Patent ID: 7215424

Claim:
An ellipsometer or polarimeter system comprising at least one multi-element lens placed at a selection from the group consisting of: at the input; and at the output; said at least one multi-element lens being comprised of at least two elements which are made of different materials, such that in use the focal length for each wavelength in a range of wavelengths is essentially the same for every other wavelength is said range of wavelengths, wherein said multi-element lens demonstrates at least some birefringence; said at least one multi-element lens being sequentially comprised of at least first and second elements which are each independently selected from the group consisting of: Ca 2 ; BaF 2 ; LiF; MgF 2 ; fused silica; a void region; a gas filled region; a liquid filled region; and a functional equivalent to a void region; said elements being cemented into a mounting fixture which comprises a tube with holes through the wall thereof through which cement is entered to secure said at least first and second elements, such that at least one of said lens elements is mounted in other than a manner specifically designed for minimizing stress thereon to minimize stress induced birefringence in said multi-element lens.