Editor's note: LED is the abbreviation of English light emitting diode, namely: light excitation diode, belonging to a kind of semiconductor component. The core of the LED is a wafer consisting of a p-type semiconductor and an n-type semiconductor. There is a transition layer between the p-type semiconductor and the n-type semiconductor, called a pn junction.
The principle of LED lighting technology
LED is the abbreviation of English light emitting diode, namely: light excitation diode, belonging to a kind of semiconductor component. The core of the LED is a wafer consisting of a p-type semiconductor and an n-type semiconductor. There is a transition layer between the p-type semiconductor and the n-type semiconductor, called a pn junction. In some PN junctions of semiconductor materials, the injected minority carriers recombine with the majority carriers to release excess energy in the form of light, thereby directly converting electrical energy into light energy. The PN junction adds a reverse voltage, and minority carriers are difficult to inject, so they do not emit light. Such a diode fabricated by the principle of injection electroluminescence is called a light-emitting diode, and is generally called an LED. When it is in the forward working state (ie, the forward voltage is applied to both ends), when the current flows from the anode of the LED to the cathode, the semiconductor crystal emits light of different colors from ultraviolet to infrared, and the intensity of the light is related to the current. For example, LEDs are like a burger. The material that can be illuminated is the â€œmeat cakeâ€ in the sandwich, and the upper and lower electrodes are the bread with meat. Through the research on the luminescent materials, people have gradually developed various LED components with higher light color and higher light efficiency, but no matter how they change, the overall illuminating principle and structure of the LEDs have not changed much.
LED light source advantages
Red LED white light LED light source, so since the birth of the 1960s, has been a great development and application. Compared with the older generation of illuminating devices such as incandescent lamps and fluorescent lamps, the advantages of LEDs are mainly reflected in the following aspects: 1. The LED has no glass casing in its structure, and it is not necessary to vacuum or rush in the lamp tube like an incandescent lamp or a fluorescent lamp. Into a specific gas, so earthquake and impact resistance is good, bringing convenience to all aspects of production, transportation and use.
As one of the latest lighting technologies, the advantages of LED are not only reflected in the quality of lighting, but also greatly exceed the incandescent lamp, fluorescent lamp, etc. in terms of production, manufacturing and ease of use.
2. The size of the LED components can be made very small, which is more convenient for the layout and design of various devices.
3. The light energy emitted by the LED is highly concentrated, concentrated in a small wavelength window, and has high purity.
4. The life of LED components is very long, generally between 50,000 and 100,000 hours. Even frequent switching will not affect the service life.
5. LED response time is very fast, in the microsecond level.
6. LED's illuminance is very strong, and the brightness attenuation is much lower than traditional light sources.
7. LED does not add "mercury" in the production process, it is very environmentally friendly.
8. LED can be driven by low-voltage DC, which requires less requirements on the use environment.
These advantages of LED lighting equipment not only lay a solid foundation for its wide application in the field of daily lighting, but also create a promising road for LEDs to enter the field of display equipment.
However, LED does not have so many advantages from the beginning, but after a long period of development, let's take a look at the development path of LED from birth to the present.
The development of LED lighting technology
When the LED just appeared, its color and efficiency were relatively low. At that time, the main luminescent material used was "GaAsP". The driving current was 20 mA, and only red light was emitted, and the luminous efficiency was only 0.1 lm/W. Therefore, the brightness could only meet the indications on some instruments and electrical appliances, and it was not obtained. Extensive use and attention.
