Author | Sensor Technology (WW_CGQJS)
In 1888, Austrian botanist Friedrich Reinitzer discovered liquid crystals, a peculiar organic compound that has two melting points. When heated to 145°C, its solid crystalline state melts into a cloudy liquid, while pure substances typically become transparent when melted. If heated further to 175°C, it appears to melt again, turning into a clear liquid. Later, German physicist Otto Lehmann observed these fatty compounds using a polarizing microscope equipped with a heating device he designed himself. He discovered that this cloudy liquid, although appearing to be a liquid, exhibited birefringence characteristic of anisotropic crystals. Thus, Lehmann named it “liquid crystal,” which is the origin of the term “LCD.” Reinitzer and Lehmann were later hailed as the fathers of liquid crystal technology. Since its discovery, the applications of liquid crystals were unknown until 1968 when they were utilized as materials in the electronics industry.
Since the first liquid crystal display was born in 1968, the development of LCD technology has gone through five stages:
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First Stage (1968—1972)
In 1968, RCA in the United States developed the dynamic scattering type LCD, and in 1972 manufactured a dynamic scattering type LCD watch, marking the practical application of LCD technology.
Second Stage (1971-1984)
In 1971, Swiss inventor twisted nematic (TN) liquid crystal display was invented, and Japanese manufacturers industrialized it. Due to the low manufacturing cost of TN-LCD, it became the mainstream liquid crystal product in the 1970s and 1980s.
Third Stage (1985-1990)
After 1985, due to the development of super twisted (STN) liquid crystal displays and the invention of amorphous silicon thin-film transistor liquid crystal display technology, LCD technology entered the stage of large-capacity display.
Fourth Stage (1990-1995)
Based on the rapid development of active matrix liquid crystal displays, LCD technology began to enter the high-quality display stage.
Fifth Stage (1996 and beyond)
LCD has become widely used in laptop computers. Since 1998, TFT-LCD products have entered the monitor market, and the three major problems that have long plagued liquid crystals—viewing angle, color saturation, and brightness—have been largely resolved.
Liquid Crystals and Their Classification
Substances that exhibit fluidity mechanically and crystalline properties optically are named liquid crystals.
Liquid crystals are divided into two major categories: solution-type liquid crystals and thermal-type liquid crystals; the liquid crystals used in display technology are all thermal-type liquid crystals.
Below temperature T1, it becomes solid (crystal), with T1 being the melting point of the liquid crystal. Above temperature T2, it becomes a clear, isotropic liquid, with T2 being the clearing point of the liquid crystal. The working temperature range of LCD is basically determined by T1 and T2.
Near crystalline phase liquid crystal molecules exhibit two-dimensional order, with molecules arranged in layers, where the long axes of the molecules are parallel and aligned in the same plane. The direction can be perpendicular to the plane or inclined relative to the plane. The thickness of the layer equals the length of the molecules, and the distance between layers can vary. Molecules can only slide back and forth or sideways within the layer but cannot move between layers. The viscosity and surface tension of near crystalline phase liquid crystals are relatively high, making them insensitive to changes in external electric, magnetic, and temperature fields.
In nematic phase liquid crystals, molecules exhibit one-dimensional order, with long axes parallel but not arranged in layers. They can slide up and down, side to side, and back and forth, maintaining parallel or nearly parallel alignment only in the direction of the long axes, with weak short-range intermolecular interactions. The arrangement and movement of nematic phase liquid crystal molecules are relatively free, making them sensitive to external electric, magnetic fields, temperature, and stress. Currently, they are the primary material for display devices.
Cholesteric phase liquid crystals are derived from cholesterol, with molecules arranged in layers, where the long axes of the molecules are parallel to the layer plane. The direction of the long axes of molecules in different layers varies slightly, with adjacent layers having a slight twist angle (about 15 degrees). The multiple layers twist into a helical shape, with the distance of a full 360º twist being called the pitch, which is roughly equivalent to the wavelength of visible light. Cholesteric phase is essentially a distorted state of nematic phase because the long axes of the molecules within the cholesteric phase are also oriented parallel to one another. However, from one layer to the next, there is a fixed angle of rotation, resulting in a helical arrangement. Therefore, by adding racemic nematic liquid crystals or mixing appropriate proportions of left-handed and right-handed cholesteric phases, the cholesteric phase can be transformed into nematic phase. A certain strength of electric or magnetic field can also convert cholesteric phase liquid crystals into nematic phase liquid crystals. Cholesteric phase is easily affected by external forces, especially sensitive to temperature, which can cause changes in pitch, and its reflected light wavelength is related to pitch. Thus, cholesteric liquid crystals change color with temperature variations.
