LCD backlight type and advantages and disadvantages (LCD, CCFL, LED)

Liquid crystal backlight display principle The biggest difference between liquid crystal and plasma is that liquid crystal must rely on a passive light source, while plasma TV is an active light-emitting display device. The mainstream LCD backlight technology currently on the market includes LED (Light Emitting Diode) and CCFL (Cold Cathode Fluorescence)

Lamp) Two types.

Cold Cathode Fluorescent Lamp (CCFL)

Traditional liquid crystal displays use CCFL (Cold Cathode Fluorescent Lamp) backlight. There are two main types of CCFL backlight designs: "side-type" and "direct-type". However, the side-type light guide design makes the photorefractive rate higher, which in turn restricts the brightness of the backlight. The larger the panel size, the brightness will be The lower it is, it is only suitable for 8-inch to 15-inch TFT LCD panels, that is, for personal viewing purposes such as Laptops and Desktops. However, when viewing large LCD TVs at home, the brightness of the side-lit type will be difficult to meet. Instead, it is Straight down.

However, the larger the size of the LCD, the higher the proportion of the cost of its backlight module, which refers to the direct-type CCFL backlight module. According to statistics, the same is the use of the direct-type CCFL backlight module. The backlight module only accounts for 23% of the overall cost in inches, but it increases to 37% by 30 inches, and it is estimated that when it reaches 57 inches, the cost of the backlight module will reach 50%. Therefore, the drop-down CCFL backlight is only suitable for use in a medium-sized LCD TV of about 30 inches, and is not suitable for use in a larger area design. At the same time, CCFL uses mercury gas discharge to produce lighting. Although the current RoHS regulations established by the European Union, as long as the dose of "mercury" is below the standard, it is still acceptable, but no one can guarantee that the standard may be raised to zero in the future (not allowed at all. Use), then CCFL will not be able to use, or must be changed to mercury-free CCFL.

Even if the mercury-free CCFL is technically feasible, the CCFL is still a gas-discharge electronic lighting with a closed fluorescent tube. The resistance of the fluorescent tube to external forces is limited. A large impact will break the fluorescent tube and make the lighting ineffective. Other solid-state electronic lighting (such as LED) has no such concerns. In addition, because the direct-drop type does not require a light guide plate and is relatively free from photorefraction problems, it does not require a brightness enhancement film, especially the brightness enhancement film is a patented technology of a few companies, and the price is expensive. Light board and brightness enhancement film, which helps to reduce costs.

However, the drop-down CCFL also has its shortcomings. In order to increase the brightness of the picture, the number of light pipes must be increased. However, the result of too close arrangement of the light pipes will not be conducive to heat dissipation. Since the distance between the left and right phases is reduced, the heat dissipation has to be increased from the thickness level. Space, however, the increase in thickness is also equivalent to partially offsetting the advantages of LCD TV: light and thin.

Incidentally, when using a CCFL light tube on a large-inch LCD TV, the length of the light tube must also increase in response to the increase in the number of inches. However, for a longer CCFL light tube, the middle position and both ends of the light tube will be The problem of brightness MURA and color MURA is easy to occur, which affects the light uniformity of the backlight. In order to maintain the light uniformity, a diffusion film must be used to enhance the light uniformity, but the diffusion film will also bring about the loss of light transmittance. To reduce the brightness, the result of the reduced brightness has to be reinforced by increasing the number of light pipes, but as mentioned earlier: adding light pipes will make it more difficult to design heat dissipation, increase the thickness of the backlight module, and even increase power consumption. It is understood that the electricity consumption of CCFL backlight modules has accounted for 90% of the total electricity consumption of LCD TVs. Therefore, changing the backlight technology is one of the current directions for changing LCD picture quality.

