The beam spread function (BSF) of sea ice is of interest for several r
easons. The BSF characterizes beam propagation through sea ice. Its eq
uivalent, the point spread function, is essentially the optical impuls
e response of the medium, which has many useful connections to radiati
ve transfer theory. In-ice measurements of the BSF over the full angul
ar range 0-180 degrees, using a novel method, were made of first-and m
ultiyear ice off the shore of Barrow, Alaska. All of the measured sea
ice BSFs were drastically different than the BSF of ocean water, and t
hey strongly indicated that sea ice is a highly scattering medium, wit
h a single scattering albedo generally >0.97 over the visible spectrum
. At pathlengths >30 cm, the BSF was found to be nearly identical to t
he computed asymptotic radiance distribution. The rapid approach to th
e asymptotic state and the high single scattering albedo of sea ice su
ggest that photon diffusion theory should accurately describe radiativ
e transfer in sea ice away from boundaries. Equating the results bf di
ffusion theory with asymptotic radiative transfer theory yields a simp
le expression that relates the asymptotic attenuation coefficient K-in
finity to the inherent optical property coefficients and the asymmetry
parameter g of the scattering phase function. It is shown that the ne
cessary optical parameters for computing g can be obtained from the me
asured BSE Thus, all the information necessary for modeling optical pr
opagation in sea ice can be obtained from the BSF measurements using t
he method described here.