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According to statistics from the Nomura Research Institute of Japan, the market size of white LEDs from April 2005 to March 2006 was about 200 billion yen. The main use was focused on mobile phone backlight modules/key pads/flashlights. Automotive interior/outdoor lighting and other various lighting areas. After 2005, the market growth of white LEDs was stuck in a stalemate. The main reason was that it was affected by the decline in mobile phone prices, which accounted for 80% of the overall white LED market . After the second half of the second quarter of 2002, the price of white LEDs continued at an annual rate of 25~. 30% fell.
Japan Nomura Research Institute predicts that the market will remain stagnant in 2006. It is generally believed that LCD TVs and computer monitors, as well as backlight modules for large displays, will continue to use LEDs. Therefore, the market for white LEDs is expected to grow again in 2007~2008. By 2010, the market will expand exponentially. At that time, the market size will surely exceed the 300 billion yen mark (Figure 1). The backlight module for large-size displays of more than 10 inches is estimated to account for about 50% of the total.
Future accession to the large-size backlight module, can affect the car headlights, the white LED general lighting market, but this does not guarantee that other Japanese Nichia LED maker can grasp the initiative. In the field of mobile phones with extremely mature market, South Korea's Samsung Electronics Machinery Co., Ltd. (SEMCO) and LG Innotek (LGIT), through the expansion of production scale, may face a competitive advantage in cost, and Korean LED manufacturers will increase their firepower and seize it fiercely. Other new application areas have forced Japanese LED chip manufacturers to establish mutual assistance mechanisms with domestic downstream application vendors, while continuously developing high value-added products.
Figure 1 Market forecast for white LED
The white LEDs shown in Fig. 2 are respectively composed of RGB three-color LEDs to form a white LED, and the combination of a blue LED and a yellow-emitting phosphor constitutes a white LED, and the recently-observed ultraviolet/near-ultraviolet LED combination RGB phosphor constitutes a white LED. In the future, in the future, white LEDs will be formed by blue LED combination RG phosphors, and RGB LEDs will be combined to form white LEDs. After 2010, near-ultraviolet LEDs with wavelengths of 380 to 400 nm will be put into practical use.
A white LED that excites RGB phosphors with short-wavelength ultraviolet/near-ultraviolet LEDs, which can improve the color rendering of LEDs. In contrast, blue LEDs excite white LEDs composed of RG phosphors, which emit blue tones when the light source illuminates the object. The extremely energetic UV/near-ultraviolet LED will only appear white light and is very close to natural color. Recently, the near-ultraviolet LED with a wavelength of about 400 nm has an output power of more than 14mW, and the light output efficiency is combined with the epitaxial package to combine the phosphors. Can be applied in the field of general lighting. At present, near-ultraviolet LEDs with wavelengths around 380~405nm have started sample shipments, and are scheduled to enter mass production in 2007.
Figure 2 Practical prediction of fluorescent white LED lighting source technology
UV/near-ultraviolet LEDs use InGaN as the light-emitting layer like blue-light LEDs. The change in In composition can change the wavelength of light. The higher the In composition fluctuation, the higher the light emission. Therefore, theoretically, an In composition with unevenness is required. However, when the wavelength is lower than 380 nm, the wavelength is lower than 380 nm. The unevenness of the In composition is lowered, and the luminous efficiency is rapidly deteriorated. The AlGaN light-emitting layer is usually used for the short wavelength, but the composition of the AlGaN is less likely to fluctuate, and as a result, the light-emitting effect is further deteriorated. If we consider both the improvement of color rendering and the reduction of luminous efficiency, as well as the excitation efficiency of the phosphor, researchers generally believe that "the optimum wavelength for white LEDs used for general illumination should be 375 nm."
Japanese LED manufacturer Nightlight Semiconductor has introduced a high-temperature SiN intermediate layer and a warm GaN buffer layer to reduce the crystal transposition. In addition, in order to improve the compositional non-uniformity in the short-wavelength region, the development of AlGaN and InGaN rich fields has been developed over the SiN monolayer. The technique (Fig. 3) is whereby the fluctuation of the band gap becomes large, and the luminous efficiency is increased by about 2 times. In theory, the luminous efficiency of ultraviolet LED is almost the same as that of blue LED. In the future, if the flip chip structure is adopted, the luminous efficiency can be increased to 30%. However, based on cost considerations, the application of ultraviolet LED is currently concentrated in medical treatment. Special fields such as sensors, resin hardening, and photocatalyst light sources.
