Abstract:
A defects evaluation system and method are provided in the present invention. Based on the principle of the microscopic scattering dark-field imaging, the present invention implements a sub-aperture scanning for the surface of spherical optical components and then obtains surface defects information with image processing. Firstly, the present invention takes full advantage of the characteristic that the surface defects of spherical optical components can generate scattering light when an annular illumination beam irradiates on the surface, to implement the sub-aperture scanning and imaging that covers the entire spherical surface. Then, a series of procedures such as the global correction of sub-apertures, the 3D stitching, the 2D projection and the digital feature extraction are taken to inspect spherical surface defects. Finally, actual size and position information of defects are evaluated quantitatively with the defects calibration data. The present invention achieves the automatic quantitative evaluation for surface defects of spherical optical components, which considerably enhance the efficiency and precision of the inspection, avoiding the influence of subjectivity on the results. Eventually, reliable numerical basis for the use and process of spherical optical components is provided.

Description:
CROSS REFERENCE OF RELATED APPLICATION 
       [0001]    This is a U.S. National Stage under 35 U.S.C 371 of the International Application PCT/CN2015/089217, filed Sep. 9, 2015, which claims priority under 35 U.S.C. 119(a-d) to CN 201410479580.7, filed Oct. 18, 2014; CN 201510535230.2, filed Aug. 27, 2015; and CN 201510536104.9, filed Aug. 27, 2015. 
       BACKGROUND OF THE PRESENT INVENTION 
     Field of Invention 
       [0002]    The present invention belongs to the technical field of machine vision inspection, relating to a defects evaluation system and method for spherical optical components. 
       Description of Related Arts 
       [0003]    Spherical optical components are widely used in many optical systems including the large-aperture space telescope, the Inertial Confinement Fusion (ICF) system and the high-power laser system. However, defects such as scratches and digs on the surface of components can not only affect the imaging quality of optical systems, but also generate unnecessary scattering and diffraction light resulting in energy loss in the high-power laser system, which may also lead to a secondary damage because of the high energy. Therefore, it is highly necessary to inspect the surface defects of the spherical optical components before put into use and to digitally evaluate defects information to provide reliable numerical basis for the use of spherical optical components. 
         [0004]    The traditional methods for inspecting the defects of spherical optical components are mostly based on the visual inspection. Using a strong light to illuminate the spherical surface, the inspector observes in different directions by naked eyes with the reflection method and the transmission method. However, the visual inspection suffers from subjectivity and uncertainty. It is greatly influenced by the proficiency of the inspector and can&#39;t provide quantitative description of defects information. Furthermore, a long-time inspection can cause eyes fatigue resulting in lower reliability. Accordingly, there remains a need for a system that can achieve the automatic evaluation for the surface defects on spherical optical components based on machine vision instead of manual visual method to considerably enhance the efficiency and precision of inspection. 
       SUMMARY OF THE PRESENT INVENTION 
       [0005]    In allusion to the deficiencies of the existing technology, the present invention aims to provide an evaluation system and method to achieve the automatic inspection of the surface defects on spherical optical components. 
         [0006]    Based on the principle of the microscopic scattering dark-field imaging, the present invention implements a sub-aperture scanning for the surface of spherical optical components and then obtains surface defects information with image processing. Firstly, the present invention takes full advantage of the characteristic that the surface defects of spherical optical components can generate scattering light when an annular illumination beam irradiates on the surface, to implement the sub-aperture scanning and imaging that covers the entire spherical surface. Then, a series of procedures such as the global correction of sub-apertures, the 3D stitching, the 2D projection and the digital feature extraction are taken to inspect spherical surface defects. Finally, actual size and position information of defects are evaluated quantitatively with the defects calibration data. 
         [0007]    Spherical surface defects evaluation system (SSDES) comprises a defect imaging subsystem and a control subsystem. The defect imaging subsystem is adapted to acquire microscopic scattering dark-field images suitable for digital image processing. The control subsystem is adapted to drive the movements of various parts of the defect imaging subsystem, to realize automatic scanning and inspection of defects on the spherical surface. The defect imaging subsystem comprises an illumination unit, a microscopic scattering dark-field imaging (MS-DFI) unit, a spatial position and posture adjustment (SPPA) unit and a centering unit. The illumination unit is adapted to provide dark-field illumination for microscopic scattering dark-field imaging of spherical surface. The MS-DFI unit is adapted to collect scatter light induced by the surface and image. The SPPA unit is adapted to achieve five-dimensional spatial position and attitude adjustment including three-dimensional translation, rotation and swing, easy to acquire sharp images at various locations on the surface of the spherical optical component. The centering unit is adapted to analyze the position of the curvature center of the component. The movement and the adjustment of the illumination unit, the MS-DFI unit, the SPPA unit and the centering unit are driven by the control subsystem. 
         [0008]    The illumination unit comprises illuminants and an illuminant support bracket. The illuminant comprises a uniform surface light source and a lens group with front fixed lens group, zoom lens group and rear fixed lens group installed in. The optical axis of the lens group intersects with the optical axis of the MS-DFI unit at the incident angle of γ ranging from 25 to 45 degrees. 
         [0009]    The illuminant support bracket comprises a top fixation board, a hollow shaft, a worm gear, a worm, a servo motor, a motor support, bearings, a rotating cylindrical part and illuminant fixation supports. The illuminant is fixed on the illuminant support bracket which is fixed on the rotating cylindrical part. The rotating cylindrical part has flexible connections with the hollow shaft by the bearings. The worm gear, installed on the rotating cylindrical part has flexible connections with the worm and achieve circular rotation by the drive of the servo motor. The servo motor is fixed on the top fixation board by the motor support and the hollow shaft is also fixed on the top fixation board, which is fixed on the Z-axis translation stage. The illuminant support bracket is applied to provide illumination for spherical surface defects in all directions. 
         [0010]    Three illuminants are in annular and uniform distribution at the angle interval of 120° by the illuminant fixation support on the rotating cylindrical part. 
         [0011]    The light path of the illumination unit is formed as follows. The zoom lens group is moved to the position in the lens group calculated according to the curvature radius of the spherical optical component. The parallel light emitted by the uniform surface light source enters into the lens group and passes through the front fixed lens group, the zoom lens group and the rear zoom lens group in turn. Finally it becomes convergent spherical wave with the aperture angle of θ l . 
         [0012]    Taking advantages of the induced scatter light by the principle that defects on the smooth surface modulate the incident light, the MS-DFI unit achieves microscopic dark-field imaging of defects and acquires dark-field images of defects. The principle is as follows. The incident light is incident onto the surface of thespherical optical component. If the spherical surface is smooth, the incident light, according to the law of reflection in geometrical optics, is reflected on the surface to form the reflected light, which can&#39;t enter the MS-DFI unit. If there is defect on the surface of the spherical optical component, the incident light is scattered to form the scatter light, which is received by the MS-DFI unit and forms the dark-field image of defects. 
         [0013]    The SPPA unit comprises an X-axis translation stage, a Y-axis translation stage, a Z-axis translation stage, a rotation stage, a swing stage and a self-centering clamp. The swing stage comprises an inner plate and a shell plate. The self-centering clamp has fixed connections with the rotation axis of the rotation stage and the base of the rotation stage is fixed on the inner plate of the swing stage. The inner plate has flexible connections with the shell plate so that the inner plate is capable of swinging by the shell plate. The sections of the inner plate and the shell plate are both in U-shape. The undersurface of the shell plate of the swing stage is fixed on the working surface of the Y-axis translation stage and the Y-axis translation stage is fixed on the working surface of the X-axis translation stage. The X-axis translation stage and the Z-axis translation stage are fixed on the same platform. 
         [0014]    The centering unit comprises a light source, a focusing lens group, a reticle, a collimation lens, a beam splitter, an objective, a plane reflector, an imaging lens and a CCD. The light beam emitted by the light source passes through the focusing lens group and irradiates the reticle with a crosshair on. Then, the light beam passes through the collimation lens, the beam splitter and the objective and irradiates on the spherical optical component. The light beam is reflected on the surface and the image of the crosshair on the reticle is indicated by the reticle image. The reflected light beam passes through the objective again and deflects at the beam splitter. Subsequently, the reflected light beam is reflected by the plane reflector and passes through the imaging lens. Finally, the light beam focuses on the CCD and the CCD acquires the image of the crosshair on the reticle. 
         [0015]    The control subsystem comprises a centering control module, an illumination control module, a five-stage translation control module and an image acquisition control module. The centering control module comprises a centering image acquisition unit and a four-stage translation control unit. The centering image acquisition unit is applied to control the CCD of the centering unit to acquire the image of the crosshair and the four-stage translation control unit is applied to control the movement of the X-axis translation stage, the Y-axis translation stage and the Z-axis translation stage and the rotation of the rotation stage during the process of centering. The illumination control module comprises an illumination rotating control unit and an illuminant zoom control unit. The illumination rotating control unit is applied to control the rotation of the illuminant support bracket of the illumination unit and the illuminant zoom control unit is applied to control the movement of the zoom lens group to change the aperture angle θ l  of the emitted convergent spherical wave. The five-stage translation control module is applied to control the movement of the X-axis translation stage, the Y-axis translation stage and the Z-axis translation stage, the rotation of the rotation stage and the swing of the swing stage during the process of inspection. The image acquisition control module comprises a sub-aperture image acquisition unit and a microscope zoom control unit. The sub-aperture image acquisition unit is applied to control the MS-DFI unit to acquire sub-aperture images and the microscope zoom control unit is applied to change the image magnification of the MS-DFI unit. 
         [0016]    The evaluation method comprises an automatic centering module, a scan-path planning module, an image processing module and a defect calibration module. The automatic centering module is adapted to automatic centering of the spherical surface, achieving accurate measurement of the curvature radius and axial consistency alignment between the rotation axis and the optical axis of the spherical optical component. The scan-path planning module is adapted to plan the optimal scan-path for the spherical surface. The image processing module is adapted to achieve spherical surface defects inspection with high precision. The defect calibration module is adapted to establish the relationship between pixels and actual size in sub-aperture images at any locations on the spherical surface in order that the actual size of defects can be obtained. The evaluation method comprises the following steps: 
         [0017]    Step1.The implementation of automatic centering of the spherical optical component by the automatic centering module. 
         [0018]    Step2. The completion of optimal scan-path planning and full-aperture scanning for the spherical optical component by the scan-path planning module. 
         [0019]    Step3. The obtainment of spherical surface defect information by the image processing module and the defect calibration module. 
         [0020]    The implementation of automatic centering of the spherical surface by the automatic centering module according to Step  1 , comprises the following steps: 
         [0021]    1-1. Initialize the centering unit. 
         [0022]    1-2. Move the spherical optical component to the initial position where the optical axis of the spherical optical component coincides with the optical axis of the centering unit approximately. 
         [0023]    1-3. The Z-axis translation stage is controlled to scan along Z-direction to find the sharpest crosshair image by use of image entropy clarity evaluation function. 
         [0024]    1-4. Judge whether the crosshair image is the surface image or the center image as follows: 
         [0025]    Move the X-axis translation stage and Y-axis translation stage slighted to observe whether the crosshair image in the field of view (FOV) is moved with the movement of translation stages or not. If the crosshair image is moved with the movement of stages, it is the center image of the spherical optical component located at the curvature center of the spherical optical component and then jump to Step 1-5. Otherwise, it is the surface image of the spherical optical component located on the surface of the spherical optical component and then jump to Step 1-9. 