In the following decades, some new luminescent materials were gradually introduced into the LEDs, and LEDs gradually began to show strong development potential. In the mid-1970s, by introducing the elements In and N, the LED can emit green light with a wavelength of 555 nm, yellow light with a wavelength of 590 nm, and orange light with a wavelength of 610 nm, while the luminous efficiency is also increased to 1 lm/W. . In the early 1980s, LED light sources using GaAlAs appeared, making the red LED's luminous efficiency reach 10 lumens per watt. After entering the 1990s, the development of two new materials, GaAlInP, which emits red and yellow light, and GaInN, which emits green and blue light, have greatly improved the light efficiency of LEDs. In 2000, the former made LEDs in the red and orange areas (wavelength 615 nm) with a luminous efficacy of 100 lm/W, while the latter made LEDs in the green area (wavelength 530 nm). Reach 50 lumens / watt. Not only does it exceed the traditional incandescent lamp, but it is also very close to the fluorescent lamp with higher luminous efficiency. According to the development speed of materials science, LEDs with luminous efficiency of 200 lm/W will also be available in a relatively fast time.
In the history of LED lighting technology, the emergence of white LEDs has become an important breakthrough in the rapid development of LEDs. At the end of the last century, inspired by the principle of fluorescent light, LED manufacturers used a blue phosphor on the high-brightness blue LED die to emit a white light. In addition, by using different phosphors, a variety of white LEDs with a color temperature of 4500 to 10 000 K and a color temperature of 2850 to 3800 K can be emitted, and the white LED has the capability of becoming a new generation of lighting equipment. At present, the luminous efficiency of white LEDs has exceeded 30 lumens / W, and some products have exceeded the level of 50 lumens / W, with a formal large-scale practical basis. After so many years of development, LED lighting technology has become a relatively mature thing. There are not only LED products that can emit various colors, but also a large number of LED products that can be used for direct lighting. In addition, in the display In the field, LED products have also undergone an evolutionary process from monochrome to color, from low-resolution text to high-resolution image display, which is increasingly affecting our lives and work.
Trends in incandescent, fluorescent and LED lighting efficiency
Since LEDs were originally produced as a kind of illuminating element, LED technology was born with a close relationship with the light. The various LED products currently available can also be roughly divided into two categories: LED indication/illumination equipment and LED display imaging equipment according to different purposes.
LED lighting equipment
The greatest value of LEDs is still reflected in the field of lighting. Since LED lighting technology completely surpasses traditional light sources, many people are optimistic about the future LED market opportunities in the lighting market. According to CIR forecasts, the global LED market will grow from 3.2 billion US dollars in 2004 to 5.6 billion US dollars in 2008, among them, The market value of high-brightness LEDs will increase from US$1.6 billion to US$2.64 billion. The ultra-high-brightness LED market will grow rapidly from 2006 and account for 22% of the global market in 2008.
The most critical reason for LEDs to have such a broad business prospect is the unique features of their high energy efficiency. As the third-generation semiconductor lighting material, LED has 100 times longer life than ordinary incandescent lamps, consumes much less energy than incandescent lamps, and has lower replacement cost. It has small size, safety, pollution-free, maintenance-free, and fast response. And other subsidiary advantages. Due to its small size and flexible configuration, theoretically, LED lighting devices can be made in any desired form. Therefore, it is foreseeable that LED light source will become the new favorite of people in the future lighting market.
As long as the package is opened, the built-in LED bulb will automatically light up, and the LED is actually widely used.
LED imaging equipment
In the mid-1990s, with the maturity of high luminous efficiency LED components, the market began to show a new type of display device - large-screen LED display, in all public places, such as stations, stock exchanges, we can see To this LED display device. The original LED display device principle is relatively simple: each LED element that can emit monochromatic light is used as the pixel point of the display device, and the display of text or simple image is realized by controlling the brightness of each pixel point. This kind of equipment is the easiest to implement and lowest cost of all LED display devices, but it is still used in many occasions because of its long life and high brightness. For example, most of the buses and subways in Beijing have LED reminders that can scroll through various text messages, station names, time, and so on.
In addition to the relatively small LED electronic display, there is a more common LED display device, which is a modular color LED display for building large-size displays.
This modular LED display device is primarily used to meet the needs of permanent or temporary large-area displays. For example, in some concerts, evening parties, and conferences, in order to better set off the atmosphere, a whole piece of display device used as a background is needed. Before the LED display appeared, this was almost an impossible task. . In terms of outdoor display, traditional neon lights can make very beautiful effects even though they have better brightness, but at best they are only lamps, not display devices, and can not achieve diversified and flexible display functions, but also more and more. The dissatisfaction of the advertisers, so the emergence of modular large-screen LED display, just to meet their needs.
