Jan Audenaert

Power and Photon budget of a Remote Phosphor LED Module

Light-emitting diodes (LEDs) are becoming increasingly important for general lighting applications. The remote phosphor technology, with the phosphor located at a distance from the LEDs, offers an increased extraction efficiency for phosphor converted LEDs compared to intimate phosphor LEDs where the phosphor is placed directly on the die. Additionally, the former offers new design possibilities that are not possible with the latter. In order to further improve the system efficiency of remote phosphor LEDs, realistic simulation models are required to optimize the actual performance. In this work, a complete characterization of a remote phosphor converter (RPC) consisting of a polycarbonate diffuser plate with a phosphor coating on one side via the bi-directional scattering distribution function (BSDF) is performed. Additionally, the bi-spectral BSDF which embraces the wavelength conversion resulting from the interaction of blue light with the RPC is determined. An iterative model to predict the remote phosphor module power and photon budget, including the recuperation of backward scattered light by a mixing chamber, is introduced. The input parameters for the model are the bi-spectral BSDF data for the RPC, the emission of the blue LEDs and the mixing chamber efficiency of the LED module. A good agreement between experimental and simulated results was found, demonstrating the potential of this model to analyze the system efficiency with errors smaller than 4%.

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.

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.

Impact of the Geometrical and Optical Parameters on the Performance of a Cylindrical Remote Phosphor LED

Remote phosphor light-emitting diode (LED) modules could offer advantages over intimate white phosphor converted LEDs in terms of phosphor operation temperature, light extraction efficiency, and angular color uniformity. Existing commercial devices show a large variety with respect to the dimensions of the mixing cavity, which raises a question about the optimization of the topology. A simplified simulation model applying a two-wavelength approach and considering the remote phosphor as one virtual surface to which three bidirectional scattering distribution functions are attributed (respectively, describing the blue-blue, blue-yellow, and yellow-yellow interactions) is developed and validated. This model has been used to analyze the impact of the cylindrical mixing cavity parameters such as the absolute reflectance, the diffuse-to-specular reflectance ratio, and the height of the mixing cavity, as well as the pitch and angular full-width at half-maximum of the LEDs on the extraction efficiency, the yellow-to-blue ratio, and the irradiance uniformity. It can be concluded that in order to increase the efficacy substantially, the recuperation of the backward emission of the converted light can only be increased by avoiding further interaction with the phosphor plate. To this extent, topologies other than cylindrical mixing cavities must be considered.

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.

Practical limitations of near-field goniophotometer measurements imposed by a dynamic range mismatch

Within near-field goniophotometry, measurement results of both an imaging luminance measurement device and a photometer detector are combined to generate the luminous intensity distribution of a light source. The simultaneous use of these two detectors may engender incorrect measurement results, due to their difference in dynamic range. In this paper, near-field and far-field based luminous intensity distribution measurements of two luminaires are presented, in order to exemplify the problem. Results demonstrate that the distributions obtained from near-field measurements may deviate from the correct intensity distribution, by an amount of up to 16% of the total luminous flux of the luminaire. A method to check for the correctness of the luminous intensity distribution from the near-field measurement, the so-called sanity check, is discussed. To conclude, some possible solutions to eliminate the dynamic range mismatch induced errors are treated.

Illumination system development using design and analysis of computer experiments

Computer assisted optimal illumination design is crucial when developing cost-effective machine vision systems. Standard local optimization methods, such as downhill simplex optimization (DHSO), often result in an optimal solution that is influenced by the starting point by converging to a local minimum, especially when dealing with high dimensional illumination designs or nonlinear merit spaces. This work presents a novel nonlinear optimization approach, based on design and analysis of computer experiments (DACE). The methodology is first illustrated with a 2D case study of four light sources symmetrically positioned along a fixed arc in order to obtain optimal irradiance uniformity on a flat Lambertian reflecting target at the arc center. The first step consists of choosing angular positions with no overlap between sources using a fast, flexible space filling design. Ray-tracing simulations are then performed at the design points and a merit function is used for each configuration to quantify the homogeneity of the irradiance at the target. The obtained homogeneities at the design points are further used as input to a Gaussian Process (GP), which develops a preliminary distribution for the expected merit space. Global optimization is then performed on the GP more likely providing optimal parameters. Next, the light positioning case study is further investigated by varying the radius of the arc, and by adding two spots symmetrically positioned along an arc diametrically opposed to the first one. The added value of using DACE with regard to the performance in convergence is 6 times faster than the standard simplex method for equal uniformity of 97%. The obtained results were successfully validated experimentally using a short-wavelength infrared (SWIR) hyperspectral imager monitoring a Spectralon panel illuminated by tungsten halogen sources with 10% of relative error.

Taking the spectral overlap between excitation and emission spectra of fluorescent materials into account with Monte Carlo simulations

Monte Carlo ray tracing is an important simulation tool in applications where fluorescence is present, e.g. in bio-medical applications and in the design of luminaires and luminescent solar concentrators. A frequently used ray tracing procedure for fluorescence is the ‘dual stage’ approach. In this approach, first, all sources are traced through the system and the rays absorbed in the fluorescent components are stored. Next, the emission from the fluorescent components is traced. This approach does not allow for subsequent re-absorption and re-emission effects in fluorescent materials with a spectral overlap between excitation and emission spectra. In this work, a ‘multi stage’ ray tracing procedure for the simulation of luminescence is presented. Herein, wavelengths are traced from short to long separately and no distinction is made regarding the origin of emission (either a fluorescent component or a source). The presented approach can be easily implemented in existing commercial ray tracing software thus reducing the programming efforts for the new ray tracing algorithm and taking advantage of the strength of the selected ray tracing package concerning the modelling of complex geometrical systems. Both techniques are compared to investigate the influence of the selected ray tracing approach on the efficiency and colour prediction of a remote phosphor LED module.