Solid-state white light sources gain increasing interest due to their advanced characteristics compared to conventional lighting solutions. New design challenges are introduced in the remote phosphor set-up by the substitution of the efficiency-droop-limited LEDs with laser diodes (LDs) that exhibit peak efficiencies at much higher operating currents. Although laser-excited remote phosphor (LRP) systems have already been employed in some commercial applications, the bottleneck in their performance is identified in the down-conversion process within the phosphor material. The high intensity exciting laser beam in combination with the temperature-dependent properties of phosphors can lead to thermally induced instabilities in the system. For this reason, an opto-thermal simulation framework is developed to investigate the optical and thermal interdependencies and derive the LRPS optimization parameters. The optical analysis is performed with commercial ray-tracing software, where the optical heat losses are computed and subsequently used as the volume heat source in thermal analysis implemented by the finite element method (F.E.M.). The question now arises as to how to properly model the phosphor material in such a simulation scheme. The LED experience has produced a variety of phosphors for lighting applications, most commonly powders in some appropriate resin matrix, which are treated simulation wise as bulk diffusers. As the low thermal conductivity of resins is deemed critical for their use in LRPS, recent research focuses on resin free materials such as glass phosphors, single crystals, polycrystalline dense ceramics, etc. The different modeling approaches of such solutions are investigated here as the scattering properties and surface topology of the samples can vary.
KEYWORDS: Scattering, Quantum efficiency, Semiconductor lasers, Monte Carlo methods, Thermal effects, Light sources and illumination, Optical simulations, Systems modeling, Laser systems engineering, Absorption
A new family of lighting products is developed as laser diodes replace LEDs in the remote phosphor configuration. The resulting lighting systems, also known as laser-excited remote phosphor systems, exhibit advanced characteristics compared to LEDs, such as significantly higher luminance and smaller étendue. However, the bottleneck in their performance is often considered to be the conversion process within the phosphor layer. The high-intensity exciting laser beam in combination with the low thermal conductivity of ceramic phosphor materials leads to thermal quenching, a phenomenon in which the emission efficiency decreases as the temperature rises. In order to investigate the thermal limitations and derive the optimization parameters for these systems, the simulation strategy proposed here effectively takes into account the interplay between the thermal and optical effects. The time-dependent heat equation is solved based on the system’s energy balance equation, while the optical effects are modeled within the geometrical optics regime using a ray tracing algorithm. The coupling is achieved considering the temperature-dependent quantum yield (or efficiency) for the phosphor material. For simulation purposes the phosphor material can be considered as a bulk diffuser; the bulk scattering properties are introduced: the absorption and scattering coefficients as well as the scattering (or phase) function. The two-term Henyey-Greenstein function is adopted as scattering function here, since it combines computational efficiency and accuracy. To conclude, an opto-thermal simulation scheme is required for the optimization of a phosphor-converted lighting source. Efficient device design can contribute to the advancement of green lighting technology, a step towards meeting the environmental challenges of our age.
The development of laser-based lighting systems has been the latest step towards a revolution in illumination technology
brought about by solid-state lighting. Laser-activated remote phosphor systems produce white light sources with
significantly higher luminance than LEDs. The weak point of such systems is often considered to be the conversion
element. The high-intensity exciting laser beam in combination with the limited thermal conductivity of ceramic
phosphor materials leads to thermal quenching, the phenomenon in which the emission efficiency decreases as
temperature rises. For this reason, the aim of the presented study is the modeling of remote phosphor systems in order to
investigate their thermal limitations and to calculate the parameters for optimizing the efficiency of such systems. The
common approach to simulate remote phosphor systems utilizes a combination of different tools such as ray tracing
algorithms and wave optics tools for describing the incident and converted light, whereas the modeling of the conversion
process itself, i.e. photoluminescence, in most cases is circumvented by using the absorption and emission spectra of the
phosphor material. In this study, we describe the processes involved in luminescence quantum-mechanically using the
single-configurational-coordinate diagram as well as the Franck-Condon principle and propose a simulation model that
incorporates the temperature dependence of these processes. Following an increasing awareness of climate change and
environmental issues, the development of ecologically friendly lighting systems featuring low power consumption and
high luminous efficiency is imperative more than ever. The better understanding of laser-based lighting systems is an
important step towards that aim as they may improve on LEDs in the near future.