Color Conversion Elements Research Articles

An Improved Model for Light Transport in the Color Conversion Element of Light-Emitting Diodes

Light transport with fluorescence in a phosphor layer based on radiative transfer theory is an efficient tool for understanding the performance of a phosphor-converted light-emitting diode. In this work, the models based on radiative transfer theory including light conversion of phosphor particles are developed for calculating the light transport in planar remote phosphor layers. The models developed include an ordinary differential approximation and an improved model based on the differential approximation and an integral formulation of the transmission light flux for a phosphor layer with Fresnel boundaries. We calculate the light extraction efficiency (LEE) of a phosphor layer as a function of various parameters, such as the thickness of the layer and the concentration of phosphor. The present models are validated by comparing the results obtained by the present methods, the double spherical harmonics method of order one and a Monte Carlo method. Comparing the results obtained by those methods, one can see that the improvement based on the differential approximation and integral formulation of transmission light flux for calculating the LEE can yield better results for optically thin cases.

Analysis of the local temperature distribution in color conversion elements of phosphor converted light-emitting diodes

Long-term stability, efficiency, and reliability of LED-based luminaires strongly depend on thermal management also inside the color conversion layer (CCE) which frequently consists of a transparent encapsulant with embedded microsized phosphor particles. Due to their limited quantum efficiency and Stokes-shift related losses these particles heat-up the CCE and need to be cooled just by heat conduction through the underlying LED chip which itself is heated by its own power loss. Significant research work has been devoted to determine the temperature distribution within the CCE using macroscopic thermal models by considering homogeneously distributed materials properties inside the CCE. By contrast, focus in this paper is to gain a deeper understand of thermal aspects on the base of microscopic thermal models considering the discontinuous set-up of the CCE. In turn, influences of degradation of the heat transfer between phosphor particles and encapsulant and between CCE and LED chip on the temperature distribution inside the CCE are studied in detail.

On the determination of the temperature distribution within the color conversion elements of phosphor converted LEDs

We present an iterative optical and thermal simulation procedure which enables the determination of the temperature distribution in the phosphor layer of a phosphor converted LED with good accuracy. Using the simulation both the highest phosphor temperatures, which are mostly relevant to material degradation as well as the temperatures of those phosphor particles which mainly contribute to converted light emission can be determined. We compare the simulations with experimental studies on the phosphor temperature. While infrared thermography only gives information on the phosphor layer surface temperature, phosphor thermometry provides temperature data on the volume temperature of the phosphor layer relevant to color conversion.

On the impact of the temperature dependency of the phosphor quantum efficiency on correlated color temperature stability in phosphor converted LEDs

Since phosphors are prone to a temperature dependent loss of luminescence intensity, an appropriate thermal management in phosphor converted light-emitting diodes (LEDs) is indispensable to maintain the desired correlated color temperature (CCT) value. This can be achieved either by improving the thermal stability of the phosphor material itself or by enhancing the thermal conductivity of the color conversion element (phosphor embedded in a transparent matrix) of the LED. Based on optical simulations we give a comprehensive discussion on the related coherences. We compare our results with the related research goals of the US Department of Energy with respect to phosphor development. We show that the thermal stability performance target of the phosphor as predicted for 2020 will be beneficial with regard to color stability.However, an improvement of the thermal conductivity of the matrix materials which encapsulate the phosphor particles would have a similar impact, even for phosphors with current state of the art performance. Besides investing in the progress of phosphor materials, research should also focus on novel concepts and materials to enhance the thermal conductivity of color conversion elements in phosphor converted LEDs.

The impact of the thermal conductivities of the color conversion elements of phosphor converted LEDs under different current driving schemes

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 thermal conductivities of the color conversion elements (CCE) of phosphor converted LEDs on their respective thermal load for different current driving schemes. In particular we show that, while for the thermal load of CCEs with low thermal conductivities also effects due to the non-linearity between the blue radiant flux and the current have to be considered, these effects are largely diminished in case of CCEs with higher thermal conductivities.

