(Chen Xiaoning) Solid-state lighting sources have emerged, but the output power of the challenging S-light diodes (LEDs) has steadily grown over the past few decades and has grown more rapidly in the past few years. Because luminous efficiency far exceeds incandescent lamps, high-power LEDs are actually used in automobiles and large-area outdoor displays. Because LEDs can efficiently produce blue, green, and red, white, and full-color optical devices, the technology is mature. Enter the direct use.
At this speed, the direct competition between LED large-area light sources and incandescent lamps and fluorescent lamps is coming soon. However, the use of LEDs for white-light indoor illumination still faces daunting challenges. Different from the light source used for display, the illumination source has spectral quality and light source stability. There are very strict standards * LED is still a big gap at this point. In addition, LED light source has to replace incandescent lamps, and its price needs to be greatly reduced. Several companies are racing to develop the first practical LED lighting source, a new indoor light source since the advent of incandescent lamps.
Since 1968, just as the number of integrated circuits containing transistors has grown following Moore's Law, the brightness output of LEDs has doubled every two years following Haitz's law. In the past two years, LED brightness output has grown faster than this, so that the largest LED source now has a brightness of 300 lm, and the largest output in 1968 is only 1 mlm. The efficiency per unit of energy input converted to light output is also Stable growth, it is reported that the luminous efficiency of orange-red LED has reached 45% (over 100Im/W), and the luminous efficiency of short-wavelength LED has reached 25%. The efficiency of incandescent lamp is only about 5% (710lm/W). 20% (3540lm/W). The advantages of high LED luminous efficiency and high brightness make it replace incandescent lamps in some common applications, such as traffic lights and car brake lights*, and full-color or white-light high-power LED applications (such as outdoor displays) become a reality. The orange-red LED device made of AlGalnP material emits light with a wavelength range of 590650 nm. The AlGaln NLED device emits 350 nm of near-ultraviolet light, 450 nm of blue light and 540 nm of green light with phosphide to convert ultraviolet light into visible light, and the LED can combine the spectrum. Get up to produce a wider range of colors of light and white light. The outdoor display made with the red, green and blue (R.GB) tri-color diode unit can already be made as large as one wall of the store.
The emergence of high-power LED devices that produce white light with high efficiency has greatly increased the practical application of solid-state lighting. The potential benefits are huge in the United States, about 21% of energy is used for lighting, 8% of which are incandescent lighting and 13% are neon lighting. The average efficiency of this kind of illumination is about 14%. If the existing lighting equipment is replaced by LED devices, the luminous efficiency is about 30%, the power consumption for lighting can be saved by half, and the total energy consumption is reduced to 10%. The life span is longer than incandescent lamps, which can reach 50,000h or more, while the life of the bulb is only 750h. However, solid-state lighting has not yet been realized. Today's LEDs are still designed for use in the "display" of direct viewing. Instead, the power of the illumination source is large enough to rely on surface reflection rather than direct viewing. In fact, there are still several obstacles that lie on the way to solid-state lighting.
Currently, LED devices have begun to achieve the output power required for illumination. At a 5W output power, a Luxeon LED device from a division of Agilent has a brightness of 150 lm, which is comparable to a 20W incandescent lamp that is more than 100 times larger than an LED device. Because the LED is small, multiple LEDs can be combined to replace the current standard light source. For example, 12 LEDs can replace an 1800lm automotive headlight* but surface illumination, as one of the basic lighting applications, except for the output power of light. In addition, more requirements have to be met. The human eye can see the light from the sun. The spectral properties of the sun are close to those emitted by the black body. Therefore, the human eye is very sensitive to the deviation of the light source spectrum from the black body spectral curve. The human eye can accurately distinguish the color of the reflected light produced by the black body radiator illuminated by different temperatures. When illuminated with a 3500K incandescent lamp or a 6500K midday sun, the colors look very similar, but a 2% deviation from the blackbody spectrum can be detected, making the color slightly distorted. The color consistency of the light is also very important, with red and green flux variations exceeding 3% and blue light flux variations exceeding % being unacceptable. At present, LED devices have considerable difficulties in meeting this color consistency requirement.
The set of spectral radiation of the color line is far from the blackbody radiation spectrum. There are three ways to convert this radiation to the blackbody spectral curve. The simplest method of the first method is to mix several monochromatic lights in an appropriate ratio to simulate the color response of a blackbody source. By mixing three or more monochromatic sources at a point in the color map, coordinates that are very close to the blackbody radiation can be obtained.
In this way, the white light obtained by mixing three monochromatic light sources has a standard color compensation coefficient (/.a) of 80, and the mixing of four monochromatic light sources can make the coefficient reach 90. In contrast, an ideal black body light source. /4 is 100, incandescent lamp is 90, and fluorescent lamp is only 80. The second method is to excite three kinds of phosphides with LEDs emitting ultraviolet light to make them emit light in the red, green and blue spectral ranges respectively. The phosphide mixes to extend the energy of the light very close to the blackbody spectrum. It has a very good ugly a. However, due to scattering and absorption, nearly half of the input energy is wasted, and the ultraviolet radiation also rapidly makes the packaging material of the LED device. Invalid.
The third method is the most commonly used, which emits a yellow-emitting phosphide by emitting a blue-light LED. A part of the blue light escapes and the yellow light mixes to obtain white light. This method is similar to the method without phosphide. There is also a loss in efficiency, but not as severe as the loss of all phosphide luminescence. The main challenge of the above methods is to reduce the variability of the light output. Light-emitting diodes change the temperature and aging in the same production process to affect the output characteristics of the LED. For every 10% increase in temperature, the light output drops by about 10%. Small changes in the thickness of the phosphide film also affect the light output of the LED.
To compensate for these changes, LED manufacturers have adopted a variety of feedback control methods. A simple approach is to use a set of three photocell detectors to measure the luminous flux emitted by each LED to correct the input current and keep the output constant. This method of God corrects the change in luminous flux of the LED due to aging and temperature changes, but does not correct the change in output wavelength due to temperature changes. One improved method is to control the temperature of the LED itself. Another method is to use a feedback circuit to measure the actual point of the LED's emitted light on the color map, and correct the input to keep the point constant. This method requires a color filter in front of each of the three detectors to match the human eye's sensitivity to color.
The Phillips Institute developed an experimental control system that demonstrates that this state of the art method achieves sufficient color consistency in 98% of the sampling tests. Once the feedback system enters commercial production, the quality of the LED output light can quickly reach the level of fluorescent or even incandescent lamps.
However, there is still a barrier that is the price of the device. Even without a feedback compensation circuit, the current price of high-power LEDs is now more than 100 times that of incandescent lamps. * The brightness of six $12 LEDs is equivalent to the brightness of a 50-cent bulb of 50 cents. Therefore, LEDs must win in the competition with existing light sources, and the price must be lowered. It is worth noting that, considering that the life of an LED is 5070 times that of an incandescent lamp, and the energy consumption is only 1/4, the lifetime of the LED light source is lower than that of an incandescent lamp. However, LEDs still need to further reduce the price to compete directly with fluorescent lamps, because fluorescent lamps also consume less energy and have a longer life. Once the feedback compensation control technology is mature, solid-state lighting becomes a reality and it is just around the corner.
(Zhang Tianshu)
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