LCD TV has corresponding parameters, LED is only the backlight mode of LCD TV, and LCD without LED is only the CCFL of backlight. Let me explain the specific differences in detail.
Types, advantages and disadvantages of LCD backlight (LCD, CCFL, LED)
The principle of liquid crystal backlight display The biggest difference between liquid crystal and plasma is that liquid crystal must rely on passive light source, and plasma TV belongs to active luminous display equipment. At present, the mainstream LCD backlight technologies on the market are LED (Light Emitting Diode) and CCFL (Cold Cathode Fluorescent Lamp).
Cold cathode fluorescent lamp (cold cathode fluorescent lamp; CCFL)
Traditional liquid crystal displays are all backlit by CCFL (cold cathode cathode fluorescent lamp). CCFL has two main backlight designs: "side-entry" and "direct-down". However, the side entry type has a high light loss rate due to the light guide design, which further limits the backlight brightness. The larger the panel size, the lower the brightness, which is only suitable for 8 inches. 15 inch TFT LCD panel is used for personal viewing, such as notebook computers and desktops. However, when the LCD TV is large in size for home viewing, it will be difficult to meet the brightness of the side-entry type, and it will be replaced by the straight-down type.
However, the larger the LCD, the higher the proportion of its backlight module, which refers to the direct CCFL backlight module. According to statistics, the direct CCFL backlight module is also used, which only accounts for 23% of the total cost at 15 inch, but it increases to 37% at 30 inch, and it is expected that the backlight module will account for 50% at 57 inch. Therefore, the direct CCFL backlight is only suitable for medium-sized LCD TVs of about 30 inches, and is not suitable for larger-area designs. At the same time, CCFL uses mercury gas discharge to generate illumination. Although the RoHS standard formulated by the European Union is acceptable as long as the dose of "mercury" is lower than the standard, there is no guarantee that the standard may be raised to zero (completely banned) in the future, CCFL will not be used at that time, or CCFL without mercury must be used.
Even if mercury-free CCFL is technically feasible, CCFL is still a kind of gas discharge electronic lighting with a closed lamp tube, which has limited resistance to external forces. Compared with other solid electronic lighting (such as LED), a large impact will break the lamp tube and make the lighting ineffective. In addition, because the direct type does not need a light guide plate, there is no light loss problem, so there is no need for brightness enhancement film, especially brightness enhancement film is a patented technology of a few manufacturers, which is expensive. The direct type can save the light guide plate and brightness enhancement film, which is helpful to reduce the cost.
However, directly dropping CCFL also has its disadvantages. In order to improve the brightness of the picture, the number of lamps must be increased. However, the result of too dense arrangement of light tubes will not be conducive to heat dissipation. As the distance between the left and right phases decreases, the heat dissipation space must be increased from the thickness level. However, the increase in thickness is equivalent to partially offsetting the advantages of LCD TV: thinness.
By the way, when CCFL light pipe is used in large-inch LCD TV, the length of light pipe must also increase with the increase of inches. However, the problems of brightness MURA and color MURA are easy to appear in the middle and both ends of long CCFL light pipe, which will further affect the light uniformity of backlight. In order to keep the uniformity of light continuously, it is necessary to use diffusion film to enhance the uniformity of light, but diffusion film will also bring the loss of transmittance and reduce brightness. The decrease of brightness can only be enhanced by increasing the number of lamps, but as mentioned above, it will be more difficult to design heat dissipation, increase the thickness of backlight module and even increase power consumption. As far as we know, the power consumption of CCFL backlight module has accounted for 90% of the total power consumption of LCD TV. Therefore, changing backlight technology is one of the directions to change LCD image quality at present.
Light emitting diode; (LED)； Light emitting diodes)
Because CCFL backlight has many side effects, the industry is also looking for various new backlight technologies. LED is one of the feasible solutions, such as Sony's Qualia series TV, which is a high-end large-size (40-inch, 46-inch) LCD TV with WLED backlight technology. At present, the research and development of LCD with LED backlight technology has reached a substantive stage, and we can already see the related products displayed at CES in 2007.
LED backlight has many advantages. First, solid-state electronic lighting has higher impact resistance than CCFL, and there is no concern about mercury gas environmental protection laws and regulations, and there is no concern about UV ultraviolet leakage. At the same time, LED can be driven as long as it is DC voltage, unlike CCFL, which needs alternating positive and negative voltages. Even if it is only a positive driving voltage, the demand level of LED is lower than CCFL. Furthermore, the brightness of LED only needs pulse width modulation; (PWM)； PWM) mode can be adjusted, and the problem of afterimage on TFT LCD display can be suppressed in the same way. However, the brightness level of CCFL is more complex, and the afterimage cannot be suppressed, so it must be suppressed in another way.
