All You Need to Know About LCD

All You Need to Know About LCD

A display LCD (liquid crystal display: ‘ LCD ‘ for short English according to abbreviationfinder) is a thin, flat screen formed by a number of pixels color or monochrome placed in front of a light source or reflector. It is often used in battery-operated electronic devices as it uses very small amounts of electrical energy.

It is present in countless devices, from the limited image that a pocket calculator shows to televisions of 50 inches or more. These screens are composed of thousands of tiny liquid crystals, which are not solids or liquids in reality but an intermediate state.

Features

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

The surface of the electrodes that are in contact with the liquid crystal materials is treated in order to adjust the liquid crystal molecules in a particular direction. This treatment is usually normally applicable consists of a thin layer of polymer that is rubbed unidirectionally using, for example, a cloth. The direction of the liquid crystal alignment is defined by the rubbing direction.

Before the application of an electric field, the orientation of the liquid crystal molecules is determined by adaptation to the surfaces. In a twisted nematic device, TN (one of the most common devices among liquid crystal devices), the surface alignment directions of the two electrodes are perpendicular to each other, thus organizing the molecules in a helical structure., or twisted. Because the material is birefringent liquid crystal, the light that passes through one polarizing filter is turned by the liquid crystal helix that passes through the liquid crystal layer, allowing it to pass through the second polarized filter.. Half of the incident light is absorbed by the first polarizing filter, but otherwise the entire assembly is transparent.

When a voltage is applied across the electrodes, a turning force orients the liquid crystal molecules parallel to the electric field, which distorts the helical structure (this can be resisted thanks to elastic forces since the molecules are confined to the surfaces). This reduces the rotation of the polarization 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 unwound and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will be mainly polarized perpendicular to the second filter, and therefore it will be blocked and the pixel will appear black. By controlling the applied voltage across the liquid crystal layer at each pixel, light can be allowed to pass through different amounts, constituting different shades of gray. The optical effect of a twisted nematic (TN) device in the voltage state is much less dependent on the thickness variations of the device than in the offset voltage state. Because of this, These devices are often used between crossed polarizers in such a way that they appear bright without voltage (the eye is much more sensitive to variations in the dark state than in the bright state). These devices can also work in parallel between polarizers, in which case light and dark are reversed states. The offset stress in the dark state of this configuration appears reddened due to small variations in thickness throughout the device. Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of a certain polarity is applied for a long period, this ionic material is attracted to the surface and the performance of the device is degraded. This is to be avoided,

When a device requires a large number of pixels, it is not feasible to drive each device directly, thus each pixel requires a separate number of electrodes. Instead, the screen is multiplexed. In a multiplexed display, the electrodes on the side of the display are grouped together with the cables (usually in columns), and each group has its own voltage source. On the other hand, the electrodes are also grouped (usually in rows), where each group gets a sink voltage. The groups have been designed so that each pixel has a unique and dedicated combination of sources and sinks. The electronic circuits or the software that controls them, activates the sinks in sequence and controls the sources of the pixels in each sink.

Specs

Important factors to consider when evaluating an LCD screen:

Resolution

The horizontal and vertical dimensions are expressed in pixels. HD displays have a native resolution of 1366 x 768 pixels (720p) and the native resolution on Full HD is 1920 x 1080 pixels (1080p).

Stitch width

The distance between the centers of two adjacent pixels. The smaller the dot width, the less granularity the image will have. The point width can be the same vertically and horizontally, or different (less common).

Size

The size of an LCD panel is measured along its diagonal, usually expressed in inches (colloquially called the active display area).

Response time

It is the time it takes for a pixel to change from one color to another

Matrix type

Active, passive and reactive.

Vision angle

It is the maximum angle at which a user can look at the LCD, it is when it is offset from its center, without losing image quality. The new screens come with a viewing angle of 178 degrees.

Color support

Number of supported colors. Colloquially known as color gamut.

Brightness

The amount of light emitted from the screen; also known as luminosity

Contrast

The ratio of the brightest to the darkest intensity.

Appearance

The ratio of the width to the height (for example, 5: 4, 4: 3, 16: 9, and 16:10).

Ports of entry

For example DVI, VGA, LVDS or even S-Video and HDMI.