The basic structure and imaging principle of LCD screens
1. Backlight Panel: The imaging principle of LCD relies on liquid crystals blocking light components to control brightness and darkness, so a light source is necessary to see images on the screen. The backlight panel provides the essential light source for the LCD screen.
2. Lower Polarizing Plate: The light emitted from the backlight panel has inconsistent directional properties and is radial. If such light passes through the twisted liquid crystal molecules, we still cannot see normal images on the screen; instead, we may see a white haze or colorful blocks, rather than the images we want to see. The lower polarizing plate is responsible for aligning the light direction before sending it to the liquid crystal layer.
3. Thin Film Substrate: The twisting angle of liquid crystal molecules is controlled by TFT.
4. Liquid Crystal: Under the control of TFT, the liquid crystal molecules twist, achieving consistent light direction control, thereby altering the brightness of the light heading towards the pixel units.
5. Color Filter: If you have memories of the 1980s, you might recall that black-and-white television screens often had a colored plastic sheet installed. With this sheet, the black-and-white TV seemed to turn into a color TV, allowing us to see faces turning pink, lips turning red, and other objects gaining color, even if sometimes the colors were not accurate. This plastic sheet is the color filter.
The liquid crystal itself has no color, so color filters are used to produce various colors. The color displayed by each liquid crystal sub-pixel depends on the color filter, not the sub-pixel itself. The backlight source emits white light, which passes through various color filters, allowing us to see the colors corresponding to the filters behind them. Thus, in LCDs, the function of color filters is to add color, corresponding to the phosphor function in CRT displays. Liquid crystal sub-pixels can only adjust grayscale by controlling the intensity of light passing through. Only a few active matrix displays use analog signals for control; most use digital signal control technology. Most digitally controlled LCDs use an 8-bit controller (some use a 10-bit controller), capable of producing 256 grayscale levels. If each sub-pixel can display 256 levels, then we can obtain 256×3 colors, allowing each pixel to display 16,777,216 colors, known as 1677.7216 million colors. Because human eyes perceive brightness changes non-linearly, they are more sensitive to lower brightness changes, so this 24-bit color depth has fully met ideal requirements. Engineers use pulse voltage adjustment methods to make color changes appear more uniform.
6. Upper Polarizing Plate: The originally consistent light direction becomes inconsistent after passing through the liquid crystal layer. If we do not realign the diffused light, we will still see a white haze on the screen, as the light twisted by the liquid crystal will not be reflected. Therefore, it is necessary to realign the diffused light using a polarizing plate that is orthogonal to the lower polarizing plate, allowing the light that has been twisted by the liquid crystal to be redirected. Different angles of light passing through the upper polarizing plate have different brightness, allowing us to see alternating light and dark images on the screen, since the redirected light has passed through the color filter, showing the images we need.
No Upper Polarizing Plate
Comparison of Effects with and without Upper Polarizing Plate
Complete Image with Polarizing Plate
The color imaging principle of LCD is similar to that of CRT, still composed of red, green, and blue primary colors. The difference is that CRT generates color light by rapidly striking the phosphors of the three primary colors with an electron beam, while LCD produces color through regular color filters. By controlling the brightness of the liquid crystal molecules under each primary color filter, different brightness levels simulate various colors found in nature. Since the color filters are located beneath the upper polarizing plate, this creates a viewing angle requirement for LCD screens, although this issue has been significantly improved recently.
We know that the arrangement of primary colors in color filters varies. Based on the position of the primary colors in the color filter, the control sequence of the liquid crystal molecules underneath must be adjusted accordingly; otherwise, the displayed image will only be a garbled screen.
The three types of primary color arrangements in the color filters shown above illustrate that the stripe arrangement is the simplest. Because the primary colors are arranged in simple vertical and horizontal lines, controlling them is relatively straightforward. However, this arrangement can lead to uneven line thickness and severe aliasing on diagonal lines. Therefore, a mosaic arrangement of green filters was developed, which can better solve the aliasing issue, but it still cannot resolve the detailed display of lines. The Dell arrangement of color filters was created to effectively address aliasing and uneven line thickness, but controlling the liquid crystal molecules in this arrangement is the most complex.
