Study On Surface Defect Testing Technology Of Microspheres Under Depth Of Field Limitation

The inertial confinement fusion experiment is an important method for the study of efficient clean energy,astrophysics and thermonuclear explosion simulation.As the core element of the inertial confinement fusion,any defect of the target shot surface may lead to the failure of the experiment.At present,among the methods for measuring the surface defects of microspheres represented by target pellets,the null interferometric microscope(NIM)is a more feasible method due to its advantages of non-contact,high precision and high resolution.However,the method uses a large numerical aperture microscopic objective lens,which results in a shallow depth of field,and only the clear imaging area within the depth of field is available for effective measurement.Aiming at the problem of limited depth of field,this paper studies the method of depth of field extension,and adopts numerical calculation of diffraction to measure the surface defects outside the depth of field of microsphere imaging area based on the experimental device of NIM.First,the diffraction formula and related fast Fourier transform algorithm are studied.Based on D-FFT(Double Fast Fourier Transform)algorithm with angular spectrum diffraction formula,the diffraction calculation between parallel planes is extended to that between inclined planes,which lays a foundation for the following calculation of surface diffraction field.Secondly,in view of the fact that the diffraction calculation from the detector to the image surface is the calculation from a plane to another curved surface,by using the method that regard a tiny plane as approximate curved surface,we establish the inclined-plane-element model,comparing with the direct-layered model.The modeling method and the solution of model parameters are analyzed in detail.And we solve the problem of spherical phase factor sampling by using a method called object-image conjugation translation.Otherwise,we use the automatic focusing algorithm and the evaluation function to optimize the diffraction distance which would ensure the accuracy of the diffraction calculation results.Finally,the defocusing process and algorithm flow of the defect are simulated.And we conducted comparative tests which would verify the feasibility of the diffraction numerical calculation method.On the one hand,we compared the measurement results of one single isolated defect under different defocusing conditions when using both NIM and diffraction numerical calculation.On the other hand,we compared the measurement results of the entire imaging field of view by these two methods.The conclusion is that diffraction numerical calculation expands the effective measurement field of view to the entire imaging field,which is three times more than the effective measurement field of view of NIM.Furthermore,compared to the direct-layered model,the reconstruction results are more accurate by using the inclined-plane-element model.When the inclined-plane-element model is used,the relative error of peak value by gaussian fitting between the recovery result of defocused isolated defect and the focusing state of that isolated defect is within 1.2%,and the relative error of full width at half maximum is within 0.8%.