  The decay behavior of the specific intensity is studied for the
spatial-frequency domain imaging (SFDI). It is shown using the radiative
transport equation that the decay is given by a superposition of different
decay modes, and the decay rates of these modes are determined by spatial
frequencies and Case's eigenvalues. This explains why SFDI can focus on shallow
regions. The fact that light with nonzero spatial frequency rapidly decays
makes it possible to exclusively extract optical properties of the top layer of
a layered medium. We determine optical properties of the top layer of a solid
phantom. This measurement is verified with different layered media of numerical
phantoms.
