Title: Characterization, modeling and optimization of remote phosphor LED modules
Authors: Acuna, Paula
Issue Date: 17-Oct-2016
Abstract: The invention of the GaN-based blue-emitting LED in the early 90’s enabled
a new generation of white light based on blue emitting LEDs. Presently,
the combination of a blue-emitting LED with a suitable phosphor material,
conventionally known as a white phosphor-converted LED, produces white
light with a higher luminous efficacy than any other existing light source.
Yet the luminous efficacy value is currently only the half of the theoretical
value. Among the loss mechanisms associated to the white phosphor-converted
LEDs, the package extraction efficiency, i.e. the efficiency to extract the white
light from the package/system, is one of the most important issues. Under the
conformal configurations of white phosphor-converted LEDs, the phosphor is
located adjacent to the LED chip, whose reflectivity is limited ( 50%). Nearly
50% of the scattered and wavelength converted light by the phosphor, after
illuminating it with blue light, is directed backwards, which turns into a loss of
25%. In an attempt to address this issue, the remote location of the phosphor
has been proposed in the mid-00s.
In this thesis, the characteristics and loss mechanisms of white phosphorconverted
LEDs with the phosphor element at a remote location from the
blue-emitting LED, are explored experimentally and by simulation models
implemented in ray tracing. The main loss mechanisms are the significant
backward scattered light by the phosphor and the absorption of this light by
the non-ideally reflective surfaces in the mixing cavity, as well as the absorption
of the converted light by the phosphor element. Novel architectures based on
these findings are designed to achieve unique luminous efficacy and color quality
The first model, a black box model, represents the phosphor element by a
bi-spectral BSDF. With the black box model, the impact of geometrical and
optical variations of the mixing cavity and the pump blue-emitting LEDs on
the light extraction ratio, the yellow-to-blue ratio and the irradiance uniformity
of the remote phosphor LED module are evaluated. In the second model, a gray box model, the scattering and the fluorescence of the phosphor element is
represented by the absorption and scattering bulk parameters, as well as by the
absorption, quantum yield and emission spectra of the phosphor. The baseline
concept consists of a cylindrical mixing cavity and a central blue-emitting LED
array. Using the gray box model, more striking configurations of white LEDs
are formulated, integrating novelties in the optical properties of the phosphor
element, such as the nanoparticle phosphors, and in the pumping source blueemitting
laser diodes.
The edge concept exhibited an increase of 5% in the luminous efficacy with
respect to the baseline at a CCT of 6800 K and can be considered as the best
performing architecture, while the phosphor mass was reduced to 5% of the
baseline phosphor mass.
In addition to the optical aspects of the white LEDs, this thesis also reports
on some thermal issues of the white phosphor-converted LEDs based on
experimental characterization. For the same CCT and test current, an intimate
pc-LED exhibited a larger the junction temperature than the remote phosphor
counterpart in a light engine. Besides, it was demostrated separately the effect
of both convection and radiation on the junction temperature increase of the
blue-emitting LED in the remote phosphor configuration.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Technologiecluster ESAT Elektrotechnische Engineering
Electrical Engineering (ESAT) TC, Technology Campuses Ghent and Aalst
ESAT - ELECTA, Electrical Energy Computer Architectures

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