Franz P. Wenzl

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.

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.

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 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.

Improvement of light extraction from high-power flip-chip light-emitting diodes by femtosecond laser direct structuring of the sapphire backside surface

We report on the structuring of the backside surface of sapphire substrates in high-power flip-chip light-emitting diodes (LEDs) by femtosecond laser direct writing. Varying the laser powers has been found to affect the sizes of the inscribed patterns on a submicron scale which facilitates the control of the structure sizes with high precision. Accordingly, since on the one hand the light extraction efficiency reveals a strong dependence on pattern sizes, and on the other hand, femtosecond laser structuring provides a simple opportunity to inscribe diverse structures along the LED surfaces, LEDs with laterally controlled light extraction efficiencies can be fabricated.

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.

White LEDs and modules in chip-on-board technology for general lighting

At present, light-emitting diode (LED) modules in various shapes are developed and designed for the general lighting, advertisement, emergency lighting, design and architectural markets. To compete with and to surpass the performance of traditional lighting systems, enhancement of Lumen output and the white light quality as well as the thermal management and the luminary integration are key factors for success. Regarding these issues, white LEDs based on the chip-on-board (COB) technology show pronounced advantages. State-of-the-art LEDs exploiting this technology are now ready to enter the general lighting segments. We introduce and discuss the specific properties of the Tridonic COB technology dedicated for general lighting. This technology, in combination with a comprehensive set of tools to improve and to enhance the Lumen output and the white light quality, including optical simulation, is the scaffolding for the application of white LEDs in emerging areas, for which an outlook will be given.

Multiple interfacing between classical ray-tracing and wave-optical simulation approaches: a study on applicability and accuracy

In this study the applicability of an interface procedure for combined ray-tracing and finite difference time domain (FDTD) simulations of optical systems which contain two diffractive gratings is discussed. The simulation of suchlike systems requires multiple FDTD↔RT steps. In order to minimize the error due to the loss of the phase information in an FDTD→RT step, we derive an equation for a maximal coherence correlation function (MCCF) which describes the maximum degree of impact of phase effects between these two different diffraction gratings and which depends on: the spatial distance between the gratings, the degree of spatial coherence of the light source and the diffraction angle of the first grating for the wavelength of light used. This MCCF builds an envelope of the oscillations caused by the distance dependent coupling effects between the two diffractive optical elements. Furthermore, by comparing the far field projections of pure FDTD simulations with the results of an RT→FDTD→RT→FDTD→RT interface procedure simulation we show that this function strongly correlates with the error caused by the interface procedure.

On the Adjustment of the Color Temperature of White Light-Emitting Diodes by Femtosecond Laser Patterning

We show that femtosecond (fs) laser microstructuring of light-emitting diode (LED) encapsulates has a high potential for the fine-tuning of the color temperatures of white LED packages. The number of scattering centers that are fabricated within the volume of the encapsulate allows the precise adjustment of the color temperature of the device to the desired value. This opens possibilities to achieve color reproducibility among individual LED packages with highest accuracy.

Surface Texturing of High-Power Flip-Chip LEDs by Femtosecond Laser Direct Structuring

We report on the femtosecond laser direct structuring of the sapphire flipside surfaces of high-power flip-chip LEDs. It is found that diameter and depth of the created submicrometer-sized holes can be manipulated by varying the laser power at a constant number of laser pulses. This method enables the control of the structure sizes with high precision. Our study shows that the light extraction from such LEDs increases strongly with increasing hole sizes.