Broadband multiple light scattering in white LED diffusers

Summary form only given. There is a strong worldwide drive to efficient general lighting using white light emitting diodes (LEDs) [1,2]. White LEDs often consist of a semiconductor diode [3,4,5] combined with luminescent phosphors [5] to convert part of the blue light to green yellow, and red. In state-of-the art white-light LEDs one exploits multiple scattering of light [1,2]. The transport of light then becomes diffusive, which serves to obtain a desirable smooth lighting without hot spots and without angular color distribution. Moreover, photons are recycled so that thin phosphor layers serve to improve cost efficiency and reduce environmental impact.Central challenges in the understanding of white LEDs arise from limitations in the physical understanding of the combined multiple light scattering and energy conversion in phosphors. LEDs are currently described by raytracing and Monte Carlo techniques [1,2]. Unfortunately, LED spectra cannot be predicted quantitatively, as optical parameters must be adjusted to be inconsistent with other data. These limitations hamper the design and development of efficient white LEDs. An improved description of multiple light scattering could be obtained by analytical theories originating from nanophotonics, invoking detailed nanostructure of a sample. We have measured diffuse optical transmission and reflectivity through diffuser plates typical of a commercial white LED (Fortimo). Using photonic diffusion theory we derive the transport mean free path [6-8]. With increasing scatterer density the mean free path decreases. To interpret the results, we use an ab initio model without free parameters by combining Mie theory with known properties, such as the scatterer size distribution taken from SEM. Our model predicts mean free paths in dramatically improved agreement with our data, without adjustable parameters [9]. We will discuss consequences for light scattering in white lighting modules.