Based on this need, the LED display device adopts a modular design. Generally, a rectangular module has a size of 0.25 square meters to 1 square meter. Through the arrangement and combination of several modules, display areas of various sizes can be formed. In addition to being assembled into flat-panel displays of different sizes, through a reasonable design, it is also possible to use a LED screen to construct a stereoscopic display, for example, a cube display with 6 sides simultaneously displayed, and 6-sided independent display through a screen control device. Or the overall display effect is very flexible to use.
Unlike basic LED display devices, modular LEDs can display a full full-color image, and viewers can get an image that is close to a traditional display at a certain viewing distance, thanks to its pixel structure. Microscopically, the modular LED display device uses a pixel structure very similar to that of a flat-panel TV, that is, each pixel is composed of LED elements that can emit R/G/B monochromatic light, and is arranged through three primary colors. Combined, each pixel can emit light of various colors. The LED display used in the U2 vertigo concert is rough due to the large dot pitch.
Due to the large-area display, the distance between each pixel of the modular LED display is much larger than that of TVs, monitors, etc. Therefore, on such devices, the resolution is not represented by resolution as in the case of a general display device, but by the dot pitch between each pixel. Generally speaking, LED screens used indoors generally have a dot pitch of 10 mm, 8 mm is medium to upper, and 6 mm is the best product, but the corresponding price is relatively high. The outdoor LED screen has a larger dot pitch, generally 10 mm, 16 mm, etc., but the outdoor screen is better than the indoor screen in waterproof and dustproof effect.
In recent years, modular LED displays have been widely used in the world. Their high display brightness and flexible matching method have been recognized by most people and have become the main varieties of outdoor display devices. And it has been widely used in activities, performances, exhibitions and other industries.
LEDs face problems in the field of lighting:
(1) First of all, the problem of luminous efficiency. The most important way to improve the luminous efficiency of LEDs is to improve the structure and manufacturing process of semiconductor luminescent materials and LED chips . Since this part of the work requires a strong theoretical research base and advanced semiconductor process equipment, it is still difficult to achieve a target of 200 lumens per watt.
(2) High power problems. As illumination, the luminous flux of a single LED output must be large enough. To increase the luminous flux of the LED, sufficient electrical power must first be injected. However, the temperature rise of the LED chip should not be too high, otherwise the performance, especially the service life, will be greatly affected. Obviously, LED devices and luminaires with larger input power must have a good heat dissipation structure in addition to larger chips. Now some famous foreign companies have designed and developed some special LED device structures, and have achieved good results.
(3) Since LED lighting needs to be composed of multiple LED tubes, its parameter dispersion is also a technical problem. In addition to pre-selection, classification, and consistency as much as possible, it is necessary to design a reasonable luminaire structure (including LED arrangement and position layout) and study appropriate driving circuits to prevent occasional energy concentration and burn some LEDs.
(4) In addition, since a plurality of LEDs constitute a lighting fixture, the LEDs are connected in parallel and in series. In the process of use, as long as there is a short circuit or open circuit of the LED, it will cause the whole small piece or the whole LED to be extinguished, which will affect the lighting effect. To this end, simple and inexpensive protection circuits must be studied to minimize this adverse effect.
