Commercial Ce3+ doped yttrium aluminum (Ce3+:YAG) and lutetium aluminum garnet (Ce3+:LuAG) powders were mixed with powdered soda-lime silicate glass with the molar composition 15.5 Na2O / 10.7 CaO / 73.8 SiO2. Then the mixtures were sintered at temperatures in the range from 800 to 1000 °C for 10 and 30 min. XRD-patterns proved that the samples contain only the respective garnet phases. During sintering at 1000 °C, a notable dissolution of the garnet phase took place as proved by the occurrence of the typical blue emission of Ce3+ in the glassy phase if excited with UV light. Dissolution of the phosphors is much lower at a processing temperature of 800 °C. Additionally, the dissolution process is strongly affected by the chemical composition of the glass. The concentration of the phosphors in the glass matrix had only a minor effect on the fluorescence intensity. The efficiency in lm/W increases with the sample thickness and is for the Ce3+:LuAG sample nearly as high as for polymer embedded samples, while it is slightly smaller for the Ce3+:YAG samples.

Besides their direct impact on the respective correlated color temperature, the extinction coefficient and the quantum efficiency of the phosphor also have tremendous impact on the thermal load of the color conversion elements of phosphor converted LEDs under operation. Because of the low thermal conductivity of the silicone matrix in which the phosphor particles are typically embedded, the by far highest temperatures within the LED assembly are reached within the color conversion element. Based on a combined optical and thermal simulation procedure we show that in particular a larger value for the extinction coefficient might have a beneficial impact on the resulting thermal load.

Typically, light emission from light-emitting diodes (LEDs) occurs under a broad range of angles. On the other hand, for a lot of applications a more directed light emission is desired. This can be realized with the use of additional optical elements, like lenses. Still, this may provide some complications in case of light sources consisting of a plurality of individual LEDs, e.g., a panel light, which is expected to illuminate a target area homogenously. Instead of a homogeneous illumination, the use of lenses is prone to give reason for an inhomogeneous light distribution in which the emission from the individual LEDs is easily distinguishable. Therefore, there is a strong request for alternative strategies of beam shaping of LED light in LED-luminaires targeting both on a directed as well as homogeneous illumination of an area.In this contribution we discuss an alternative approach in this regard: Firstly, a collimator is designed, which strongly directs the light emitted from a single LED light source. Subsequently, a foil with an optical structure, that can be fabricated in a cost-effective way by soft-lithography and which diffuses the collimated light again, is applied on the collimator. The optical structure and the respective amount of light diffusion are designed in a way that the desired radiation patterns both from a single as well as a plurality of LED sources can be realized. In addition, we show that the realization of a desired radiation profile is not the only advantage of such an approach. A key benefit of this concept is the possibility to reduce the angle dependent inhomogeneity of the CIE chromaticity coordinates of the emitted light, which typically occurs in case that only a collimator is used.

For a systematic approach to improve the white light quality of phosphor converted light-emitting diodes (LEDs) for general lighting applications it is imperative to get the individual sources of error for correlated colour temperature (CCT) reproducibility and maintenance under control. In this regard, it is of essential importance to understand how geometrical, optical and thermal properties of the colour conversion elements (CCEs), which typically consist of phosphor particles embedded in a transparent matrix material, affect the constancy of a desired CCT value. In this contribution we use an LED assembly consisting of an LED die mounted on a printed circuit board by chip-on-board technology and a CCE with a globe-top configuration on the top of it as a model system and discuss the impact of the CCE size, the substrate reflectivity as well as the thermal load of the CCEs on CCT maintenance and the respective radiant fluxes. It turns out that optimized solutions for CCT maintenance and high radiant fluxes in regard of one of the relevant influence factors do not need to be optimized solutions in regard of another one.

For a systematic approach to improve the white light quality of phosphor converted light-emitting diodes (LEDs) for general lighting applications it is imperative to get the individual sources of error for color temperature reproducibility under control. In this regard, it is imperative to understand how compositional, optical and materials properties of the color conversion element (CCE), which typically consists of phosphor particles embedded in a transparent matrix material, affect the constancy of a desired color temperature of a white LED source. In this contribution we use an LED assembly consisting of an LED die mounted on a printed circuit board (PCB) by chip-on-board technology and a CCE with a glob-top configuration as a model system and discuss the impact of potential sources for color temperature deviation among individual devices. Parameters that are investigated include imprecisions in the amount of materials deposition, deviations from the target value for the phosphor concentration in the matrix material, deviations from the target value for the particle sizes of the phosphor material, deviations from the target values for the refractive indexes of phosphor and matrix material as well as deviations from the reflectivity of the substrate surface. From these studies, some general conclusions can be drawn which of these parameters have the largest impact on color deviation and have to be controlled most precisely in a fabrication process in regard of color temperature reproducibility among individual white LED sources.

The generation of white light by Light Emitting Diodes (LED) is presented. Unlike thermal emitters (e.g. incandescent bulbs) or UV excited fluorescent light-sources (e.g. compact fluorescent bulbs), the emission is in the visible range only, allowing high conversion efficiencies. We detail the losses in electrical to white light conversion, give an insight on development challenges to estimate the efficacy increase in the following years. The emitted spectrum can be tuned to adapt the spectral sensitivity of human eye to maximize the efficacy of solid state light sources. As efficacy and light quality is generally a trade-off, we review recent light quality metrics to introduce optimization processes in white light generation.

