Guy Durinck

Absolute determination of photoluminescence quantum efficiency using an integrating sphere setup

An integrating sphere-based setup to obtain a quick and reliable determination of the internal quantum efficiency of strongly scattering luminescent materials is presented. In literature, two distinct but similar measurement procedures are frequently mentioned: a two measurement and a three measurement approach. Both methods are evaluated by applying the rigorous integrating sphere theory. It was found that both measurement procedures are valid. Additionally, the two methods are compared with respect to the uncertainty budget of the obtained values of the quantum efficiency. An inter-laboratory validation using the two distinct procedures was performed. The conclusions from the theoretical study were confirmed by the experimental data.

Extended adding-doubling method for fluorescent applications

In this paper a fast, yet accurate method to estimate the spectral and angular distribution of the scattered radiation of a fluorescent material is described. The proposed method is an extension of the adding-doubling algorithm for non-fluorescent samples. The method is validated by comparing the spectral and angular transmittance and reflectance characteristics obtained with the extended algorithm with the results obtained using Monte Carlo simulations. The agreement using both methods is within 2%. However, the adding-doubling method achieves a reduction of the calculation time by a factor of 400. Due to the short calculation time, the extended adding-doubling method is very useful when fluorescent layers have to be optimized in an iterative process.

Estimation of the effective phase function of bulk diffusing materials with the inverse adding-doubling method.

The accuracy of optical simulations including bulk diffusors is heavily dependent on the accuracy of the bulk scattering properties. If no knowledge on the physical scattering effects is available, an iterative procedure is usually used to obtain the scattering properties, such as the inverse Monte Carlo method or the inverse adding-doubling (AD) method. In these methods, a predefined phase function with one free parameter is usually used to limit the number of free parameters. In this work, three predefined phase functions (Henyey-Greenstein, two-term Henyey-Greenstein, and Gegenbauer kernel (GK) phase function) are implemented in the inverse AD method to determine the optical properties of two strongly diffusing materials: low-density polyethylene and TiO₂ particles. Using the presented approach, an estimation of the effective phase function was made. It was found that the use of the GK phase function resulted in the best agreement between calculated and experimental transmittance, reflectance, and scattered radiant intensity distribution for the LDPE sample. For the TiO₂ sample, a good agreement was obtained with both the two-term Henyey-Greenstein and the GK phase function.

Determination of the bulk scattering parameters of diffusing materials

Diffusors are widely used optical components having numerous applications. They are commonly used to homogenize light beams and to create particular intensity distributions. The angular scattering profile of bulk scattering diffusing materials is determined by three bulk scattering parameters that are, however, not commonly available. This hampers an accurate implementation of bulk diffusors in ray tracing simulations. In this paper, the bulk scattering parameters of a concentration series of milk diluted with water were determined with the inverse adding-doubling method. Using these values as input, the macroscopic angular scattering profile was simulated using ray tracing software. The simulation results were compared to experimental data, and a good agreement between measured and simulated data was found. The method was also proven to be successful when applied to commercial diffusors.

The use of the adding-doubling method for the optical optimization of planar luminescent down shifting layers for solar cells

To enhance the efficiency of solar cells, a luminescent down shifting layer can be applied in order to adapt the solar spectrum to the spectral internal quantum efficiency of the semiconductor. Optimization of such luminescent down shifting layers benefits from quick and direct evaluation methods. In this paper, the potential of the adding-doubling method is investigated to simulate the optical behavior of an encapsulated solar cell including a planar luminescent down shifting layer. The results of the adding-doubling method are compared with traditional Monte Carlo ray tracing simulations. The average relative deviation is found to be less than 1.5% for the absorptance in the active layer and the reflectance from the encapsulated cell, while the computation time can be decreased with a factor 52. Furthermore, the adding-doubling method is adopted to investigate the suitability of the SrB4O7:5%Sm2 + ,5%Eu2 + phosphor as a luminescent down shifting material in combination with a Copper Indium Gallium Selenide solar cell. A maximum increase of 9.0% in the short-circuit current can be expected if precautions are taken to reduce the scattering by matching the refractive index of host material to the phosphor particles. To be useful as luminescent down shifting material, the minimal value of the quantum yield of the phosphor is determined to be 0.64.