Figure 3 Adding SiN to form uneven In composition
LED Lighting Simulation Analysis Technology ‧ When analyzing an LED illumination optical system, it is necessary to prepare various setting methods according to the purpose of analysis.
‧ When analyzing the LED body, it is necessary to faithfully make the optical and structural actual model detail model, so that the simulation analysis close to the real image can be performed.
‧ When analyzing LED applications such as LCD, the structure of the LED is not important. Usually, the intensity distribution on the light-emitting surface and the distribution of the light distribution can be set. At this point, you can use the simple light source and far field distribution settings, or use the light data source.
The former is mainly used in general lighting systems, regardless of time and precision. LighTools determines the radiation direction of the light according to the distribution of light distribution, that is, the high-intensity direction to emit more light, so as long as the LED manufacturer provides The light distribution database can be easily set. The latter ray data source has data for each ray, that is, data related to the starting position and direction of ray tracing, and it uses the defined near-field illuminating distribution as a light source, and an optical system in which the light source and other parts are closely arranged. Perform a detailed analysis. The above light source is used by Radiant Imaging's data library or LED data provided by LED manufacturers. LighTools is a lighting design analysis software developed by Optical Research Associates of the United States. It is widely used in the design of LEDs, light guides for LED backlight modules (Fig. 4), automotive lighting, etc. It has the 3D CAD function of optical system modeling. Lighting analysis function.
Figure 4 Simulation analysis of LED backlight module
The illumination analysis function is based on geometric optics to trace the light of the light source, calculate the illuminance, luminance, light distribution, chromaticity distribution, and the recent optimization design function of the lighting analysis software, which can make the completed model guide better. As a result, the illumination analysis software analyzes the light emitted by the light source defined by the above method, traces it along the optical system mode, and finally calculates the distribution of illumination, luminance, light distribution, etc. according to the light reaching the evaluation surface. (Figure 5). It is generally believed that the number of trials and experiments can be reduced through simulation analysis, thereby effectively reducing development costs and shortening development time.
Figure 5 RGB three LED analysis
The above design analysis software uses an internal optimization engine, which converges earlier than global optimization, and can set a large number of design parameters. For best results analysis software, designers will be asked for the most appropriate design. The initial and incremental components of the parameters are set to give full play to the designer's experience and knowledge. In other words, in the past, the manual analysis of manually changing parameters repeatedly requires very cumbersome and lengthy operations to be opened, and designers can participate in design development.
As mentioned above, one of the mainstream white LEDs is a white LED composed of a blue LED combined with a yellow phosphor. The LighTools analysis software is very effective for color simulation analysis using LEDs and phosphors to change the luminescent color. In addition, when the phosphor is modeled, as long as the excitation spectrum and the fluorescence spectrum of the specified material (phosphor) are defined, and then the light is incident on the completed phosphor, the absorption, the scattering, and the appearance of the fluorescence can be simulated. In addition to the addition of optical real-time perspective (photo real rendering) function, LighTools analysis software combines ray tracing simulation analysis results with recracing CG images. Through the additional functions set by the texture of the material surface, the designer can obtain visual evaluation. Very realistic perspective image (Figure 6).
Figure 6 Simulation analysis results of automotive LEDs
LED heat dissipation technology
General lighting fixtures or backlight modules for large LCD TVs face tricky heat dissipation problems if they are LED-based. The conventional heat dissipation method is to bond an aluminum substrate with an LED to a heat pipe, and heat the heat to the heat sink through the heat dissipation path. However, such a structure has a certain heat dissipation effect. limit. In view of this, Japanese manufacturers Great Plastics using conventional plastic injection molding technology, the development of nano-molding manufacturing techniques (NMT: Nano Moulding Technology) The aluminum substrate and the resin integrally molded to prepare new LED heat sink (FIG. 7).
The specific method firstly presses a film-shaped copper heat pipe into the aluminum substrate provided with holes, and then applies high temperature and pressure to expand the two, thereby improving the adhesion between the aluminum substrate and the copper tube, and simultaneously forming the inside of the hole. The copper film is then introduced into the cooling water from the hole to break the vacuum to lower the boiling point, so that the thermal energy generated by the LED is transferred to the cooling component such as the heat dissipating fin to dissipate heat. Since the heat dissipating fin is away from the LED heat source, it can effectively prevent thermal energy from accumulating on the LED substrate. Internal issues.
