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Liquid Crystal Display


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Liquid Crystal Display What are liquid crystals? Most substances exist in three states - solid, liquid and gaseous. The differences between these materials are in an internal arrangement that is temperature and pressure dependent. At low temperatures when the material is in a solid state, atoms, ions or molecules can not move freely. Their only movements are the thermal vibrations around the equilibrium position. If the temperature increases, more energy gets into the system, resulting in stronger vibrations. Finally, at the temperature between the solid state and the liquid state, the bonds are relaxed enough to free movement of the molecules that come into contact with one another and change the direction of movement. However, the thermal energy is not so high that it is..

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Liquid Crystal Display

What are liquid crystals?

Most substances exist in three states - solid, liquid and gaseous. The differences between these materials are in an internal arrangement that is temperature and pressure dependent.

At low temperatures when the material is in a solid state, atoms, ions or molecules can not move freely. Their only movements are the thermal vibrations around the equilibrium position.

If the temperature increases, more energy gets into the system, resulting in stronger vibrations.

Finally, at the temperature between the solid state and the liquid state, the bonds are relaxed enough to free movement of the molecules that come into contact with one another and change the direction of movement. However, the thermal energy is not so high that it is enough to overcome the bonds and therefore there is still a positional arrangement.

Upon another temperature rise, the bonds between the molecules overcome and the substance passes into the gaseous state.

However, some organic substances occur in more states than solid and liquid. These substances are referred to as liquid crystals and their molecules are often called mesogens. Their different phases between the solid state and the liquid state are liquid crystalline phases or mesophases. The molecules of these substances are mostly long and narrow. The liquid crystalline phase has some liquid and solid phase properties. It is liquid as a liquid but has optical and electromagnetic properties as a solid.

A very important feature of liquid crystals is that the oriented arrangement of their molecules causes many interesting optical phenomena. There is a change in the polarization of the light passing through them, depending on the position of the material molecules. Another significant feature is behavior in the electric field. Molecules are neutral. However, the size of the el. the charges in the individual parts of the molecule may vary greatly. If one part of the molecule has a positive charge and the other part is negative (which is the majority) then the molecule becomes a dipole. In the electric field, the dipole has a tendency to turn in its direction. Both of these effects are used in the LCD.

Distribution of liquid crystals

When dividing liquid crystals we distinguish two basic types:  with a NEMATIC arrangement and SMEKTICAL arrangement.

DISPLAY

The most common LCD application is in displays.

The LCD consists of the following parts:

The first layer - the glass plate - is covered with a thin layer of metal oxide, which acts as an electrode. This film is arranged in columns and rows (passive matrix display) or in individual shapes (active matrix display). The electrodes are used to adjust the voltage between the parts we want to make visible.

A polymeric alignment layer (usually a polyamide) follows. This layer passes through a scratch process that leaves a series of parallel grooves in it. These grooves help align the LC molecules in the right direction to ensure the TN effect.

Two equal plates are prepared and one of them is covered internally with a spacer layer of polymer beads. This layer provides a constant gap between the plates where the liquid crystal is placed.

Both plates are joined and their edges are bonded with epoxy. The corner is left open so that LC can be injected between them. Once the display is filled with liquid crystals, the corner is sealed and polarizing layers are applied to the surface of the glass in the corresponding direction.

In the Twisted Nematic display, the grooves in the alignment layers are perpendicular to one another and the polarizers are parallel to each other. For Super Twisted Nematic, the alignment layers are positioned depending on the twist angle setting from 180 ° to 270 °.

The display is completed by connecting the outlets that supply the voltage to the specified parts of the display.

Reflective LCD

Ambient light is used to illuminate the display. Behind the rear polarizer is a reflective layer that reflects the light. Without the presence of external lighting, it is not visible. (When light is disconnected, the light passes, reflects from the reflection layer, and appears white instead of the reflective layer.)

Transmitting (transmissive) LCD

This display has a transparent rear polarizer and does not reflect ambient light. To be visible, it must be backlit. This means that these displays are best readable in the dark, unlike the previous type.

Translucent LCD

It is a combination of both previous types, it has a partially reflexive rear polarizer. Backlight is used only when ambient light is insufficient to save energy.

Backlight of displays

Three types of backlit displays are used: EL - Electroluminiscent. It's a very thin light-emitting plate. Available in different colors. Power consumption is low but requires an AC voltage of 80 - 100 V. In addition, its lifetime is more than 10 times lower than the LED. (3000 - 5000 hours) LED - Light Emitting Diode. Their benefits are long lifetime and do not require high voltage. the disadvantage is the high energy consumption compared to EL. CFL - Cold Cathode Fluorescent Lamp (Cold Cathode Fluorescent Lamp). It has low power consumption and gives clear white light. the supply voltage is 270-300 V AC. They are used in graphical LCDs and have a longer service life than EL - 10,000 - 15,000 hours.

