Liquid crystal display structure diagram

Each pixel of a liquid crystal display consists of the following parts: a layer of liquid crystal molecules suspended between two transparent electrodes (indium tin oxide), and two polarizing filters whose polarizing directions are perpendicular to each other on the outer sides of the two sides. Without the liquid crystal between the electrodes, light passing through one of the polarizing filters would be polarized exactly perpendicular to the second polarizer and thus be completely blocked. But if the polarization direction of light passing through one polarizing filter is rotated by the liquid crystal, then it can pass through the other polarizing filter. The rotation of the liquid crystal on the polarization direction of light can be controlled by an electrostatic field, thereby realizing the control of light.

Liquid crystal molecules are easily affected by an external electric field to generate induced charges. A small amount of charge is added to the transparent electrode of each pixel or sub-pixel to generate an electrostatic field, and the molecules of the liquid crystal will be induced by the electrostatic field to induce an electric charge and generate electrostatic torsion, which will change the original rotational arrangement of the liquid crystal molecules. The magnitude of the rotation through the light. Change the angle so that it can pass through the polarizing filter.

Before the charge is applied to the transparent electrode, the alignment of the liquid crystal molecules is determined by the alignment of the electrode surface, and the chemical surface of the electrode acts as a seed for the crystal. In the most common TN liquid crystal, the upper and lower electrodes of the liquid crystal are arranged vertically. The liquid crystal molecules are arranged in a spiral, and the light passing through one polarizing filter rotates in the polarization direction after passing through the liquid crystal chip, so that it can pass through the other polarizing plate. A small portion of the light is blocked by the polarizer during this process and appears gray from the outside. After the charge is applied to the transparent electrode, the liquid crystal molecules will be almost completely aligned in parallel with the direction of the electric field, so the polarization direction of the light passing through a polarizing filter is not rotated, so the light is completely blocked. At this point the pixel looks black. By controlling the voltage, the degree of distortion of the arrangement of the liquid crystal molecules can be controlled to achieve different grayscales.

Some liquid crystal displays turn black under the action of alternating current. The alternating current destroys the helical effect of the liquid crystal. When the current is turned off, the liquid crystal display will become brighter or transparent. This type of liquid crystal display is commonly used in notebook computers and cheap liquid crystal displays. Another type of liquid crystal display that is often used in high-definition liquid crystal displays or large-scale liquid crystal televisions is that when the power is turned off, the liquid crystal display is in an opaque state.

In order to save power, the liquid crystal display adopts the method of multiplexing. In the multiplexing mode, the electrodes at one end are connected together in groups, and each group of electrodes is connected to a power supply, and the electrodes at the other end are also connected in groups, and each group is connected to the power supply. On one end, the grouping design ensures that each pixel is controlled by an independent power supply, and the electronic device or the software driving the electronic device controls the display of the pixel by controlling the on/off sequence of the power supply.

Metrics for verifying LCD monitors include the following important aspects: display size, response time (sync rate), array type (active and passive), viewing angle, supported colors, brightness and contrast, resolution and aspect ratio, and Input interfaces (such as vision interfaces and video display arrays).

Brief History

In 1888, Austrian chemist Friedrich Leinitzer discovered liquid crystals and their special physical properties.

The first operable liquid crystal display was based on Dynamic Scattering Mode (DSM), which was developed by a group led by George Hellman of Radio Corporation of America. Hellmann founded Optech, a company that developed a series of liquid crystal displays based on this technology.

In December 1970, the spin-nematic field effect of liquid crystals was registered as a patent in Switzerland by Zander and Helfrich at the Hoffmann-Leroc Central Laboratory. But in 1969 the previous year, James Ferguson discovered the spin-nematic field effect of liquid crystals at Kent State University in Ohio, USA, and registered the same patent in the United States in February 1971. In 1971, ILIXCO produced the first liquid crystal display based on this characteristic, which replaced the poorer DSM type liquid crystal display. It was only after 1985 that the discovery had commercial value. In 1973, Japan's Sharp Corporation used it for the first time to make digital displays of electronic calculators. In the 2010s, LCD monitors have become the primary display device for all computers.

