Study On Surface Defect Classification And Detection Technology Of Super Smooth Optical Elements

At present,the super-smooth optical elements are widely used in the high-precision fields such as the lens of lithography machine,laser devices,etc.,so the quality requirements of its surface are more and more stringent.However,due to the influence of processing technology,manufacturing environment and other factors,there may be pits,bubbles,dust and other defects on the surface of optical elements.Different types of defects will cause various unknown consequences to the optical elements themselves and the performance of the entire optical system,so it is very urgent to detect the types of defects on the surface of optical elements.The light scattering method based on polarization bidirectional reflectance distribution function?polarization BRDF?is an effective nondestructive testing method.This method studies the light scattering characteristics of defects by analyzing and processing the received scattered light signals,which provides a new idea for the classification of surface defects of super-smooth optical elements.In order to realize the classification and detection of super-smooth optical element surface defects,the current research status of super-smooth surface defects detection and bidirectional reflection distribution function?BRDF?is described in this dissertation.The classification detection of the three kinds of scattering models which contain pits on the surface,sub-surface defects and micro-roughness were studied.Firstly,the relevant statistical parameters of rough surface,the basic concept and polarization characteristics of BRDF are introduced.The above three kinds of scattering models that affect the surface quality are studied by polarization BRDF,and the related concepts of angle distribution scattering are discussed.Secondly,the scattering distribution of defects on the super-smooth optical elements surface is simulated.Based on the mathematical model of polarization BRDF with pits on the surface,sub-surface defects and micro-roughness,the relationship of BRDF with scattering angle and scattering azimuth under ss,sp,pp and ps polarization states is simulated by using the Matlab software.The results show that the difference of BRDF_ppwith scattering angle or scattering azimuth can be used to distinguish the scattering signals generated by several scattering models.At last,the experimental device is built by using the designed detection scheme,and the scattering measurement experiments are carried out for several known types of defects.Based on the angular distribution scattering principle,the measured experimental data are calculated to obtain the roughness value,and the results are compared with those of Horos scattering instrument.From the analysis of experimental data,it can be concluded that the variation trend of the BRDF_ppvalues of pits which on the surface and micro roughness with the scattering angle is consistent with the simulation results.The calculated roughness is almost the same as the measured value of Horos,while the data of subsurface defects fluctuate violently.The results show that it is feasible to use the designed detection device to distinguish the scattered signals caused by the pits and micro roughness on the surface,which can provide the research basis for the processing and detection of super-smooth optical elements with higher precision in the future.