Impact of extinction coefficient of phosphor on thermal load of color conversion elements of phosphor converted LEDs

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.

Improvement of Color Temperature Constancy of Phosphor Converted LEDs by Adaption of the Thermo-Optic Coefficients of the Color Conversion Materials

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.

The impact of the non-linearity of the radiant flux on the thermal load of the color conversion elements in phosphor converted LEDs under different current driving schemes

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.

Color temperature constancy of phosphor converted LED modules: a combined thermal and optical simulation study on the materials requirements

Color temperature constancy and color maintenance are key issues in the context of the utilization of light-emitting diodes (LEDs) for general lighting applications. For a systematic approach to improve the white light quality of phosphor converted LEDs and to fulfill the demands for color temperature constancy, 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 correlated color temperature of a white LED source. In this contribution we discuss, based on combined optical and thermal simulations, the dependencies of the quantum efficiency of the phosphor and the thermal conductivity of the matrix material on the temperature increase within the CCE. We show that even small improvements of these parameters may have a notable beneficial impact on the thermal load of the CCEs and therefore also temperature related insufficiencies of the device performance.

The Impact of Light Scattering on the Radiant Flux of Phosphor-Converted High Power White Light-Emitting Diodes

We present an optical ray-tracing study which investigates the influence of the refractive indexes of the matrix and the phosphor materials as well as the phosphor particle sizes of the color conversion elements (CCE) on the radiant flux of phosphor-conversion-based white light-emitting diodes. We show that an appropriate combination of those parameters is a crucial factor in order to optimize device efficacy.

The Impact of Inhomogeneities in the Phosphor Distribution on the Device Performance of Phosphor-Converted High-Power White LED Light Sources

We present a study by optical ray-tracing in order to determine the impact of an inhomogeneous phosphor distribution in the color conversion elements (CCE) of phosphor-conversion- based white LED light sources. It turns out that in particular the color temperature and its angular variation, but also the flux-output are highly sensitive towards phosphor distribution variations.

The Effect of the Phosphor Particle Sizes on the Angular Homogeneity of Phosphor-Converted High-Power White LED Light Sources

Based on optical ray tracing, we discuss the effect of the phosphor particle sizes on the angular homogeneity of the light emitted from phosphor-converted LEDs. Since the blue LED and the yellow-converted light have rather different emission characteristics, which have to be harmonized to one another by the scattering processes within the color conversion element, the phosphor particle size turns out to be an essential parameter in order to attain angular homogeneity. This can be attributed, on the one hand, to the number of scattering processes within a specific unit volume for a given phosphor concentration, and on the other hand, to the specific scattering functions, both of which depend on the phosphor particle diameter.

A detailed study on the requirements for angular homogeneity of phosphor converted high power white LED light sources

We present a simulation procedure based on optical ray-tracing in order to optimize the angular homogeneity of the light emitted from the color conversion element (CCE) in a phosphor conversion-based white LED. The blue LED and the yellow CCE light have rather different emission characteristics; so the geometry of the CCE as well as its phosphor concentration have to be carefully adjusted in order to achieve equal irradiance and/or radiant intensity distributions on a photo-detector surrounding the LED. The simulations identify the optimal CCE geometries and material compositions in order to obtain angular homogeneity for a broad range of color temperatures.

Tailoring of the Color Conversion Elements in Phosphor-Converted High-Power LEDs by Optical Simulations

Ray-tracing simulations are used to identify the demands for angular homogeneity of the white light emitted from phosphor-converted white light-emitting diodes having color conversion elements (CCEs) of constant thickness. The simulations reveal that a constant thickness of the CCE by itself is not sufficient for a homogeneous white light emission. Rather the height and the broadness of the CCE as well as the phosphor concentration have to be precisely adjusted in order to assure a homogeneous white light emission.