Display principle

In-vehicle information system for automobiles

JR East Yamanote Line operation information screen

In the absence of voltage, the light will travel along the gap of the liquid crystal molecules and turn 90 degrees, so the light can pass. But after adding the voltage, the light goes straight along the gap of the liquid crystal molecules, so the light is blocked by the filter plate.

Liquid crystal is a substance with flow characteristics, so only a very small force can be applied to make the liquid crystal molecules move. Taking the most common nematic liquid crystal as an example, the liquid crystal molecules can easily turn the liquid crystal molecules by the action of the electric field. The optical axis of the liquid crystal is quite consistent with its molecular axis, so it can produce optical effects. When the electric field applied to the liquid crystal is removed and disappears, the liquid crystal will use its own elasticity and viscosity to restore the liquid crystal molecules very quickly. The state before the electric field is applied.

Transmissive and Reflective Displays

Liquid crystal displays can be transmissive or reflective, depending on where the light source is placed.

Transmissive LCDs are illuminated by a light source behind one screen, while viewing is on the other side (front) of the screen. This type of LCD is mostly used in applications that require high-brightness displays, such as computer monitors, PDAs, and cell phones. The power consumption of the lighting devices used to illuminate the liquid crystal display tends to be higher than that of the liquid crystal display itself.

Reflective liquid crystal displays, commonly found in electronic clocks and calculators, (sometimes) reflect external light back to illuminate the screen by a diffuse reflective surface at the back. This type of LCD has a higher contrast ratio, because the light passes through the liquid crystal twice, so it is cut twice. Not using lighting devices significantly reduces power consumption, so devices that use batteries will last longer on batteries. Because small reflective liquid crystal displays consume so little power that a photovoltaic cell is enough to power them, they are often used in pocket calculators.

Transflective liquid crystal displays can be used as both transmissive and reflective types. When the external light is sufficient, the liquid crystal display works as a reflective type, and when the external light is insufficient, it can also be used as a transmissive type.

color display

A Subpixel Structure of Color Liquid Crystal Display

Pixel zoom on LCD

The LCD technology also changes the brightness according to the magnitude of the voltage, and the color displayed by each sub-picture element of the LCD depends on the color screening process. Since the liquid crystal itself has no color, color filters are used to generate various colors instead of sub-picture elements. The sub-picture elements can only adjust the gray scale by controlling the intensity of light passing through. Only a few active matrix displays use analog signal control, and most Digital signal control technology is used. Most digitally controlled LCDs use an eight-bit controller that can generate 256 grayscales. Each sub-element can represent 256 levels, so you can get 2563 colors, and each element can represent 16,777,216 colors. Because the human eye's perception of brightness does not change linearly, and the human eye is more sensitive to changes in low brightness, this 24-bit chromaticity cannot fully meet the ideal requirements. Engineers use the method of pulse voltage adjustment to make the color Changes look more uniform.

In a color LCD, each pixel is divided into three cells, or sub-pixels, with additional filters to label red, green, and blue. The three sub-pixels can be independently controlled, and the corresponding pixels can generate thousands or even millions of colors. Older CRTs display colors in the same way. The color components are arranged according to different pixel geometries as needed.

Active and passive arrays

Liquid crystal displays, which are commonly used in electronic watches and pocket computers, are composed of a small number of segments, and each segment has a single electrode contact. An external dedicated circuit provides electrical charge to each control unit, and this display structure can be cumbersome when there are many display units (eg liquid displays). Small monochrome displays, such as passive array liquid crystal displays on PDAs or older notebook computer displays, that apply Super Twisted Nematic (STN) or Dual Layer Super Twisted Nematic (DSTN) technology (DSTN corrects the color deviation of STN) .