Color temperature constancy and color temperature maintenance are key issues in the context of the utilization of light-emitting diodes (LEDs) for general lighting applications. For a systematic improvement, it is imperative to understand how compositional, optical and thermal properties of the color conversion elements (CCE), which typically consist of phosphor particles embedded in a transparent matrix material, affect the constancy of a desired color temperature of a white LED source under operation. In particular, thermal stress, like a distinct thermal load of the CCEs under operation may also cause notable color shifts. In order to gain a better understanding of the thermal behavior of CCEs under operation, in this contribution we give by means of a combined optical and thermal simulation procedure a comprehensive discussion on the impact of different CCE shapes and sizes on their thermal responses.

We present an approach to diminish temperature induced color shifts of phosphor converted LEDs. In particular, we discuss the impact of the thermo-optic coefficients of the materials constituting the color conversion elements (CCEs) on the constancy of the CIE chromaticity coordinates. While silicones have a comparably large thermo-optic coefficient, phosphors, e.g., Ce:YAG have a much smaller one. Hence, increasing temperature will lead to an increase in the differences of the refractive indexes of phosphor and matrix. This will enhance light-scattering which for itself would give reason for a more yellowish emission. Thus, on the one hand the thermo-optic coefficient of the silicone matrix has to be considered for as a potential source of failure for color temperature deviation upon device operation. However, on the other hand adjusting the thermo-optic coefficient of the silicone matrix by an appropriate materials engineering to the temperature induced luminescence loss of a phosphor can be applied to counterbalance a shift of the chromaticity coordinates of the white light in this regard.

For a systematic approach to improve the white light quality of phosphor converted light-emitting diodes (LEDs) for general lighting applications it is imperative to get the individual sources of error for correlated color temperature (CCT) reproducibility and maintenance under control. In this regard, it is of essential importance to understand how geometrical, optical and thermal properties of the color conversion elements (CCE), which typically consist of phosphor particles embedded in a transparent matrix material, affect the constancy of a desired CCT value. In this contribution we use an LED assembly consisting of an LED die mounted on a printed circuit board by chip-on-board technology and a CCE with a glob-top configuration on the top of it as a model system and discuss the impact of the CCE shape and size on CCT constancy with respect to substrate reflectivity and thermal load of the CCEs. From these studies, some general conclusions for improved glob-top design can be drawn.

For further improvements to the reliability and the white light quality of phosphor‐converted light‐emitting diodes (LEDs), it is imperative to understand how the compositional, optical, and thermal properties of the materials constituting the color‐conversion elements (CCEs) affect their respective thermal loads. By means of a combined optical and thermal simulation procedure, a comprehensive discussion is given on the underlying coherences of the absorption profile of the blue LED light, the phosphor concentration, the quantum efficiency of the phosphor, and the thermal conductivities of the CCEs. Some general strategies of material composition and design are deduced in order to minimize the thermal load of the CCEs, which is a prerequisite for correlated color temperature maintenance and long‐term material reliability of phosphor‐converted white LEDs.

For a systematic approach to improve the reliability and the white light quality of phosphor converted light-emitting diodes (LEDs) it is imperative to gain a better understanding of the individual parameters that affect color temperature constancy and maintenance. By means of a combined optical and thermal simulation procedure, in this contribution we give a comprehensive discussion on the impact of different current driving schemes on the thermal load of the color conversion elements (CCEs) of phosphor converted LEDs. We show that on the one hand a decreasing duty cycle under pulse width modulation driving conditions may cause a notable temperature variation and on the other hand also effects due to the non-linearity between the blue radiant flux and the current have to be considered for the thermal load of the CCEs.

Based on combined optical and thermal simulations, we comprehensively discuss the underlying coherences of light absorption, quantum efficiency and thermal conductivity on the temperature increase within the phosphor layer of white LEDs.

For a systematic approach to improve the white light quality of phosphor converted light-emitting diodes (LEDs) for general lighting applications it is imperative to get the sources of error for color constancy under control. In this context, it is essential to gain a deeper insight how the individual components of an LED package may contribute to color deviation. Typically, both monochromatic and phosphor converted light-emitting diodes are finally encapsulated by a pristine silicone layer in order to prevent mechanical damage of the LED packages. In this contribution we focus on the shapes of such encapsulation layers and discuss, based on an optical simulation procedure, their impact on the color temperatures of phosphor converted white LEDs as well as the ramifications of manufacturing imprecision of these shapes on the constancy and reproducibility of a desired color temperature.

Based on optical ray-tracing simulations we discuss the effect of the extinction coefficient and the quantum efficiency of the phosphors on the angular homogeneity of the white light emitted from phosphor converted light-emitting diodes. In particular variations of the extinction coefficient are prone to affect diverse CIE chromaticity coordinates for different viewing angles. Contrarily, the impact of the quantum efficiency on angle dependent variations of the chromaticity coordinates turns out to be of minor importance.

For a systematic approach to improve the white light quality of phosphor converted LEDs and to fulfil the demands for colour temperature reproducibility and constancy, it is imperative to understand how variations of the extinction coefficient and the quantum efficiency of the phosphor particles as well as variations of the excitation wavelength of the blue LED die affect the correlated colour temperature of the white LED source. Based on optical ray tracing of a phosphor converted white LED package we deduce permissible values for the variation of a given extinction coefficient and a given quantum efficiency of a phosphor material in order to maintain acceptable colour variations. These quantitative valuations of the required constancy of the optical properties of the phosphors will in particular provide some benchmarks for the synthesis of improved phosphor materials aiming at solid state lighting applications. (C) 2012 Elsevier B. V. All rights reserved.