  In billiard systems with a flux line semiclassical approximations for the
density of states contain contributions from periodic orbits as well as from
diffractive orbits that are scattered on the flux line. We derive a
semiclassical approximation for diffractive orbits that are scattered once on a
flux line. This approximation is uniformly valid for all scattering angles. The
diffractive contributions are necessary in order that semiclassical
approximations are continuous if the position of the flux line is changed.