         [0026]    1-5. Move the crosshair image to the center of FOV by the X-axis translation stage and the Y-axis translation stage. After the movement, the optical axis of the spherical optical component coincides with the optical axis of the centering unit. 
         [0027]    1-6. Find the position of the rotation axis by rotation measurement in optical alignment as follows: 
         [0028]    The spherical optical component can rotate around the rotation axis of the rotation stage under the self-centering clamp. Every 30° rotation of the rotation stage, CCD acquires a crosshair image. The positions of the crosshair images in the FOV of CCD vary with different rotation angles. The trajectory formed by the center of the crosshair is close to a circle, the center of which is the position of the rotation axis. 
         [0029]    1-7. Obtain the trajectory center by the least square circle fitting method and the max deviation between the trajectory center and the crosshair center is calculated. 
         [0030]    1-8. Judge whether the max deviation is lower than the max permissible error. If the max deviation is lower than the max permissible error, the axial consistency alignment is considered completed. Otherwise, the optical axis of the spherical optical component is not coincident with the rotation axis, therefore the center of the crosshair image is moved to the fitting trajectory center by adjusting the self-centering clamp and then jump to Step 1-5. 
         [0031]    1-9. Move the Z-axis translation stage to image at theoretical curvature center obtained by initialization. The Z-axis translation stage is controlled to scan along Z-direction to find the sharpest crosshair image and then jump to Step 1-5. At the same time, Z-direction displacement from the position of the surface image to the position of the center image is recorded to get the real curvature radius of the spherical optical component, which is the displacement of the Z-axis translation stage. 
         [0032]    The completion of optimal scan-path planning and full-aperture scanning for the spherical optical component by the scan-path planning module according to Step 2, comprises the following steps: 
         [0033]    2-1. With the fiducial position obtained in the process of axial consistency alignment in Step 1, the spherical optical component is moved by the SSPA unit below the MS-DFI unit. Then the MS-DFI unit acquires the sub-aperture located at the vertex of the spherical surface. For the convenience of the following statement, spherical coordinate system X s Y s Z s  is defined here, whose origin O s  is located at the curvature center of the spherical optical component and z-axis Z s passes through the vertex of the spherical surface. To achieve the full-aperture sampling, two-dimension movements along the meridian and parallel scanning trajectory is required, combining the swing around X s  and the rotation around Z s . 
         [0034]    2-2. The spherical optical component is driven to swing around X s  with swing angle β 1 , one sub-aperture image is acquired on the meridian. After that, rotating around Z s  with rotation angle α 1  is implemented to acquire another sub-aperture image on the parallel. 
         [0035]    2-3. Every time after the rotation around Z s  with the same rotation angle α 1 , one sub-aperture is acquired so that multiple sub-apertures on the parallel are obtained. 
         [0036]    2-4. After the completion of sub-aperture acquisition on the parallel, the spherical optical component is driven to swing around X s  again with swing angle β 2 , then one sub-aperture is acquired on meridian. 
         [0037]    2-5. Every time after the rotation around Z s  with the same rotation angle α 2 , one sub-aperture is acquired so that multiple sub-apertures on the parallel are obtained. Full-aperture sampling is finished with several times repetition of such a process that the spherical optical component is driven to swing around X s  with swing angle β 2  to acquire multiple sub-apertures on next parallel after the completion of sub-aperture acquisition on this parallel. 
         [0038]    According to Step 2, the completion of optimal scan-path planning and full-aperture scanning for the spherical optical component by the scan-path planning module is characterized by that the sub-aperture plan model is established firstly. In this model, sub-aperture A and sub-aperture B are two adjacent sub-apertures on meridian C. Sub-aperture Aa is adjacent to sub-aperture A on parallel D 1  where sub-aperture A is located. Similarly, sub-aperture Bb is adjacent to sub-aperture B on parallel D 2  where sub-aperture B is located. Besides, the bottom intersection of sub-aperture A and sub-aperture Aa is indicated by P cd , the top intersection of sub-aperture B and sub-aperture Bb is indicated by P cu . So the sufficient conditions for the realization of sub-aperture no-leak inspection is that the arc length           is less than or equal to the arc length z, 999  . Under such a constraint, planning result can be solved and obtained by establishing the relationship between swing angle β 1 , swing angle β 2  and rotation angle α 1 , rotation angle α 2 . The solution procedure of swing angle β 1 , swing angle β 2 , rotation angle α 1  and rotation angle α 2  is as follows: 
         [0039]    {circle around (1)} Validate relevant parameters about the spherical optical component, including the curvature radius, aperture of the spherical optical component and the size of the object field of view of the MS-DFI unit. 
         [0040]    {circle around (2)} Specify the initial value of swing angle β 1  and swing angle β 2  according to the above three parameters. After that, calculate the value of rotation angle α 1  and rotation angle α 2  according to the same overlapping area between adjacent sub-apertures on one parallel. Then, figure out arc length           and arc length          . 
         [0041]    {circle around (3)} Compare arc length           and arc length           to determine whether the given initial value of swing angle β 2  is appropriate or not. If          &gt;         , reduce the value of swing angle β 2  by 5% and go back to Step {circle around (2)}. Otherwise, sub-aperture plan for covering the entire spherical surface is finished. 
         [0042]    The obtainment of spherical surface defects information by the image processing module and the defect calibration module according to Step 3, comprises the following steps: 
         [0043]    3-1. The imaging sub-aperture image is a2D image, which is obtained when the surface of the spherical optical component is imaged by the MS-DFI unit in the image plane. Due to the information loss along the direction of optical axis during the optical imaging process, 3D correction of sub-apertures should be conducted firstly to recover the information loss of surface defects of the spherical optical component along the direction of optical axis during the optical imaging process.3D correction of sub-apertures means that the imaging process of the MS-DFI unit is simplified to be a pin-hole model and imaging sub-aperture images are transformed into  3 D sub-aperture images with geometrical relationship. 
         [0044]    3-2. For convenience of feature extraction, 3D sub-aperture images obtained after 3D correction of sub-apertures are projected onto a 2D plane with full-aperture projection to obtain the full-aperture projective image. 
         [0045]    3-3. Feature extraction at low magnification is conducted on the full-aperture projective image; then 3D sizes of defects is obtained with inverse-projection reconstruction; finally, actual sizes and positions of surface defects on the spherical optical component are obtained taking advantages of the defect calibration data got with the calibration module. 
         [0046]    3-4. Defects are inspected at high magnification to guarantee the micron-scale inspection precision. First, the imaging magnification of the MS-DFI unit is zoomed to high magnification; then, according to the positions obtained by Step 3-3, surface defects are moved to the center of the object field of view to acquire images at high magnification; Finally, feature extraction at high magnification is conducted and micron-scale evaluation results of defects are obtained taking advantages of the defect calibration data got with the calibration module. 
         [0047]    3-5. Evaluation results are output in the form of 3D panoramic preview of the spherical surface, electronic report and defect location map. 
         [0048]    According to Step 3-1, imaging sub-aperture images are obtained when the surface of the spherical optical component is imaged by the MS-DFI unit in the image plane. The detailed description is as follows: 
         [0049]    3-1-1. According to the optimal scan-path planned by the scan-path planning module in Step 2, one point p on the surface of the spherical optical component is moved to the point p′ by the SPPA unit. 
         [0050]    3-1-2. The MS-DFI unit acquires sub-apertures at low magnification. Point p′ is imaged to be image point p″ in the imaging sub-aperture image by the MS-DFI unit. 
         [0051]    3-1-3. During the process of digital image acquisition, the image-plane coordinate system X c Y c  is transformed into the image coordinate system X i Y i  and the imaging sub-aperture image is obtained. X-axis X c  and y-axis Y c  compose the image-plane coordinate system X c Y c , whose origin O c  is located at the intersection of the optical axis of the MS-DFI unit and the imaging sub-aperture image. X-axis X i  and y-axis Y i  compose the image coordinate system X i Y i  coordinate system, whose origin O i  is located at the top left corner of the digital image. 
         [0052]    According to Step 3-2, the full-aperture projective image is obtained. The detailed description is as follows: 
         [0053]    3-2-1. 3D sub-aperture images are transformed into spherical sub-aperture images by global coordinate transformation. 
         [0054]    3-2-2. Spherical sub-aperture images are projected vertically onto the plane to obtain projective sub-aperture images. 
         [0055]    3-2-3. Projective sub-aperture images are stitched and sizes and positions of defects are extracted in the plane. Precise inspection for surface defects of the spherical optical component can be achieved by inverse-projection reconstruction. The way of direct stitching for parallel circle and annulus stitching for meridian circle is used for image stitching of projective sub-aperture images. The process of image stitching of projective sub-aperture images is as follows: 
         [0056]    {circle around (1)} Projective sub-aperture images are denoised to remove the effect of background noise on stitching accuracy. 
         [0057]    {circle around (2)} After denoising, image registration according to overlapping area is carried out to on adjacent projective sub-aperture images on the same parallel circle. 
         [0058]    {circle around (3)} Adjacent projective sub-aperture images after registration on the same parallel circle are stitched to obtain the annulus image of one parallel circle. 
         [0059]    {circle around (4)} The minimum annulus image containing all overlapping areas is extracted. 
         [0060]    {circle around (5)} The image registration points of the minimum annulus image are extracted to acquire the best registration location, so that the image stitching of projective sub-aperture images is finished. 
         [0061]    According to Step 3-3, feature extraction at low magnification is conducted on the full-aperture projective image; then, 3D sizes of defects is obtained with inverse-projection reconstruction; finally, actual sizes and positions of surface defects of the spherical optical component are obtained taking advantages of the defect calibration data got with the defect calibration module. The detailed description is as follows: 
         [0062]    3-3-1. Extract features of the 2D full-aperture image after image stitching of projective sub-aperture images to obtain sizes and positions of defects. 
         [0063]    3-3-2. Obtain 3D sizes and positions in pixels of surface defects of the spherical optical component by inverse-projection reconstruction. 
         [0064]    3-3-3. Taking advantages of the defect calibration data got with the defect calibration module, convert 3D sizes and positions in pixels to actual sizes and positions. 
         [0065]    The defect calibration data according to Step 3-3 and Step 3-4 comprises defect length calibration data and defect width calibration data. The process of defect length calibration is to establish the relationship between actual lengths of line segments at any locations on the spherical surface and corresponding pixels in spherical sub-aperture images. The defect length calibration data is obtained as follows: 
         [0066]    Firstly, a standard line segment d l  is taken in the object plane and its length is measured by a standard measuring instrument. Standard line segment d l  is imaged by the MS-DFI unit and its image d p  can be obtained in the imaging sub-aperture image. 
         [0067]    Then, this imaging sub-aperture image is transformed into a 3D sub-aperture image by 3D correction, in which the spherical image of standard line segment d l , namely a short arc d c  on the spherical surface can be obtained. The size of d c  is quantified in pixels and its corresponding arc angle d θ  is obtained. Since the curvature radius R of the spherical optical component can be determined accurately during the process of centering, the corresponding actual size of d c  can be deduced by d=Rd θ . By establishing the relationship between d c  and d, the relationship between the pixels in the 3D sub-aperture image and the actual size is calibrated, namely the calibration coefficient k=d/d c . If substituting the equation d=Rd θ , we have k=Rd θ /d c . Continuing to substitute the equation d c =R pixel d θ , we can finally deduce calibration coefficient by k=R/R pixel , where R pixel  is the curvature radius in pixels of the 3D spherical surface image, called pixel curvature radius for short. To extract the length of surface defects on one spherical optical component, feature extraction is firstly implemented to get each pixel&#39;s position coordinates of defects. Then the continuous defects are discretized into a plurality of line segments described by a series of line equations l i : y i =k i x i +b i  based on position coordinates, where i=1,2,3 . . . n. After the process of inverse-projection reconstruction for each line segment, the corresponding arc C i  of line segment l i  on the spherical surface with the curvature radius R pixel  is obtained. And the length of defects in pixels can be figured out with the surface integral equation: 
         [0000]    
       
