A fast evaluation method for pitch deviation and out-of-flatness of a planar scale grating

A planar scale grating with a pitch of 1 μm used in an interferential scanning-type planar encoder, which produces multi-axis position signals based on interference between first-order diffracted beams from the grating, is evaluated by a 100 mm-aperture Fizeau interferometer. The out-of-flatness of the grating is first evaluated from the wavefront of the zero-order diffracted beam from the grating. The grating is then tilted to align the axes of the first-order diffracted beams with that of the interferometer so that the X- and Y-directional pitch deviations of the grating can be evaluated from the wavefronts of the first-order diffracted beams.