LEDs have gradually replaced traditional cold cathode fluorescent lamps (CCFLs) in more and more applications due to their high brightness, long life, uniform color and space saving. From small mobile phone liquid crystal displays (LCDs) to large 70-inch LCD TV panels, LEDs are rapidly becoming the technology of choice for backlighting.
Historically, LCD backlights have used different light sources, including incandescent, fluorescent, and LED. The selection criteria used by OEMs to determine the backlighting technology that best suits their application include cost, brightness, uniformity, performance, lamp life, robustness, size, integration, and environmental factors that have recently become more of a concern.
LED has been a choice for small LCD backlight technology, especially for mobile phones, PDAs, MP3 players and other diagonal less than 5 inches below the handheld device. For larger screen sizes, CCFLs have traditionally been more cost effective and have been providing backlighting for most large and medium industrial displays. However, due to the increasing performance and the steadily decreasing cost of high-brightness LEDs (HB LEDs), its brightness is already comparable to that of CCFLs used for large display backlights, and the transition to LEDs is underway.
Advantages of HB LED
HB LEDs have many important advantages over CCFLs in backlight applications. HB LEDs provide higher brightness than CCFLs, and LED backlights last longer when properly integrated into the system. In addition, HB LEDs work efficiently over a wider temperature range, especially at low temperatures. In applications where it is difficult to provide the high voltages required for CCFLs, LEDs have unique advantages because they work well at low DC voltages. Other advantages of HB LEDs include higher and higher light output per unit of power input and the ability to optimize color gamut. Finally, the wide dimming capability of LEDs is also an important advantage in some applications.
As the price of high-brightness HB LEDs continues to decline due to increased usage, many experts predict that HB LEDs will eventually reach the same price as CCFLs. This cost balancing has prompted OEMs to choose HB LEDs for larger displays such as GPS systems, portable DVD players, notebooks, desktop displays, and 6.5-20.1-inch LCDs used in the industry. LEDs are also being used in large flat panel LCD TVs ranging in size from 32 inches up to 70 inches. Table 1 shows the comparison of LEDs with other common sources.
Control uniformity
When multiple LEDs are used to provide backlighting for larger LCDs, the illumination of the LED sources must be spread over a larger range, and it is important to ensure that the brightness and wavelength of each color remain uniform to avoid bright spots and dark blocks. This is especially critical for direct projection of LED backlight units (BLUs), even small color differences can reduce display uniformity. In order to better control the uniformity, the wavelengths of the various colors of each backlight must be strictly matched, green and blue are 5nm, because they are the two most important colors.
During the LED manufacturing process, there may be a slight difference in performance from the average given in the technical data sheet. For this reason, LED manufacturers dispense components based on luminous flux, color, and forward voltage (V1). Fine dispensing of brightness and color can be used to achieve better consistency. For brightness, 1/4-pack and 15% of each tube spread are the standard for brightness.
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