Thin film wave plates are constructed by the oblique deposition
of dielectric material onto glass substrates. Two basic designs incorporating
biaxial columnar thin films have been proposed. The singlecolumn-
angle plate provides different sensitivities of retardance to
angular tilting, but suffers from thickness (and hence retardance) wedging.
The double-column-angle plate is relatively insensitive to tilt and
substantially free of wedging. However, both basic designs are susceptible
to the ingress of atmospheric moisture, which has the effect of reducing
the retardance. In this paper we describe modifications to the
basic designs that seal the wave plates against the uptake of atmospheric
moisture. The modifications eliminate wedging in the singlecolumn-
angle plate, but the characteristic angular sensitivities of the basic
designs are retained.