Although LED backlight has many advantages, it also has disadvantages. First, luminous efficiency. As far as the same power consumption is concerned, LED is not as good as CCFL, so the heat dissipation problem will be more serious than CCFL. In addition, LED is a point light source, and it is more difficult to control the uniformity of light compared with CCFL line light source. In order to achieve the uniformity of light as much as possible, it is necessary to carefully select the types of LEDs produced and use a large number of LEDs with the same characteristics (wavelength and brightness) for the same purpose. Fortunately, the luminous efficiency of LED is still improving, which can reach above 100 ml/W at present, so that the color saturation can be better and the WLED arrangement of backlight can be looser, so that the problems of power consumption and heat dissipation can be alleviated. After the manufacturing yield continues to improve and mature, the cost of LED with consistent brightness characteristics in fermentation will also be reduced.
Just changing the backlight technology may not be enough to trigger the LCD revolution, so let's look at the development of other LCD technologies. Organic light emitting diode (OLED) refers to organic light emitting diode. Organic light emitting diode display technology is different from the traditional liquid crystal display mode. It does not need backlight, and uses very thin organic material coating and glass substrate. These organic materials will emit light when current passes through them. In addition, the organic light-emitting diode display screen can be made lighter and thinner, with a larger viewing angle, and can significantly save electricity. But at present, its life and price are the bottlenecks that limit its development in LCD.
Organic light emitting diode (OLED) is another striking panel application technology, and the realization cycle of small-size panel is earlier. According to the customer's plan, in 2008? There will be more models coming out in 2009, but the sub-panels are still the main ones. Even if the models and shipments are significantly increased compared with the present, the market share will not exceed 10%. Originally, organic light-emitting diodes are better than TFT-LCD because of its thinness, contrast, viewing angle and power saving. Concerned by the industry, it is thought that it will replace TFT-LCD, and it was also put into research and development in early years. However, on the one hand, the technology of organic light-emitting diodes itself has encountered bottlenecks, and life problems need to be overcome; On the other hand, the continuous progress of TFT-LCD technology can now provide excellent contrast and perspective, which leads to the fact that the demand for organic light-emitting diodes has never been greatly improved, and the market is small and the supply exceeds demand, which is limited by price competition; The original investment operators can't escape the fate of dissolution and downsizing. In the past, Taiwan Province Shenghua Technology invested in the establishment of Shengyuan to invest in the research and development of organic light-emitting diodes. See organic light-emitting diodes and TFT-LCD can not be compared, especially the cost gap is large. In terms of specifications, TFT-LCD can easily achieve a viewing angle of 170 degrees, a contrast ratio of 500: 1, and increase brightness. Can also be made very thin, the response speed is slightly slower, but it can reach the acceptable range of human eyes. Therefore, Sheng Yuan was also closed, leaving only a few R&D personnel to return to Shenghua to develop materials. If the life and price of organic light-emitting diodes can be greatly improved in the future, there is still a chance; At present, it is limited to products with particularity and emphasis on novelty; I haven't seen it for a long time.
And amoled (Active Matrix/Organic Light Emitting Diode) Active Matrix Organic Light Emitting Diode Panel (AMOLED) is called the next generation display technology, including Samsung Electronics, Samsung SDI and LG Philips. At present, apart from Samsung Electronics and LG Philips, which mainly focus on developing large-size AMOLED products, both Samsung SDI and AUO focus on developing small and medium-sized products. Judging from the current performance of finished products, if the cost of AMOLED can be effectively controlled, the traditional LCD panel technology will be greatly challenged.
One of the advantages of AMOLED is that it does not need backlight.
One of the advantages of AMOLED: greater color saturation.
One advantage of AMOLED is that it can reach the viewing angle of 180 degrees of IPS or VA panel.
One advantage of AMOLED is that it can effectively solve the dynamic blur problem of LCD panel.
Among the above four advantages of organic light-emitting diodes, we pay special attention to the fourth product feature, because in all desktop liquid crystal displays on the market at present, the problem of dynamic blur of LCD screen can not be solved. The dynamic picture blur of LCD usually refers to the phenomenon of blurred edge contour in the process of picture transformation. There are two reasons for the blur of dynamic picture, one is the response time of LCD and the residual light of phosphor, and the other is TFT driving, just like image control in Hold mode.
