LED (Light EmitTIng Diode) means light emitting diode. It is a solid-state semiconductor device that can directly convert electricity into light. The heart of the LED is a semiconductor chip. One end of the chip is attached to a bracket, one end is the negative electrode, and the other end is connected to the positive electrode of the power supply, so that the entire chip is encapsulated by epoxy resin. The semiconductor wafer is composed of two parts. One part is a P-type semiconductor, in which holes are dominant, and the other end is an N-type semiconductor, where mainly electrons. But when the two semiconductors are connected, a "PN junction" is formed between them. When electric current acts on the chip through the wire, the electrons will be pushed to the P region, where the electrons recombine with holes, and then emit energy in the form of photons. This is the principle of LED light emission. The wavelength of light, which is the color of light, is determined by the material that forms the PN junction. Although it can also be used as a display screen, it is usually used for large screen displays, such as large billboards outside the building.
LED development history
Fifty years ago, people already knew the basic knowledge of semiconductor materials that can produce light. The first commercial diode was produced in 1960. LED is the abbreviation of English light emitTIng diode (light emitting diode), its basic structure is a piece of electroluminescent semiconductor material, placed on a lead frame, and then sealed with epoxy resin around, that is, solid package, so it can To protect the internal core wire, so the anti-vibration performance of LED is good.
At first, LEDs were used as indicator light sources for instrumentation. Later, LEDs of various light colors were widely used in traffic lights and large-area display screens, resulting in good economic and social benefits. Take the 12-inch red traffic signal as an example. In the United States, a 140-watt incandescent lamp with long life and low light efficiency was originally used as the light source. It produces 2000 lumens of white light. After passing through the red filter, the light loss is 90%, leaving only 200 lumens of red light. In the newly designed lamp, Lumileds uses 18 red LED light sources, including circuit losses, and consumes a total of 14 watts to produce the same light effect. Automotive signal lights are also an important field of LED light source applications.
For general lighting, people even need white light sources. In 1998, the white LED was successfully developed. This LED is made by encapsulating a GaN chip and yttrium aluminum garnet (YAG). The GaN chip emits blue light (Î»p = 465nm, Wd = 30nm). The Ce3 + -containing YAG phosphor produced by high-temperature sintering emits yellow light after being excited by this blue light, with a peak value of 550nm. The blue LED substrate is installed in a bowl-shaped reflective cavity, covered with a thin layer of resin mixed with YAG, about 200-500nm. The blue light emitted by the LED substrate is partially absorbed by the phosphor, and the other part of the blue light is mixed with the yellow light emitted by the phosphor to obtain white light. Now, for InGaN / YAG white LEDs, by changing the chemical composition of the YAG phosphor and adjusting the thickness of the phosphor layer, various colors of white light with a color temperature of 3500-10000K can be obtained. This method of obtaining white light through a blue LED has a simple structure, low cost, and high technology maturity, so it is used the most.
In the 1960s, scientists and technicians developed LED light-emitting diodes using the principle of semiconductor PN junction light emission. The LED developed at the time was made of GaASP and its light emission color was red. After nearly 30 years of development, the LEDs that everyone is very familiar with can now emit red, orange, yellow, green, blue and other colors. However, the white LEDs required for lighting have only developed in recent years.
In recent years, large LED display screens have appeared in people's lives, such as large billboards outside buildings, large outdoor TVs, etc., which are manufactured using the principle of LED dot matrix.
The abbreviation of LCD (Liquid Crystal Display), called in Chinese: liquid crystal display. The LCD is constructed by placing a liquid crystal cell between two parallel glass substrates, a TFT (thin film transistor) on the lower substrate glass, and a color filter on the upper substrate glass, which controls the liquid crystal molecules by changing the signal and voltage on the TFT Rotate the direction, so as to control whether the polarized light of each pixel is emitted or not to achieve the display purpose. Now that LCD has replaced CRT as the mainstream, the price has dropped a lot, and it has been fully popularized. The LCD can only be displayed through the backlight panel. At present, the more common TFT and SLCD on the market belong to the category of LCD.
