  We study light transport in phosphor plates of white light-emitting diodes
(LEDs). We measure the broadband diffuse transmission through phosphor plates
of varying YAG:Ce$^{3+}$ density. We distinguish the spectral ranges where
absorption, scattering, and re-emission dominate. Using diffusion theory, we
derive the transport and absorption mean free paths from first principles. We
find that both transport and absorption mean free paths are on the order of the
plate thickness. This means that phosphors in commercial LEDs operate well
within an intriguing albedo range around 0.7. We discuss how salient parameters
that can be derived from first principles control the optical properties of a
white LED.
