Analysis of LCD backlight types and advantages and disadvantages
LCD backlight display principle The biggest difference between LCD and plasma is that LCD must rely on passive light sources, while plasma TVs are active light-emitting display devices. Currently, the mainstream LCD backlight technologies on the market include LED (Light Emitting Diode) and CCFL (Cold Cathode Fluorescent Lamp).
Cold Cathode Fluorescent Lamp (CCFL)
Traditional LCD displays use CCFL (cold cathode fluorescent lamp) backlight. There are two main types of CCFL backlight designs: "side-type" and "straight-type". However, the side-type type has a higher light refractive index due to the light guide design, which limits the brightness of the backlight. The larger the panel size, the lower the brightness. The lower it is, it is only suitable for 8-inch to 15-inch TFT LCD panels, which are used for personal viewing such as laptops and desktops. However, when used on large-size LCD TVs for home viewing, the brightness of the side-entry type will be difficult to satisfy. Instead, Straight drop.
However, the larger the size of the LCD, the higher the cost proportion of the backlight module. This refers to the straight-type CCFL backlight module. According to statistics, the same type of LCD uses the straight-type CCFL backlight module. CCFL backlight modules account for only 23% of the overall cost at 15 inches, but increase to 37% at 30 inches, and it is estimated that at 57 inches, the cost of the backlight modules will reach 50 %. Therefore, the direct-fall CCFL backlight is only suitable for use in medium-sized LCD TVs of about 30 inches, and is not suitable for use in larger-area designs. At the same time, CCFL uses mercury gas discharge to generate lighting. Although the current RoHS regulations established by the European Union are still acceptable as long as the "mercury" dose is below the standard, no one can guarantee that the standard may be raised to zero content in the future (completely inaccurate ), CCFL will no longer be able to be used, or a mercury-free CCFL will have to be used.
Even if mercury-free CCFL is technically feasible, CCFL is still a gas discharge electronic lighting with a closed light pipe. The resistance of the light pipe to external forces is limited, and a large impact will cause the light pipe to collapse. If it breaks, the lighting will fail. Compared with other solid-state electronic lighting (such as LED), there is no such concern. In addition, since the straight-down type does not require the use of a light guide plate and has less problems with light refraction, it does not require a brightness enhancement film. In particular, the brightness enhancement film is a patented technology of a few manufacturers and is expensive. The straight-down type can save the need for a light guide plate. Light panels and brightness-enhancing films, which help reduce costs.
However, the straight-down CCFL also has its shortcomings. In order to improve the brightness of the picture, the number of light pipes must be increased. However, the result of too dense arrangement of the light pipes will be detrimental to heat dissipation. Since the distance between the left and right is reduced, We have to increase the heat dissipation space from the thickness level. However, the increase in thickness also partially offsets the advantages of LCD TV: thinness and lightness.
By the way, when CCFL light pipes are used on large-inch LCD TVs, the length of the light pipe must also increase with the increase in inch size. However, for longer CCFL light pipes, the length of the light pipe must be longer. The middle position and both ends will easily produce brightness MURA and color MURA problems, which will affect the light uniformity of the backlight. In order to continue to maintain light uniformity, a diffusion film must be used to enhance the light uniformity, but the diffusion film will also cause The loss of light transmittance reduces the brightness. The result of the reduced brightness has to be enhanced by increasing the number of light pipes. But as mentioned above: adding light pipes will make it more difficult to design heat dissipation and increase the thickness of the backlight module. Even the power consumption has increased. According to our understanding, the power consumption of CCFL backlight modules has accounted for 90% of the overall power consumption of LCD TVs. Therefore, changing backlight technology is one of the current directions for changing LCD image quality.
Light Emitting Diode (LED)
Since CCFL backlight has many side effects, the industry is also looking for various new backlight implementation technologies, and LED is feasible One solution, such as Sony's Qualia series TVs, is a high-end large-size (40-inch, 46-inch) LCD TV. The backlight part is composed of WLED, which is called WLED backlight technology.
The research and development of LCD Monitor with LED backlight technology has now reached a substantial stage. We can already see related product displays at the 2007 CES exhibition.
LED backlight has many advantages. First, it is a solid-state electronic lighting, which is more resistant to impact than CCFL. It has no environmental regulations concerns about mercury gas and no concerns about UV ultraviolet leakage. At the same time, it has better color performance. The saturation and lifespan exceed CCFL. In addition, LED can be driven only by forward voltage, unlike CCFL which requires AC positive and negative voltages. Even in terms of forward driving voltage, the demand level of LED is lower than that of CCFL. Furthermore, the brightness of LEDs can be adjusted simply by using Pulse Width Modulation (PWM), and the same method can be used to suppress the image sticking problem on TFT LCD displays. However, the brightness adjustment of CCFL is more complicated. And the afterimage cannot be suppressed and must be suppressed in another way.
Although LED backlight has many advantages, it also has its disadvantages. The first is the 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 dot type. Compared with the linear light source of CCFL, it is more difficult to control the light uniformity of the light source. In order to achieve the light uniformity as much as possible, the characteristics of the produced LED must be carefully selected, and a large number of characteristics (wavelength, brightness) must be consistent LEDs are used in the same backlight, and the cost of this selection is also quite high. Fortunately, the luminous efficiency of LEDs is still improving and can now reach over 100 ml/W. This allows for better color saturation and allows for a looser arrangement of WLEDs in backlights, thereby alleviating power consumption and heat dissipation problems. , 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.
Simply changing the backlight technology may not be enough to trigger a revolution in LCD, so let’s look at the development of other LCD technologies. OLED (Organic Light Emitting Diode) is 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 organic material coating and a glass substrate. When current passes through, these organic materials will emit light. Moreover, OLED display screens can be made lighter and thinner, have wider viewing angles, and can significantly save power. However, its lifespan and price are currently bottlenecks limiting its development in LCD. ;