Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

This paper presents a polynomial dimensional decomposition (PDD) method for global sensitivity analysis of stochastic systems subject to independent random input following arbitrary probability distributions. The method involves Fourier-polynomial expansions of lower-variate component functions of a stochastic response by measure-consistent orthonormal polynomial bases, analytical formulae for calculating the global sensitivity indices in terms of the expansion coefficients, and dimension-reduction integration for estimating the expansion coefficients. Due to identical dimensional structures of PDD and analysis-of-variance decomposition, the proposed method facilitates simple and direct calculation of the global sensitivity indices. Numerical results of the global sensitivity indices computed for smooth systems reveal significantly higher convergence rates of the PDD approximation than those from existing methods, including polynomial chaos expansion, random balance design, state-dependent parameter, improved Sobol’s method, and sampling-based methods. However, for non-smooth functions, the convergence properties of the PDD solution deteriorate to a great extent, warranting further improvements. The computational complexity of the PDD method is polynomial, as opposed to exponential, thereby alleviating the curse of dimensionality to some extent. Mathematical modeling of complex systems often requires sensitivity analysis to determine how an output variable of interest is influenced by individual or subsets of input variables. A traditional local sensitivity analysis entails gradients or derivatives, often invoked in design optimization, describing changes in the model response due to the local variation of input. Depending on the model output, obtaining gradients or derivatives, if they exist, can be simple or difficult. In contrast, a global sensitivity analysis (GSA), increasingly becoming mainstream, characterizes how the global variation of input, due to its uncertainty, impacts the overall uncertain behavior of the model. In other words, GSA constitutes the study of how the output uncertainty from a mathematical model is divvied up, qualitatively or quantitatively, to distinct sources of input variation in the model [1].

Reliability Engineering and System Safety

A lighting device comprising first and second groups of solid state light emitters, which emit light having peak wavelength in ranges of from 430 nm to 480 nm, and first and second groups of lumiphors which emit light having dominant wavelength in the range of from 555 nm to 585 nm. In some embodiments, if current is supplied to a power line, a combination of (1) light exiting the lighting device which was emitted by the first group of emitters, and (2) light exiting the lighting device which was emitted by the first group of lumiphors would have a correlated color temperature which differs by at least 50 K from a correlated color temperature which would be emitted by a combination of (3) light exiting the lighting device which was emitted by the second group of emitters, and (4) light exiting the lighting device which was emitted by the second group of lumiphors.

Lighting device and lighting method

Even though light-emitting diodes (LEDs) may have a very long life, poorly designed LED lighting systems can experience a short life. Because heat at the p-n-junction is one of the main factors that affect the life of the LED, by knowing the relationship between life and heat, LED system manufacturers can design and build long-lasting systems. In this study, several white LEDs from the same manufacturer were subjected to life tests at different ambient temperatures. The exponential decay of light output as a function of time provided a convenient method to rapidly estimate life by data extrapolation. The life of these LEDs decreases in an exponential manner with increasing temperature. In a second experiment, several high-power white LEDs from different manufacturers were life-tested under similar conditions. Results show that the different products have significantly different life values.

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Life of LED-based white light sources

Light emitting diodes (LEDs) are on the verge of a breakthrough in general lighting, due to their rapidly improving efficiency. Currently, white LEDs with high color rendering are mainly based on wavelength conversion by one or more phosphor materials. This Review first describes how to quantify the quality of a light source, discussing the color rendering index (CRI) and alternative color quality indices. Then, six main criteria are identified and discussed, which should be fulfilled by a phosphor candidate to be considered for actual application in LEDs. These criteria deal with the shape and position of the emission and the excitation spectra, the thermal quenching behavior, the quantum efficiency, the chemical and thermal stability and finally with the occurrence of saturation effects. Based on these criteria, the most common dopant ions (broad-band emitting Eu 2+ , Ce 3+ and Mn 2+ , line-emitting rare earth ions,...) and host compounds (garnets, sulfides, (oxy)nitrides,...) are evaluated. Although many phosphor materials have been proposed in literature in recent years, the number of phosphors effectively fulfilling all six requirements is relatively small.

/pdf/selecting-conversion-phosphors-for-white-light-emitting-20y8tlis4y.pdf

Selecting Conversion Phosphors for White Light-Emitting Diodes

https://www.lrc.rpi.edu/programs/solidstate/pdf/narendran2004_jcrystalgrowth.pdf

Solid-state lighting: failure analysis of white LEDs

A light emitting apparatus includes a source of light for emitting light; a down conversion material receiving the emitted light, and converting the emitted light into transmitted light and backward transmitted light; and an optic device configured to receive the backward transmitted light and transfer the backward transmitted light outside of the optic device. The source of light is a semiconductor light emitting diode, a laser diode (LD), or a resonant cavity light emitting diode (RCLED). The down conversion material includes one of phosphor or other material for absorbing light in one spectral region and emitting light in another spectral region. The optic device, or lens, includes light transmissive material.

High Efficiency Light Source Using Solid-State Emitter and Down-Conversion Material

A light emitting apparatus having a light source for emitting short wavelength radiation and an optic device configured to receive the radiation emitted from the light source. A device directs at least some of the short wavelength radiation emitted from the light source into the optic device and a down conversion material receives at least some of the short wavelength radiation directed into the optic device in one spectral region and emits the radiation in another spectral region.

High-efficient light engines using light emitting diodes

It is a truth, universally ignored, that a single metric of color rendering must be in want of another. Evi- dence presented here, together with those from an earlier study, strongly suggest that the quest for a single metric to quantify color rendering will be in vain. Rather, the strengths of color rendering index (CRI) and of gamut area index (GAI) y seem to counteract the weaknesses of one another, such that together they can be used to guide lighting practitioners in choosing a source that will pro- vide good color rendering of most objects in most appli- cations. The present study was conducted to determine whether sources, both warm and cool, with high levels of both CRI (above 80) and GAI (above 80 and less than 100) were judged better than ones with high levels of just CRI or just GAI. The results support the conclusion that a two-metric system of color rendering is needed for general illumination applications. � 2010 Wiley Periodicals,

Color rendering: Beyond pride and prejudice

A light emitting apparatus includes a lamp base, a light-transmissive bulb envelope, a light source for emitting light, and a heat sink coupled to the light source. A solid state LED light bulb may further include a down conversion material. The down conversion material is disposed within the bulb envelope, remote from the light source and between the light source and the lamp base. The heat sink may include at least one metal fin and, additionally or alternatively, include a mesh disposed over at least an outer portion of the bulb envelope. A solid state light bulb may include a light guide for directing the light emitted by the light source. The solid state light bulb configurations place the light source and heat sink at the apex of the light bulb envelope, distant from the lamp base, in order to dissipate heat produced by the light source into the environment. In addition, at least part of the heat sink is outside the light bulb envelope to maximize the heat dissipation.

Jean Paul Freyssinier

Papers

Solid-state lighting: failure analysis of white LEDs

High Efficiency Light Source Using Solid-State Emitter and Down-Conversion Material

High-efficient light engines using light emitting diodes

Color rendering: Beyond pride and prejudice

Solid state light source light bulb