The biggest difference between liquid crystal and plasma is that liquid crystal must rely on passive light source, while plasma TV belongs to active luminescence display equipment.At present, the main liquid crystal backlighting technologies in the market include LED(light-emitting diode) and CCFL(cold cathode fluorescent lamp).LCD LCD is..Liquid Crystal Display is short for Liquid Crystal Display. The structure of LCD is a Liquid Crystal placed between two parallel pieces of glass. There are many small vertical and horizontal wires between the two pieces of glass.
Liquid crystal itself does not emit light, can only produce color changes, need a backlight to see the contents of the display.The difference between traditional laptop screens, which use cold cathode fluorescent tubes (CCFL) as a backlight, and LED backlit screens, which use light-emitting diodes (leds), is that.White LED is point light source, CCFL tube is strip light source.Small white leds are powered by direct current (dc) power, which can be used in tandem, but if you have more than a few watts, you need to consider the appropriate drive circuit to improve efficiency.CCFL tube must have “high pressure plate” matching use.Just there are several kinds of LCD backlight way, including the LED (Light Emitting Diode) with CCFL (Cold Cathode Fluorescent Lamp) or is called the CCFT (Cold Cathode Fluorescent Tube).
CCFL(cold cathode fluorescent lamp) backlight is the main backlight product of LCD TV.It works when the high voltage on both ends of tube, tube within a few electronic high-speed impact after the electrode produce secondary electron emission, began to discharge, the tube of mercury or inert gas electronic after the impact, the excitation radiation 253.7 nm ultraviolet light, ultraviolet excitation of tu the phosphors on the tube wall and produce visible light.CCFL lamp life is generally defined as: at 25℃ ambient temperature, rated current drive lamp, brightness reduced to 50% of the initial brightness of the length of time for the lamp life.At present, the nominal life of LCD TV backlight can reach 60,000 hours.CCFL backlight is characterized by low cost, but the color performance is not as good as LED backlight.
LED backlight USES LED as backlight source, which is the most promising technology to replace the traditional cold cathode fluorescent tube in the future.Leds are made of thin layers of doped semiconductor material, one with an excess of electrons, and the other without them, creating positively charged holes through which electrons and holes combine as electricity passes, releasing excess energy in the form of light radiation.Leds with different luminescence characteristics can be obtained by using different semiconductor materials.Leds already in commercial use can provide red, green, blue, green, orange, amber and white.The mobile phone mainly USES white LED backlight, while the LED backlight used in LCD TV can be white, red, green and blue. In high-end products, multi-color LED backlight can also be applied to further improve the color expression, such as six primary colors LED backlight.The advantage of LED backlighting is that the thickness is thinner, about 5 cm, and the color gamut is very wide, which can reach 105% of the NTSC color gamut. The luminous flux of black can be reduced to 0.05 lumens, which makes the contrast ratio of LCD TV as high as 10,000:1.At the same time, the LED backlight source has another 100,000 hours of life.At present, the main problem restricting the development of LED backlight is the cost, because the price is much higher than the light source of cold fluorescent lamp, LED backlight source can only appear in high-end LCD TVS abroad.
Advantages of LED backlight source
1. The screen can be made thinner. If we look at some LCDS, we can see that there are several filament CCFL tubes arranged.Backlighting, on the other hand, is a flat light-emitting material, requiring no additional devices.
2. Better picture effect CCFL backlit screen generally has different brightness in the middle and around, and some white when the screen is completely black
CCFL fluorescent lamps, like fluorescent lamps, age over time, so traditional laptop screens will turn yellow and dark after two or three years, while LED backlit screens will last longer, at least two or three times longer.
We all know that fluorescent lamps need high voltage to bombade mercury vapor, so the power consumption of CCFL screen is large, generally 14 inches of power consumption in more than 20 watts.Leds are semiconductors that work at low voltage, are simple in structure, and consume little power, making them especially good for laptop battery life.
5. More environmentally friendly the mercury in CCFL lights will cause great pollution to the environment, and it will be very difficult to recycle harmlessly.
Working principle of CCFL cold cathode fluorescent lamp
The physical composition of CCFL cold cathode fluorescent lamp is that the inert gas Ne+Ar mixture containing trace mercury vapor (mg) is sealed in a glass tube and the fluorescent substance is coated on the inner wall of the glass.CCFL cold cathode fluorescent tubes emit light by striking fluorescent powder on the wall with ultraviolet light excited by gaseous mercury through electrodes at both ends of the tube.The wavelength is determined by the properties of the fluorescent material.
Defect of CCFL cold cathode fluorescent lamp
The CCFL light source that liquid crystal TV USES commonly at present, no matter look from give off light principle or from physical structure, look with the daylight tube that we use everyday very close.This kind of light source has the advantages of simple structure, low temperature rise on the surface of the tube, high brightness on the surface of the tube and easy processing into various shapes.But the service life is short, contains the mercury, the color gambit is narrow, only can achieve the NTSC 70% ~ 80%.For large – size TV screens, CCFL voltage and extended pipe processing is difficult.
First, the biggest headache is the short life span.CCFL backlight service life is generally 15,000 hours to 25,000 hours, the longer the use of LCD(especially laptop LCD), the more obvious the brightness decline, in the use of 2-3 years, the LCD screen will be dark, yellow, this is the short life of CCFL defects caused by.
Second, limits the LCD color play.Each pixel in the LCD is composed of R, G and B rectangular color blocks, and the color performance of the LCD completely depends on the performance of the backlight module and the color filter film.The three primary colors of the filter film are the same as the white light emitted by CCFL (the composition of the three primary colors), but the CCFL backlight module can not actually meet the design requirements, only about 70% of the NTSC standard.
Third, the structure is complex and the brightness output uniformity is poor.Because cold cathode fluorescent lamp is not a plane light source, so in order to achieve uniform brightness output of backlight, the backlight module of LCD needs to be equipped with many auxiliary devices such as diffuser plate, light guide plate and reflector plate.
Fourth, large volume, power consumption is not ideal.The volume of LCD can not be further reduced because CCFL backlight must contain diffuser plate, reflector plate and other complex optical devices.In terms of power consumption, LCDS using CCFL as a backlight are also unsatisfactory, as 14-inch LCDS require 20W or more power.
Of course, the last two years domestic and foreign manufacturers in view of the shortcomings of the traditional CCFL made some improvements, seems to have reached a very high level, manufacturers publicity is said the magic, but these improvements are limited, and cannot completely eliminate the CCFL backlight congenital technical defects.
At present, the backlight is mainly CCFL tube, the cost may be slightly lower, the technology is more mature.LED backlighting is also limited to small screen products such as mobile phone, MP3, MP4, etc. For large screen products, it is still a direction of efforts. However, it is more energy-saving, which is its advantage