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How Does a Liquid Crystal Display LCD Screen Work

A Liquid crystal display (LCD) is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals. Liquid crystals do not emit light directly.

LCDs are available to display arbitrary images (as in a general-purpose computer display) or fixed images which can be displayed or hidden, such as preset words, digits, and 7-segment displays as in a digital clock. They use the same basic technology, except that arbitrary images are made up of a large number of small pixels, while other displays have larger elements.

LCDs are used in a wide range of applications including computer monitors, televisions, instrument panels, aircraft cockpit displays, and signage. They are common in consumer devices such as video players, gaming devices, clocks, watches, calculators, and telephones, and have replaced cathode ray tube (CRT) displays in most applications.

They are available in a wider range of screen sizes than CRT and plasma displays, and since they do not use phosphors, they do not suffer image burn-in. LCDs are, however, susceptible to image persistence.

LCD Screens are uniquely modern in style, and the liquid crystals that make them work have allowed humanity to create slimmer, more portable technology than we’ve ever had access to before. From your wrist watch to your laptop, a lot of the on the go electronics that we tote from place to place are only possible because of their thin, light LCD display screens.

Liquid crystal display (LCD) technology still has some stumbling blocks in its path that can make it unreliable at times, but on the whole the invention of the LCD Screen has allowed great leaps forward in global technological progress.

Although liquid crystals are not really liquid, their molecules behave more like a liquid than they do like a solid, which earns them their name. The crystals in an LCD exist in a kind of a unique middle ground between solid form and liquid form, which gives them the movement and flexibility of a liquid; but can also let them remain in place, like a solid.

Heat can quickly melt a solid to liquid, allowing it to move, whereas cool will make the liquid solidify almost instantly. The sensitivity of liquid crystals to temperature can be an advantage, or a disadvantage. It allows for the highly successful use of liquid crystals in devices like thermometers, where temperature responsiveness is a boon; but this same property can unfortunately make LCD screens unreliable in extreme climates.

In an LCD screen, electric currents work at a microscopic level to control the amount of light that passes through the liquid crystal molecules that make up the moving layer of the screen, which is sandwiched between clear glass panels.

The currents can force the naturally twisted molecules to unwind or coil tighter, thereby changing the amount of light that can pass from the bulb behind the glass to the eye of the viewer. It may help you understand this process by imagining that light filters through an LCD screen the same way that sunlight filters through the leaves of a tree. Imagine that the tree is being blown in the wind, and you will see that the amount and placement of the light that comes through the leaves changes. This is very similar to the dynamic that powers an LCD screen, except that the sun is a small light bulb, the leaves are molecules of liquid crystal, and the wind is made up of electric currents sent by the computer and designed to create a specific light pattern that your eye will interpret as words or images.

Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters (parallel and perpendicular), the axes of transmission of which are (in most of the cases) perpendicular to each other. Without the liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer.

Before an electric field is applied, the orientation of the liquid-crystal molecules is determined by the alignment at the surfaces of electrodes. In a twisted nematic device (still the most common liquid-crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist.

This induces the rotation of the polarization of the incident light, and the device appears gray. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.

The optical effect of a twisted nematic device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, these devices are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). These devices can also be operated between parallel polarizers, in which case the bright and dark states are reversed. The voltage-off dark state in this configuration appears blotchy, however, because of small variations of thickness across the device.

Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).

Displays for a small number of individual digits and/or fixed symbols (as in digital watches and pocket calculators) can be implemented with independent electrodes for each segment. In contrast full alphanumeric and/or variable graphics displays are usually implemented with pixels arranged as a matrix consisting of electrically connected rows on one side of the LC layer and columns on the other side, which makes it possible to address each pixel at the intersections.

The general method of matrix addressing consists of sequentially addressing one side of the matrix, for example by selecting the rows one-by-one and applying the picture information on the other side at the columns row-by-row.

The LCD screen is more energy efficient and can be disposed of more safely than a CRT. Its low electrical power consumption enables it to be used in battery-powered electronic equipment. It is an electronically modulated optical device made up of any number of segments filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in color or monochrome.

Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters (parallel and perpendicular), the axes of transmission of which are (in most of the cases) perpendicular to each other. Without the liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer.

Before an electric field is applied, the orientation of the liquid-crystal molecules is determined by the alignment at the surfaces of electrodes. In a twisted nematic device (still the most common liquid-crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. This induces the rotation of the polarization of the incident light, and the device appears gray. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.

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