1. Optical apparatus for measuring the thickness of an object, comprising: an objective lens disposed above the plane of said object and with its optical axis perpendicular to said plane; an illumination source having a range of emitted wavelengths, said source being disposed above said lens and essentially in the focal plane of said lens such that said lens generates a collimated beam therefrom, and said source being laterally offset from said optical axis, such that said collimated beam illuminates said object at a non-normal angle of incidence; a first polarizing element disposed between said source and said lens; a detector element disposed essentially in the image plane of said object generated by said lens, and laterally offset from said optical axis; and a second polarizing element disposed between said lens and said detector.