  Scattering by infinite hexagonal ice prisms is calculated using Maxwell's
equations in the discrete dipole approximation for size parameters up to x=400.
Birefringence is included in the calculations. Applicability of the geometric
optics approximation is investigated. Excellent agreement between wave optics
and geometric optics is observed for large size parameter in the outer part of
the 22 degree halo feature. For smaller ice crystals halo broadening is
predicted, and there is appreciable "spillover" of the halo into shadow
scattering angles <22 degrees. Ways to retrieve ice crystal sizes are suggested
based on the full width at half-maximum of the halo, the power at <22 deg, and
the halo polarization.