According to the experimental results, if the heat sink is equipped with a thermoelectric conversion module (Peltier device, TE cooler), the surface temperature of the LED substrate is 7 ° C lower than the conventional method. If the diameter of the copper heat pipe is increased, it can be further improved. Thermal characteristics. The aluminum substrate is integrally formed with the copper heat pipe, which not only reduces the manufacturing cost, but also greatly reduces the structural size of the module. The newly developed integrated LED heatsink is suitable for general lighting fixtures, backlight modules for large LCD TVs, and LED lighting fixtures for vehicles. Further copper heat pipe with high flexibility, which means that the lighting designer, design freedom of the free surface of the LED relative increase in the future.
Figure 7 Aluminum substrate and copper tube integrated radiator
LED feature evaluation new scheme <br> With the expansion of LED application field, the importance of light extraction for various needs has once again received attention. The US SphereOptics Company takes the full beam measuring instrument "SLM system" as the main body and puts high in LEDEX JAPAN 2006. New solution for easy evaluation of LED characteristics (Figure 8). The LM system is composed of an integrating sphere, a beam splitter, a correction light source power supply, a self-absorption correction lamp, and software. The main features of the LM system, such as measuring an LED array, are self-absorption correction functions that automatically correct the substrate to cause light absorption. High precision measurement of the light source body. In addition, the built-in software can easily measure the LED light source by pressing the operation button, and the measurement includes the spectral intensity spectrum, the total radiation intensity, the full beam, the chromaticity, the color temperature, the color evaluation number Ra, and the like.
Among the integrating spheres, “ENITH Type†has the highest reflectance for visible light, followed by “OptoWhite Typeâ€, and “GOLD Type†is suitable for optical characteristics measurement of UV LEDs. The above “ENITH Type†integrating sphere is coated with PTFE powder attached to the inner wall of the integrating sphere. It can support the measurement of the characteristics of the light source with a wavelength of 250~2500nm. The reflectivity is up to 99% in the wavelength range of 350~1500nm. The heat resistant temperature is 250°C. With high energy intensity, it is mainly used in the development of UV LEDs; the internal wall of the "OptoWhite Type" integrating sphere is coated with barium sulfate powder, and the reflectance in the wavelength range of 400 to 1000 nm is 98%.
The coating of the inner wall of the integrating sphere and the roundness of the integrating sphere determine the performance of the integrating sphere. The above integrating spheres can uniformly reflect the light, indicating that the coating and roundness of the inner wall of the integrating sphere are extremely accurate. In addition, SphereOptics has proposed a new solution for the LED evaluation system. The specific content is to add full beam, luminosity, light distribution characteristics and LIV characteristic evaluation function to the LED measurement system composed of multi-frequency spectroscope and integrating sphere. Ocean Optics has introduced the "USB4000" system for fiber-optic multi-frequency splitters. The light-receiving components use linear semiconductor CCD matrix components with a total of 3,468 components, which can achieve 16-bit A/D resolution. Evident Technologies has launched the "EviDot/EviTag" fluorescent semiconductor nanocrystal system, which has a very high luminous intensity and a very small quantum dot size distribution with a sharp luminescence spectrum. High quantum efficiency can be achieved, and multiple fluorescent colors can be excited with a single excitation source.
The main purpose of the new schemes proposed by the above-mentioned companies is to completely solve the problems caused by the light absorption of the substrate when the optical ball is measured by the integrating sphere, and the accuracy of the operation, the speed is slow, and the measurement items are limited. A new generation of full-featured integrating spheres consisting of a simple LED feature evaluation new solution is in line with the actual operational needs of LED-related manufacturers.
Figure 8 Full beam measurement system structure
Optical characteristic measurement system
When the LED application is in a special field, it is necessary to fully grasp the optical characteristics of the LED. For example, when designing a white LED backlight module for a mobile phone, it is necessary to set the number and configuration of the LED according to the luminous intensity, light distribution, and chromaticity of the LED, in order to Uniform distribution of light typically uses reflective and diffuse optical films, where brightness and chromaticity uniformity become very important. In other words, in order to ensure that the applied product can fully exert its characteristics in accordance with the design specifications, measurement and evaluation of optical characteristics are required. To grasp the optical characteristics, we must pay attention to the following three major elements, namely:
➀ Wavelength Diffusion ➠Spatial Diffusion ➂ Time Diffusion This also needs to pay attention to the representative measurement light of light intensity, that is, the measurement of illuminance, luminosity and total beam. The main reason is that the LED has a diversity of light distribution such as monochromatic and white, and the light has a spatial characteristic after passing through the lens. Since the electrical response speed of the LED is very fast and easy to obtain pulsed illumination, it is necessary to consider the above three factors when evaluating the optical characteristics of the LED.
