Diffusion coefficient depends on time, not on absorption

The recent controversy over whether the photon diffusion coefficient depends on absorption is addressed by use of the analytical solution of the photon transport equation in an infinite homogeneous scattering medium. The diffusion coefficient is found to be independent of absorption but temporally dependent. After a long period of time, the photon diffusion coefficient approaches D = w1/3mu(s)', which supports a claim made by Furutsu and Yamada [Phys. Rev. E 50, 3634 (1994)]. At early times, the diffusion coefficient is smaller than D = 1/3mu(s)', but this reduction cannot be expressed as D = 1/3(mu(s)' + mu(a)), since the time-dependent diffusion coefficient is found to be unrelated to absorption. (C) 2002 Optical Society of America.