The use of white or color tunable LEDs (light-emitting diodes), which can replace a large light source apparatus and
light-guiding fiber bundle, enable the miniaturization of the whole endoscope system and remove constraints on the
design of its shape. We have developed a novel white LED for a new experimental prototype LED-illuminated gastrointestinal endoscope
having the color rendering in the clinically important red range at around 600 nm.
Near ultraviolet (n-UV) LED-based white light source (LS) can provide the excellent
illuminant properties that show high-luminous efficacy (Le>80 lm/W) of radiation and
high-color rendering index (CRI or Ra>99). Recent progress and future advantages of the n-UV
white LED technology on the basis of our direct flip-chip bonding and stacked multi-layer
structure methods will be described. Particularly, the fabrication process, and electrical and
illuminant properties in the low-correlated color temperature (CCT or Tc~3452K) white LED
with a high-luminous flux (~500 lm/PKG) are discussed, and its lighting and medical
applications such as ecological street lighting and gastrointestinal digestive endoscope will be
introduced. Furthermore, a color tunable white LS which is composed of multi-phosphor
conversion LEDs and its characterization are described. Differences on the quality of light
between the n-UV white LED and the conventional blue-YAG white LED will be clarified.
KEYWORDS: Light emitting diodes, LED lighting, Light sources and illumination, Endoscopes, RGB color model, Luminescence, Radium, Near ultraviolet, Blue light emitting diodes, Mouth
We have for the first time developed warm white LEDs lighting using a combination of near ultraviolet LED and three-band (red,
green and blue) white phosphors. This LED has the average color-rendering index Ra=96. Moreover, special color-rendering index R9
(red) and R15 (face color of Japanese) are estimated to be 95 and 97, respectively. We will describe the results of evaluation on the
medical lighting applications such as operation, treatment and endoscope experiments, application to the LED fashions and application
to the Japanese antique art (ink painting) lighting.
A near-ultraviolet (UV)-based white light-emitting diode (LED) lighting system linked with a semiconductor InGaN LED and compound phosphors for general lighting applications is proposed. We have developed for the first time a novel type of high-color rendering index (Ra) white LED light source, which is composed of near-UV LED and multiphosphor materials showing orange (O), yellow (Y), green (G), and blue (B) emissions. The white LED shows the superior characteristics of luminous efficacy and high Ra to be about 40 lm/W and 93, respectively. Luminous and chromaticity characteristics, and their spectral distribution of the present white LED can be evaluated using the multipoint LED light source theory. It is revealed that the OYGB white LED can provide better irradiance properties than that of conventional white LEDs. Near-UV white LED technologies, in conjunction with phosphor blends, can offer superior color uniformity, high Ra, and excellent light quality. Consequently we are carrying out a "white LEDs for medical applications" program in the second phase of this national project from 2004 to 2009.
The near-ultraviolet (nUV) white LED approach is analogous to three-color fluorescent lamp technology, which is based on the conversion of nUV radiation to visible light via the photoluminescence process in phosphor materials. The nUV light is not included in the white light generation from nUV-based white LED devices. This technology can thus provide a higher quality of white light than the blue and YAG method. A typical device demonstrates white luminescence with Tc=3,700 K, Ra > 93, K > 40 lm/W and chromaticity (x, y) = (0.39, 0.39), respectively. The orange, yellow, green and blue OYGB) or orange, yellow, red, green and blue (OYRGB) device shows a luminescence spectrum broader than of an RGB white LED and a better color rendering index. Such superior luminous characteristics could be useful for the application of several kinds of endoscope. We have shown the excellent pictures of digestive organs in a stomach of a dog due to the strong green component and high Ra.
“The light for the 21st century” METI national (Akari) project, which is based on the high-efficient white light-emitting diode (LED) lighting technologies using near ultraviolet (UV) LED and phosphor system, has started from 1998 and its first phase programme finished in 2004. The near UV white LED system linked with semiconductor technologies on GaN LED and compound phosphors for the general lighting applications has for the first time been proposed in the world. In particular, we have demonstrated high-efficient near UV LED having external quantum efficiencies more than 43% around an emission wavelength of 400 nm. Basic illumination properties, and applications of the high luminous efficacy (>40 lm/W) and the high general color rendering index (Ra>90) white LED sources are described. The near UV white LED technologies in conjunction with phosphor blends can offer superior color uniformity, high Ra and the excellent light quality, and the second phase programme has been performed in the MEXT national project “White LEDs for medical applications” from 2004 to 2009 for 5 years.
