What is LCD? Working and Types

02 April

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LCD is a liquid crystal display. The LCD structure is to place a liquid crystal cell between two parallel glass substrates. LCD has replaced CRT as the mainstream, and the price has dropped a lot, and it has become fully popular.

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&#; LCD Features

(1) Low-voltage and micropower consumption

(2) The appearance is small and exquisite, the thickness is only 6.5~8mm

(3) Passive display type (no glare, no irritation to human eyes, and no eye fatigue)

(4) The amount of display information is large (because the pixels can be made small)

(5) Easy to colorize (reproduce very accurately on the chromatogram)

(6) No electromagnetic radiation (safe for the human body, conducive to information confidentiality)

(7) Long life 

&#; LCD advantages

(1) Since CRT displays rely on the electromagnetic field generated by the deflection yoke to control the electron beam, and because the electron beam cannot be positioned absolutely on the screen, CRT displays often have different degrees of geometric distortion and linear distortion. However, LCD does not have any geometric distortion or linear distortion due to its principle problems, which is also a major advantage.

(2) Compared with traditional CRT, the liquid crystal also performs well in terms of environmental protection. This is because there are no high-voltage components like CRT inside LCD, so it will not cause X-ray excessive due to high pressure, so its radiation index is generally lower than that of CRT.

(3) The biggest advantage of LCD compared with traditional CRT is power consumption and volume. For traditional 17-inch CRT, its power consumption is almost all above 80W, while the power consumption of 17-inch LCD is mostly around 40W. Calculated in this way, LCD has obvious advantages in energy saving.

&#; LCD types

According to the different backlight sources, LCDs can be divided into two types: CCFL displays and LED displays.

Misunderstanding:

Many users believe that liquid crystal displays can be divided into LEDs and LCDs. To some extent, this understanding is misguided by advertisements.

LCD display

The LED display on the market is not a true LED display. To be precise, it is an LED-backlit liquid crystal display. The liquid crystal panel is still a traditional LCD display. South Korea&#;s Samsung was once convicted by the British Advertising Association as violating the country&#;s advertising laws because its &#;LED TV&#; LCD TVs were suspected of misleading consumers. For liquid crystal displays, the most important key is its LCD panel and backlight type, while the LCD panels of displays on the market generally use TFT panels, which are the same. The difference between LEDs and LCDs is only their backlight types: LED backlight and CCFL backlight (that is, fluorescent lamps) are diodes and cold cathode lamps, respectively.

LCD is the acronym for Liquid Crystal Display. The LED display refers to a type of liquid crystal display (LCD), that is, a liquid crystal display (LCD) with LED (light-emitting diode) as the backlight source. It can be seen that LCD includes LEDs. Corresponding to the LED display is actually a CCFL display.

(1) CCFL

CCFL refers to a liquid crystal display (LCD) with CCFL (cold cathode fluorescent lamp) as the backlight source.

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The advantage of CCFL display is good color performance, but the disadvantage is higher power consumption.

(2) LED

LED refers to a liquid crystal display (LCD) that uses LEDs (light-emitting diodes) as a backlight source and generally refers to WLEDs (white light LEDs).

The advantages of LED displays are small size and low power consumption. Therefore, using LEDs as a backlight source can achieve high brightness while taking into account lightness and thinness. The main disadvantage is that the color performance is worse than that of CCFL monitors, so most professional graphics LCDs still use traditional CCFL as the backlight source.

&#; LCD working principle

The principle of liquid crystal display is that liquid crystals will exhibit different light characteristics under the action of different voltages. Liquid crystals are physically divided into two categories, one is passive liquid crystal. This type of liquid crystal itself does not emit light and requires an external light source. According to the position of the light source, it can be divided into two types: reflective and transmissive. The cost of a passive liquid crystal display is lower, but the brightness and contrast are not large. The effective viewing angle is small, and the color saturation of the color passive liquid crystal display is relatively small, so the color is not bright enough. 

LCD basic structure

The other type is active liquid crystal, mainly TFT (Thin Film Transistor). Each liquid crystal is actually a transistor that can emit light. A liquid crystal display is composed of many liquid crystals that are arranged in an array. In a monochrome liquid crystal display, a liquid crystal is a pixel, and in a color liquid crystal display, each pixel is composed of three liquid crystals, red, green, and blue. At the same time, there is an 8-bit register on the back of each liquid crystal. The value of the register determines the respective brightness of the three liquid crystal cells. However, the value of the register does not directly drive the brightness of the three liquid crystal cells but is accessed through a "palette". It is unrealistic to equip each pixel with a physical register. In fact, only one row of registers is equipped. These registers are connected to each row of pixels in turn and load the content of the row. They drive all pixel rows once and display a  complete frame.

(1) Passive-matrix type

The display principles of TN-LCD, STN-LCD, and DSTN-LCD are basically the same. The difference is that the twist angle of the liquid crystal molecules is somewhat different. Let's take a typical TN-LCD as an example to introduce its structure and working principle.

In a TN-LCD liquid crystal display panel with a thickness of less than 1 cm, it is usually plywood made of two large glass substrates with a color filter, an alignment film, etc., and two polarizing plates on the outside. The color filter is a filter composed of three colors of red, green, and blue, which are regularly fabricated on a large glass substrate. Each pixel is composed of three color units (or called sub-pixels). 

