Advantages of LED lighting street lamps

LED is the abbreviation of Light Emitting Diode. It is made of several thin layers of doped semiconductor materials, one layer with excess electrons, and the other layer lacks electrons to form positively charged "holes". When working, current passes through, electrons and holes combine with each other, and excess energy is released in the form of light radiation. The shape and size of LED backlight panels vary depending on the shape and size of LCD display panel lights. LED backlight panels are basically rectangular or long strips. It has two basic structures: side-emitting and bottom-emitting. The side-emitting structure is mainly used for narrow strip-shaped backlight panels (generally the length is greater than 2 times the width); while the bottom-emitting structure is mainly used for backlight panels with similar length and width. When the light-emitting diode is lit, the light enters the transparent organic glass, so that the entire emitting surface can be seen with bright light, which is called the edge light effect. The top of the organic glass is made into micro-beads, which can make the light of the entire emitting surface more uniform. There is a layer of milky white transparent plastic film on the top of the organic glass, which can make the emitted light softer. The two sides of the backlight board are sealed with silver light-shielding tape. Each LED light-emitting unit column has two diodes connected in series, and several columns form an LED array (depending on the length of the backlight board).

Compared with the existing CCFT technology, light-emitting diodes have a lot of advantages. LED is a flat light source. The most basic LED light-emitting unit is a square with a side length of 3 to 5 mm. It is very easy to combine together to form a surface light source of a given area. If it is used as the backlight source of the LCD screen, the auxiliary optical components required can be made very simple, and the screen brightness uniformity is more outstanding. In addition, LED backlight has a better color gamut and stronger color expression than cold cathode fluorescent lamp backlight, which can play a good role in compensating for the LCD technology with insufficient display colors. At present, the color gamut range of LED backlight even exceeds the highest level of AdobeRGB and NTSC color standards, which can make LCD displays have extremely excellent color performance (compared with the NTSC specification, the color gamut range of LCD displays using LED backlight can reach 105% of it, while the color gamut of LCD displays using CCFT backlight is only 78% of NTSC at most). Secondly, LED uses a low-voltage power supply of 6 to 24V, and the design of the power supply module is also quite simple. This LCD screen using LED backlight saves 48% of electricity than the commonly used CCFT (cold cathode fluorescent tube) LCD screen, and does not contain heavy metal mercury that is harmful to health and the environment.

Take Toshiba's Libretto U100 as an example. It uses 32 independent control mechanisms of LEDs on a 7.2-inch LCD screen to realize the backlight system, which can display images and texts vividly and clearly, with a power consumption of only 1.3W, while the CCFT cold cathode fluorescent tube required for the same size LCD screen consumes more than 2.5W. Compared with LCD monitors, the higher refresh rate of LED monitors makes LED have better performance in video - the reaction speed of a single element of the LED display is 1,000 times that of the LCD screen, and it can be viewed even in strong light and can adapt to temperatures as low as -40°C. In addition to low power consumption and environmental protection, LED has another major advantage, that is, its service life is up to 100,000 hours. Even if it is used continuously for 10 hours a day, it can be used continuously for 27 years.

However, LEDs also have some serious inherent defects, among which the most troubling issue for the industry should be the luminous efficiency of LEDs. Most of the LED devices used as backlight sources are white light types, and white light LED devices have low luminous efficiency at the beginning, resulting in generally low screen brightness. For example, in current LED technology, the brightness of LED components varies regardless of who the screen manufacturer is. At present, the luminous efficiency of CCFT cold cathode fluorescent lamps can reach 50 to 100 lm/W (lumens/watt), while the luminous efficiency of white light LED devices is only 20 lm/W or even lower when they first started. This problem leads to color shift, inconsistent color purity and poor quality true "white", which means that LEDs are not suitable as backlight sources for LCD displays at the beginning. To eliminate this problem, LED manufacturers "partition" or decompose the chip into blocks with similar colors and brightness. This treatment helps to a certain extent, but there are still deficiencies and there are still shifts in brightness and color. Although manufacturers later developed color correction circuits that can correct this problem to a certain extent, the uniformity problem still exists. Fortunately, the technology in this field has been progressing very rapidly. The luminous efficiency of white light LEDs has been increasing by 60% every year. So far, the luminous efficiency of white light LED devices has exceeded 50lm/W and has begun to reach a practical level. The industry hopes to increase the luminous efficiency of white light LEDs to a high level of 75lm/W before 2007, and the latest color correction system can also control the deviation in color and brightness within a reasonable range. The most representative color correction system is Lighthouse's M4 system, which allows each pixel (brightness and color) in each screen panel to be controlled individually, ensuring pure white and achieving better color balance/color control effects. The deviation in color and brightness between pixels is controlled within a range of no more than 1%. In contrast, most LED screens that only use graded or partitioned block LEDs generally have a deviation of 30%.

