A improved method for accurately measuring the wavefront aberration of aspheric surfaces with a lateral shearing interferomety and Ronchi test method is described. For Ronchi test, the phase of the Ronchi interferogram can be retrieved very accurately with the aid of a phase -reduction algorithm originally designed for phase shift interferometry. But in conventional Ronchi test method, wavefronts are diffracted by Ronchi grating with the high diffraction orders. So S/N ratio of phase of interferogram is decreased in actual test. However, by using the sinusoidal grating, the phase measurement indicated that the repeatability is about 5 times better than Ronchi grating.
In lateral shearing interferometry, wavefronts are sheared in one direction, one direction derivatives of wavefront are acquired and wavefront is retrieved by numerical integration of derivatives of wavefront. But to get accurate wavefront, derivatives of x,y directions of wavefront in image sensor coordinate are needed. So new method of getting the two direction derivatives of wavefront is described by grating location ±45˚. In this method, measurement results indicated that coarse sinusoidal grating, which benefited from the accuracy of phase reduction and qualified for integration in retrieving wavefronts, has good accuracy and repeatability in aspheric surface measurements.
