Explore the difference between LCD and LED

LCD liquid crystal display is the abbreviation of Liquid Crystal Display. The LCD is constructed by placing a liquid crystal cell in two parallel glass substrates, a TFT (thin film transistor) on the lower substrate glass, and a color filter on the upper substrate glass. The signal and voltage change to control the direction of rotation of the liquid crystal molecules, so as to control the emission of polarized light at each pixel point for display purposes. Now that LCD has replaced CRT as the mainstream, the price has dropped a lot and has been fully popularized.

The liquid crystal itself does not emit light, it can only produce color changes, and a backlight is required to see the displayed content. Traditional notebook screens use cold cathode fluorescent tubes (CCFLs) as backlights, while LED backlight screens use light-emitting diodes, which is the difference between the two.

The white LED is a point source, and the CCFL tube is a strip source. Small white LEDs use DC power supply. When used in multiples, they can be serialized and combined. However, if the number is more than watts, it is necessary to consider a suitable driving circuit to improve efficiency. The CCFL tube must be used with a "high voltage board".

There are only a few LCD backlights, including LEDs and CCFLs or CCFTs.

CCFL (Cold Cathode Fluorescent Lamp) backlight is the most important backlight product of LCD TVs. Its working principle is that when a high voltage is applied to both ends of the lamp tube, a small number of electrons in the lamp tube collide with the electrode at high speed to generate secondary electron emission, and discharge begins. After the mercury or inert gas in the tube is struck by electrons, the excitation radiation is 253.7 nm. The ultraviolet light generated by the ultraviolet light excites the phosphor coated on the inner wall of the tube to generate visible light.

The life of a CCFL lamp is generally defined as: at a ambient temperature of 25 ° C, the lamp is driven at a rated current, and the brightness is reduced to 50% of the initial brightness for the length of the lamp life. At present, the LCD TV backlight has a nominal life of 60,000 hours. CCFL (Cold Cathode Fluorescent Lamp) backlights are characterized by low cost, but the color performance is not as good as LED backlighting.

LED backlights use LEDs as backlight sources, which is the most promising technology to replace traditional cold cathode fluorescent tubes in the future. Light-emitting diodes are made up of several layers of very thin doped semiconductor materials, one with an excess of electrons and the other with electrons to form positively charged holes. When working, current flows through, electrons and holes combine with each other. The energy is released in the form of light radiation. Light-emitting diodes of different luminescent properties can be obtained by using different semiconductor materials.

Light-emitting diodes that have been put into commercial use can provide colors such as red, green, blue, cyan, orange, amber, and white. The main use of the mobile phone is white LED backlight, and the LED backlight source used on the LCD TV can be white or red, green and blue. In high-end products, multi-color LED backlight can also be applied to further improve the color. Expressive, such as six primary color LED backlight sources.

The advantage of using LED backlight is that the thickness is thinner, about 5 cm, the color gamut is also very wide, can reach 105% of NTSC color gamut, and the black luminous flux can be reduced to 0.05 lumens, so that the contrast ratio of LCD TV is up to 10000:1. . At the same time, the LED backlight source has a lifetime of 100,000 hours. At present, the problem of restricting the development of LED backlight is mainly cost. Since the price is much higher than that of the cold fluorescent tube, the LED backlight source can only appear in high-end LCD TVs abroad.

Advantages of LED backlight source

1. The screen can be made thinner. We pay attention to some LCD monitors. You can see that there are several filament CCFL tubes arranged in order. In order to make the screen shine evenly, you need to add some other devices, so you can't do very much. Thin; and the backlight is different, the LED backlight itself is a flat luminescent material, no need to add other devices.

2. The picture is better. The CCFL backlight screen is generally different in the middle brightness and the surrounding area. At the same time, the screen is whitish when it is completely black.

3. It will not turn yellow and dark. CCFL fluorescent lamp is the same as fluorescent lamp. It will age after a long time, so the traditional notebook screen will turn yellow and dark after two or three years, and the life of LED backlight screen will be much longer, at least two. three times.

4. More power-saving Everyone knows that the fluorescent lamp has a high voltage to bombard the mercury vapor, so the power consumption of the CCFL screen is large, and the power consumption of the 14-inch is generally 20 watts or more. LED is a kind of semiconductor, working under low voltage, simple structure, low power consumption, especially beneficial to the notebook's endurance.

5. Mercury in more environmentally friendly CCFL lighting has a great pollution to the environment, and harmless recycling has great difficulties.

How CCFL Cold Cathode Fluorescent Lamps Work

The physical structure of a CCFL cold cathode fluorescent lamp is to enclose an inert gas Ne+Ar mixed gas in a glass tube containing a trace amount of mercury vapor (several mg) and coating the phosphor on the inner wall of the glass. The CCFL cold cathode fluorescent tube passes through the electrodes at both ends of the tube, so that the ultraviolet rays excited by the gaseous mercury in the tube collide with the phosphor on the tube wall to emit light. Its wavelength is determined by the properties of the phosphor material.

Defects of CCFL cold cathode fluorescent tubes

At present, the CCFL light source commonly used in LCD TVs is very close to the fluorescent tubes that we use every day, both in terms of light-emitting principle and physical structure. The light source has the advantages of simple structure, small temperature rise of the surface of the lamp tube, high brightness of the surface of the lamp tube, and easy processing into various shapes. However, the service life is short, mercury is contained, and the color gamut is narrow, which can only reach 70% to 80% of NTSC. For large-size TV screens, it is also difficult to increase the voltage of the CCFL and lengthen the tube.

First, the most headache is the short life. The service life of CCFL backlights is generally 15,000 hours to 25,000 hours. The longer the LCD is used (especially the LCD screen of notebook computers), the more obvious the brightness decreases. After 2 to 3 years, the LCD screen will become dark and yellow. This is caused by the shortcomings of CCFL's short life.

Second, it limits the color of the LCD display. Each pixel in the liquid crystal display is composed of three rectangular blocks of R, G, and B, and the color performance of the liquid crystal display depends entirely on the performance of the backlight module and the color filter film. The three primary colors of the color filter are as uniform as the white light emitted by the CCFL (the composition of the three primary colors), but the CCFL backlight module can not meet the design requirements, and can only reach about 70% of the NTSC standard.

Third, the structure is complicated and the brightness output uniformity is poor. Since the cold cathode fluorescent lamp is not a planar light source, in order to achieve uniform brightness output of the backlight, the backlight module of the LCD needs to be equipped with a plurality of auxiliary devices such as a diffusion sheet, a light guide plate, a reflection plate, etc., but when displaying an all white or black screen, the screen The difference in edge and center brightness is significant.

Fourth, the volume is large and the power consumption is not ideal. Since the CCFL backlight must contain complicated optical components such as a diffusion plate and a reflection plate, the volume of the LCD cannot be further reduced. In terms of power consumption, LCDs using CCFLs as backlights are also unsatisfactory. CCFL backlights for 14-inch LCDs often consume 20W or more.

Of course, in the past two years, domestic and foreign manufacturers have made some improvements to the drawbacks of traditional CCFLs. They seem to have reached a very high level. Manufacturers’ propaganda is even more fascinating, but these improvements are limited and cannot completely eliminate CCFL backlights. Innate technical flaws.

From the current situation of backlighting or CCFL tube, the cost may be slightly lower and the technology is more mature. LED backlighting, but also limited to mobile phones, MP3, MP4 and other small screen products, for large-screen products, is still an effort, but more power-saving, is its advantages.