Research On Mirrorless Imaging Algorithm Based On Pinhole Point Diffraction Interferometer

The pinhole-type diffraction interferometer uses the ideal spherical wave as the reference wave,which is incomparable to the conventional commercial Fizeau interferometer.The measurement precision can reach sub-nanometer level.The interference imaging system in the point diffraction interferometer has a high manufacturing cost and inevitably introduces an error(coherent noise,etc.).Therefore,in the pinhole-type diffraction interferometer,removing the interference imaging system and studying the lensless imaging technology is an important way to solve this difficulty.The lensless imaging technique is based on the wavefront acquired by the CCD,which is transmitted to the testing sphere or the conjugate position of the testing sphere to retrieve the surface information of the testing sphere.Therefore,we propose a lensless imaging algorithm which removes the interference imaging system and reconstructs the interferogram from the CCD directly.By constructing the virtual lens,the virtual imaging plane is established.The wavefront retrieval at CCD is simplified to the inversion of CCD wavefront to virtual imaging plane.In the simulation phase,the process of lensless imaging algorithm is simulated.The distance error factor in the lensless imaging algorithm is analyzed.The influence of distance error in the lensless imaging algorithm on the wavefront retrieval of spherical elements with different NA is discussed.This paper presents a distance iterative method based on the lensless imaging algorithm.The diffraction distance is obtained by iterating the number of pixels occupied by each retrieved interferogram.Finally,an accurate diffraction distance is obtained and the interferogram is accurately retrieved.In the experimental stage,the interferogram with the imaging system and the lensless interferogram are acquired,and the lensless interferogram is retrieved according to the lensless imaging algorithm.Comparing the wavefront solved by the pinhole-type diffraction interferometer,the lensless interferogram,the ZYGO interferometer,the results show that the proposed method can effectively eliminate the edge diffraction and center diffraction fringes of the interferogram while suppressing the coherent noise,which proves the feasibility of the lensless imaging algorithm.