         
           
             
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         [0000]    where ds refers to the curve differential element. After substituting the calibration coefficient k, the actual length of defects can be obtained by: 
         [0000]    
       
         
           
             
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         [0068]    The defect width calibration data is obtained as follows: 
         [0069]    Firstly, in the 3D coordinate system, a standard line segment is taken in the object plane and its actual width is measured by a standard measuring instruments. The standard line segment is imaged by the MS-DFI unit and its image can be obtained in the imaging sub-aperture image. 
         [0070]    Then, this imaging sub-aperture image is transformed into a 3D sub-aperture image by 3D correction, in which the spherical image of the standard line segment can be obtained. For the spherical image, the arc length in pixels along width direction is the width of defects in pixels. Since the defects are located in the center of FOV during the process of image acquisition at high magnification, information loss along the direction of the optical axis can be ignored. Thus, the actual width of defects is equal to that of the standard line segment. 
         [0071]    Finally, a piecewise fitting for the corresponding discrete points of actual width and width in pixels of defects is used to obtain the best fitting curve, which is as the calibration transfer function (CTF). With the CTF, the actual width at any locations on the spherical surface can be calculated from the width in pixels. 
         [0072]    The present invention achieves the automatic quantitative evaluation for surface defects of spherical optical components, which not only liberates the inspectors from the heavy work of visual inspection, but also considerably enhance the efficiency and precision of the inspection, avoiding the influence of subjectivity on the results. Eventually, reliable numerical basis for the use and process of spherical optical components is provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0073]      FIG. 1  is a perspective view of according to a preferred embodiment of the present invention. 
           [0074]      FIG. 1  illustrates a block diagram of surface defects evaluation system and method for spherical optical components in accordance with the first and the second embodiment of the present invention; 
           [0075]      FIG. 2  illustrates a schematic diagram of all parts of surface defects evaluation system and method for spherical optical components in accordance with  FIG. 1  in more detail; 
           [0076]      FIG. 3  illustrates a schematic diagram of the structure of the illumination unit in accordance with  FIG. 1 ; 
           [0077]      FIG. 4  illustrates a schematic diagram of the illumination light path in accordance with the first embodiment of the present invention; 
           [0078]      FIG. 5  illustrates a graph of the relationship between the curvature radius of the convex spherical optical component and the aperture angle of the illuminant in the case of the incident angle of 40° in accordance with  FIG. 4 ; 
           [0079]      FIG. 6  illustrates a schematic diagram of the principle of the microscopic scattering dark-field imaging of the present invention; 
           [0080]      FIG. 7  illustrates a schematic diagram of the structure of the centering unit in accordance with the first embodiment of the present invention; 
           [0081]      FIG. 8A  illustrates a schematic diagram of the light path in the case of Z-direction deviation between the positions of the reticle image and the curvature center of the convex spherical optical component in accordance with  FIG. 7 ; 
           [0082]      FIG. 8B  illustrates an image of the crosshair captured by CCD in the case of Z-direction deviation between the positions of the reticle image and the curvature center of the spherical optical component in accordance with  FIG. 7 ; 
           [0083]      FIG. 9A  illustrates a schematic diagram of the light path in the case of X-direction and Y-direction deviation between the positions of the reticle image and the curvature center of the convex spherical optical component in accordance with  FIG. 7 ; 
           [0084]      FIG. 9B  illustrates an image of the crosshair captured by CCD in the case of X-direction and Y-direction deviation between the positions of the reticle image and the curvature center of the convex spherical optical component in accordance with  FIG. 7 ; 
           [0085]      FIG. 10  illustrates a block diagram of the control subsystem in accordance with  FIG. 1 ; 
           [0086]      FIG. 11A  illustrates a schematic diagram of portions of the control subsystem when SSDES operates in centering mode in accordance with  FIG. 10 ; 
           [0087]      FIG. 11B  illustrates a schematic diagram of portions of the control subsystem when SSDES operates in inspection mode in accordance with  FIG. 10 ; 
           [0088]      FIG. 12  illustrates a flowchart of the automatic centering module in accordance with  FIG. 1 ; 
           [0089]      FIG. 13A  illustrates a graph of image entropy clarity evaluation function in accordance with  FIG. 12 ; 
           [0090]      FIG. 13B  illustrates a schematic diagram of fitting the trajectory center of the crosshair in accordance with  FIG. 12 ; 
           [0091]      FIG. 14  illustrates a schematic diagram of sub-aperture scanning in accordance with  FIG. 1 ; 
           [0092]      FIG. 15  illustrates a schematic diagram of the sub-aperture plan model in accordance with  FIG. 14 . 
           [0093]      FIG. 16  illustrates a flowchart of the scan-path planning module in accordance with  FIG. 14 ; 
           [0094]      FIG. 17  illustrates a flowchart of the image processing module in accordance with  FIG. 1 ; 
           [0095]      FIG. 18  illustrates a schematic diagram of the imaging process of the sub-aperture in accordance with  FIG. 17 ; 
           [0096]      FIG. 19  illustrates a schematic diagram of the 3D correction of the sub-aperture, image stitching of spherical sub-aperture images and full-aperture projection in accordance with  FIG. 17 ; 
           [0097]      FIG. 20  illustrates a schematic diagram of inverse-projection reconstruction of projective sub-aperture images in accordance with  FIG. 17 ; 
           [0098]      FIG. 21  illustrates a flowchart of full-aperture projection in accordance with  FIG. 17 ; 
           [0099]      FIG. 22  illustrates a flowchart of image stitching of projective sub-apertures in accordance with  FIG. 21 ; 
           [0100]      FIG. 23  illustrates a schematic diagram of the process of defect length calibration; 
           [0101]      FIG. 24  illustrates a schematic diagram of the process of defect width calibration; 
           [0102]      FIG. 25  illustrates a graph of the calibration transfer function for width in accordance  FIG. 24 ; 
           [0103]      FIG. 26  illustrates a schematic diagram of the illumination light path in accordance with the second embodiment of the present invention; 
           [0104]      FIG. 27  illustrates a graph of the relationship between the curvature radius of the concave spherical optical component and the aperture angle of the illuminant in the case of the incident angle of 40° in accordance with  FIG. 26 ; 
           [0105]      FIG. 28  illustrates a schematic diagram of the structure of the centering unit in accordance with the second embodiment of the present invention; 
           [0106]      FIG. 29A  illustrates a schematic diagram of the light path in the case of Z-direction deviation between the positions of the reticle image and the curvature center of the concave spherical optical component in accordance with  FIG. 28 ; 
           [0107]      FIG. 29B  illustrates an image of the crosshair captured by CCD in the case of Z-direction deviation between the positions of the reticle image and the curvature center of the concave spherical optical component in accordance with  FIG. 28 ; 
           [0108]      FIG. 30A  illustrates a schematic diagram of the light path in the case of X-direction and Y-direction deviation between the positions of the reticle image and the curvature center of the concave spherical optical component in accordance with  FIG. 28 ; 
           [0109]      FIG. 30B  illustrates an image of the crosshair captured by CCD in the case of X-direction and Y-direction deviation between the positions of the reticle image and the curvature center of the concave spherical optical component in accordance with  FIG. 28 ; 
           [0110]      FIG. 31  illustrates a block diagram of surface defects evaluation system and method for spherical optical components in accordance with the third embodiment of the present invention; 
           [0111]      FIG. 32  illustrates a schematic diagram of all parts of surface defects evaluation system and method for spherical optical components in accordance with  FIG. 31  in more detail; 
           [0112]      FIG. 33  illustrates a flowchart of the image processing module in accordance with  FIG. 31 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0113]    Hereinafter, the present invention will now be described in detail with the combination of the accompany drawings and the embodiments. 
         [0114]    The present invention is capable to evaluate the surface defects of both convex and concave spherical optical components. The first embodiment applies to the surface defects evaluation of the convex spherical optical components. The second embodiment applies to the surface defects evaluation of the concave spherical optical components. The third embodiment applies to the surface defects evaluation of the small-aperture spherical optical component. In this case, the evaluation method is much more simplified because only single sub-aperture image is required to obtain dark-field image covering the full-aperture. 
         [0115]    Embodiments of the present invention will be described in detail with reference to the above drawings. In principle, the same components are indicated by the same reference numbers in all drawings for describing the embodiments. 
       First Embodiment 
       [0116]    Hereafter, a first embodiment of the present invention will be described in detail with reference to  FIGS. 1 to 25 , which describes surface defects evaluation system and method for convex spherical optical components. 