Each row or column on the display has an independent circuit, and the position of each pixel is also specified by a row and column at the same time. This type of display is called a "passive array", because each pixel must also be remembered before updating. In their respective states, there is no stable charge supply per pixel at this time. As the number of pixels increases, so does the relative number of rows and columns. This display method becomes more difficult to use. Liquid crystal displays made with passive arrays are characterized by very slow response times and low contrast ratios.

Current high-resolution color displays, such as computer monitors or televisions, are active arrays. Thin film transistor liquid crystal displays are added to polarizers and color filters. Each pixel has its own transistor, allowing manipulation of a single pixel. When a column line is turned on, all row lines will be connected to a whole column (Row) of pixels, and each row line will be driven with the correct voltage, this column line will be turned off and the other column (Row) will be turned on. In a complete screen update operation, all column lines will be opened in time series. An active array display of the same size will appear brighter and sharper than a passive array display, and has a shorter response time.

quality control

Some LCD panels contain defective transistors that cause permanent bright and dark spots. Unlike IC, the LCD panel can still display normally even if there are dead pixels, which can avoid the waste of discarding the LCD panel that is much larger than the IC area due to only a few dead pixels. Panel manufacturers have different criteria for determining dead pixels.

Because of their larger size, LCD panels are more prone to defects than IC circuit boards. For example, a 12-inch SVGA LCD has 8 dead pixels, while a 6-inch wafer has only 3 defects. However, 3 scraps on a wafer that can be partitioned into 137 ICs is not very bad, and discarding this LCD panel means 0% output. Due to the fierce competition among manufacturers, the current standard of quality control has been raised. If the LCD screen has four or more dead pixels, it is easier to detect, so customers can ask for a new one. The location of the dead pixels of the LCD screen is also not negligible. Manufacturers often lower standards by destroying pixels in the center area of ​​the display. Some manufacturers offer a zero dead pixel guarantee.

power consumption

Active matrix liquid crystal displays have less electrical power than CRTs. In fact, it has become the standard display for portable devices, from PDAs to notebook computers. But the efficiency of LCD technology is still too low: even if you display the display white, less than 10% of the light emitted from the background light source passes through the display, and the rest is absorbed. Therefore, the current power consumption of the new plasma display is lower than that of the liquid crystal display of the same area.

PDAs such as Palm and CompaqiPAQ often use reflective displays. This means that ambient light enters the display, passes through the polarized liquid crystal layer, hits the reflective layer, and is reflected back to display an image. It is estimated that 84% of the light is absorbed in the process, so only one-sixth of the light is active, which, while still in need of improvement, is enough to provide the contrast needed for visual video. One-way reflective and reflective displays make it possible to use liquid crystal displays with minimal energy consumption under different lighting conditions.

Zero power display

1. The polarizer polarizes the incident light in the vertical direction;

2. Transparent electrodes with indium tin oxide (ITO) on glass substrates. The shape of the transparent electrode will determine the address of the dark color without the light passing through after turning on the power of the liquid crystal display. Vertical stripes are etched on the substrate, so that the alignment direction of the sub-liquid crystals will be in the same direction as the polarized incident light;

3. Twisted nematic (TN) liquid crystal;

4. The glass substrate with a common transparent electrode film (ITO), the horizontal stripes are etched on the substrate, so that the alignment direction of the liquid crystal becomes horizontal;

5. Horizontally deflected polarizer, which can block or allow light to pass through;

6. Reflective surfaces reflect light back to the observer.

In 2000, a zero-power display was developed that does not require electricity when in standby, but this technology is currently not in mass production. Another zero-power thin LCD technology was developed by France's Nemoptic, which was mass-produced in Taiwan in July 2003. This technology targets low-power mobile devices such as e-books and laptops. Zero-power LCDs are also competing with e-paper.

TFT-LCD

Main articles: Thin-film transistor liquid crystal displays and TFTs

TFT-LCD is the abbreviation of Thin film transistor liquid crystal display (thin film transistor liquid crystal display).