Study On The Distribution Of Scattered Light Field In Ultra-smooth Surface Defects

Ultra-smooth surface optical elements are mainly used in photolithography,micro-optics,aerospace and many other advanced instruments and equipment systems.At the same time,their surface quality also directly affect the performance of optical systems,which puts forward more stringent requirements for high-precision detection technology of optical element surface quality.Based on the theory of angular resolution scattering detection,this paper studies the variation law between the surface defects of optical elements and its scattered light field and realizes the quantitative measurement of the surface defects through the variation law between them.Firstly,the quantization process of scattered light modulated by surface defects of optical elements and the measurement and analysis methods of light scattering are studied in this paper,and the basic theory of angular resolution scattering and the statistical parameters of optical surfaces are systematically discussed.Secondly,the principle,system parameter and technical index of scattering measuring instrument are described.At the same time,the standard defect samples of the experimental measurement are introduced.Then,the standard defect samples were measured experimentally,and the statistical parameters of the surface microstructure of the samples were studied through the experimental data.The results show that the root mean square roughness of the defect has a certain degree of mutation compared with the smooth surface.Especially,the change of the roughness at the pitting point is about one order of magnitude.The corresponding power spectral density shows obvious dominant peaks in the sampling direction.And with the increase of defect size,the dominant peaks increase gradually and the peak separation decrease.Finally,the regulations of the spatial scattering field distribution in different defects are analyzed.The scattering field distribution has obvious diffraction characteristics.According to the diffraction characteristics,the size of the defects are calculated by inversion.Two algorithms are used in inversion calculation.One is to calculate the fringe spacing directly byimage processing of the diffraction fringes.The other is a fitting algorithm based on the diffraction light field of defects.This algorithm is mainly proposed for the diffraction light field of small size defects.And it solves the problem that the diffraction light field with small size(less than 10um)defects which can not calculate the fringe spacing to obtain the defect size.The algorithm is used to calculate the standard pits,scratches and discrete random defects.The results accord with the real size of defects and the relative error is less than 5%.At the same time,the contour of the point is drawn and its equivalent circle is given.In summary,the scattering light field distribution of super smooth surface defects is mainly studied in this paper,and the recognition of defect types and the inversion calculation of defect size are realized by the distribution law.