         [0117]      FIG. 1  illustrates a block diagram of surface defects evaluation system and method for spherical optical components in accordance with the first and the second embodiment of the present invention. The SSDES  100  comprises a defect imaging subsystem  200 , a control subsystem  700 . The defect imaging subsystem  200  is adapted to acquire microscopic scattering dark-field images suitable for digital image processing. The control subsystem  700  drives the movements of the illumination unit  300 , the MS-DFI unit  400 , the SPPA unit  500  and the centering unit  600 , to acquire images of the inspected surface of the spherical optical component. 
         [0118]    Referring to  FIG. 1 , the defect imaging subsystem  200  comprises an illumination unit  300 , an MS-DFI unit  400 , a SPPA unit  500  and a centering unit  600 . The illumination unit  300  is adapted to provide dark-field illumination for the MS-DFI unit  400 . The MS-DFI unit  400  is adapted to collect scatter light induced by the surface and image. The SPPA unit  500  is adapted to achieve five-dimensional spatial position and attitude adjustment including three-dimensional translation, rotation and swing, easy to acquire sharp images at various locations on the surface of the spherical optical component. The centering unit  600  is adapted to analyze the position of the curvature center of the component. The movement and the adjustment of the illumination unit  300 , the MS-DFI unit  400 , the SPPA unit  500  and the centering unit  600  are driven by the control subsystem  700 . 
         [0119]    The illumination unit  300  is adapted to provide dark-field illumination for the MS-DFI unit  400 . Common parallel light source is not suitable for dark-field illumination because the incident light that doesn&#39;t pass through the curvature center of the component is reflected by the spherical surface, passes through the MS-DFI unit  400  and becomes a bright-field reflective spot finally, destroying the dark-field illumination condition. Therefore, the illumination unit  300 , which is provided for surface defects inspection for spherical optical components, emits illumination light with the aperture angle varying with the curvature radiuses, providing dark-field illumination for the convex spherical optical component. 
         [0120]      FIG. 3  illustrates a schematic diagram of the structure of the illumination unit  300  in accordance with  FIG. 1 . The illumination unit  300  comprises illuminants and an illuminant support bracket  310 . The illuminant comprises a uniform surface light source  320  and a lens group  330  with front fixed lens group  331 , zoom lens group  332  and rear fixed lens group  333  installed in. The optical axis of the lens group  330  intersects with the optical axis of the MS-DFI unit  405  at the incident angle of y ranging from 25 to 45 degrees. 
         [0121]    The illuminant support bracket  310  comprises a top fixation board  311  a hollow shaft  312 , a worm gear  313 , a worm  314 , a servo motor  315 , a motor support  316 , bearings  317 , a rotating cylindrical part  318  and illuminant fixation supports  319 . The illuminant is fixed on the illuminant support bracket  319  which is fixed on the rotating cylindrical part  318 . The rotating cylindrical part  318  has flexible connections with the hollow shaft  312  by the bearings  317 . The worm gear  313 , installed on the rotating cylindrical part  318  has flexible connections with the worm  314  and achieve circular rotation by the drive of the servo motor  315 . The servo motor  315  is fixed on the top fixation board  311  by the motor support  316  and the hollow shaft  312  is also fixed on the top fixation board  311 , which is fixed on the Z-axis translation stage  530 . 
         [0122]    The illuminant support bracket  310  is applied to provide illumination for spherical surface defects in all directions. Three illuminants  301   a ,  301   b  and  301   c are in annular and uniform distribution at the angle interval of 120° by the illuminant fixation support  319  on the rotating cylindrical part  318 . The servo motor  315  is driven by the illumination rotating control module  721  to achieve annular illumination. 
         [0123]      FIG. 4  illustrates a schematic diagram of the illumination light path in accordance with the first embodiment of the present invention. The parallel light emitted by the uniform surface light source  320  passes through the lens group  330  and becomes convergent spherical wave with the aperture angle of θ l . The detailed process is as follows. The zoom lens group  332  is moved to the position in the lens group  330  calculated according to the curvature radius of the convex spherical optical component  201 . The parallel light emitted by the uniform surface light source  320  enters into the lens group  330  and passes through the front fixed lens group  331 , the zoom lens group  332  and the rear zoom lens group  333  in turn. Finally it becomes convergent spherical wave with the aperture angle of θ l . 
         [0124]      FIG. 5  illustrates a graph of the relationship between the curvature radius of the convex spherical optical component and the aperture angle θ l  of the illuminant in the case of the incident angle γ of 40° in accordance with  FIG. 4 . It can be found that with the curvature radius increasing, the aperture angle θ l  decreases and the illumination range received by the surface also decreases. The aperture angle θ l  is less than or equal to 15°. 
         [0125]    Taking advantages of the induced scatter light by the principle that defects on the smooth surface modulate the incident light, the MS-DFI unit  400  achieves microscopic dark-field imaging of defects and acquires dark-field images of defects. The MS-DFI unit  400  is the machine vision module of the SSDES  100 . 
         [0126]      FIG. 6  illustrates a schematic diagram of the principle of the microscopic scattering dark-field imaging of the present invention. The incident light  210  is incident onto the surface of the convex spherical optical component  201 . If the spherical surface is smooth, the incident light  210 , according to the law of reflection in geometrical optics, is reflected on the surface to form the reflected light  212 , which can&#39;t enter the MS-DFI unit  400 . If there is defect  203  on the surface of the spherical optical component, the incident light  210  is scattered to form the scatter light  211 , which is received by the MS-DFI unit  400  and forms the dark-field image of defects. 
         [0127]    The SPPA unit  500  is adapted to achieve adjustment of positions and attitude of the convex spherical optical component  201 .  FIG. 2  illustrates a schematic diagram of all parts of surface defects evaluation system and method for spherical optical components in accordance with  FIG. 1  in more detail. Referring to  FIG. 2 , the SPPA unit  500  comprises an X-axis translation stage  510 , a Y-axis translation stage  520 , a Z-axis translation stage  530 , a rotation stage  540 , a swing stage  550  and a self-centering clamp  560 . The swing stage  550  comprises an inner plate and a shell plate. The self-centering clamp  560  has fixed connections with the rotation axis of the rotation stage  540  and the base of the rotation stage  540  is fixed on the inner plate of the swing stage  550 . The inner plate has flexible connections with the shell plate so that the inner plate is capable of swinging by the shell plate. The sections of the inner plate and the shell plate are both in U-shape. The undersurface of the shell plate of the swing stage  550  is fixed on the working surface of the Y-axis translation stage  520  and the Y-axis translation stage  520  is fixed on the working surface of the X-axis translation stage  510 . The X-axis translation stage  510  and the Z-axis translation stage  530  are fixed on the same platform. The illumination unit  300 , the MS-DFI unit  400  and the centering unit  600  are all fixed on the Z-axis translation stage  530 . 
         [0128]    The centering unit  600  provides hardware basis for the automatic centering of the convex spherical optical component  201 .  FIG. 7  illustrates a schematic diagram of the structure of the centering unit  600  in accordance with the first embodiment of the present invention. The light beam emitted by the light source  601  of the centering unit  600  passes through the focusing lens group  602  and irradiates the reticle  603  with a crosshair on. Then, the light beam passes through the collimation lens  604 , the beam splitter  605  and the objective  606  and irradiates on the convex spherical optical component  201 . The light beam is reflected on the surface and the image of the crosshair on the reticle  603  is indicated by the reticle image  610 . The reflected light beam passes through the objective  606  again and deflects at the beam splitter  605 . Subsequently, the reflected light beam is reflected by the plane reflector  607  and passes through the imaging lens  608 . Finally, the light beam focuses on the CCD  609  and the CCD  609  acquires the image of the crosshair on the reticle  603 . 
         [0129]    Referring to  FIG. 7 , if the incident light beam after passing through the objective  606  focuses on the surface of the convex spherical optical component  201 , the incident light beam and the reflected light beam are symmetric about the optical axis of the centering unit  615 , so the reflected light beam becomes parallel light beam again after passing through the objective  606  the second time and the CCD  609  can acquire sharp crosshair image, which is called the surface image of the crosshair because the image is located on the spherical surface. The position of the surface image in the FOV of the CCD  609  doesn&#39;t vary with the slight movement of the convex spherical optical component  201  in X-direction or Y-direction. If the centering unit  600  is moved down to a certain position by the Z-axis translation stage  530 , the incident light beam after passing through the objective  606  focuses on the curvature center of the convex spherical optical component  202 . In this case, the reticle image  610  is located at the curvature center of the convex spherical optical component  202  and the reflected light beam coincides with the incident light beam. The CCD  609  can also acquire sharp crosshair image, which is called the center image of the crosshair because the image is located at the curvature center of the spherical surface. Therefore, the CCD  609  can acquire sharp crosshair images twice, which are named the surface image and the center image respectively. Thus according to the position and clarity of the crosshair image acquired by CCD  609 , the position of the curvature center of the convex spherical optical component  202  can be obtained as follows: 
