A pathlength matched prism assembly is constructed from Polarizing Beam Splitter optical components having varying degrees of precision by arranging them in pathlength matched positions and fixing them to a baseplate or frame. Gaps between the optical components are sealed by the frame or adhesive sealant. Planar optical elements are inserted between the optical components and space between the components and elements is filled with an optical coupling fluid having an index of refraction that closely matches the index of refraction of both components and elements. An expansion compensation device is attached to the prism assembly to compensate of expansion and contraction of the optical coupling fluid. The prism assembly is best suited for use in HDTV and High Definition video projectors.