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Optoelectronics/Displays??

Utilising LEDs for LCD backlighting (Part 1)

Posted: 09 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:LED? LCD backlight? TFT-LCDs? CRT? backlight unit?

With the rapid increase of modern multimedia requirements, a wide range of display devices have become available. Thin-film-transistor liquid crystal displays (TFT-LCDs) are one of the most popular display devices. They range from small to large1C3 in devices such as mobile phones, notebooks and netbooks (NBs), car navigation systems and televisions. Because an LCD cannot radiate by itself, it has to rely on an external source to provide the illumination. Generally, a backlight unit (BLU) positioned on the back of an LCD cell is used to supply sufficient and uniform brightness and light for transmissive and transflective LCDs. Around 2004, LCD technology became suitable for televisions. Due to the strong market demand for thin flat-panel televisions and the limits on other display technology from technological bottlenecks, LCD televisions have gradually penetrated the television market. The penetration rate of LCD televisions exceeded 50% for the first time in 2008 and they have substantially replaced the cathode ray tube (CRT).

However, LCD televisions still have many shortcomings, such as poor viewing angles, response times, contrast ratios and colour gamut compared to plasma televisions. Therefore some people have always thought that the LCD television would just be a transitional product, unless its shortcomings could be overcome or its features improved. Currently, everyone agrees that the LED light source has been the savior of LCD technology. LEDs can effectively overcome the shortcomings of LCDs and enhance their quality. Thus LCDs have successfully consolidated their mainstream status, especially in LCD televisions. The shipment proportion of LED and cold cathode fluorescent lamp (CCFL) LCD televisions based on a survey by Displaybank in February 2012 is shown in figure 1.4 For LCD televisions, the LED penetration rate is increasing and it exceeded 50% in the last quarter of 2011.

Figure 1: Shipment proportion of LED and CCFL LCD televisions.4

Over the last decade, organic light-emitting diode (OLED) display technology has been the biggest threat to LCD technology. Although the LCD technology using LEDs has improved and can be further enhanced in the future, as long as the OLED display technology manufacturing costs decline, LCD technology is still in danger of being replaced by OLED technology. This is because OLEDs are self-emissive and have many other advantages over LED LCD technology, such as wider viewing angles, faster response times, slim body, light weight and they can flex.

Types of LED LCD backlighting units (BLUs)
Technical considerations for the light source: The most significant part of a BLU is the light source. The best light source for a specific BLU is determined by factors such as spectral content, luminous flux and efficiency, operating temperature range and stability over that range, and dimmability. The types of light source used in BLUs include: LEDs, CCFLs, hot cathode fluorescent lamps (HCFLs), external electrode fluorescent lamps (EEFLs), flat fluorescent lamps (FFLs) and electroluminescent (EL) devices.3 The general properties of these light sources for BLUs are listed in table 1.5 CCFLs were once considered the best light sources for LCDs, even though they have many shortcomings.

Table 3: General characteristics of light sources for BLUs5.


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