Characteristics Of White Light-emitting Diodes Research Articles

The effects of SiO2@Y2O2S:Eu3+ phosphor on the optic characteristics of white light-emitting diodes

The study <span>investigated the effectiveness of coating silica nanoparticles (SiO<sub>2</sub>) and resin-silica nanocomposites poly methyl methacrylate (PMMA)-silicaover the surface of Y<sub>2</sub>O<sub>2</sub>S:Eu<sup>3+</sup> red phosphors. The purpose of this surface coating is to enhance the optical properties, including the photoluminescence (PL) and long-term stability, of Y<sub>2</sub>O<sub>2</sub>S:Eu<sup>3+</sup>. Two methods used to coat the phosphor with 5-mm silica nanoparticles are dip-coating and sol-gel (Stöber) methods. SiO<sub>2</sub> nanoparticles were formed via hydrolysis and condensation reactions, while radical polymerization was performed to fabricate the poly (1-vinyl-2-pyrrolidone). The Y<sub>2</sub>O<sub>2</sub>S:Eu<sup>3+ </sup>surface coating of PMMA-silica composite was performed via two reactions. One is the reaction of SiO<sub>2</sub> nanoparticles with methyl methacrylate (MMA) monomer, and the other is MMA-tetraethyl orthosilicate (TEOS) reaction. The results showed that by using the latter method, Y<sub>2</sub>O<sub>2</sub>S:Eu<sup>3+</sup> yielded better PL and long-term stability. Additionally, surface coating with the PMMA-SiO<sub>2</sub> nanocomposite using the second technique resulted in 5% enhancement in PL and stabilized the cathode luminescence (CL) intensity of Y<sub>2</sub>O<sub>2</sub>S:Eu<sup>3+</sup>, compared to those properties of uncoated Y<sub>2</sub>O<sub>3</sub>S:Eu<sup>3+</sup> phosphor particles.</span>

Optical Simulation Study of the Improvement of Color-rendering Characteristics of White Light-emitting Diodes by Using Red Quantum-dot Films

Optical Simulation Study of the Improvement of Color-rendering Characteristics of White Light-emitting Diodes by Using Red Quantum-dot Films

COLOR AND SPECTRAL CHARACTERISTICS OF WHITE LIGHT EMITTING DIODES AND THEIR VARIATION DURING AGING

The relation between numerical values of photometric characteristics (total luminous flux TLF, correlated color temperature CCT, color rendering index CRI) of white light emitting diodes (LED) and the variation of the spectral shape of their radiation during aging has been investigated. All the measurements were made on internationally adopted test methods, taking into account environmental conditions, electrical parameters and evaluated measurement uncertainty. Every piece of test and measurement equipment has actual verification or calibration with traceability to national and international references. It was demonstrated that in the luminescence spectra consisting of the “blue” band around 450 nm originating from the semiconductor heterostructure, and the broad “yellow” band from luminophor, the last band is nonelementary and consists of at least two bands: the “green” one around 530 nm and the “orange” one around 580 nm. The most unstable “green” band has the highest impact on photometric characteristics. As a consequence, further investigation should be performed on how instability of elementary bands and its quantity will link not only with photometric characteristics, but with production conditions and material properties of LED heterostructure and luminophor itself. In particular, for improvement of the color stability of white LED, the parameters of luminophor forming the “green” band should be stabilized. А unified method for accelerated testing of LED products and method for long-time lifetime prediction shall be developed, taking into account not only depreciation of TLF, but also shift of other photometric and spectral characteristics of white LED.

Optimization of the methods for measuring color characteristics of light-emitting diodes in laboratory conditions

In this work the features of measuring color characteristics of light-emitting diodes are described. Methods allowing increasing the accuracy of measurements in the laboratory conditions are shown. The results of determination of the color characteristics of white light-emitting diodes using the described technique are analyzed.

Improvement of Electro-Optic Characteristics of White Light-Emitting Diodes by Using Transparent Ceramic-Based Remote Phosphors

Improvement of Electro-Optic Characteristics of White Light-Emitting Diodes by Using Transparent Ceramic-Based Remote Phosphors

Analysis of the spectral characteristics of white light-emitting diodes under various thermal environments

An empirical functional form was suggested for the analysis of the emission spectrum of high-power light-emitting diode (LED) consisting of a sharp blue peak from the LED chips and a broad yellow peak from the phosphor layer. The peak positions, half widths, shape parameters, and amplitudes of these two peaks were reliably obtained as a function of the temperature, and the results were discussed qualitatively in relation with the junction temperature. The adoption of an inert liquid was found to have significantly reduced the LED temperature and the color shift of the emitted light. The phenomenological approach used in this study may be helpful in the simulation of the LED spectrum under various thermal conditions, and may thus be helpful in the improvement of the device performance.

Phosphor-Concentration-Dependent Characteristics of White LEDs in Different Current Regulation Modes

Phosphor-concentration-dependent characteristics of white light-emitting diodes (LEDs) under different current regulation conditions were investigated. It is found that the phosphor conversion efficiency of white LEDs driven under constant current is lower than that under pulse current. In addition, white LEDs driven under constant current exhibit higher junction temperature than under pulse current, and the difference is phosphor concentration dependent. For both pulse and constant current modes, white LEDs show relatively stable optical characteristics at relatively higher drive currents, when relatively higher phosphor concentrations are used. At relatively higher phosphor concentrations, the correlated color temperature and the chromaticity coordinates have also been observed to be relatively stable for white LEDs in both constant and pulse current modes.

Lighting theory and luminous characteristics of white light-emitting diodes

A near-ultraviolet (UV)-based light-emitting diode (LED) lighting system linked with a semiconductor InGaN LED and compound phosphors for general lighting is proposed. We have developed for the first time a novel type of high-color rendering index (Ra) 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 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 LED can be evaluated using the multipoint LED light source theory. It is revealed that the OYGB LED can provide better irradiance properties than that of conventional LEDs. Near-UV 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.