The external quantum efficiency (EQE, ηe) of conventional near-ultraviolet (NUV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure is limited by high dislocation density and by the narrow escape cone due to total internal reflection at the GaN/air or sapphire/air interface. We have fabricated the NUV and violet InGaN-MQW-LEDs with the high EQE on the patterned-sapphire substrate (PSS) using a single growth process by metal-organic vapor phase epitaxy (MOVPE). The PSS with parallel grooves along the <11-20>GaN direction or the <1-100>GaN direction was fabricated by a standard photolithography and subsequent reactive ion etching (RIE). In this study, fabricated the LED on the PSS with parallel grooves along the <11-20>GaN direction. The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has dislocation density of 1.5x108 cm-2. The LEPS-NUV (or violet)-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the LEPS-NUV-LED (the emission peak wavelength λp: 382 nm) was operated at a forward-bias current of 20 mA at room temperature (RT), the output power (Po) and the EQE were 15.6 mW and 24%, respectively. When the LEPS-violet-LED (λp: 405 nm) was operated at a forward-bias current of 20 mA at RT, the Po and the EQE were 26.3 mW and 43%, respectively. Furthermore, we obtained the Po of approximately 61 mW at 50 mA and 111 mW at 100 mA, respectively. It was revealed that the PSS is very effective in reducing the dislocation density and for increasing the extraction efficiency due to the multiple scattering of the emission light at the GaN/patterned sapphire interface.
The light extraction process in GaN-based light emitting diodes (LEDs) is studied in this paper. In order to increase the light extraction efficiency of large area LEDs, several novel LED geometries are discussed. The light propagation in the LEDs is simulated numerically by using the finite-difference time-domain (FDTD) method. It is shown that the following improvements in the GaN-based LEDs are very effective for increasing the light extraction: (1) To fabricate GaN micro-pyramid array on the surface of the LED, which guides the generated light to the surface; (2) To make inverted V-shaped groove formation on the GaN layer, which restricts the average length of ray path in the LEDs and refracts the waveguide-mode light to the surface; (3) To separate the LED epilayer from its substrate and then mount it on a metal mirror base, which is used to reflect the backside light to the LED surface. The FDTD simulation results show clearly that these improved geometries guide most of the internal luminescence to escape from the LED, and increase greatly the external light-extraction efficiency of GaN-based LEDs.
We have performed theoretical studies on the luminous characeristics of white LED light source which composed of multi phosphors and near ultraviolet (UV) LED for general lighting. White LED source for general lighting applications requires the conditions that have high-flux, high luminous efficacy of radiation (> 100 lm/W) in addition to high color rendering index (Ra > 90) and variable color temperatures. Recently, we have proposed a novel type white LED based on multi phosphors and near UV LED system in order to high-Ra (>93). We will describe the excellent luminescence properties of white LED consisting of orange (O), yellow (Y), green (G) and blue (B) phosphor materials, and near UV LED. The color spectral contributions of individual phosphor-coated LED are theoretically analyzed using our multi LED lighting theory calculated the maximum luminous efficacy can be estimated to be approximately 300 lm/W having a high Ra of about 90 taking into account individual radiation spectrum. Illuminance distribution of white LED is in fairly good agreement with the experimental data.
The growth of homoepitaxial GaN, AlGaN layers, and GaN/AlGaN multiple quantum wells (MQWs) on Ga- and N-faces of bulk GaN single crystal substrates prepared by pressure-controlled solution growth (PC-SG) has been performed by radio-frequency molecular-beam epitaxy (RF-MBE). It was determined that homoepitaxial GaN layers grown on both Ga- and N-faces had good crystallinity with narrow full-width at half maximum (FWHM) of 150 and 94 arcsec for the (0002) plane and 119 and 106 arcsec for the (10-12) plane in x-ray rocking curve measurements, respectively. Crack-free AlGaN epilayers with Al mole fraction up to 30% were obtained on both faces. AlGaN epilayers on Ga-faces with higher Al mole fraction than those on N-faces under the same Al flux condition were obtained. Furthermore, phase separation existed only in the AlGaN epilayers grown on N-faces. The 5 K photoluminescence spectra for the GaN/AlGaN MQW structures grown on Ga-faces at peak energy of 3.419 to 3.686 eV can be obtained by varying the well thickness from 18 to 2 ML.
The light for the 21st century Japanese national project, which is based on the high-efficient ultraviolet (UV) light- emitting diode (LED) and phosphor systems, has started from 1998 in Japan. The proposal of UV white LED system for general lighting applications has for the first time in the world been done at August 1997 before start up the project. The outline and purpose of this project are introduced. This is also the brief summary of this national project based on white LEDs lighting, which has been carried out by the cooperation between universities and companies in the fiscal year 2000. In particular, we have demonstrated high- power UV LED having an external quantum efficiency of 24% at an emission wavelength of 382 nm. With increasing forward- bias current, the output power increased linearly and was estimated to be above 38 mW at 50 mA. Basic illumination properties of the lighting sources using phosphor-based UV white LEDs are described.
