Precision profile measurement of aspheric surfaces by improved Ronchi test

We present an improved version of Ronchi test that is particularly intended for the precision profile measurement of aspheric surfaces of mold cores. A sinusoidal grating is used, instead of conventional rule grating, to obtain high quality lateral shear interferograms with only +1, 0, and -1 order diffraction terms. Undesirable higher order diffraction terms are inherently removed for better signal-to-noise ratio of measured interferograms. Further, the relative phases of interferograms are accurately computed by adopting phase-shifting technique along two orthogonal shear directions. Finally, the 3-dimensional profile of the aspheric surface under inspection is reconstructed using the wavefront extracted by a dual-path wavefront integration algorithm. As compared with the conventional Ronchi test, this improved version of Ronchi test provides a great improvement in measurement stability.