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 single-column-angle plate provides differentsensitivities of retardance to angular tilting, but suffersfrom 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 ofreducing the retardance. In this paper wedescribe modifications to the basic designs that seal the wave plates against the uptake of atmospheric moisture. The modifications eliminate wedging in the single-column-angle plate, but the characteristic angular sensitivities of the basic designs areretained.
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