LCD is short for liquid crystal display. LCDs are devices that use liquid crystals to create images. Liquid crystal images are being used in watch faces, laptop computer screens, camcorder viewers, virtual reality helmet displays, and television screens.
The two main LCDs are passive and active. In passive LCDs, once the image has been made, it cannot be changed. In active LCDs, the image can be manipulated even over short periods of time by using electricity. Most LCDs are of the active type.
The most common way to build an active LCD is by sandwiching liquid crystals between two clear plate electrodes. Electrodes are able to conduct electrical current. When electrical current is applied to the electrodes, the liquid crystal molecules change orientation. The orientation of liquid crystal molecules controls the amount, color and direction of vibration of the light that passes through them. As the electrical current is turned on and off, the liquid crystal alternates between being transparent (able to see through) and opaque (scatters so much light that you cannot see through).
The most popular version of this liquid crystal sandwich is the twisted nematic, or TN, cell. In the TN cell, the liquid crystal is placed between the electrodes. Then one of the electrodes is twisted in-plane by 90 degrees. The electrode sandwich is then placed between polarizers. These polarizers control which direction of light vibration can pass through the whole system. In a TN cell, the polarizers are arranged so that light that can get though the first one is vibrating in the wrong direction to get through the second one unless the liquid crystal helps it. When the electrical current is off, light is vibrating in the same direction as the liquid crystals are lying. The twisted liquid crystal guides the light, rotating
it by 90 degrees, until it reaches the next polarizer and can pass though. When the electrical current is turned on, the liquid crystal molecules are reoriented. The light is not guided through from one polarizer to the next. You see a dark spot where this happens. If these sandwiches are made very small and are individually controlled, each sandwich becomes a picture element or pixel. This is a common way to build LCD watch faces. The dark digits are made up of pixels where the electrical current supplied by the watch battery is preventing the liquid crystal molecules from guiding light to your eye.
The pixels can be made extremely small. If there are many of them, small enough so that your eye can not tell them apart, your eye then blends all the pixels into an image. An LCD television may have more than 300,000 pixels in a picture. The more pixels, the more detailed the image can be.
Although liquid crystals may be used in their pure form, current research is focused on mixing liquid crystals with other materials such as polymers. This allows the liquid crystal to be dispersed as droplets. Because the droplets are small, they require less energy to re-orient. They are also very efficient at scattering light which can make displays brighter and useful over wider viewing angles.
See also LED.