         [0130]      FIG. 8A  illustrates a schematic diagram of the light path in the case of Z-direction deviation between the positions of the reticle image  610   a  and the curvature center of the convex spherical optical component  202  in accordance with  FIG. 7 . In this case, the reflected light beam doesn&#39;t coincide with the incident light beam so that the CCD  609  acquires fuzzy crosshair image, as is illustrated in  FIG. 8B . Besides,  FIG. 9A  illustrates a schematic diagram of the light path in the case of X-direction and Y-direction deviation between the positions of the reticle image  610   b  and the curvature center of the convex spherical optical component  202  in accordance with  FIG. 7 . In this case, the optical axis of the convex spherical optical component  205  doesn&#39;t coincide with the optical axis of the centering unit  615 . The reflected light beam focuses on the CCD  609  so that the CCD  609  acquires sharp crosshair image which is not located in the center of the FOV, as is illustrated in  FIG. 9B . Therefore according to the states of crosshair images on the CCD  609 , the 3D-position of curvature center of the convex spherical optical component  202  can be determined. 
         [0131]    The control subsystem  700  is adapted to drive the movements of various parts of the defect imaging subsystem  200 , to realize automatic scanning and inspection of defects on the spherical surface. 
         [0132]      FIG. 10  illustrates a block diagram of the control subsystem  700  in accordance with  FIG. 1 . The control subsystem  700  comprises a centering control module  710 , an illumination control module  720 , a five-stage translation control module  730  and an image acquisition control module  740 . 
         [0133]    Referring to  FIG. 10 , the centering control module  710  comprises a centering image acquisition unit  711  and a four-stage translation control unit  712 . The centering image acquisition unit  711  is applied to control the CCD  609  of the centering unit  600  to acquire the image of the crosshair and the four-stage translation control unit  712  is applied to control the movement of the X-axis translation stage  510 , the Y-axis translation stage  520  and the Z-axis translation stage  530  and the rotation of the rotation stage  540  during the process of centering. 
         [0134]    Referring to  FIG. 10 , the illumination control module  720  comprises an illumination rotating control unit  721  and an illuminant zoom control unit  722 . The illumination rotating control unit  721  is applied to control the rotation of the illuminant support bracket  310  of the illumination unit  300  and the illuminant zoom control unit  722  is applied to control the movement of the zoom lens group  332  to change the aperture angle θ l  of the emitted convergent spherical wave. 
         [0135]    Referring to  FIG. 10 , the five-stage translation control module  730  is applied to control the movement of the X-axis translation stage  510 , the Y-axis translation stage  520  and the Z-axis translation stage  530 , the rotation of the rotation stage  540  and the swing of the swing stage  550  during the process of inspection. 
         [0136]    Referring to  FIG. 10 , the image acquisition control module  740  comprises a sub-aperture image acquisition unit  741  and a microscope zoom control unit  742 . The sub-aperture image acquisition unit  741  is applied to control the MS-DFI unit  400  to acquire sub-aperture images and the microscope zoom control unit  742  is applied to change the image magnification of the MS-DFI unit  400 . 
         [0137]    The SSDES  100  operates in two modes, which are centering mode and inspection mode.  FIG. 11A  illustrates a schematic diagram of portions of the control subsystem  700  when SSDES  100  operates in centering mode in accordance with  FIG. 10 . When the convex spherical optical component  201  is located below the centering unit  600  by the SPPA unit  500 , the SSDES  100  operates in centering mode. In this mode, the control subsystem  700  achieves automatic centering by the centering image acquisition unit  711  and the four-stage translation control unit  712 . The four-stage translation control unit  712  drives the movement of the Z-axis translation stage  530  to make the centering unit  600  focus automatically and accurately along the Z-direction, the movement of the X-axis translation stage  510  and the Y-axis translation stage  520  to realize translation of the convex spherical optical component  201 , and rotation of the rotation stage  540 . 
         [0138]      FIG. 11B  illustrates a schematic diagram of portions of the control subsystem  700  when SSDES  100  operates in inspection mode in accordance with  FIG. 10 . When the convex spherical optical component  201  is located below the MS-DFI unit  400  by the SPPA unit  500 , the SSDES  100  operates in inspection mode. In this mode, the control subsystem  700  completes full-aperture defects inspection of the convex spherical optical component  201  by the illumination control module  720 , five-stage translation control module  730  and image acquisition control module  740 . The illumination control module  720  comprises an illumination rotating control unit  721  and an illuminant zoom control unit  722 . The illumination rotating control unit  721  is applied to achieve all-direction illumination for surface defects of the convex spherical optical component  201  and the illuminant zoom control unit  722  is applied to achieve dark-field illumination for surface defects of the convex spherical optical component  201 . The five-stage translation control module  730  is applied to drive the convex spherical optical component  201  to precisely adjust the spatial position and posture of the convex spherical optical component  201  for the purpose of full-aperture scanning and inspection. The image acquisition control module  740  comprises a sub-aperture image acquisition unit  741  and amicroscope zoom control unit  742 . The sub-aperture image acquisition unit  741  is applied to acquire the sub-aperture images for the image processing module  1100  and the microscope zoom control unit  742  is applied to automatically change the imaging magnification of the MS-DFI unit  400 . 
         [0139]    The control subsystem  700  is the hub of the SSDES  100  connecting the defect imaging subsystem  200  and the evaluation method  800 . The control subsystem  700  not only precisely controls the defect imaging subsystem  200 , but also delivers images obtained by the defect imaging subsystem  200  and the information of position and state to the evaluation method  800  to process. The control subsystem  700  achieves high-speed delivery and high-efficiency collaborative processing of information between the defect imaging subsystem  200  and the evaluation method  800 , realizes automatic scanning of the convex spherical optical component  201  and increases the inspection efficiency of SSDES  100 . 
         [0140]    The evaluation method  800  comprises an automatic centering module  900 , a scan-path planning module  1000 , an image processing module  1100  and a defect calibration module  1400 . 
         [0141]    The automatic centering module  900  is adapted to achieve automatic centering, accurate measurement of the curvature radius and consistency alignment between the rotation axis  565  and the optical axis of the spherical optical component  205 . The scan-path planning module  1000  is adapted to plan the optimal scan-path for the spherical surface in order that the whole surface can be inspected without omission by sub-apertures as few as possible. The image processing module  1000  is adapted to achieve spherical surface defects inspection with high precision. The defect calibration module  1400  is adapted to establish the relationship between pixels and actual size in sub-aperture images at any locations on the spherical surface in order that the actual size of defects can be obtained. 
         [0142]    The evaluation method  800  comprises the following steps: 
         [0143]    Step1: The implementation of automatic centering of the spherical optical component by the automatic centering module  900 ; 
         [0144]    Step2: the completion of optimal scan-path planning and full-aperture scanning for the spherical optical component by the scan-path planning module  1000 ; 
         [0145]    Step3: the obtainment of spherical surface defect information by the image processing module  1100  and the defect calibration module  1400 . 
         [0146]    The implementation of automatic centering of the spherical surface by the automatic centering module  900  according to Step 1, comprises the accurate measurement of the curvature radius of the convex spherical optical component  201  and axial consistency alignment between the rotation axis  565  and the optical axis of the spherical optical component  205 , providing fiducial position for planning optimal scan-path in Step 2.  FIG. 12  illustrates a flowchart of the automatic centering module  900  in accordance with  FIG. 1 . Referring to  FIG. 12 , the automatic centering module  900  comprises the following steps: 
         [0147]    1-1. Initialize the centering unit  600 . 
         [0148]    1-2. Move the convex spherical optical component  201  to the initial position where the optical axis of the spherical optical component  205  coincides with the optical axis of the centering unit  615  approximately. 
         [0149]    1-3. The Z-axis translation stage  530  is controlled to scan along Z-direction to find the sharpest crosshair image by use of image entropy clarity evaluation function.  FIG. 13A  illustrates a graph of image entropy clarity evaluation function in accordance with  FIG. 12 . 
         [0150]    1-4. Judge whether the crosshair image is the surface image or the center image as follows: 