Light Emitting Diode (Light Emitting Diode; LED)

Since the CCFL backlight has many side effects and doubts, the industry is also looking for a variety of new backlight implementation technologies, and LED is one of the feasible solutions, such as Sony’s Qualia series TVs, which are high-end large-size (40 inches, 46 inches) LCD TV, whose backlight part is made of WLED, is called WLED backlight technology. The LCD Monitor research and development of LED backlight technology has also reached a substantive stage. We can already see related product displays at the 2007 CES exhibition.

LED backlight has many advantages. First, solid-state electronic lighting. Its resistance to impact is higher than that of CCFL. There is no concern about environmental protection regulations of mercury gas, no concern about leakage of UV ultraviolet rays, and it surpasses color saturation and lifespan. CCFL, in addition, LEDs can be driven as long as they are driven by a positive voltage. Unlike CCFL, which requires alternating positive and negative voltages, even if only the positive drive voltage is used, the demand level of LEDs is lower than that of CCFL. Furthermore, the brightness of the LED can be adjusted only by Pulse Width Modulation (PWM), and the same method can be used to suppress the afterimage problem on the TFT LCD display. However, the brightness adjustment of CCFL is more complicated. And the afterimage cannot be suppressed, it must be suppressed in another way.

Although LED backlight has many advantages, it also has its disadvantages. The first is luminous efficiency. In terms of the same power consumption, LED is not as good as CCFL, so the heat dissipation problem will be more serious than CCFL. In addition, LED is a point-type light source, which is similar to CCFL's linear type. The light source is more difficult to control the uniformity of light than the actual light source. In order to achieve as much light uniformity as possible, the characteristics of the produced LEDs must be carefully selected, and a large number of LEDs with the same characteristics (wavelength, brightness) are used for the same backlight Among them, the cost of this selection is also quite high. Fortunately, the luminous efficiency of LEDs is still improving. At present, it can reach more than 100 ml/W. In this way, the color saturation can be better, and the WLED arrangement of the backlight can be more relaxed, thereby alleviating the problems of power consumption and heat dissipation. And after the manufacturing yield rate continues to improve and mature, the cost of carefully selecting LEDs with consistent brightness characteristics will also be reduced.

Changing the backlight technology alone may not be enough to trigger a revolution in LCD, so let's look at other LCD technology developments. OLED (Organic Light Emitting Diode) is an organic light emitting diode. OLED display technology is different from traditional LCD display methods. It does not require a backlight and uses a very thin coating of organic materials and a glass substrate. When a current passes through, these organic materials will emit light. Moreover, the OLED display screen can be made lighter and thinner, with a larger viewing angle, and can significantly save power. However, its current life and price are the bottlenecks restricting its development in LCD.

OLED is another panel application technology that has attracted attention, and the realization of small-size panels is earlier. According to customer plans, there will be more models coming out from 2008 to 2009, but sub-panels will still be the main ones, and even if the models and shipments have increased significantly compared with today, the market share will not exceed 10%. OLED was originally thinner and had better conditions than TFT-LCD in terms of contrast, viewing angle, and power saving. It has always been valued by the industry as it will replace TFT-LCD, and it has also invested in research and development in the early years. However, on the one hand, OLED technology has encountered bottlenecks, and the life problem needs to be overcome; on the other hand, TFT-LCD technology continues to improve, and now it can also provide excellent contrast and viewing angles, resulting in the OLED demand has not been greatly increased, and the market is small and oversupply , Is limited to price competition; the businesses that originally invested can hardly escape the fate of dissolution and downsizing. In the past, Taiwan Shenghua Technology invested in the establishment of Shengyuan to invest in OLED research and development. Seeing that OLED and TFT-LCD cannot compete, especially the cost difference is large. In terms of specifications, TFT-LCD can easily achieve 170-degree viewing angle, 500:1 contrast, and brightness. It can be increased or made thinner. Although the reaction speed is relatively inferior, it can reach the range acceptable to the human eye. Therefore, Shengyuan has also been closed, leaving only a few R&D personnel to return to Shenghua to develop materials. In the future, if the lifespan and price of OLED can be greatly improved, there is still a chance; at this stage, it is limited to products with special characteristics and emphasizes the need to be innovative; the time point for large quantities has not yet been seen.

