TV Light Guide Plate - LCD Backlight Type And Analysis Of Advantages And Disadvantages

The principle of LCD backlight display 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. At present, the mainstream LCD backlight technology on the market includes LED (Light Emitting Diode) and CCFL (Cold Cathode Fluorescent Lamp).

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", but 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 is 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, 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 vertical type CCFL backlight module. According to statistics, the vertical type CCFL backlight module is also used. In inches, the backlight module only accounts for 23% of the overall cost, but it increases to 37% when it is 30 inches, and it is estimated that when it is 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 set 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 the CCFL will not be available, or you must switch to a 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 cause the fluorescent tube to rupture and cause the lighting to fail. Other solid-state electronic lighting (such as LED) has no such concerns. In addition, since the direct-drop type does not require a light guide plate and is relatively free from photorefractive damage, it does not need 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 contribute to cost reduction.

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 dense 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 (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 up of WLED, is called WLED backlight technology. The LCD Monitor research and development of LED backlight technology has also reached a substantial stage. We can already see related product displays at the 2007 CES exhibition.

LED backlight has many advantages. First, solid electronic lighting. Its resistance to impact is higher than that of CCFL. There is no environmental protection regulatory concerns of mercury gas, no concerns about UV ultraviolet leakage, and both color saturation and life. Beyond CCFL, 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 it is only a positive drive voltage, 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 the LCD revolution, so let's take a 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 very thin organic material coatings and glass substrates. When 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 that limit its development in LCD.