         [0151]    Move the X-axis translation stage  510  and Y-axis translation stage  520  slighted to observe whether the crosshair image in the field of view (FOV) is moved with the movement of translation stages or not. If the crosshair image is moved with the movement of stages, it is the center image of the convex spherical optical component  201  and then jump to Step 1-5. Otherwise, it is the surface image of the convex spherical optical component  201  and then jump to Step 1-9. 
         [0152]    1-5. Move the crosshair image to the center of FOV by the X-axis translation stage  510  and the Y-axis translation stage  520 . After the movement, the optical axis of the convex spherical optical component  205  coincides with the optical axis of the centering unit  615 . 
         [0153]    1-6. Find the position of the rotation axis  565  by rotation measurement in optical alignment as follows: 
         [0154]    The convex spherical optical component  201  can rotate around the rotation axis of the rotation stage  540  under the self-centering clamp  560 . Every 30° rotation of the rotation stage  540 , CCD  609  acquires a crosshair image. The positions of the crosshair images in the FOV of CCD  609  vary with different rotation angles. The trajectory formed by the center of the crosshair is close to a circle.  FIG. 13B  illustrates a schematic diagram of fitting the trajectory center of the crosshair in accordance with  FIG. 12 . Referring to  FIG. 13B , the center  910  is the position of the rotation axis  565 . 
         [0155]    1-7. Obtain the trajectory center by the least square circle fitting method and the max deviation between the trajectory center and the crosshair center is calculated. 
         [0156]    1-8. Judge whether the max deviation is lower than the max permissible error. If the max deviation is lower than the max permissible error, the axial consistency alignment is considered completed. Otherwise, the optical axis of the spherical optical component  205  is not coincident with the rotation axis  565 , therefore the center of the crosshair image is moved to the fitting trajectory center  910  by adjusting the self-centering clamp  560  and then jump to Step 1-5. 
         [0157]    1-9. Move the Z-axis translation stage  530  to image at theoretical curvature center obtained by initialization. The Z-axis translation stage  530  is controlled to scan along Z-direction to find the sharpest crosshair image and then jump to Step 1-5. At the same time, Z-direction displacement from the position of the surface image to the position of the center image is recorded to get the real curvature radius of the convex spherical optical component  201 , which is the displacement of the Z-axis translation stage  530 . 
         [0158]    During the process of centering, the self-centering clamp  560  is adjusted to move the center of the crosshair image to the trajectory center in order that the optical axis of the convex spherical optical component  205  coincides with the rotation axis  565 . The X-axis translation stage  510  and the Y-axis translation stage  520  are adjusted to move the crosshair image to the center of the FOV of the CCD  609  in order that the optical axis of the convex spherical optical component  205  coincides with the optical axis of the centering unit  615 . After the above adjustment, the optical axis of the convex spherical optical component  205 , the rotation axis  565  and the optical axis of the centering unit  615  are in consistency. In this case, the position of the convex spherical optical component  201  is the fiducial position for planning optimal scan-path. 
         [0159]      FIG. 14  illustrates a schematic diagram of sub-aperture scanning in accordance with  FIG. 1 . Referring to  FIG. 14A-14F , the completion of optimal scan-path planning and full-aperture scanning for the spherical optical component by the scan-path planning module  1000  according to Step 2, comprises the following steps: 
         [0160]    2-1. With the fiducial position obtained in the process of axial consistency alignment in Step 1, the convex spherical optical component  201  is moved by the SSPA unit  500  below the MS-DFI unit  400 . Then the MS-DFI unit  400  acquires sub-aperture  1010  located at the vertex of the spherical surface  1009 , as is illustrated in  FIG. 14A . For the convenience of the following statement, spherical coordinate system X s Y s Z s  is defined here, whose origin O s    1004   s is located at the curvature center of the convex spherical optical component  201  and the z-axis Z s    1003   s  passes through the vertex of the spherical surface  1009 . To achieve the full-aperture sampling, two-dimension movements along the meridian and parallel scanning trajectory is required, combining the swing around X s    1001   s  and the rotation around Z s    1003   s . 
         [0161]    2-2. The convex spherical optical component  201  is driven to swing around X s    1001   s  with swing angle β 1    1007   a , one sub-aperture  1020  is acquired on meridian  1005 , as is illustrated in  FIG. 14B . After that, rotating around Z s    1003   s  with rotation angle α 1    1008   a is implemented to acquire another sub-aperture  1020   a  on parallel  1006   a , as is illustrated in  FIG. 14C . 
         [0162]    2-3. Every time after the rotation around Z s    1003   s  with the same rotation angle α 1    1008   a , one sub-aperture is acquired so that multiple sub-apertures on parallel  1006   a  are obtained, as is illustrated in  FIG. 14D . 
         [0163]    2-4. After the completion of sub-aperture acquisition on parallel  1006   a , the convex spherical optical component  201  is driven to swing around X s    1001   s  again with swing angle β 2    1007   b , then one sub-aperture  1030  is acquired on meridian  1005 . 
         [0164]    2-5. Every time after the rotation around Z s    1003   s with the same rotation angle α 2    1008   b , one sub-aperture is acquired so that multiple sub-apertures on parallel  1006   b are obtained, as is illustrated in  FIG. 14F . Full-aperture sampling is finished with several times repetition of such a process that the convex spherical optical component  201  is driven to swing around X s    1001   s  with swing angle β 2    1007   b to acquire multiple sub-apertures on next parallel after the completion of sub-aperture acquisition on this parallel. 
         [0165]      FIG. 15  illustrates a schematic diagram of the sub-aperture plan model in accordance with  FIG. 14 . Referring to  FIG. 15 , the sub-aperture plan model is established firstly in order that the whole surface can be inspected without omission by sub-apertures as few as possible. In this model, sub-aperture  1020  and sub-aperture  1030  are two adjacent sub-apertures on meridian  1005 . Sub-aperture  1020   a is adjacent to sub-aperture  1020  on parallel  1006   a where sub-aperture  1020  is located. Similarly, sub-aperture  1030   a is adjacent to sub-aperture  1030  on parallel  1006   b where sub-aperture  1030  is located. Besides, the bottom intersection of sub-aperture  1020  and sub-aperture  1020   a  (the intersection far from the vertex of the spherical surface  1009 ) is indicated by P cd    1040   a , the top intersection of sub-aperture  1030  and sub-aperture  1030   a  (the intersection near the vertex of the spherical surface  1009 ) is indicated by P cu    1040   b . So the sufficient conditions for the realization of sub-aperture no-leak inspection is that the arc length            1045   b is less than or equal to the arc length            1045   a . Under such a constraint, planning result can be solved and obtained by establishing the relationship between swing angle β 1    1007   a , swing angle β 2    1007   b  and rotation angle α 1    1008   a , rotation angle α 2    1008   b .The solution procedure of swing angle β 1    1007   a , swing angle β 2    1007   b , rotation angle α 1    1008   a  and rotation angle α 2    1008   b is as follows: 
         [0166]    {circle around (1)} Validate relevant parameters about the convex spherical optical component  201 , including the curvature radius, aperture of the convex spherical optical component  201  and the size of the object field of view of the MS-DFI unit  400 . 
         [0167]    {circle around (2)} Specify the initial value of swing angle β 1    1007   a  and swing angle β 2    1007   b  according to the above three parameters. After that, calculate the value of rotation angle α 2    1008   a  and rotation angle α 2    1008   b  according to the same overlapping area between adjacent sub-apertures on one parallel. Then, figure out arc length            1045   b  and arc length            1045   a.    
         [0168]    {circle around (3)} Compare arc length            1045   b  and arc length            1045   a to determine whether the given initial value of swing angle β 2    1007   b is appropriate or not. If          &gt;          reduce the value of swing angle β 2    1007   b by 5% and then jump to Step {circle around (2)}. Otherwise, sub-aperture plan for covering the entire spherical surface is finished. 
         [0169]      FIG. 17  illustrates a flowchart of the image processing module  1100  in accordance with  FIG. 1 . Referring to  FIG. 17 , the obtainment of spherical surface defects information by the image processing module  1100  and the defect calibration module  1400  according to Step 3, comprises the following steps: 
         [0170]    3-1. The imaging sub-aperture image is a 2D image, which is obtained when the surface of the convex spherical optical component  201  is imaged by the MS-DFI unit  400  in the image plane. Due to the information loss along the direction of optical axis during the optical imaging process, 3D correction of sub-apertures should be conducted firstly to recover the information loss of surface defects of the convex spherical optical component  201  along the direction of optical axis during the optical imaging process. 
         [0171]    3-2. For convenience of feature extraction, 3D sub-aperture images obtained after 3D correction of sub-apertures are projected onto a 2D plane with the full-aperture projection to obtain the full-aperture projective image. 
         [0172]    3-3. Feature extraction at low magnification is conducted on the full-aperture projective image; then 3D sizes of defects is obtained with inverse-projection reconstruction; finally, actual sizes and positions of surface defects on the convex spherical optical component  201  are obtained taking advantages of the defect calibration data got with the calibration module  1400 . 
