US Patent 7414730 High precision interferometer apparatus employing a grating beamsplitter

Measurement of a distance change between a reference surface and a target is provided. A substrate has a first surface facing the target and including a grating. The grating and target combine to form an optical interferometer responsive to changes in distance between the grating and the target. A second surface of the substrate coincides with the reference surface and faces away from the target. Thickness information pertaining to the substrate is combined with results from the optical interferometer to provide a measurement of distance change between reference surface and target. The substrate is preferably a rigid material having picometer level dimensional stability.