Hold is the main reason for blurring the dynamic picture.
The so-called "hold mode" display mode is to display an image within a certain period of time, which is equivalent to a vertical period (16.7ms) in a TV picture. Generally speaking, it is quite clear to all that the response time of LCD is very important for dynamic picture display, because for LCD TV, the conversion time of a picture is about16.7 ms. However, there is another case where the blur will not disappear even if the response time of LCD is 0ms (which is unlikely and difficult). This is because the LCD screen uses the "hold mode" method to display images. According to some experimental reports, we can know that the animation displayed on the screen in "Hold" mode will shake left and right on the retina. With the accumulation of this jitter, the dynamic picture will be blurred. Just like improving the response time of liquid crystal, it is necessary to develop a display method that shortens the "hold" time. According to the above situation, the dynamic picture on the LCD screen is vague, which can't be expressed by the liquid crystal response time of white to black and black to white. It has been used for a long time.
Improve dynamic picture blur caused by holding time.
If the response time is 0 ms (holding time is 100%), the MPRT is 16.7 ms (frequency is 60Hz). When the retention time is 50%, MPRT is about 8.3ms；; When the holding time is 25%, the MPRT is 4.2 ms, and the MPRT of ordinary LCD is lower than 8 ms. If it is an LCD with high quality requirements for commercial products, the MPRT can be estimated to be lower than 4 ms As mentioned above, MPRT includes two major elements: liquid crystal response time and holding time. Therefore, if the image display quality is to be achieved, it is desirable that the liquid crystal response time can be less than the above value. Among the methods to improve the response time of liquid crystal, there are OCB, IPS, VA, overdrive and other high-speed dynamic modes. Now, LCD TVs that pay attention to image quality have put these methods into production. There are two ways to improve the dynamic picture blur caused by holding time. One is to turn off the backlight according to the picture frequency, and the other is the double-speed display mode using motion compensation technology. The first concrete method is to use the flicker of backlight and the insertion of black signal. Among these two technologies, the most interesting one is the dynamic compensation technology. Intermittent display methods such as backlight blanking and black signal insertion can improve the blur of dynamic pictures and are relatively simple to implement. But in the case of large screen and high brightness, it is easy to flicker. In contrast, the dynamic compensation double-speed display method can improve the dynamic picture blur without increasing the picture flicker, but it is not easy to realize until now because it requires a large-scale signal processing circuit.
Japanese manufacturers announced that they would improve the image quality by shortening the holding time.
In the past two years, quite a number of manufacturers have published related technologies and products to improve image quality by shortening the holding time. For example, there is a 32-inch WXGA LCD TV with dynamic compensation and high-speed display technology produced by Japanese manufacturers. The method is to use dynamic compensation technology to increase the frequency of picture signal and driving picture from general 60Hz to 90Hz, shorten the holding time to about 70%, and shorten the extinction by scanning backlight to 70% and 50%. On the premise of not increasing the flicker of the picture, the problem of blurred dynamic picture is improved. Because the backlight is turned off at 90Hz, it is not easy for human eyes to feel the flicker of the picture. In addition, other manufacturers also adopt motion dynamic compensation technology to improve the dynamic picture quality by increasing the picture frequency to 120Hz.
A friend of the author bought a 15 inch liquid crystal display (LCD) and has been addicted to it ever since. But recently, he found that the display screen began to turn yellow and the brightness dropped obviously. No matter how to adjust it, it didn't help. After many reconnaissance, the "culprit" was found-the backlight tube was broken. At present, CCFL (cold cathode fluorescent lamp) with short life is used as the backlight of mainstream LCD, which is a big injury to LCD. Fortunately, people have now found its successor-the leader.
Defects of traditional CCFL backlight
Before deeply understanding the LED backlight technology, it is necessary for us to know what problems exist in the current backlight technology. As we know, liquid crystal is a substance between liquid and crystal. The wonder of liquid crystal is that it can change its molecular arrangement state by current, and control the luminous flux by applying different voltages to liquid crystal, thus displaying various images. But liquid crystal itself does not emit light, so all liquid crystals need backlight illumination. At present, the backlight of LCD is almost CCFL (cold cathode fluorescent lamp).