Although LED has many advantages, it is difficult to popularize it in a short time. For example, the cost of LED backlight system is higher than that of cold cathode fluorescent tube. At present, the price of LED backlight module components is about 5 times that of CCFT. The larger the screen size, the higher the cost of using LED backlight technology. The reason why Toshiba U100 adopts this display technology is that it is only used as a commemorative product for the 20th anniversary of Toshiba notebook. Fortunately, the LED industry also has a similar "Moore's Law" in the microprocessor industry - Haitz's Law, named after Roland Haitz, a former technical scientist of Agilent (a leading manufacturer in the LED field). Its content is that the price of LED will be 1/10 of the original every 10 years, and the performance will be improved by 20 times. If this law can be fulfilled continuously, and with the increase in production capacity, the cost of LED backlight will decline rapidly. It is expected that the price of LED backlight can be reduced to about 2 times that of CCFT backlight in the next one or two years, and it is only one step away from large-scale popularization.

Laptops use LCD liquid crystal displays to display colors and graphics. Whether it uses super twisted nematic (STN) technology or thin film transistor (TFT) technology, it needs to use white backlight for illumination. The white light spectrum contains all colors, and the color filter of the display will select the colors required to be displayed from the white light spectrum, and finally form the colorful and realistic picture we see with our eyes. From its principle, we can know that indicators such as LCD brightness and contrast depend to a large extent on the performance of the backlight source. The backlight tube has a limited life due to its own characteristics and is a consumable, which directly determines the service life of the LCD display device. Whether it is a desktop LCD monitor or a notebook screen, almost all large-area LCD monitors use CCFT (Cold Cathode Fluorescent Tube) backlight.

Although CCFT is technically quite mature, and its performance and stability have been proven. However, cold cathode fluorescent lamps are tubular light sources. To evenly distribute the light emitted to every area of ​​the panel, quite complex auxiliary components are required, and the thickness of the screen is also difficult to control. Moreover, as the panel increases, multiple light sources must be used, which requires that the light from these CCFTs must also be matched. More importantly, the color gamut of cold cathode fluorescent lamps is relatively narrow, which will seriously affect the color spectrum displayed on LCD monitors, resulting in almost all LCD monitors failing to meet the AdobeRGB color gamut standard for flat printing. CCFT backlights allow LCD monitors to reproduce less than 80% of the colors that can be transmitted by NTSC signals. At the same time, the energy utilization efficiency of CCFT backlights is low. In the process of transmitting light energy from the backlight to the screen, the light energy loss is very serious, and ultimately only about 6% of the light energy can be truly utilized. In order to achieve higher brightness and contrast, manufacturers must increase the output power of the light source or increase the number of lamps, and the consequence of this is an increase in the power consumption of the entire machine. This will not have much impact on desktop LCD monitors or LCD TVs, but it will have a great impact on notebook LCD screens. In particular, these CCFTs require high-voltage AC drive, and the requirements for the power transformer and rectifier group are relatively complex. In addition, the service life of cold cathode fluorescent lamps is not long. After a few years of use, the screens of many LCD products will turn yellow and the brightness will become significantly dim.

LCD is the component with the highest power consumption in laptop computers. In order to maximize battery life, the industry is actively developing low-power LCD screens, but CCFT backlight sources are obviously contrary to this.

How to reduce the power consumption of notebook computer display screens and enhance environmental protection? Some notebook computer manufacturers, represented by Sony and Toshiba, have long explored the use of white LED light-emitting diodes to replace CCFT as the backlight source of notebook computer displays, and have made major breakthroughs. At the beginning of this year, Toshiba announced the launch of a new generation of white light LEDs, TL10W02-D, with the highest luminous flux in the industry. In May, in commemoration of the 20th anniversary of the birth of notebook computers, it launched the first Libretto U100 with LED backlight technology and a 7.2-inch LCD screen. Sony also uses LED backlight technology in its newly launched TX series notebooks.