Frequently used with color and multicolor transflector.	Good (No backlight)	Fair (No backlight)	Good (Backlit)	Very Good (Backlit)
Transmissive Negative Image	Backlit segments on dark background	Cannot be read by reflection.	Poor (Backlit)	Good (Backlit)	Very Good (No backlight)	Excellent (Backlit)
Transmissive Positive Image	Dark segments on backlit background	Designed for very low light conditions, yet able to be read in bright ambient lights.	Good (No backlight)	Good (Backlit)	Very Good (Backlit)	Excellent (Backlit)

COLORS OF LCDs

An LCD that can show colors must have three subpixels with red, green, and blue color filters to create each color pixel. Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. Combining the subpixels produces a possible palette of 16.8 million colors (256 shades of red x 256 shades of green x 256 shades of blue), as shown below. Color displays take an enormous number of transistors. For example, a typical laptop computer supports resolutions up to 1024 by 768. If you multiply 1024 columns by 768 rows by 3 subpixels, you get 2,359,296 transistors etched onto the glass! If there is a problem with any of these transistors, it creates a "bad pixel" on the display. Most active matrix displays have a few bad pixels scattered across the screen. (more)

WHY LCDS ARE USED

The reasons LCDs are being so widely used are simple. LCD's are lightweight, small in size, and energy efficient. Because LCDs consist of only two thin plates of glass, they are small enough to fit anywhere. Also, compared to televisions, they are much thinner and lighter because there is no need for a bulky picture tube. The amount of energy used compared to televisions is favorable to LCDs as well. (more)

PROBLEMS WITH LCDS

Although LCDs are extremely useful, they also have some problems. One problem is that LCDs cannot be easily viewed at an angle. Another problem is contrast. In digital TVs and monitors, the problem is cost. But with today's rate of technological innovation, these problems will probably soon be solved, making LCDs even more dominating in display technology. (more)

REAL WORLD APPLICATIONS

Liquid crystal displays are actually common in the real world. In fact, you have probably already looked at several LCDs today alone. As it turns out, LCDs are useful for visual displays. For instance, there is a good chance that your alarm clock face is a liquid crystal display. You also, quite possibly, have an LCD in your car-CD panels inside cars, and stereos can also be LCDs. Digital watch faces are another example of real world uses of liquid crystal displays. If you have not seen any of those examples above, then you most certainly have seen this next example: computer monitors. Laptop and desktop computer monitors are often LCDs.

Now, there are two types of LCDs. These types are called active and passive LCDs. Active LCD's can show colors other than black and white. This is due to the pixels that make up a computer monitor, digital clock faces, and other liquid crystal displays. Pixels are the tiny dots that you can see if you put your face really close to the television or computer screen. In active LCDs, each of these little pixels can be programmed individually to display color. However, in passive LCDs only entire lines of pixels can be programmed. So, passive LCDs are not made to display colors. All of these examples and many more surround you with LCDs everyday of your life, and soon there will be even more. (more)

LCD technology is constantly evolving. LCDs today employ several variations of liquid crystal technology, including super twisted nematics (STN), dual scan twisted nematics (DSTN), ferroelectric liquid crystal (FLC), and surface stabilized ferroelectric liquid crystal (SSFLC). Display size is limited by the quality control problems faced by manufacturers. Simply put, to increase display size, manufacturers must add more pixels and transistors. As they increase the number of pixels and transistors, they also increase the chance of including a bad transistor in a display. Manufacturers of existing large LCDs often reject about 40% of the panels that come off the assembly line. The level of rejection directly affects LCD price because the sales of the good LCDs must cover the cost of manufacturing both the good and bad ones. This is why we don't have big-screen LCD TVs hanging on our walls, they would simply be too expensive. Only advances in manufacturing can lead to affordable displays in bigger sizes. In a future article, I will look at how LCD technology is changing the way projection systems work. (more)

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

An LCD is a device that uses these four facts in a surprising way. To create an LCD, take two polarized pieces of glass. A special polymer that creates microscopic grooves in the surface is rubbed on the side of the glass that does not have the polarizing film on it. The grooves must be in the same direction as the polarizing film. Add a coating of nematic liquid crystals to one of the filters. The grooves will cause the first layer of molecules to align with the filter's orientation. The second piece of glass is added with the polarizing film at a right angle to the first piece. Each successive layer of TN molecules will gradually twist until the uppermost layer is at a 90° angle to the bottom, matching the polarized glass filters. As light strikes the first filter, it is polarized. The molecules in each layer then guide the light they receive to the next layer. As the light passes through the liquid crystal layers, the molecules 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 will vibrate 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.

If you apply an electric charge to liquid crystal molecules, they untwist! When they 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. Building a simple LCD is easier than you think. Your start with the sandwich of glass and liquid crystals described above and add two transparent electrodes to it. For example, imagine that you want to create the simplest possible LCD with just a single rectangular electrode on it. The layers would look like this:

The LCD needed to do this job is basic. It has a mirror (A) in back, which makes it reflective. Then, we add a piece of glass (B) with a polarizing film on the bottom side, and a common electrode plane (C) made of indium tin oxide on top. A common electrode plane covers the entire area of the LCD. Above that is the layer of liquid crystal substance (D). Next comes another 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. The electrode is hooked up to a power source, such as a battery. When there is no current, light entering through the front of the LCD will simply hit 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. That makes the LCD show the rectangle as a black area.

Today, LCDs are everywhere we look, but they didn't sprout up over night. It took a long time to get from the discovery of liquid crystals to the multitude of LCD applications we now enjoy. Liquid crystals were first discovered in 1888, by Austrian botanist Friedrich Reinitzer. Reinitzer observed that when he melted a curious cholesterol-like substance (cholesteryl benzoate), it first became a cloudy liquid and then cleared 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 ingenious variations and improvements on the technology, taking the LCD to amazing levels of technical complexity. And there is every indication that we will continue to enjoy new LCD developments in the future! (more)

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