How to distinguish elastically scattered light from Stokes shifted light for solid-state lighting?

We have studied the transport of light through phosphor diffuser plates that are used in commercial solid-state lighting modules (Fortimo). These polymer plates contain YAG:Ce+3 phosphor particles that both elastically scatter and Stokes shift light in the visible wavelength range (400–700 nm). We excite the phosphor with a narrowband light source and measure spectra of the outgoing light. The Stokes shifted light is spectrally separated from the elastically scattered light in the measured spectra, and using this technique, we isolate the elastic transmission of the plates. This result allows us to extract the transport mean free path l tr over the full wavelength range by employing diffusion theory. Simultaneously, we determine the absorption mean free path l abs in the wavelength range 400 to 530 nm where YAG:Ce+3 absorbs. The diffuse absorption ( μ a = 1 l abs ) spectrum is qualitatively similar to the absorption coefficient of YAG:Ce+3 in powder, with the diffuse spectrum being wider than the absorption coefficient. We propose a design rule for the solid-state lighting diffuser plates.