If a panel has a resolution of ×, it actually has × transistors and sub-pixels. The upper left corner (gray rectangle) of each sub-pixel is an opaque thin film transistor, and the color filter can produce the three primary colors of RGB. Each interlayer contains electrodes and grooves formed on the alignment film, and the upper and lower interlayers are filled with multiple layers of liquid crystal molecules. In the same layer, although the position of the liquid crystal molecules is irregular, the long axis orientation is parallel to the polarizer. On the other hand, between different layers, the long axis of the liquid crystal molecules is continuously twisted 90 degrees along the plane parallel to the polarizer. Among them, the orientation of the long axis of the two layers of liquid crystal molecules adjacent to the polarizing plate is consistent with the polarization direction of the adjacent polarizing plate. The liquid crystal molecules near the upper interlayer are arranged in the direction of the upper groove, and the liquid crystal molecules in the lower interlayer are arranged in the direction of the lower groove. Finally, it is packaged into a liquid crystal box and connected with the driver IC, the control IC, and the printed circuit board.

IPS LCD and TN LCD

Under normal circumstances, when light is irradiated from top to bottom, usually only one angle of light can penetrate through the upper polarizing plate into the groove of the upper interlayer, and then passing through the lower polarizing plate through the passage of the twisted arrangement of liquid crystal molecules. This is a complete path of light penetration. The interlayer of the liquid crystal display is attached with two polarizing plates, and the arrangement and light transmission angle of the two polarizing plates are the same as the groove arrangement of the upper and lower interlayers. When a certain voltage is applied to the liquid crystal layer, due to the influence of the external voltage, the liquid crystal will change its initial state, and will no longer be arranged in a normal way, but will become an upright state. Therefore, the light passing through the liquid crystal will be absorbed by the second layer of the polarizing plate and the entire structure will appear opaque, resulting in black color on the display screen. When no voltage is applied to the liquid crystal layer, the liquid crystal is in its initial state and will twist the direction of the incident light by 90 degrees, so that the incident light from the backlight can pass through the entire structure, resulting in white on the display. In order to achieve the color you want for each individual pixel on the panel, multiple cold cathode lamps must be used as the backlight of the display.

(2) Active-matrix type

The structure of the TFT-LCD liquid crystal display is basically the same as that of the TN-LCD liquid crystal display, except that the electrodes on the upper interlayer of the TN-LCD are changed to FET transistors, and the lower interlayer is changed to a common electrode.

There are many differences between the working principle of TFT-LCD and TN-LCD. The imaging principle of the TFT-LCD liquid crystal display is to use the "back-through" illumination method. When the light source is irradiated, it first penetrates upward through the lower polarizing plate and transmits light with the help of liquid crystal molecules. Since the upper and lower interlayer electrodes are changed to FET electrodes and common electrodes when the FET electrodes are turned on, the arrangement of the liquid crystal molecules will also change, and the purpose of the display is achieved by shielding and transmitting light. But the difference is that because the FET transistor has a capacitance effect and can maintain a potential state, the previously transparent liquid crystal molecules will remain in this state until the FET electrode is energized next time to change its arrangement.

What are Liquid Crystals?

LCD panels can be categorized as flat-panel displays. What makes them distinct from other display technologies is the layer of liquid crystal material within. In this thin layer, liquid crystal molecules are aligned between two glass substrates. On the inner surfaces of each of those substrates lie electrodes that control charge carriers like electrons that then interact with the liquid crystals, creating an electric field that runs through them; this, in turn, can change the alignment of the crystals, also changing the overall behavior of the molecules. On the opposite sides of the substrate, polarizers are used to control the levels of light passage, affecting the overall image of the display.

How Do Liquid Crystal Displays Work?

Unlike CRT monitors, LCD monitors cannot illuminate themselves, and so they require a light source: the backlight. This backlight is most frequently made of the well-known LEDs which stand for light-emitting diodes. Sourced from the backlight, light is moved through the back polarizer and back substrate, into the liquid crystals. Now, the light waves can behave in a variety of ways. Backlight used in LCD displays can be LED (Light Emitting Diode) backlight or CCFL (Cold Cathode Fluorescent Lamp) backlight. LED backlights use less power which becomes more popular, while CCFL is lower cost for large size LCD displays such as large LCD TV. Recently, quantum dots technology is used to increase the LCD contrast.

Electrodes are the controlling factors of the liquid crystal behavior, and thus also the light behavior. By conducting or not conducting a current into the crystal layer, the light may or may not be able to pass through the liquid crystals in a manner that will allow passage through the polarizer. Because of this role, electrodes in LCDs are often made of indium tin oxide (ITO). ITO has good conducting properties and can also make for a transparent electrode which is essential to the appearance of displays today.

How the electrodes affect the liquid crystal alignment can vary depending on the method of alignment used (twisted nematic, multi-domain, in-plane switching). For example, twisted nematic liquid crystals are oriented in a twist when no electric field is present which then polarizes the light passing through the layer; when the electrodes apply the field in full, the twist will straighten out, no longer polarizing the light, and so no light passes. In each of these alignment types, the electrodes are placed differently within the structure, altering the properties of the display, such as width of viewing angle, power consumption, and response time. Despite these different alignment methods, the liquid crystal layer&#;s purpose remains the same: to polarize the light so that the polarized light passes through to the surface of the display. By polarizing the light transmitted from the backlight, the liquid crystal molecules play a role in how much of the light passes through the polarizing filters, whether it be all, none, or a partial amount.

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