Brief history

  • 1887: Friedrich Reinitzer (1858-1927) discovered that cholesterol extracted from carrots is a liquid crystal (that is, he discovers the existence of two melting points and the generation of colors), and published his findings at a meeting of the Chemical Society Vienna on May 3, 1888 (F. Reinitzer: Zur Kenntniss de Cholesterins, Monatshefte für Chemie (Wien / Vienna) 9, 421-441 (1888)).
  • 1904: Otto Lehmann publishes his work Liquid Crystals.
  • 1911: Charles Mauguin describes the structure and properties of liquid crystals.
  • 1936: The Marconi Wireless Telegraph company patents the first practical application of the technology, the “liquid crystal light valve”.
  • 1960 to 1970: Pioneering work on liquid crystals was carried out in the 1960s by the Royal Radar Establishment, in the city of Malvern, UK. The RRE team supported ongoing work by George Gray and his team at the University of Hull, who eventually discovered cyanobiphenyl in liquid crystals (which had the correct stability and temperature properties for application in LCD displays).
  • 1962: The first major publication in English on the topic Molecular structure and properties of liquid crystals, by Dr. George W. Gray.
    Richard Williams from RCA found that there were some liquid crystals with interesting electro-optical characteristics and he realized the electro-optical effect by generating band patterns in a thin layer of liquid crystal material by applying a voltage. This effect is based on a hydrodynamic instability formed, what is now called ” Williams domains ” inside the liquid crystal.
  • 1964: In the fall of 1964 George H. Heilmeier, while working in the RCA laboratories on the effect discovered by Williams, he noticed the color switching induced by the readjustment of dichroic dyes in a homeotropically oriented liquid crystal. The practical problems with this new electro-optical effect caused Heilmeier to continue working on the effects of dispersion in liquid crystals and, finally, the realization of the first functioning liquid crystal display based on what he called the dynamic mode dispersion (DSM). Applying a voltage to a DSM device initially changes the transparent liquid crystal into a milky, cloudy, state layer. DSM devices could operate in transmission and reflection mode, but require considerable current flow to operate.
  • 1970: The 4 of December of 1970, the patent of the effect of the field “twisted nematic” liquid crystals was filed by Hoffmann-LaRoche in Switzerland (Swiss Patent No. 532,261), with Wolfgang Helfrich and Martin Schadt (who worked for the Central Research Laboratories) where they are listed as inventors. Hoffmann-La Roche, then licensed the invention, gave it to the Swiss manufacturer Brown, Boveri & Cie, which produced watch devices during the 1970’s, and also to the Japanese electronics industry that soon produced the first digital quartz wristwatch. with TN, LCD screens and many other products. James Fergason in Kent State University filed an identical patent in the US on April 22, 1971. In 1971 the Fergason company ILIXCO (now LXD Incorporated) produced the first LCD displays based on the TN effect, which soon replaced the poor quality DSM types due to improvements in lower operating voltages and lower power consumption. Energy.
  • 1972: The first liquid crystal active matrix display was produced in the United States by Peter T. Brody.

A detailed description of the origins and complex history of liquid crystal displays from an insider’s perspective from the earliest days has been published by Joseph A. Castellano in Liquid Gold, The Story of Liquid Crystal Displays and the Creation of an Industry

The same story seen from a different perspective has been described and published by Hiroshi Kawamoto (“The History of Liquid-Crystal Displays”, Proc. IEEE, Vol. 90, No. 4, April 2002), this document is available at public at the IEEE History Center.

Color in devices

In color LCD screens each individual pixel is divided into three cells, or sub-pixels, colored red, green, and blue, respectively, by increasing filters (pigment filters, tint filters, and metal oxide filters). Each sub-pixel can be independently controlled to produce thousands or millions of possible colors for each pixel. CRT monitors use the same ‘sub-pixel’ structure through the use of phosphor, although the analog electron beam used in CRTs does not give an exact number of sub-pixels.

Color components can be arranged in various pixel geometric shapes, depending on monitor usage. If the software knows what type of geometry is being used on a particular LCD, it can be used to increase the resolution of the monitor through sub-pixel display. This technique is especially useful for anti-aliasing text.

All You Need to Know About LCD

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