LCD liquid crystal display principle It uses the electro-optic effect of liquid crystal to control the transmittance and reflectance of the liquid crystal cell through a circuit, thereby generating beautiful images with different gray levels and up to 16.7 million colors. The main imaging device of an LCD display is a liquid crystal panel. The size of the LCD projector depends on the size of the liquid crystal panel. The smaller the liquid crystal panel, the smaller the size of the projector.
The LCD liquid crystal display can be divided into two types of single-chip and three-chip according to the number of internal liquid crystal panels. Most modern liquid crystal display screens use a 3-piece LCD panel. The three-piece LCD display uses red, green, and blue liquid crystal panels as the control layers for the red, green, and blue light, respectively. The white light emitted by the light source passes through the lens group and converges to the dichroic mirror group. The red light is first separated and projected onto the red liquid crystal panel. The image information expressed in transparency under the "recording" of the liquid crystal panel is projected into the image. Red light information. Green light is projected onto the green liquid crystal panel to form the green light information in the image. Similarly, blue light passes through the blue liquid crystal panel to generate blue light information in the image. The three colors of light converge in the prism and projected by the projection lens A full-color image is formed on the projection screen. The three-chip LCD display has higher image quality and higher brightness than the single-chip LCD display. The LCD display is small in size, light in weight, simple in manufacturing process, high in brightness and contrast, and moderate in resolution. The market share of LCD displays now accounts for more than 70% of the overall market share, which is the current market share The highest and most widely used display screen.
Main technical parameters of LCD
LCD liquid crystal display contrast is a very important parameter. The accessories such as control IC, filter and orientation film used in LCD manufacturing are related to the contrast of the panel. For the average user, a contrast ratio of 350: 1 is enough, but in The contrast level in the professional field cannot meet the needs of users. Relative to CRT monitors easily reaching a contrast ratio of 500: 1 or higher, only high-end LCD monitors can achieve this level. The LCD monitors of first-line brands such as Samsung, Asus, and LG on the market today can all reach a level of 1000: 1 contrast.
The LCD emits light by means of an additional light source. The earliest liquid crystal display has only two lamps at the top and bottom. Up to now, the lowest popular type is also four lamps, and the high-end is six lamps. The four-lamp design is divided into three placement forms: one is that there is a lamp on each of the four sides, but the disadvantage is that there will be a black shadow in the middle, and the solution is to arrange the four lamps from top to bottom, The last one is the "U" -shaped display, which is actually two lamps produced by the two lamps in disguise. The six-lamp design actually uses three lamps. The manufacturer bends the three lamps into a "U" shape and places them in parallel to achieve the effect of six lamps.
3 Signal response time Response time refers to the response speed of the LCD to the input signal, that is, the response time of the liquid crystal from dark to bright or from bright to dark. Brightness from 10%-> 90% or 90%-> 10% of the time), usually in milliseconds (ms). To make this clear, we need to start with the human eye's perception of dynamic images. There is a "visual residue" phenomenon in the human eye, and the high-speed motion picture will form a short impression in the human brain. Animation, movies, etc. until now the latest game is the application of the principle of visual residue, allowing a series of gradual images to be displayed quickly and continuously in front of the human eye, forming a dynamic image. The acceptable screen display speed is generally 24 frames per second, which is the origin of the 24 frames per second of the movie. If the display speed is lower than this standard, people will obviously feel the pause and discomfort of the screen. According to this index calculation, the display time of each screen needs to be less than 40ms. In this way, for a liquid crystal display, a response time of 40ms becomes a hurdle, and a display higher than 40ms will have a noticeable flickering phenomenon, which makes people feel dazzled. If you want the image to be flash-free, it is best to achieve 60 frames per second.
