Optical and Thermal Management of Phosphor-Converted LEDs

Phosphor conversion generates over 95% of the light source lumens and affects efficacy, spectral tuning and quality of Solid State Lighting (SSL) devices as much as the efficient generation of suitable primary light from the pumping semiconductor chips.  In SSL, a large number of applications such as low/mid-power distributed area illumination may not need the highest luminance sources that typically bring along most stringent requirements on materials, for example, as with high-brightness spotlights or automotive headlights. The cumulative efficiency of handling generated photons matters everywhere.  Basic properties of phosphors like refractive index and temperature-dependent quantum efficiency, in a series of physical processes and according to conditions surrounding the converter (for example optical scattering and reflection off of various interfaces, transmission and heat conductivity through materials), determine the overall amount of light being extracted and in use.  Modifying the light scattering, reflectance and transmission in different conversion media types and adding to it options for reducing the converter temperature has the cumulative effect that allows for optimizing the light generation process. Crucial to the overall efficiency are losses both from light recycled back into the LED, from various reflective surfaces, and weak volume absorption, including self-absorption of the phosphor emission.  The influence of these factors will be demonstrated through examples of thermal-optical modeling and actual measurements on specific packaging configurations for a range of forward currents I_f.  For example, advantages gained through controlling the scattering in ceramics versus powder-in-silicone and using thermally conductive materials will be elaborated.  The gains can be realized in either efficacy or luminous flux.