LEDs face problems in the field of display technology: (1) Like the main problems encountered in the field of lighting, LEDs are also facing the problem of luminous efficiency as a new type of backlight. At present, the conventional CCFL cold cathode fluorescent lamp has a large power consumption and a general light-emitting quality, but its luminous efficiency can reach 50-100 lumens/watt, and the luminous efficiency of the white LED device is only 20 lm/W or even lower at the beginning, which is It is doomed that the LED is not suitable as a backlight for LCD displays at the beginning. However, the luminous efficiency of white LEDs has increased by 60% per year. So far, the luminous efficiency of white LED devices has exceeded 50 lm/W and has begun to reach practical levels. (2) The cost of the LED backlight system is higher than that of the cold cathode fluorescent tube. At present, the price of LED backlight module components is about 5 times that of CCFL backlight. The larger the screen size, the higher the cost of adopting LED backlight technology. However, the LED industry also has a similar "Moore's Law" in the microprocessor industry - Haitz's Law, named after Roland Haitz, a former technical scientist at Agilent (leading in the LED field). The content is that the price of LED will be 1/10 of the original 10 years, and the performance will be 20 times higher. If this law can be continuously fulfilled, and as capacity increases, the cost of LED backlights will fall rapidly. It is expected that the price of LED backlights will be reduced to about 2 times that of CCFT backlights in the next one or two years, and it is only one step away from mass popularization.
Large-size TVs use a lot of LED components, so heat dissipation and price are more difficult problems.
(3) In the RGB-LED backlight, the LED components that emit each of the primary colors use different luminescent materials, so the performance degradation will be inconsistent after long-term use, which may also result in a larger display effect at that time. Deviation.
to sum up:
As one of the most important 100 inventions of human beings in the 20th century, LED technology has finally ushered in a long-distance application after more than 30 years of development. I believe that through the introduction of this article, you will also be where LEDs will be. Several aspects have had a certain understanding of the impact on our lives. No matter where LED technology goes, this promising technology will definitely bring great changes to our lives.
LED technology in the field of display
Due to the excellent light-emitting characteristics of LEDs, LED components can be made not only as direct display devices, but also in recent years, LEDs have been gradually introduced into existing flat panel display technologies, especially liquid crystal display technology, and it is very likely that the next LED will be monopolized by LEDs. Industry. LED backlights have better color saturation, very fast response time, and weak light leakage. If you increase the contrast, regional control and other means, the performance is much better than the cold cathode fluorescent lamp (CCFL). Moreover, cold cathode fluorescent lamps contain harmful substances such as mercury, and LEDs are more environmentally friendly than others. LEDs have become popular in small-sized panels used in many mobile devices. Backlights for large-size panels such as laptops and LCD TVs will be the next heavyweight application for the LED industry.
At present, the application of LED technology in the field of liquid crystal mainly uses LED light-emitting elements to replace the previous CCFL fluorescent light source as a backlight for liquid crystal display devices. If you want to subdivide again, you can divide it into white LED backlight and RGB-LED backlight according to the color of the light source emitted by the LED.
White LED backlight technology
In the above, we have mentioned that one of the breakthroughs in the development of LEDs is to achieve the goal of emitting white light. In the imaging principle of the liquid crystal display device, the white light emitted by the backlight passes through the liquid crystal layer and then passes through the R/G/B color filter film to become an independent primary color. In this process, the key to determining the final color of the liquid crystal display device is not the liquid crystal layer, but the light quality of the backlight. The higher the purity of each primary color of RGB in the spectrum of the backlight, the more pure the original color can be restored at the end, and only the pure RGB three primary colors can be restored to produce a pure and true color effect.
The traditional CCFL light source is not ideal in terms of illuminating quality, so it can't restore very pure color. If it is measured by the gamut range, it is generally about 72% of the NTSC ratio, even by using a modified CCFL light source. It can only reach the color gamut range of about 90% of NTSC, which causes the vivid and true colors in the real world to be restored on the LCD TV, thus affecting the image quality. By adopting a white LED backlight with high luminous quality, the color gamut of the LCD TV can easily reach the NTSC ratio of about 100%, which has an obvious effect on the color effect.