4 Viewing angle
The viewing angle of the LCD is a headache. When the backlight passes through the polarizer, liquid crystal and alignment layer, the output light has directionality. That is to say, most of the light is emitted vertically from the screen, so when viewing the LCD from a large angle, you can not see the original color, or even see all white or all black. In order to solve this problem, manufacturers have also started to develop wide-angle technology, so far there are three more popular technologies, namely: TN + FILM, IPS (IN-PLANE -SWITCHING) and MVA (MULTI-DOMAIN VERTICAL alignment).
The difference between LCD and LED
The biggest difference between liquid crystal and plasma is that liquid crystal must rely on passive light sources, and plasma TVs are active light-emitting display devices. At present, the mainstream liquid crystal backlight technology on the market includes LED (light emitting diode) and CCFL (cold cathode fluorescent lamp). LCD is LCD .. LCD is the abbreviation of Liquid Crystal Display. The structure of LCD is to place liquid crystals between two parallel glasses. There are many vertical and horizontal thin wires between the two glasses. The rod-shaped crystal molecules are controlled to change direction and refract light to produce a picture. The liquid crystal itself does not emit light, but can only produce color changes, and requires a backlight to see the displayed content. Conventional notebook screens use cold cathode fluorescent tubes (CCFL) as the backlight, and LED-backlit screens use light-emitting diodes, which is the difference between the two. The white LED is a point light source, and the CCFL tube is a strip light source. Small white LEDs use DC power supply, and can be connected in series and parallel when used multiple times, but those with several watts or more should consider a suitable driving circuit to improve efficiency. CCFL pipes must be used with â€œhigh-voltage platesâ€.
There are only several LCD backlighting methods, including LED (Light Emitting Diode) and CCFL (Cold Cathode Fluorescent Lamp) or CCFT (Cold Cathode Fluorescent Tube).
CCFL (Cold Cathode Fluorescent Lamp) backlight is currently the most important backlight product for LCD TVs. Its working principle is that when a high voltage is applied to both ends of the lamp tube, a small number of electrons in the lamp tube hit the electrode at high speed to produce secondary electron emission and start to discharge. After the mercury or inert gas in the tube is impacted by the electrons, the excitation radiation emits 253.7nm The ultraviolet light generated excites the phosphor coated on the inner wall of the tube to produce visible light. CCFL lamp life is generally defined as: at 25 â„ƒ ambient temperature, the lamp is driven at a rated current, the working time length of the brightness is reduced to 50% of the initial brightness is the lamp life. At present, the nominal life of LCD TV backlight can reach 60,000 hours. CCFL (Cold Cathode Fluorescent Lamp) backlight is characterized by low cost, but the color performance is not as good as LED backlight.
LED backlight uses light emitting diodes as the backlight light source, which is the most promising technology to replace traditional cold cathode fluorescent tubes in the future. The light-emitting diode is made of several thin layers of doped semiconductor materials, one layer has excessive electrons, and the other layer lacks electrons to form positively charged holes. When working, current passes through, and the electrons and holes combine with each other. The energy is released in the form of light radiation. Light emitting diodes with different light emitting characteristics can be obtained by using different semiconductor materials. The light-emitting diodes that have been put into commercial application can provide red, green, blue, cyan, orange, amber, white and other colors. The white LED backlight is mainly used on mobile phones, and the LED backlight light source used on LCD TVs can be white, or red, green, and blue primary colors. In high-end products, multi-color LED backlight can also be used to further improve color Expressive, such as six primary colors LED backlight light source. The advantage of using LED backlight is that the thickness is thinner, about 5 cm, and the color gamut is very wide, which can reach 105% of the NTSC color gamut. The luminous flux of black can be reduced to 0.05 lumens, which makes the LCD TV contrast ratio as high as 10000: 1 . At the same time, the LED backlight light source also has a life of 100,000 hours. At present, the problem that restricts the development of LED backlight is mainly cost. Since the price is much higher than that of cold fluorescent tube light sources, LED backlight light sources can only appear in high-end LCD TVs abroad.
Advantages of LED backlight
1. The screen can be made thinner. Let's pay attention to some LCD monitors. You can see that several filament-shaped CCFL lamps are arranged. In order to make the screen shine evenly, you need to add some other devices, so you can't do it very well. Thin; and the backlight is different, the LED backlight itself is a planar luminescent material, without the need to add other devices.
