Determination of volume scattering parameters that reproduce the luminance characteristics of diffusers.

The extension of a well-known inverse technique, inverse adding-doubling (IAD), is investigated for determining the volume scattering properties of diffusers for display and lighting applications. The luminance characteristics of volume scattering diffusers are vital for these applications. Through a simulation study, it is shown that fitting solely to the scattered (angular) intensity information with the extended IAD method, results in a volume scattering characterization that also reproduces the correct (spatial and angular) luminance characteristics for a wide range of samples. The gap between the simulation work and the experimental application of the investigated fitting procedure is bridged by considering the effect of experimental error in the scattered intensity distributions. This does not significantly alter the presented conclusions.

Determination of the optimal amount of scattering in a wavelength conversion plate for white LEDs.

Luminescent materials are widely used in white LEDs to convert part of the blue LED light into light with a longer wavelength, resulting in white light when both colors are well mixed. One way to integrate the luminescent material in the LED package is to deposit a thin luminescent layer on a planar carrier or disperse luminescent particles in the carrier material and then position the resulting wavelength conversion plate above one or more LEDs. It is very important that these wavelength conversion plates have the right properties to ensure homogeneous white light with a high efficiency and desired correlated color temperature (CCT). Key properties are the absorption and emission spectrum and the scattering and absorption coefficients. These properties strongly influence the color of the resulting light, but also the efficiency and the angular uniformity. This work describes an extensive study of the effect of the scattering and absorption coefficients in terms of the desired CCT. A computationally efficient extended Adding-Doubling method is used for the simulation of the light distribution and conversion in the planar wavelength conversion element. Ultimately an optimal combination with a high efficiency and low angular color deviation is desired. Different systems are investigated and optimal coefficients are found. With these findings a more targeted approach can be used in the manufacturing of wavelength conversion plates for white LEDs. The addition of scatterers or non-scattering luminescent particles can be used to obtain optimal scattering properties of the wavelength conversion plate.

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.

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%.

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.

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.

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.