Wolfgang Nemitz

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.

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

Abstract 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.

Direct junction temperature measurement in high-power LEDs

The light quality and long-term stability of phosphor converted light-emitting diodes (LEDs) for luminaires depend on the temperature distribution inside the LED chip and the color conversion element. Therefore, a reliable and accurate method to establish the LEDs junction temperature is required to further improve and optimize high quality LED luminaires. In this paper we describe the development and application of an innovative junction temperature measurement method which is based on a precise and universally applicable calibration procedure, allowing to use the calibrated LED itself as a temperature sensor under the respective operation condition of interest. This method is based on an extremely fast pulse measurement procedure allowing to record pairs of forward current and voltage drop values periodically every microsecond starting from the first microsecond of a pulse. From these experiments we reap two kinds of result: (1) Independent of the pulse shape in our experiments we observe a constant relation of current-to-voltage drop which we interpret as a constant junction conductivity. Depending on the type of LED (but independent of the packaging technology) we obtain a constant junction conductivity throughout several ten microseconds which we understand as a proof that the junction temperature did not change during this very first pulse phase. (2) The junction conductivity obtained in this moment is a measure for the junction temperature so that a calibration can be made by comparison with an independent steady-state temperature measurement made at zero-current condition. The method has been successfully applied to thermally characterize high-power LED modules as a 3 × 3 LED arrays with color conversion glob tops built-up on an insulated metal substrate (IMS).

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.

Simulation based experimental study of the temperature and power density distribution in a high-power LED

Temperature variations strongly influence the performance of light-emitting diodes (LEDs). The electrical properties of the LED, the power and the quality of the emitted light, and their long-term stability depend on the operating temperatures. The required electrode structures of LEDs lead to an inhomogeneous current distribution, heat generation, and light emission, which additionally exhibits a time-dependence. A high-power LED chip is studied on a microscopic scale by electro-thermal 2D-simulations, and by thermography. Temperature variations in the active LED region caused by the electrode structure are shown, and their steady-state and transient consequences are discussed.

Thermal performance study of direct attached high-power LEDs using an innovative submount technique

The long-term stability of the optical properties of high-power LEDs like spectral emissivity and total luminous efficacy is still one of the big technical challenges in solid-state lighting. A basic requirement to guarantee the promised lifetime is to keep the maximum temperature during operation reliably below the specified value. With this respect, the thermal performance of innovative LED-sub mounts based on a thin bismaleimide triazine (BT) substrate with copper filled thermal vias for the direct attachment of high-power LEDs is investigated. The influence of the arrangement of the thermal vias and the copper structure as well as the impact of the degree of imperfectness of solder joints on the junction-to-case thermal resistance are revealed by thermal simulation. The thermal model is verified experimentally using a demonstrator set-up with a thermal resistance of ca. 11 K/W.

Foil based optical elements for beam shaping and color homogenization of phosphor converted white LED sources

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

Thermal and optical aspects of glob-top design for phosphor converted white LED light sources

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.

Optical design of freeform micro-optical elements and their fabrication combining maskless laser direct write lithography and replication by imprinting

Abstract. Today, freeform micro-optical structures are desired components in many photonic and optical applications, such as lighting and detection systems, due to their compactness, ease of system integration, and superior optical performance. The high complexity of a freeform structure’s arbitrary surface profile and the need for high throughput upon fabrication require sophisticated approaches for their integration into a manufacturing process. In this paper, we discuss a smart fabrication process of freeform micro-optical elements that ranges from their design by optical simulations to their cost-efficient fabrication by maskless laser direct write lithography (MALA) and replication from the as-fabricated master by imprinting. Aided by profilometry and optical microscopy, the fidelity of the fabricated freeform micro-optical elements to the design is characterized. Finally, the light intensity distribution on a target plane affected by the freeform micro-optical element illuminated with a light-emitting diode is determined and compared with the predicted one.

Time dependent and temperature dependent properties of the forward voltage characteristic of InGaN high power LEDs

Estimating the junction temperature and its dynamic behavior in dependence of various operating conditions is an important issue, since these properties influence the optical characteristics as well as the aging processes of a light-emitting diode (LED). Particularly for high-power LEDs and pulsed operation, the dynamic behavior and the resulting thermal cycles are of interest. The forward voltage method relies on the existence of a time-independent unique triple of forward-voltage, forward-current, and junction temperature. These three figures should as well uniquely define the optical output power and spectrum, as well as the loss power of the LED, which is responsible for an increase of the junction temperature. From transient FEM-simulations one may expect an increase of the temperature of the active semiconductor layer of some 1/10 K within the first 10 μs. Most of the well-established techniques for junction temperature measurement via forward voltage method evaluate the measurement data several doz...