Impact of the accurateness of bidirectional reflectance distribution function data on the intensity and luminance distributions of a light-emitting diode mixing chamber as obtained by simulations

Abstract. The reliability of ray tracing simulations is strongly dependent on the accuracy of the input data such as the bidirectional reflectance distribution function (BRDF). Software developers offer the possibility to implement BRDF data in different ways, ranging from simple predefined functions to detailed tabulated data. The impact of the accuracy of the implemented reflectance model on ray tracing simulations has been investigated. A light-emitting diode device including a frequently employed diffuse reflector [microcellular polyethylene terephthalate (MCPET)] was constructed. The luminous intensity distribution (LID) and luminance distribution from a specific viewpoint were measured with a near-field goniophotometer. Both distributions were also simulated by use of ray tracing software. Three different reflection models of MCPET were introduced, varying in complexity: a diffuse model, a diffuse/specular model, and a model containing tabulated BRDF data. A good agreement between the measured and simulated LID was found irrespective of the applied model. However, the luminance distributions only corresponded when the most accurate BRDF model was applied. This proves that even for diffuse reflective materials, a simple BRDF model may only be employed for simulations of the LID; for evaluation of luminance distributions, more complex models are needed.

Experimental observation on a frequency spectrum of a plate mode of a predominantly leaky nature

The problem of normal propagation modes of a plate submerged in a fluid is usually treated by considering continuous leaky Lamb waves or by considering transient waves. Angular plate resonances are associated with modes obtained by the first approach, whereas frequency plate resonances are associated with modes obtained using the second method. The dispersion curves for these two kinds of mode are almost identical, except for certain modes at large phase speed. In an experiment one is never dealing with one of these extreme situations because the applied signal is never infinitely long and the beam used to insonify the plate is never infinitely wide. In this paper we report on the manifestation in the transmission frequency spectrum, of a plate mode of a predominantly leaky nature. The extra mode, which has never been reported on, is observed between the cutoff frequencies of the symmetrical transient modes S1 and S2 of a submerged aluminium plate. The modes are identified by means of an Argand diagram.

Simulating the spatial luminance distribution of planar light sources by sampling of ray files

Ray files offer a very accurate description of the optical characteristics of a light source. This is essential whenever optical components are positioned in close proximity (near-field) of the light source in order to perform accurate ray tracing simulations. However, a ray file does not allow for a direct simulation of the spatial luminance distribution, i.e. luminance map, by off-the-shelf ray tracers. Simulating luminance maps of light sources or luminaires is especially important in general lighting in order to predict their general perception when viewed by the observer, and more specific, the perception of glare of luminaires having a non-uniform luminance distribution. To enable the simulation of luminance maps while maintaining the high accuracy offered by a ray file, a sampling method is presented. To validate the approach, near-field goniophotometer measurements of two planar light sources were performed. From these measurement data, ray files were extracted to which the sampling method was applied in order to obtain a set of surface sources. This approach was validated by comparing measured luminance images with simulated luminance images. A good agreement was found, validating the presented method.

A hybrid tool for spectral ray tracing simulations of luminescent cascade systems

To perform adequate simulations of luminescent cascade systems, a hybrid method combining a commercial ray tracer and a programming tool is presented. True Monte Carlo algorithms for luminescent materials, treating each ray individually, are adapted to allow wavelength conversion of ray sets. Two solutions for the wavelength conversion of ray sets are discussed: a random approach, where absorption events are randomly selected to create emission events, and a combined approach, where information from multiple absorption events is combined to create emission events. Both methods are applied to simulate the performance of a virtual remote phosphor light-emitting diode module. When using the combined approach, the required computation time to achieve sufficient accuracy is a factor 2 lower, compared to the time required when applying the random approach.

Bayesian deconvolution method applied to experimental bidirectional transmittance distribution functions

Optical simulations are a common tool in the development of luminaires for lighting applications. The reliability of the virtual prototype is strongly dependent on the accuracy of the input data such as the emission characteristics of the light source and the scattering properties of the optical components (reflectors, filters and diffusers). These scattering properties are characterized by the bidirectional scatter distribution function (BSDF). Experimental determination of the BSDF of the materials is however very sensitive to the characteristics of the measuring instrument, i.e. the dimensions of the illumination spot, the detector aperture, etc. These instrumental characteristics are reflected in the instrument function. In order to eliminate the influence of the instrument function the use of a Bayesian deconvolution technique is proposed. A suitable stopping rule for the iterative deconvolution algorithm is presented. The deconvolution method is validated using Monte Carlo ray tracing software by simulating a BSDF measurement instrument and a virtual sample with a known bidirectional transmittance distribution function (BTDF). The Bayesian deconvolution technique is applied to experimental BTDF data of holographic diffusers, which exhibit a symmetrical angular broadening under normal incident irradiation. In addition, the effect of applying deconvolved experimental BTDF data on simulations of luminance maps is illustrated.