In recent years, the industry has begun to use LED instead of CCFL and EL as the backlight of LCD (short for backlight illumination), compared with CCFL and EL. LEDs have the following advantages:
1) The LCD color can be made more realistic, and the LED backlight can provide 130% NTSC color scale, while the CCFL is only 70%. The expansion of the color scale makes the LCD image more saturated and more realistic;
2) The thickness of the LCD can be made thinner. In the 18-inch LCD module, the thickness of the LED backlight is 4 mm to 6 mm, and the CCFL is 8 mm to 12 nm.
3) Long life, up to 50,000 hours;
4) In line with environmental requirements, LED does not contain mercury,
5) LED backlights are not disturbed compared to EL backlights. Therefore, LED backlights are widely used in PC, TV, car audio, mobile phones, communication equipment, personal digital assistants (PDAs) and watches, etc. It has become the leading product in the LCD backlight market.
In 2002, the market demand for LEDs accounted for about 60% of the total demand of the backlight market. At present, there are green, red, blue and white LEDs as backlights for LCDs. Due to price factors, green LEDs are mainstream, accounting for 80% of LED backlight products. %, their rated current is 2mA ~ 20mA, the brightness is 600mcd. Due to the high cost of white LEDs, it is currently mainly used for backlighting of color screen mobile phones and color screen PDAs, as well as lighting of automobile instruments.
Luminescence mechanism and characteristics of white LED
1 Luminescence mechanism
The single-chip white LED is a CaN light-emitting diode containing an active layer of InGaN. It mainly has two kinds of light-emitting mechanisms: one is to combine blue LED and yellow phosphorus, and the white light is generated by the mixture of yellow light emitted by blue light and phosphorus; The species is white light produced by a combination of ultraviolet LEDs and red, blue, and green phosphorous.
2 Features
The main characteristics of white LED are: forward voltage drop is 3.5 V; luminous efficiency is more than 20lm/W, which is better than incandescent bulb, next to fluorescent lamp (601m/W ~ 100lm/W). In 2004, luminous efficiency can be improved to 60lm/w, close to the level of fluorescent lamps, which can be used in the lighting market in large quantities; the luminous flux is 231m; the package size is small. Nichia introduced the SMD white LED in 2003, model NSCW215, which is a side-view SMD white LED with a height of 0.8/1mm and a brightness of 600mcd at 20mA. Toyoda Gosei introduced the SMD white LED with a size of 3.2 mm × 2.8 mm, model TG white, with a current of 20 mA, a brightness of 100 mcd, and a luminous efficiency of 4.5 lm/W to 5 lmW.
Citizen has developed the world's smallest white LED with Nichia's white LED die, which is 0.55 mm thick. Nichia's non-SMD type white LED measures 11.2mm (width) × 7.2mm (length) × 6mm (height) and has a long life of more than 50,000 hours. The application prospects of white LEDs in the lighting market are attractive. For this reason, LED manufacturers around the world have stepped up development of high-power white LEDs. For example, Nichia has developed high-power InGaN LEDs with powers of 1 W to 2 W, which are 10 of existing LEDs. Times. The Solid State Lighting and Display Center of the University of California plans to develop a white LED with a luminous efficiency of 200 lm/W by 2007.
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