Because cold cathode fluorescent lamp is not a plane light source, in order to realize the uniform brightness output of backlight, the backlight module of LCD needs to be equipped with various auxiliary devices such as diffuser, light guide plate and reflector. Even so, it is very difficult to obtain uniform brightness output like CRT. When most liquid crystal displays display all-white or all-black pictures, the brightness difference between the edge and center of the screen is very obvious.
CCFL, as an LCD backlight, has a headache besides its complicated structure and poor brightness output uniformity-short service life. The brightness of most CCFL backlights decreases obviously after 2 ~ 3 years of use (life span is 15000 ~ 25000 hours). Many liquid crystal displays (especially the LCD screen of notebook computers) will turn yellow and dark after several years of use, which is caused by the short attenuation period of CCFL.
At the same time, since CCFL backlight must include complex optical devices, such as diffusers and reflectors, the volume of LCD cannot be further reduced. In terms of power consumption, LCD using CCFL as backlight is not satisfactory. CCFL backlight of 14 inch LCD often needs to consume 20W or more power. For notebook computers and portable devices, their endurance will be greatly tested.
In order to solve these problems of CCFL, almost all LCD manufacturers began to look for better LCD backlight. Because LED has the characteristics of ultra-low energy consumption, long working life and simple structure, it has quickly gained the favor of LCD manufacturers. So what is an LED? What are the highlights?
In fact, LED is not a cutting-edge technology product. It can be seen everywhere in our daily life: colorful billboards on the roadside, different color indicator lights on household appliances, backlight lighting of mobile phone buttons, headlights of cars and so on. , all use LED as light source.
After its birth in 1960s, LED was recognized as the terminator of fluorescent tubes, light bulbs and other lighting equipment. Some people even think that LED will usher in a new era of lighting and will eventually appear in all occasions where lighting is needed. The working principle of LED is completely different from our common incandescent lamp and fluorescent lamp. LED is essentially a semiconductor device.
The core part of LED is a wafer composed of P-type semiconductor and N-type semiconductor, and a thin layer with special conductivity will appear at the interface between P-type semiconductor and N-type semiconductor, which is commonly called PN junction transistor. PN junction can produce resistance to the diffusion movement of majority carriers in p-type and n-type semiconductors. When DC voltage is applied to the PN junction, the current flows from the anode to the cathode of the LED, and the minority carriers recombine with the majority carriers in the PN junction, and the excess energy will be converted into light and released. According to this principle, LED realizes electro-optical conversion. According to the different physical properties of semiconductor materials, LED can emit light in different bands and colors from ultraviolet to infrared.
Little knowledge: p-type semiconductor and n-type semiconductor
If a small amount of trivalent elements such as boron, indium, gallium or aluminum are added to semiconductor materials such as silicon or germanium, it will become a hole-conducting semiconductor, that is, a P-type semiconductor. In p-type semiconductors, holes (positively charged) are called majority carriers; Electrons (with negative charges) are called minority carriers.
If a small amount of pentavalent elements such as phosphorus, antimony and arsenic are added to semiconductor materials such as silicon or germanium, it will become a semiconductor dominated by electron conduction, that is, an N-type semiconductor. In n-type semiconductors, electrons (negatively charged) are called majority carriers; Holes (positively charged) are called minority carriers.
Because LEDs can only emit light with a single wavelength, they cannot emit white light as easily as incandescent lamps. This is why the LED indicator light is only blue, red, green and other colors, but not white. The inability to emit white light is not a problem for applications such as indicator lights, but it is an insurmountable obstacle for LCD backlight. In order to realize LED as the successor of display backlight as soon as possible, LED manufacturers have begun to focus on white LED products. Niya Chemical of Japan is a pioneer in this field, and it proposed a solution in 1996, that is, coating yellow phosphor on blue LED to realize white light output. Nichia Chemical has made a leading position in the field of white LED due to its early start and mature technology. According to statistics, the products adopting Japan-Asia chemical scheme occupy 80% market share.
What benefits will LED bring as LCD backlight? First of all, the LCD volume of LED backlight will be further reduced. LED backlight is composed of many grid-like semiconductors, and each "grid" has an LED semiconductor, so LED backlight successfully realizes the planarization of light source. Planar light source not only has excellent brightness uniformity, but also does not need complicated optical path design, which makes the thickness of LCD thinner and has higher reliability and stability. Thinner LCD panel means that notebook computers have better mobility. For example, Sony's recently launched VAIO TX notebook uses an LED backlight LCD screen with a thickness of only 4.5 mm ..