Main Working Modes of Liquid Crystal Display
Various working modes have derived from the basic principles of liquid crystal display, mainly including TN mode, STN mode, FLC mode, and liquid crystal-polymer mode. Currently, twisted nematic liquid crystals (TN) are on the verge of obsolescence, while super twisted nematic (STN) and active matrix (TFT) technologies have matured and become widespread.
Twisted Nematic (TN) Liquid Crystal Display
Twisted nematic (TN) liquid crystal is a nematic liquid crystal with a 90º twist. TN liquid crystal displays emerged in the 1970s. Besides possessing the basic characteristics required for liquid crystal displays, they also feature high contrast, simple manufacturing technology, and low cost. Currently, most of the liquid crystal displays used in portable calculators, watches, and instruments are of this type. Most domestic LCD manufacturers also primarily produce this type of product.
Twisted nematic (TN) liquid displays are formed by sandwiching twisted nematic (TN) liquid crystal material between two ITO glass plates, with the liquid crystal thickness generally at 5µm, depending on the birefringence of the liquid crystal material. A layer of alignment coating is applied to the upper and lower ITO glass substrates, using the interaction between the liquid crystal molecules and the alignment layer surface to align the liquid crystal molecules parallel to the surface with a tilt angle of 2-3º, as shown in the figure. The friction direction of the upper and lower substrates is set at 90º, twisting the liquid crystal molecules by 90º. A small amount of chiral agent is added to determine the twisting direction of the liquid crystal molecules. Polarizers are attached to the outer sides of the glass substrates, with the optical axis of the polarizers aligned with the friction direction of the glass substrates, resulting in a normally white display. When the incident light’s polarization plane rotates 90º with the liquid crystal molecules, the polarized light passes through the polarizer, resulting in a bright state. When a voltage is applied, the liquid crystal molecules align with the electric field direction, keeping the polarization plane unchanged, preventing polarized light from passing through the output polarizer, resulting in a dark state. Therefore, the liquid crystal display acts as an electrically controlled optical valve. However, under optimal parameter conditions, the maximum number of scanning lines for twisted nematic (TN) liquid displays can only reach 32, resulting in a small information capacity. Moreover, since they can only produce black-and-white, monochrome, low-contrast (20:1) liquid displays with a narrow viewing angle of only 30º, their application range is significantly limited. Currently, they are only used in electronic watches, calculators, and simple handheld gaming devices.
Thin Film Transistor (TFT) Liquid Crystal Display
Thin Film Transistor (TFT) liquid crystal displays are formed by introducing thin film transistor switches into twisted nematic (TN) liquid crystal displays, overcoming the disadvantages of passive matrix displays such as cross-interference, low information capacity, and slow writing speed, greatly improving display quality, making it applicable to high-resolution full-color displays in computers. Currently, the thin film transistors (TFT) used are based on the structure of amorphous silicon thin film transistors (α-Si TFTAM-LCD).
On the lower glass substrate, a TFT array is built, with each pixel’s ITO electrode connected to the TFT’s drain electrode, the gate connected to the scanning bus, and the source connected to the signal bus. When a scanning signal voltage is applied, the source electrode turns on, applying the signal voltage to the storage capacitor, charging it. During the frame frequency, the signal voltage on the storage capacitor is applied to the liquid crystal pixels, putting them in the on state. During the next addressing, the size of the signal voltage determines whether to charge or discharge. In this way, each pixel is isolated by the thin film transistor switch component, preventing cross-interference while ensuring the liquid crystal response speed meets the frame frequency speed, while the size of the stored information determines the grayscale level. Currently, the grayscale can reach 256 levels, allowing for 16.7 million colors, achieving nearly full-color display. Since the industry formed in the 1990s, the production lines for thin film transistor (TFT) liquid crystal displays have evolved from the first generation to the sixth generation, with each upgrade significantly increasing the area of the glass substrate, improving yield, and reducing costs. For instance, the glass substrate size for the seventh generation thin film transistor (TFT) liquid crystal display production line will reach 1870*2200mm, currently allowing for the production of 94cm (37-inch) LCD TVs, with the maximum size for laptop screens at 38.1cm (15 inches) and monitor screens reaching up to 63.5cm (25 inches). Another trend in the development of thin film transistor (TFT) liquid crystal displays is miniaturization, lightweight, and low power consumption. Based on the development of new materials, innovations in manufacturing processes, increased equipment precision and automation, and advances in software technology, the speed of product updates for thin film transistor (TFT) liquid crystal displays is very rapid.