         [0173]    3-4. Defects are inspected at high magnification to guarantee the micron-scale inspection precision. First, the imaging magnification of the MS-DFI unit  400  is zoomed to high magnification; then, according to the positions obtained by Step 3-3, surface defects are moved to the center of the object field of view to acquire images at high magnification; Finally, feature extraction at high magnification is conducted and micron-scale evaluation results of defects are obtained taking advantages of the defect calibration data got with the calibration module  1400 . 
         [0174]    3-5. Evaluation results are output in the form of 3D panoramic preview of the spherical surface, electronic report and defect location map. 
         [0175]      FIG. 18  illustrates a schematic diagram of the imaging process of the sub-aperture in accordance with  FIG. 17 . According to Step 3-1, imaging sub-aperture images are obtained when the surface of the convex spherical optical component  201  is imaged by MS-DFI unit  400  in the image plane. Referring to  FIG. 17 , the detailed description is as follows: 
         [0176]    3-1-1. According to the optimal scan-path planned by the scan-path planning module  1000  in Step 2, one point p  1201  on the surface of the convex spherical optical component  201  is moved to the point p′  1202  by the SPPA unit  500 , as is illustrated by Procedure  1261  in  FIG. 18 . 
         [0177]    3-1-2. The MS-DFI unit  400  acquires sub-apertures at low magnification. Point p′  1202  is imaged to be image point p″  1211  in the imaging sub-aperture image  1210  by the MS-DFI unit  400 , as is illustrated by Procedure  1263  in  FIG. 18 . 
         [0178]    3-1-3. During the process of digital image acquisition, the image-plane coordinate system X c Y c  is transformed into the image coordinate system X i Y i  and imaging sub-aperture image  1210  obtained, as is illustrated by Procedure  1263  in  FIG. 18 . Referring to  FIG. 18 , X-axis X c    1001   c  and y-axis Y c    1002   c  compose the image-plane coordinate system X c Y c , whose origin O c    1004   c  is located at the intersection of the optical axis of the MS-DFI unit  405  and the imaging sub-aperture image  1210 . X-axis X i    1001   i  and y-axis Y i    1002   i  compose the image coordinate system X i Y i , whose origin O i    1004   i  is located at the top left corner of the digital image. 
         [0179]    As is illustrated by Procedure  1264  in  FIG. 19 , 3D correction of sub-apertures according to Step 3-1 means that the imaging process of MS-DFI unit  400  is simplified to be a pin-hole model and imaging sub-aperture image  1210  are transformed into 3D sub-aperture image  1220  with geometrical relationship. 
         [0180]      FIG. 19  illustrates a schematic diagram of the 3D correction of the sub-aperture, image stitching of spherical sub-aperture images and full-aperture projection in accordance with  FIG. 17 .  FIG. 21  illustrates a flowchart of full-aperture projection in accordance with  FIG. 17 . According to Step 3-2, the full-aperture projective image is obtained. Referring to  FIG. 19 and 21 , the detailed description is as follows: 
         [0181]    3-2-1. 3D sub-aperture image  1220  is transformed into spherical sub-aperture image  1230  by global coordinate transformation, as is illustrated by Procedure  1265  in  FIG. 19 . 
         [0182]    3-2-2. Spherical sub-aperture image  1230  is projected vertically onto the plane to obtain projective sub-aperture image  1240 , as is illustrated by Procedure  1266  in  FIG. 19 . In this way, data volume describing one sub-aperture is reduced so that computations of the following feature extraction can be largely simplified. 
         [0183]    3-2-3. In terms of inspection for surface defects of the convex spherical optical component  201  involving multiple sub-apertures, perfect stitching should be carried out before extracting sizes and positions of defects. Since it is difficult to extract sizes and positions of defects in three-dimensional space, spherical sub-aperture image  1230  is projected vertically onto the plane to obtain projective sub-aperture image  1240 . Projective sub-aperture images are stitched and sizes and positions of defects are extracted in the plane. Precise inspection for surface defects of the convex spherical optical component  201  can be achieved by inverse-projection reconstruction. 
         [0184]    The way of direct stitching for parallel circle and annulus stitching for meridian circle is used for image stitching of projective sub-aperture images.  FIG. 22  illustrates a flowchart of image stitching of projective sub-apertures in accordance with  FIG. 21 . Referring to  FIG. 22 , the process of image stitching of projective sub-aperture images is as follows: 
         [0185]    {circle around (1)} Projective sub-aperture images are denoised to remove the effect of background noise on stitching accuracy. 
         [0186]    {circle around (2)} After denoising, image registration according to overlapping area is carried out on adjacent projective sub-aperture images on the same parallel circle. 
         [0187]    {circle around (3)} Adjacent projective sub-aperture images after registration on the same parallel circle are stitched to obtain the annulus image of one parallel circle. 
         [0188]    {circle around (4)} The minimum annulus image containing all overlapping areas is extracted. 
         [0189]    {circle around (5)} The image registration points of the minimum annulus image are extracted to acquire the best registration location, so that the image stitching of projective sub-aperture images is finished. 
         [0190]    Referring to  FIG. 19 , during the process of vertical projection, spherical sub-aperture image  1230  after global coordinate transformation has difference in deformation and compression of defects. Thus, during the following process of feature extraction at low magnification, inverse-projection reconstruction is needed to recover the deformation and compression due to the vertical projection of spherical sub-aperture image  1230 . 
         [0191]    According to Step 3-3, feature extraction at low magnification is conducted on the full-aperture projective image; then, 3D sizes of defects is obtained within verse-projection reconstruction; finally, actual sizes and positions of surface defects of the spherical optical component are obtained taking advantages of the defect calibration data got with defect calibration module  1400 . The detailed description is as follows. 
         [0192]    3-3-1. Extract features of the 2D full-aperture image after image stitching of projective sub-aperture images to obtain sizes and positions of defects. 
         [0193]    3-3-2. Obtain 3D sizes and positions in pixels of surface defects of the convex spherical optical component  201  by inverse-projection reconstruction, as is illustrated by Procedure  1267  in  FIG. 20 . 
         [0194]    3-3-3. Taking advantages of the defect calibration data got with the defect calibration module  1400 , convert 3D sizes and positions in pixels to actual sizes and positions. 
         [0195]    The defect calibration data according to Step 3-3 and Step 3-4 comprises defect length calibration data and defect width calibration data. The sizes and position coordinates of defects are quantified in pixels after image processing module  1100 , thus the defect calibration module  1400  is needed to establish the relationship between actual sizes of line segments at any locations on the spherical surface and corresponding pixels in sub-aperture images for purpose of actual lengths, widths and position coordinates of defects. 
         [0196]    The process of defect length calibration is to establish the relationship between actual lengths of line segments at any locations on the spherical surface and corresponding pixels in spherical sub-aperture images.  FIG. 23  illustrates a schematic diagram of the process of defect length calibration. Referring to  FIG. 23 , The defect length calibration data is obtained as follows: 
         [0197]    Firstly, a standard line segment d l    1420  is taken in the object plane  1250  and its length is measured by a standard measuring instrument. Standard line segment d l    1420  is imaged by MS-DFI unit  400  and its image d p    1410  can be obtained in the imaging sub-aperture image  1210 . 
         [0198]    Then, this imaging sub-aperture image  1210  is transformed into a 3D sub-aperture image  1220  by3D correction, in which the spherical image of standard line segment d l    1420 , namely a short arc d c    430  on the spherical surface can be obtained. The size of d c    1430  is quantified in pixels and its corresponding arc angle d θ   1440  is obtained. Since the curvature radius R of the convex spherical optical component  201  can be determined accurately during the process of centering, the corresponding actual size of d c    1430  can be deduced by d=Rd θ . By establishing the relationship between d c  and d, the relationship between the pixels in the 3D sub-aperture image  1220  and the actual size is calibrated, namely the calibration coefficient k=d/d c .If substituting the equation d=Rd θ , we have k=Rd θ /d c .Continuing to substitute the equation d c =R pixel d θ , we can finally deduce calibration coefficient by k=R/R pixel , where R pixel  is the curvature radius in pixels of the 3D spherical surface image, called pixel curvature radius for short. Thus it can be seen that the calibration coefficient k varies with the curvature radius R and calibration should be carried out again if the curvature radius R changes. 
         [0199]    To extract the length of surface defects on one spherical optical component, feature extraction is firstly implemented to get each pixel&#39;s position coordinates of defects. Then the continuous defects are discretized into a plurality of line segments described by a series of line equations l i : y i =k i x i +b i  based on position coordinates, where i=1,2,3 . . . n. After the process of inverse-projection reconstruction for each line segment, the corresponding arc C i  of line segment l i  on the spherical surface with the curvature radius R pixel  is obtained. And the length of defects in pixels can be figured out with the surface integral equation: 
         [0000]    
       