Secondly, in terms of luminous life, LED backlight technology also leaves CCFL far behind. The general service life of ordinary CCFL backlights is about 30,000 hours, while the luminous life of some top CCFL backlights is only about 60,000 hours. Such a long service life means that the brightness of LCD will drop obviously after 2 ~ 3 years of use, and the CCFL backlight module of LCD has to be replaced. The LED backlight does not have this problem at all. At present, the service life of white LED backlight has reached 654.38+10,000 hours, and there is potential for further improvement. Even if it is used continuously for 24 hours, this life is enough for 5 years!
In terms of color performance, LED backlight is far superior to CCFL. Due to color purity and other problems, the original CCFL backlight does not perform well in color scale. Therefore, LCD is not as good as CRT in grayscale and color transition. According to the test, CCFL backlight can only reach 78% NTSC color zone, while LED backlight can easily obtain NTSC color zone above 100%. In terms of color expression and color gradation transition, LED backlight also has significant advantages.
Little knowledge: NTSC standard
In the field of video, NTSC (National Television System Committee) standard is generally used as an index to measure the color reproduction ability of video equipment. This index refers to the saturation that the display device can display on various colors in the whole color space, that is, the degree of blue, green and red that can be displayed. Traditional LCD TVs and monitors can only cover 65% ~ 75% of the NTSC standard, which is reflected in the great difference between the green, yellow and red parts and the standard values.
It is no exaggeration to say that with the introduction of LED backlight technology, LCD can compete with CRT in color performance for the first time. In addition, due to the planar light source characteristics of LED, LED backlight can also realize the color and chromaticity adjustment in different areas that CCFL cannot achieve, thus achieving more accurate color reproduction and meeting the requirements of printing, publishing and graphic design.
Although LED backlight technology has great advantages, there are still some problems to be solved at this stage. The first challenge of LED backlight technology is cost. Because white LED devices are monopolized by several oligarchs, the manufacturing cost of LED backlight remains high. At present, the price of products with LED backlight is still significantly higher than CCFL backlight products, and the output of white LED devices can not meet the needs of large quantities. To realize the rapid popularization of LED backlight, it is necessary to break through the patent blockade of white LED.
In addition to the cost problem, the LED backlight technology at this stage is not satisfactory in terms of luminous efficiency. At present, the luminous efficiency of CCFL is about 60 lm/W (lumen/watt), while that of large LED backlight is only 30 lm/w. The main reason is that with the increase of chip area, the current density of LED will be uneven, which will inevitably lead to low luminous efficiency and large heat generation. Because the luminous efficiency gap of large-size backlight reaches 50%, the power consumption of LED backlight used in large-size panel will be twice that of ordinary CCFL! This is also the reason why the finished large-size LED backlight LCD is equipped with active cooling system.
However, with the increasing investment of major manufacturers in LED backlight research, the above two key problems that plague LED backlight are expected to be solved soon. Riya Chemical, the leading manufacturer of white LED, has been increasing the output of its white LED products since 2004. At the same time, other manufacturers' white LED products have also started mass production, and the price of white LED is also falling at a faster rate. I believe that the price of LED backlight module will be the same as that of CCFL backlight module in two years. As for the luminous efficiency of LED, it has been improved obviously recently. The luminous efficiency of the new generation of white LED is increased to 50 lm/W, and the gap with CCFL is only10lm/w. In the next 3-5 years, the luminous efficiency of LED may exceed 80 lm/w. ..
Small information: the pace of finished LED backlight technology
As early as 2004, Sony took the lead in completing LED backlight technology, and introduced a 23-inch LCD monitor and a 46-inch LCD TV with LED backlight. Although these two products have the defects of high power consumption, high calorific value and high price, the advantages of LED in display quality are fully reflected.
At the SID 2005 conference held in May 2005, LG- Philips and Samsung Electronics both exhibited their own LED backlight flat panel displays. Among them, LG- Philips also proposed a backlight solution of mixed LED and CCFL for the first time. Through this design, not only the power consumption of LED backlight is successfully reduced, but also the contrast of LCD is improved to 10000: 1!
Undoubtedly, LED backlight technology will replace CCFL and become the mainstream backlight of LCD in the near future. With the help of LED backlight technology, the color reproduction and service life of LCD will be greatly improved. By then, LCD will be one step closer to perfection.
Well, having said so much, I hope it can be adopted! !