Color displays

Their design is almost the same as for monochrome displays. Each display point contains a red, green, and blue filter that is placed side by side on top of the glass. Color display REQUIRES backlight. By letting the light into the color filters and blending it, we get the resulting color.

This is the so-called TFT LCD. It is controlled by switching on the element known as thin-film transistor, which is located in each pixel.

Paper LCD is not a fiction

Developers at Royal Philips Electronics have found a way to apply the active LCD screen to almost any surface. The result is durable and resilient, so the screen may be a piece of cloth that you put into your pocket when you finish working (almost) nothing. The essence of the discovery is a mixture of liquid crystals and polymer. Then, the mixture is irradiated with ultraviolet radiation, which aligns the liquid crystals in the pattern of the bee honey, and the polymer shrinks over them in a transparent cover layer. Liquid crystals are then activated in the same way as today's LCD displays.

The system should allow researchers not only to produce displays with completely new features that can be applied to almost any material - from fabric to stone. Thoughts are small application devices that will allow you to conjure up a screen of any size, for example, on a wall. (You are also looking forward to all-inclusive graphs - with a miniature microchip driven message, the bag will probably break :-)

Glossary of Terms

Anode - High voltage component of the CRT monitor, attracting rays ejected by an electron gun

Aperture grille - A grid consisting of thin wires pointing from top to bottom to ensure that the electron beam more accurately irradiates the desired spot of the CRT screen. Monitors using this technology are often referred to as trinitron or diamondtrons.

BNC - Connects a monitor that separates vertical and horizontal signals and red, green and blue signals. It gives a slightly better signal than D-Sub.

Point spacing - Values in millimeters that indicate the distance between the individual CRT monitor points or between individual LCD monitor elements.

Color purity - Balanced color quality across the screen.

CRT - (Cathode Ray Trube) CRT technology is used in most monitors and TVs.

Degauss - A way to get rid of the monitor of an overloaded electromagnetic field.

D-Sub - A 15-pin connector that can be found on the monitor's graphics card.

Phosphorus - The material inside the CRT screen, which brightens when the electron beam strikes it.

Convergence - A bad light will come on if the rays do not fall directly to the desired location.

LCD - (liquid crystal display) LCD technology is used on most flat displays.

Refresh Rate - Measures in Hertz and indicates how many times per second each point on the screen is redrawn. For CRT monitors, it should be at least above 75 Hz, with LCD panels at 60 Hz.

Resolution - The screen resolution shows the number of points in both the horizontal and the vertical direction, such as the resolution of 1024 x 768 pixels. CRT monitors can resolve the resolution, but it should always match the screen size. LCD displays work with fixed resolution.

Shade - Lightly curved metal plate with holes where the beam is directed to reach the correct position.

TFT - (Thin Film Transistor) TFT is an active matrix LCD screen. It is currently the most used type of LCD display.

Derating coil - Electromagnetic electronically deflecting electron beam within CRT monitors.

History of liquid crystals

LCs were discovered about 150 years ago. In the middle of the 19th century, Virchow, Mettenheimer and Valentin discovered that the nerve fibers they examined created a liquid substance when introduced into the water, behaving strangely when viewed using polarized light. They did not believe it was a different phase, but they are still considered to be the discoverer of LC. In addition to this discovery, of course, there were others - a number of materials were discovered that are interesting at the temperature around the melting point and that their optical properties vary with temperature.

Later, in 1877, Otto Lehmann used a polarizing microscope with sample temperature control to examine transitions between the phases of different substances. He found that one of the substances in the transition from the liquid to the solid phase forms an interfacial.

In 1888, Austrian botanist Friedrich Reinitzer examined the properties of the organic substance based on cholesterol during melting. He found that the substance melted at 145.5°C formed the interfacial and was only liquid at 178.5°C.

This phase was called the liquid crystal phase. After 1888, the era of research and development of liquid crystals began. A great deal of theoretical work has been created.

In 1922, in Paris, Georges Freidel performed many experiments and was the first to find that the liquid crystal molecules were orienting in the direction of the el. field. He proposed a classification scheme for the separation of liquid crystals for nematic, smectic and cholesteric, which is used today.

Then Carl Oseen in Sweden worked on the elastic properties of LC. The results of his research were used in the continuum theory of Englishman F. C. Frank. This theory is today one of the basic theories of LC.

During World War II and beyond, no one was interested in LC for a long time. Only after 1950. Microscopic LC theory and continuum theory for static and dynamic systems were formulated. In 1968, scientists from RCA presented a liquid crystal display. The interest in LC, of course, has risen immediately due to the possibilities offered by LC as well as the commercial interest in the LCD. Even so, there are still unresolved issues in this area today.

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