. . . "For example, the coefficient of thermal expansion of a plastic material such as polycarbonate may be, e.g., 6.6???10-5 /???C., and the coefficient of thermal expansion of a glass material such as BK-7 may be, e.g., 7.1???10-6 /???C. Due to the difference in the rates of expansion of these materials, the beam splitter planar surface 28 and the path length compensator surface 62 may experience varyi" .