Modeling of Light Scattering, Absorption and Emission in a PhosphorConverted Light-Emitting Diode for the Prediction of Spectral Power Distribution

Speaker :
Mr Jiaqi WANG
Department of Mechanical and Aerospace Engineering, HKUST
Date : 28 Aug 2018 (Tue)
Time : 3:00 pm
Venue : Room 4504 , HKUST (4/F., Lift #25/26)


Owing to their high luminous efficiencies, environmental-friendliness, flexibility in the electrical control, low cost as well as long lifetime, phosphor-converted white light emitting diodes (pc-WLEDs) have been widely utilized in general lighting recently. People currently use the trial and error approach to determine the optimal phosphor recipe to achieve the target optical and chromatic performance of these WLEDs. This method is very time-consuming and causes the development of novel packaging structures to be difficult. The determination of phosphor recipes and the design of phosphor layers will be greatly facilitated if the output spectral power distributions (SPDs) of pc-WLEDs can be numerically predicted. The prediction of SPDs requires a profound understanding of photon transportation through the encapsulation layer consisting of phosphor and silicone. During past years, great efforts have been made to study the light scattering and absorption processes of phosphor clusters. However, few SPDs have been predicted since the scattering performance of phosphor particles with non-spherical morphologies or unknown refractive indexes (RIs) could not be effectively characterized and modeled, and the dependence of the phosphor’s absorption on the excitation wavelength was rarely considered. Besides, previous research has only been limited to single phosphor based pc-WLEDs and the modeling of WLEDs with multiple phosphor is still a challenging issue, making the phosphor layer design to achieve high color rendering performance difficult.

In this thesis, a novel approach has been proposed to characterize the scattering performance of phosphors with arbitrary RIs and morphologies employing a laser particle analyzer (LPA). The characterization effectiveness was validated by transmittance measurements of phosphor plates. To obtain the spectral absorption coefficients of phosphors, an experimental apparatus was built based on a commercial goniophotometer to test the spectral transmittance of phosphor plates. Using the obtained absorption and scattering input parameters and employing the Monte-Carlo ray-tracing algorithm, SPDs of single YAG:Ce phosphor based pc-WLEDs were predicted and the error was evaluated in terms of spectral, chromatic, radiometric and photometric aspects. To enhance the light emission in the red region and improve the color rendering index (CRI), the red phosphor is usually blended with the yellow phosphor in the encapsulant. Thus, it is necessary to predict SPDs of multiple phosphor based WLEDs. In this work, two kinds of multiple phosphor based WLEDs, where two phosphors were mixed or decked, were studied. In the prediction of both structures, the re-absorption of fluorescent yellow light by the red phosphor, resulting from the spectral overlap between red phosphor’s excitation and yellow phosphor’s emission spectra, was modeled. In the decked multiple phosphor based WLEDs, the blue photons’ back-scattering effect was modeled between two decked phosphor layers. The SPD prediction capability was examined by experimental measurements and the agreement between the predictions and the experimental results was evaluated, where the error from the single phosphor package serves as the base error.

Employing the SPD prediction model for multiple phosphors, the influence of the phosphor configuration on the optical performance of WLEDs was investigated. SPDs of two decked structures with reverse phosphor orders and one mixed structure were predicted under the same material consumption and the same color coordinate conditions, respectively. Results concluded from the prediction were also verified by experiments.

(Supervisor: Prof. Ricky Shi-Wei Lee)