2. The picture effect is better. CCFL backlight screen generally has a different brightness in the middle and around, and at the same time, the screen is white when it is completely black.
3. No yellowing and darkening CCFL fluorescent lamps are the same as fluorescent lamps, and will age over time, so traditional notebook screens will yellow and darken after two or three years, and the life of LED backlight screens will be much longer, at least two three times.
4. More power saving Everyone knows that fluorescent lamps require a very high voltage to bombard mercury vapor, so the power consumption of the CCFL screen is large, generally the power consumption of 14 inches is more than 20 watts. LED is a kind of semiconductor, working under low pressure, simple structure, low power consumption, especially conducive to the endurance of the notebook.
5. More environmentally friendly Mercury in CCFL lights has great pollution to the environment, and harmless recycling has great difficulties
The working principle of CCFL cold cathode cold fluorescent lamp
The physical structure of CCFL cold cathode fluorescent lamp tube is to enclose a inert gas Ne + Ar mixed gas in a glass tube, which contains trace mercury vapor (several mg), and apply phosphor on the inner wall of the glass. The CCFL cold cathode fluorescent lamp tube emits light through the electrodes at both ends of the lamp tube, so that the ultraviolet light excited by the gaseous mercury in the lamp tube collides with the phosphor powder on the wall of the lamp tube, thereby emitting light. Its wavelength is determined by the properties of the phosphor material.
The defects of CCFL cold cathode fluorescent tubes The current CCFL light sources commonly used in LCD TVs are very close to the fluorescent tubes we use every day, no matter from the luminescence principle or the physical structure. This light source has the excellent characteristics of simple structure, small surface temperature rise of the lamp tube, high brightness of the lamp tube surface, and easy processing into various shapes. However, the service life is short, it contains mercury, and the color gamut is narrow, which can only reach 70% to 80% of NTSC. For large-size TV screens, it is also difficult to increase the voltage of the CCFL and process the lengthened tubes.
First, the most troublesome problem is the short service life. The service life of the CCFL backlight is generally 15,000 hours to 25,000 hours. The longer the LCD (especially the LCD screen of a notebook computer), the more the brightness decreases. The LCD screen will become dark and yellow after 2 to 3 years of use This is due to the shortcomings of the CCFL's short service life.
Second, it restricts the display of the color of the LCD. Each pixel in the LCD is composed of three rectangular color blocks of R, G, and B, and the color performance of the LCD completely depends on the performance of the backlight module and the color filter. The three primary colors of the color filter film default that the white light emitted by the CCFL is as uniform as sunlight (the components of the three primary colors), but the CCFL backlight module cannot actually meet the design requirements, and can only reach about 70% of the NTSC standard.
Third, the structure is complex and the brightness output uniformity is poor. Since the cold cathode fluorescent lamp is not a planar light source, in order to achieve uniform brightness output of the backlight, the LCD backlight module needs to be equipped with many auxiliary devices such as a diffuser, a light guide plate, a reflective plate, etc. The difference in brightness between the edges and the center is obvious.
Fourth, the volume is large and the power consumption is not ideal. Since CCFL backlights must contain complex optical devices such as diffusers and reflectors, the size of LCDs cannot be further reduced. In terms of power consumption, LCDs that use CCFL as a backlight are also unsatisfactory. CCFL backlights for 14-inch LCDs often consume 20W or more of power.
Of course, in the past two years, domestic and foreign manufacturers have made some improvements to the disadvantages of traditional CCFL, and they seem to have reached a very high level. Manufacturers â€™propaganda is even more amazing, but these improvements are limited and cannot completely eliminate CCFL backlights Inherent technical flaws
From the current situation that the backlight is still dominated by CCFL tubes, the cost may be slightly lower and the technology is relatively mature. LED backlighting is still limited to small-screen products such as mobile phones, MP3, MP4, etc. For large-screen products, it is still a direction of effort, but it is more advantageous to save power.
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