And AMOLED (Active Matrix/Organic Light Emitting Diode) active matrix organic light emitting diode panel (AMOLED) is called the next generation of display technology, including Samsung Electronics, Samsung SDI, LG Philips all attach great importance to this new display technology. At present, apart from Samsung Electronics and LG Philips focusing on the development of large-size AMOLED products, Samsung SDI and AUO are all focusing on small and medium sizes. From the current product performance of finished products, if the cost of AMOLED can be effectively controlled, then the traditional LCD panel technology will be greatly challenged.

One of the advantages of AMOLED: no need for backlight

One of the advantages of AMOLED: greater color saturation

One of the advantages of AMOLED: it can reach the 180-degree viewing angle of IPS or VA panels

One of the advantages of AMOLED: Effectively solve the problem of LCD panel dynamic blur

Among the above four OLED advantages, we pay special attention to the fourth product feature, because all desktop LCD monitors currently on the market cannot solve the problem of liquid crystal screen dynamic blur. The dynamic picture blur of the LCD screen usually refers to the phenomenon of blurred edge contours during the screen change. There are two reasons for the dynamic picture blur phenomenon. One is the response time of the liquid crystal and the afterglow of the phosphor, and the other One is TFT drive, like the image control of the Hold method.

Hold is the main cause of blurry dynamic pictures

The so-called "Hold mode" display mode is to display a Frame image within a certain period of time. In a TV screen, this Hold time is equivalent to a vertical period (16.7 milliseconds). Generally speaking, everyone is quite clear that the LCD response time It is very important for dynamic picture display, because for LCD TV, the conversion time of a picture is about 16.7ms, so the response time of LCD TV can be shorter than 16.7ms, for the picture performance of dynamic picture It’s very important. However, there is another situation that even if the response time of the liquid crystal is 0ms (which is unlikely and difficult), the blur will not disappear. This is because the LCD screen uses the "Hold Method" method to display images. According to some experimental reports, we can know that the animation displayed on the screen using the "Hold" method will shake left and right on the retina. Such shaking accumulates over time, and the dynamic picture feels blurred. As with improving the response time of liquid crystals, it is necessary to develop a display method that shortens the "Hold" time. According to the above-mentioned situation, the dynamic picture blur of the liquid crystal screen cannot be expressed by the measurement used for a long time, that is, the liquid crystal response time from white to black and black to white change time.

Improve the blur of the dynamic picture caused by the Hold time

If the response time is an ideal control liquid crystal panel (Hold time 100%) with a response time of 0ms, the MPRT is 16.7ms (frequency is 60Hz). When the Hold time is 50%, the MPRT is about 8.3ms; when the Hold time is 25%, the MPRT is 4.2ms. The MPRT of a general LCD is less than 8ms; if it is an LCD with high image quality requirements for commercial products, the MPRT can be estimated to be less than 4ms. As mentioned above, MPRT contains two major elements: liquid crystal response time and hold time. Therefore, if the display quality of the image is to be achieved, the liquid crystal response time is hoped to be smaller than the above value. Among the methods to improve the response time of liquid crystals, there are high-speed dynamic modes such as OCB, IPS, and VA, as well as over-drive driving and so on. Now, LCD TVs that value image quality have put these methods into production. There are two ways to improve the blur of the dynamic picture caused by the Hold time. One is to turn off the backlight source according to the screen frequency, and the other is the double-speed display method using motion compensation technology. The first specific method is to use the flicker of the backlight and the insertion of the black signal. Among these two technologies, the most interesting is the dynamic compensation technology. Intermittent display methods such as backlight turning off and black signal insertion can improve the blur of the dynamic picture and are relatively simple to implement. But in the case of a large screen and high brightness, it is easy to cause flickering of the screen. In contrast, the dynamic compensation double-speed display method can improve the dynamic picture blur without increasing the picture flicker, but it has not been easy to implement until now because it requires a large-scale signal processing circuit.