When evaluating the optical characteristics of LEDs, high optical measurement techniques are required. Therefore, high reproducibility measurement and measurement values ​​are important issues. In order to completely solve this problem, LED measurement standardization is actively underway. The measurement of the amount of light, compared with the standard-added standard LED, has become a standard work method, the Japanese regional electric ball industry association and other lighting relations four major groups, issued a common standard "General Rules for White LED Metering for Lighting" JEL311 (2006) The second edition is currently moving towards JIS. The International Commission on Illumination (CIE) released the technical information "Measurement of LED" CIE127 (1997), and is currently working on the revision of the revised edition and the development of the ISO/CIE specification.
The vivid color tone is the main feature of the monochrome LED, and its spectral distribution is mostly similar to the Gaussian curve and the Lorenz curve with a half amplitude of several tens of nm. When performing such narrow-bandwidth LED photometry, it is necessary to add the spectral distribution of the LED and the spectral response characteristics of the photoreceiver. A white light LED is formed for the blue light and the yellow phosphor, and since it is a narrow bandwidth and strong blue light, it is also necessary to pay special attention. The MCPD series LED spectrometer of Fig. 9 can correct the Polychromator including the light receiving unit according to each wavelength, and it can measure the spectral characteristics of the LED with high precision.
Figure 9 Appearance of high-precision LED spectrometer
LEDs are point-emitting components that, when combined with optical components such as lenses, make it easy to control the spatial diffusion of light (light distribution). LED components with the same luminous intensity (density) can have different light distribution angle characteristics through light intensity control in a specific direction. Generally, when the angle of the point source is required, first, the illuminance of the vertical plane is measured for the radiation axis, and then the illuminance is multiplied by the square of the distance from the measuring surface to the illuminating point, that is, the angle measurement of the general point source, and most of the ray is used in the inverse The method is based on the premise that the optical density of the measuring surface must be equal. However, in fact, the LED with the light distribution angle is controlled to be smaller, the optical density of the measurement surface is not necessarily equal, and the conventional method is not easy to measure the luminosity correctly. Therefore, CIE advocates "CIE averaging LED luminosity", and the new metering amount has been adopted by JEL311. The MCPD series of Figure 10 uses a photometric measurement accessory that achieves the CIE averaging LED luminosity measurement advocated by CIE.
Figure 10 CIE averaging LED photometric accessory appearance
The full beam refers to the sum of the light intensities output by the illuminant, and the total beam obtained for supplying electric power is regarded as one of the LED performance requirements. The full beam measurement of the illuminant is typically performed by placing the source at the center of the integrating sphere (Fig. 11), and measuring the integrated internal wall light intensity. Some surface-encapsulated and small-angled LEDs have no light distribution on the back of the light-emitting point. In this case, an LED must be placed close to the wall of the integrating sphere to prevent direct light from entering the metering section. As for the size of the integrating sphere, it is necessary to consider measuring the surface area of ​​the LED (module) and the amount of heat generated. In addition, when the inside of the integrating sphere is lit, the emitted light is absorbed and a so-called self-absorption phenomenon occurs. Therefore, the integrating sphere with the correction light source is used. The necessary means of measuring LEDs.
Figure 11 Actual sphere outside the sphere
When using LED to design LED modules or lamps, the mastery of light distribution is very important. For example, the application of LED backlight module requires the illuminance angle distribution (light distribution) and chromaticity angle distribution of white LED. In this case, it is necessary to use the double control capable of automatic control. The Goniometer is designed to measure the spectral distribution at various angles, and then analyze the spectral distribution at each angle to evaluate the angular distribution of luminosity and chromaticity (Figure 12).
Figure 12 Correlated color temperature angular distribution characteristics of white LED
Conclusion
LEDs have low power consumption, long life, and high start-up reactivity, and are considered to be the next generation of primary lighting sources. The introduction of the blue LED in 1996, the white LED composed of RGB three primary colors, and the white LED composed of blue phosphor combined with blue light are widely used in backlighting modules for mobile phones and LCD TVs. Due to the continuous innovation of LED luminous efficiency and output power, and the practical use of UV LEDs, it is generally believed that LED lighting fixtures in the near future are expected to become an indispensable part of daily life.
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