Precision profile measurement of aspheric surfaces by improved Ronchi test

With the intention of precision profile measurement of aspheric surfaces, the Ronchi test is revisited to devise a special scheme of multiple-beam lateral shearing interferometry. A holographic sinusoidal grating is used instead of conventional rule gratings so that only 0 and ± 1 lower order diffraction beams induce necessary lateral shear with high quality. in addition, relative fringe phases of wavefront slopes are automatically obtained by applying phase shifts along two orthogonal directions in sheared interferograms. Finally an elaborate in-line calibration scheme is incorporated to identify the system scale factors with which the 3-D profiles of test surfaces are accurately reconstructed from the measured wavefront slopes. Experimental results prove that the proposed technique is useful, especially for aspheric surfaces with a short focal length.