KEYWORDS: Light emitting diodes, LED lighting, Lamps, Light sources and illumination, Solar cells, Sensors, Light sources, Light, Photovoltaics, Control systems
We describe the lighting characteristics and systems of the power energy-saving type street lamp which consists of white light-emitting diodes (LEDs), and a solar-cell and battery system. The prototype street lamp has been constructed by two LED light sources, each of which includes a total of 700 units of 10 cd-class white LEDs. The white LED lighting system is mainly divided into three components which are the control, the electric-power supply and LED lighting divisions. The illuminance is normally 80 lx. When a person approaches within 2 m near the lamp, the body sensor catches the situation. The illuminance then increases to about 660 lx, which is about 50 times brighter than that of a white incandescent lamp. The color rendering index is estimated to be 85 which is similar to that of three color fluorescent tube. The illuminance distribution can be analyzed by our recently developed 'multi sources of LED light' theory.
The radiative recombination mechanism of InGaN single-quantum- well (SOW) blue light-emitting diodes (LEDs) and InGaN double heterostructure (DH) ultraviolet (UV) LEDs has extensively been investigated by means of the dependence of photoluminescence (PL) and time-resolved PL (TRPL) spectra on an external-electric field. Two emission components are found in the luminescence spectra from each LED on the condition of reverse-bias at 77 K. It is also found that the luminescence intensity of the LEDs decreases dramatically with increasing reverse-bias voltage at room temperature (RT). The model based on field ionization of excitons cannot explain the present experimental phenomena. It is therefore suggested that the free-carrier recombination process is dominant at RT. We have also suggested that these experimental results on the blue and UV LEDs can be explained by the same recombination model. Finally, on the basis of both the experimental ecidence in In0.08Ga0.92N epitaxial layers and strong electron-phonon interaction, the radiative recombination mechanism on InxGa1-xN ternary alloys has been discussed.
We have attempted the grown of GaN by RF-MBE which crystalline quality is a match for the by MOCVD, performing homoepitaxial growth using MOCVD-GaN as a substrate. We confirmed that homoepitaxial GaN had Ga polarity by (1 x 1) RHEED streaky pattern after coolingd own. 10 min-BHF-etching was the most effective for cleaning the surface of GaN substrate, and as the result crystalline quality of homoepitaxial GaN was improved. Thermal annealing of GaN substrate was also affect for the improvement of crystalline quality of homoepitaxial. From XRD measurement, FWHM of diffraction spectrum from homoepitaxial GaN almost equaled to that from GaN substrate. So, the crystalline quality of homoepitaxial GaN was not inferior to that of GaN substrate. Large compression strain in c plane of homoepitaxial GaN indicated that homoepitaxy prevented 3D growth and/or formation of defects. From PL measurement, we observed radiative recombination of free excitons clearly from homoepitaxial GaN.
The basic illumination characteristics of an efficient white LEDs lighting source, which is composed of cannon-ball type 10 cd-class InGaN-based white LEDs, are described. It is noted that the temperature and forward-bias current dependence under full wave rectification of AC 100 V gives a significant effect to both electroluminescence properties and luminous efficacy. We have obtained a maximum luminous intensity of 95000 cd/m2 and a luminous efficacy of about 27 lm/W at an electric power of 20 W (at a constant current of 10 mA). The developed white LEDs lighting source indicates two injection electroluminescence peaks at 465 and 555 nm, which are originated from a blue LED and a YAG:Ce3+ phosphor, respectively. The illuminance distribution of the LED lighting source was analyzed using our `multi sources of LED light' theory. We have performed the design of a street lighting as the practical application using the 700 LEDs array lighting source.
White illumination characteristics of ZnS-based phosphor materials exited by an InxGa1-xN-based single quantum well-structure ultraviolet (UV) light-emitting diode (LED) have extensively been investigated. In order to evaluate white luminescence, two kinds of ZnS-based white phosphors have been employed. When an UV LED was operated at a current of 10 mA, chromaticity (x, y) color temperature (Tc) and general color rendering index (Ra) of the white luminescence are obtained to be (x,y) equals (0.29, 0.33), Tc equals 7700 K and Ra equals 70, respectively, for ZnS:Ag + (Zn,Cd)S:Cu,Al phosphors, whilst (x,y) equals (0.31, 0.34), Tc equals 6900 K and Ra equals 83, respectively, for white phosphor material including ZnS:Cu,Al, Sr and Y materials. The value of chromaticity slightly changed with increasing forward current of the UV light source. As a result, it is possible to obtain stable white luminescence spectrum. The dependence of the luminescence brightness on the thickness of phosphor shows a tendency to saturate for reflection brightness, but for transmission brightness its dependence has a peak due to light scattering effect. The reflection brightness was higher than the transmission brightness. It is revealed that the white luminescence light of stable chromaticity and high brightness using reflection light can be obtained.
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