Another advantage of using white LEDs is that it can effectively reduce the thickness of the LCD panel, which is very suitable for the pursuit of thinness and lightness in mobile devices. When introducing the advantages of LED technology, one thing is that the volume of LEDs can be made very small. In the liquid crystal display device, the shape and size of the LED backlight panel vary depending on the shape and size of the liquid crystal panel. The LED backlight panel is basically rectangular or elongated. It has two basic structures: side light and bottom light. The side-emitting structure is mainly used for a strip-shaped backlight board (generally having a width greater than 2 times); and the bottom-emitting structure is mainly used for a backlight board having a length and a width. When the LED is lit, the light is incident on the transparent plexiglass, so that the entire illuminating surface can see the bright light, which is called the edge light effect. The top of the plexiglass is made of micro-beads to make the light on the entire luminous surface more uniform. The top of the plexiglass has a creamy white plastic film that softens the light. The sides of the backlight panel are sealed with silver shading tape. Each LED lighting unit has two diodes connected in series, and several columns form an LED array (depending on the length of the backlight panel). Taking notebook products as an example, after using white LEDs as the backlight of the liquid crystal display, the LED backlights are arranged in multiple groups at the bottom to replace the traditional CCFL thin tubes. On the SONY VAIO TX notebook, 40 are used. LED lighting element. After replacing the thin lamp tube with the LED, the original light guide plate is thick and wedge-shaped, and the light guide plate can be made uniform in thickness and can reduce the overall thickness. Another advantage of LEDs - power saving, is also favored in applications. For mobile products such as notebooks, the use of LEDs can increase the life of the entire machine, thereby increasing the durability of the mobile. However, for TV sets, the use of LED backlights at this stage does not significantly reduce power consumption, mainly because TVs have higher requirements for brightness. In order to meet the demand for brightness, it is necessary to increase the number of LED components, resulting in overall power. There was no significant reduction. However, it is believed that with the advent of higher luminous efficiency LED components, LCD TVs will be able to achieve high-brightness display with a smaller number of LED components, and the power consumption will drop significantly.
RGB-LED backlight technology
Since RGB three primary colors can be used to match any color in nature, in addition to white LEDs, in the field of liquid crystal display, there is also an LED backlight technology that can emit R/G/B three kinds of monochromatic light.
The RGB-LED backlight is a wide color gamut that cannot be achieved by conventional CCFL sources through LED components that emit high-purity red, green, and blue light. At present, mainstream RGB-LED backlights can reach the NTSC color gamut range of 105%, and as long as more powerful LED devices are used, more than 120% of the NTSC color gamut can be achieved. This will be a very effective means of improving image quality for TVs that rely on reduced images. It is foreseeable that RGB-LED will become an important development breakthrough for LCD TVs in the next few years.
In addition to better color gamut performance, the use of RGB-LED light source can also effectively enhance the contrast of the TV, to achieve more accurate color gradation and more layered picture. Since the entire backlight is composed of a plurality of tiny LED lighting units, precise brightness control can be achieved for each of the lighting devices. It is possible to correct the brightness of the light in a small area according to the characteristics of the original picture. For example, in a picture with strong contrast between light and dark, the LED backlight of the dark area can be completely turned off, and the LED backlight of the bright area achieves high brightness output, thereby bringing The contrast enhancement effect will be unmatched by LCD TVs that used CCFL light sources in the past.
At present, in the field of TV sets, many manufacturers have introduced commercial products using RGB-LED backlights, such as Sony's latest 70-inch LCD TV 70X300A, which uses RGB-LED backlights. LED can be quickly turned off, flexible control in sub-area, and adjustable brightness. It not only gets rid of the shortcomings of the traditional LCD TV dark picture, but also shows the beautiful color effect, which has been opened with any existing LCD TV. Sufficient distance.
Since the R/G/B primary colors can be directly emitted, more manufacturers plan to eliminate the high-cost color filter in LCD TVs. However, this technology has not yet entered the real commercial stage, and it still needs market and time tests.
In general, LED technology can be used in the field of lighting, display auxiliary fields, or directly used as display devices, and can obtain performance that cannot be compared with traditional devices through its own advantages. However, if there is a good side to everything, there will of course be a bad side. As a new generation of lighting technology, LED is not perfect, there are still some problems in the above several application fields.
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