  We have investigated the transport of light through slabs that both scatter
and strongly absorb, a situation that occurs in diverse application fields
ranging from biomedical optics, powder technology, to solid-state lighting. In
particular, we study the transport of light in the visible wavelength range
between $420$ and $700$ nm through silicone plates filled with YAG:Ce$^{3+}$
phosphor particles, that even re-emit absorbed light at different wavelengths.
We measure the total transmission, the total reflection, and the ballistic
transmission of light through these plates. We obtain average single particle
properties namely the scattering cross-section $\sigma_s$, the absorption
cross-section $\sigma_a$, and the anisotropy factor $\mu$ using an analytical
approach, namely the P3 approximation to the radiative transfer equation. We
verify the extracted transport parameters using Monte-Carlo simulations of the
light transport. Our approach fully describes the light propagation in phosphor
diffuser plates that are used in white LEDs and that reveal a strong absorption
($L/\ell_{\mathrm{a}} > 1$) up to $L/\ell_{\mathrm{a}} = 4$, where $L$ is the
slab thickness, $\ell_{\mathrm{a}}$ is the absorption mean free path. In
contrast, the widely used diffusion theory fails to describe this parameter
range. Our approach is a suitable analytical tool for industry, since it
provides a fast yet accurate determination of key transport parameters, and
since it introduces predictive power into the design process of white light
emitting diodes.
