LCD

Liquid crystal displays (LCDs) emerged as a popular form of electronic display starting in the 1970s. They use very little power (a thousand times less energy than a LED “light-emitting diode” uses—even, for some new designs, zero power) and are easily visible even in direct sunlight because they scatter light rather than producing it. That is, they produce dark areas that may be seen in illuminated environments by reflection, like print on paper, instead of acting as glowing lights that might be overpowered by bright ambient light.

At the heart of a LCD is a material called a liquid crystal. First discovered in 1889, liquid crystals are fluid substances whose molecules tend to arrange themselves in ordered structures, like those of solid crystals. In a liquid crystal in its “relaxed” or undisturbed state the molecules are parallel to one another and are arranged in planes, each slightly offset from the last, creating a twisted or spiral structure. As light passes through a liquid crystal, it is twisted and exits at a slightly different orientation from where it entered.

In a LCD, a thin layer of liquid crystal is sandwiched between two pieces of glass. Each piece of glass is polarized so that it will only allow light of a certain direction to pass through. Normally, because the two glasses are not aligned, light passing through one section of glass would be blocked by the other; however, the sandwiched layer of liquid crystal twists the light enough to pass through the second section of glass. Here, in the LCD’s “off” position, the display is clear.

To turn a LCD “on,” a voltage must be applied to the liquid crystal. As the electrical field moves through the liquid, its molecules will line up with the direction of the field, “untwisting” the twisted planes. Light will now pass through the liquid crystal unaffected and be blocked by the second section of polarized glass. In

this position the LCD appears black because it scatters all the light that enters.

In order to show numbers, letters, or pictures on the LCD, tiny metallic segments must be baked onto the surface of the polarized glass. Each segment can be turned on or off independently, forming numbers and letters. While many calculators use a simple seven-segment display format, more complex devices, for example, such as hand-held video games and laptop computers, form whole screens, called dot-matrix screens, out of tiny LCD squares. LCD screens have been developed for desktop and laptop computers as well, offering the advantages of small footprint, low power consumption, a flat screens, and lower cost than plasma screens. Digital cameras utilize LCD displays for viewing images as well. Certain types of liquid crystals are also temperature sensitive and have been used in thermometers.

See also LED.