Structure and Principle of LCD Backlight Source
Classification of Backlight and Structure of Lamps
Backlight (hereinafter referred to as B/L) is classified into Direct Light Type and Side Light Type based on the arrangement of lamps. Side Light Type requires a light guide plate to direct the light emitted from the side lamps to the front of the B/L, while Direct Light Type allows light emitted directly from the lamps to the front of the B/L, thus not requiring a light guide plate.
The lamps used in TFT-LCD backlight sources are cathode fluorescent lamps (CFL), which emit visible light when an external voltage is supplied to the cathode, scanning the phosphor to produce visible light. The structure of CFL generally consists of a glass tube, electrodes, sealed gas (Hg, Ar, Ne), and phosphor. CFL generates visible light by scanning the ultraviolet rays produced from the sealed mercury onto the phosphor coated on the inner wall of the glass tube. To facilitate the easy ignition of a small amount of mercury and to suppress the evaporation of cathode material, argon is sealed inside the glass tube. CFLs are classified into two types based on the electron emission mechanism: CCFL (Cold Cathode Fluorescent Lamp) and HCFL (Hot Cathode Fluorescent Lamp).
1. Lamp: Receives high voltage from the inverter to generate visible light. Primarily uses CCFL (Cold Cathode Fluorescent Lamp) and HCFL (Hot Cathode Fluorescent Lamp).
2. Lamp Housing: Reflects the light emitted from the lamp to the light guide plate. Made of materials such as brass, aluminum, and thin films coated with Ag, etc.
3. Light Guide Panel: Generally made of acrylic (PMMA) using injection molding or casting methods to guide the incoming light and evenly distribute it.
4. Reflector: Primarily made of polyethylene terephthalate (PET) material to reduce light loss entering the light guide plate, providing reflection functionality.
5. Diffuser Down: Generally made of polyethylene terephthalate (PET) material shaped into spheres using acrylic resin, evenly diffusing the light emitted from the light guide plate while also focusing the light.
6. Bottom Prism: Generally made of polyethylene terephthalate (PET) material, shaped into prisms to focus light, increasing brightness by 1.55 times at the surface.
7. Top Prism: Functions similarly to the Bottom Prism, increasing brightness by 1.33 times at the surface of the Bottom Prism.
Prisms are arranged in a cross pattern to collect light in both the X and Y axis directions.
8. Diffuser Up (Protector Film): Shares the same structure as Diffuser Down, primarily serving to protect the prism, also referred to as a protective film. A transparent diffuser is used, which may slightly reduce the light collected by the Top Prism, but is used to minimize adverse effects on prism characteristics.
Prospects of LCD Technology
In recent years, various non-liquid crystal flat displays such as OLED, DMD, and FED have matured and entered the market, posing challenges to LCD displays due to their deficiencies, such as low brightness and difficulty in scaling to large screens. Recently, some have claimed that OLED will replace liquid crystal displays.
In fact, due to the different advantages and disadvantages of various displays, it is generally impossible for them to completely replace one another. However, it is entirely realistic for one type of display to replace or impact another in certain aspects. LCD displays must face these challenges and competition. This challenge and competition pose both a threat to the LCD display industry and a driving force for its development.
In the future, LCD displays will strive to make significant breakthroughs in the following major areas to address challenges from other types of displays:
1. Improve display brightness and contrast through methods such as developing reflective displays, improving backlight sources, increasing aperture ratios, and enhancing polarizer transmittance.
2. Improve materials, device structures, and processes to enhance the response speed of liquid crystal displays. At the same time, efforts will be made to develop new types of liquid crystal display modes with fast response times to better meet video display requirements.
3. The narrow working temperature range is a significant defect determined by liquid crystal materials, which can only be overcome by improving the liquid crystal materials themselves. Currently, liquid crystal materials that can operate from -50 to 90 degrees Celsius have been developed. Additionally, the development of auxiliary heating systems will ensure that the working temperature range of liquid crystals will be significantly widened.
4. To achieve large screen displays, liquid crystal displays have opened up a new path—projection displays. Based on the original transmission-type amorphous silicon TFT projection displays, there has been a transition to polysilicon TFT projection displays in recent years. While polysilicon can improve aperture ratios by over 10-15%, significantly enhancing display brightness and clarity, it is still not ideal. To compete with PDP and other large screen displays, liquid crystal displays have developed a new type of “liquid crystal on silicon” (LCOS), integrating large-scale integrated circuits as substrates with liquid crystal to create reflective micro-liquid crystal displays. This allows for projection of large screens exceeding 50 to 100 inches using external light sources.
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