         
           
             
               L 
               pixel 
             
             = 
             
               
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                   i 
                   = 
                   1 
                 
                 n 
               
                
               
                   
               
                
               
                 ( 
                 
                   
                     ∫ 
                     
                       C 
                       i 
                     
                   
                    
                   ds 
                 
                 ) 
               
             
           
         
       
     
         [0000]    where ds refers to the curve differential element. After substituting the calibration coefficient k, the actual length of defects can be obtained by: 
         [0000]    
       
         
           
             
               L 
               real 
             
             = 
             
               
                 ∑ 
                 
                   i 
                   = 
                   1 
                 
                 n 
               
                
               
                 
                   k 
                   i 
                 
                  
                 
                   ( 
                   
                     
                       ∫ 
                       
                         C 
                         i 
                       
                     
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                     ds 
                   
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         [0200]    The purpose of width calibration is to establish the relationship between actual length of standard line segments at any locations on the spherical surface and corresponding pixels in 3D sub-aperture images. When MS-DFI unit  400  works at low magnification, the width in micron-scale is difficult to be calibrated accurately due to its small FOV and low resolution. So the width calibration results obtained at low magnification could not be used for evaluation, but only for reference. Defects width should be calibrated and evaluated at high magnification. At low magnification, the method similar to that for the process of length calibration is applied to the process of width calibration.  FIG. 24  illustrates a schematic diagram of the process of defect width calibration. At high magnification, referring to  FIG. 24 , since defects width is in micron-scale and defects are located in the center of the FOV,the defect width calibration data is obtained as follows: 
         [0201]    Firstly, in the 3D coordinate system, a standard line segment is taken in the object p 1 ane  1250  and its actual width  1420   w is measured by a standard measuring instruments. The standard line segment is imaged by the MS-DFI unit  400  and its image can be obtained in the imaging sub-aperture image  1210  with imaging width  1410   w  in pixels. 
         [0202]    Then, this imaging sub-aperture image  1210  is transformed into 3D sub-aperture image  1220  by 3D correction, in which the spherical image of the standard line segment can be obtained. For the spherical image, the arc length  1430   w in pixels along width direction is the width of defects in pixels. 
         [0203]    Since the defects are located in the center of FOV during the process of image acquisition at high magnification, information loss along the direction of the optical axis can be ignored. Thus, the actual width of defects is equal to the width of the standard line segment  1420   w.    
         [0204]      FIG. 25  illustrates a graph of the calibration transfer function for width in accordance  FIG. 24 . 
         [0205]    Finally, a piecewise fitting for the corresponding discrete points  1450  for actual width and width in pixels of defects is used to obtain the best fitting curve, which is as the calibration transfer function (CTF)  1460 . With the CTF  1460 , the actual width at any locations on the spherical surface can be calculated from the width in pixels. 
       Second Embodiment 
       [0206]    Hereafter, a second embodiment of the present invention will be described in detail with reference to  FIGS. 26 to 30 , which describes surface defects evaluation system and method for concave spherical optical components. 
         [0207]    Surface defects evaluation system and method for concave spherical optical components described in the second embodiment of the present invention is similar to that for convex spherical optical components described in the first embodiment of the present invention. In order to avoid confusion and repetition, parts in  FIGS. 26 to 30  which are relevant to parts in  FIGS. 1 to 25  are indicated by the same reference numbers. Emphasis in the second embodiment is also put on parts different from the first embodiment. 
         [0208]      FIG. 26  illustrates a schematic diagram of the illumination light path in accordance with the second embodiment of the present invention. The parallel light emitted by the uniform surface light source  320  passes through the lens group  330  and becomes convergent spherical wave with the aperture angle of θ l . The detailed process is as follows. The zoom lens group  332  is moved to the position in the lens group  330  calculated according to the curvature radius of the concave spherical optical component  1501 . The parallel light emitted by the uniform surface light source  320  enters into the lens group  330  and passes through the front fixed lens group  331 , the zoom lens group  332  and the rear zoom lens group  333  in turn. Finally it becomes convergent spherical wave with the aperture angle of θ l . 
         [0209]      FIG. 27  illustrates a graph of the relationship between the curvature radius of the concave spherical optical component and the aperture angle θ l  of the illuminant in the case of the incident angle γ of 40° in accordance with  FIG. 26 . It can be found that with the curvature radius increasing, the aperture angle θ l  decreases and the illumination range received by the surface also decreases. The aperture angle θ l  is less than or equal to 12°. Comparing  FIG. 27  with  FIG. 5 , it can be seen that the aperture angle formed by illuminants&#39; irradiating on the concave spherical optical component is smaller than that formed by illuminants&#39; irradiating on the convex spherical optical component with the same curvature radius, the aperture angle decreases with the curvature radius increasing more sharply and the critical curvature radius corresponding with the aperture angle of 0° is smaller. 
         [0210]      FIG. 28  illustrates a schematic diagram of the structure of the centering unit  600  in accordance with the second embodiment of the present invention. The light path during the process of centering for the concave spherical optical component  1501  is similar to that during the process of centering for the convex spherical optical component  201 . According to the position and clarity of the crosshair image, the relative position of the curvature center of the concave spherical optical component  1502  to the reticle image  1710  can be obtained as follows: 
         [0211]      FIG. 29A  illustrates a schematic diagram of the light path in the case of Z-direction deviation between the positions of the reticle image  1710   a  and the curvature center of the concave spherical optical component  1502  in accordance with  FIG. 28 . In this case, the reflected light beam doesn&#39;t coincide with the incident light beam so that the CCD  609  acquires fuzzy crosshair image, as is illustrated in  FIG. 29B . Besides,  FIG. 30A  illustrates a schematic diagram of the light path in the case of X-direction and Y-direction deviation between the positions of the reticle image  1710   b  and the curvature center of the concave spherical optical component  1502  in accordance with  FIG. 28 . In this case, the optical axis of the concave spherical optical component  1502  doesn&#39;t coincide with the optical axis of the centering unit  615 . The reflected light beam focuses on the CCD  609  so that the CCD  609  acquires sharp crosshair image which is not located in the center of the FOV, as is illustrated in  FIG. 30B . Therefore according to the states of crosshair images on the CCD  609 , the 3D-position of curvature center of the concave spherical optical component  1502  can be determined. The second embodiment of the present invention describes surface defects evaluation system and method for concave spherical optical components. The evaluation method for concave spherical optical components is the same as that described in the first embodiment. The illumination unit  300  and the centering unit  600  is different from those for convex spherical optical components due to the difference in surface shape. 
       Third Embodiment 
       [0212]    Hereafter, a third embodiment of the present invention will be described in detail with reference to  FIGS. 31 to 33 , which describes surface defects evaluation system and method for the small-aperture spherical optical components. Similarly, in order to avoid confusion and repetition, parts in  FIGS. 31 to 33  which are relevant to parts in  FIGS. 1 to 25  are indicated by the same reference numbers. Emphasis in the third embodiment is also put on parts different from the first embodiment. 
         [0213]    The small-aperture spherical optical component  1801  is characterized by that its aperture is smaller than the illumination aperture of the illumination unit  300  and the object field of view of the MS-DFI unit  400 . Thus, the MS-DFI unit  400  needs to acquire only one sub-aperture located at the vertex of the spherical surface  1009  (as is illustrated in  FIG. 15 ), which is the full-aperture image covering the whole surface of the small-aperture spherical optical components. Referring to  FIGS. 31 to 33 , surface defects evaluation system and method for small-aperture spherical optical components described in the third embodiment doesn&#39;t need the scan-path planning module and the image processing module  2000  only need to process one single sub-aperture. Correspondingly, the evaluation method  1900  is easier than that applied to the first and the second embodiment. 
         [0214]      FIG. 31  illustrates a block diagram of surface defects evaluation system and method for small-aperture spherical optical components in accordance with the third embodiment of the present invention.  FIG. 32  illustrates a schematic diagram of all parts of surface defects evaluation system and method for small-aperture spherical optical components in accordance with  FIG. 31  in more detail. Referring to  FIG. 31 and 32 , the evaluation method  1900  comprises an automatic centering module  900 , an image processing module  2000  and a defect calibration module  1400 . The evaluation method  1900  comprises the following steps: 
         [0215]    Step1. The implementation of automatic centering of the spherical optical component by the automatic centering module  900 . 
         [0216]    Step2. The obtainment of spherical surface defect information by the image processing module  2000  and the defect calibration module  1400 . 
         [0217]      FIG. 33  illustrates a flowchart of the image processing module  2000  in accordance with  FIG. 31 . Referring to  FIG. 33 , the obtainment of spherical surface defects information by the image processing module  2000  and the defect calibration module  1400  according to Step 2, comprises the following steps: 
         [0218]    2-1. The imaging sub-aperture image is a 2D image, which is obtained when the surface of the small-aperture spherical optical component  1801  is imaged by the MS-DFI unit  400  in the image plane. 3D correction of sub-apertures should be conducted firstly to recover the information loss of surface defects of the small-aperture spherical optical component  1801  along the direction of optical axis during the optical imaging process. 
         [0219]    2-2. For convenience of feature extraction, 3D sub-aperture images obtained after 3D correction of sub-apertures are projected onto a 2D plane with the single sub-aperture projection to obtain the single sub-aperture projective image. 
         [0220]    2-3. Feature extraction at low magnification is conducted on the single sub-aperture projective image; then 3D sizes of defects is obtained with inverse-projection reconstruction; finally, actual sizes and positions of surface defects on the small-aperture spherical optical component  1801  are obtained taking advantages of the defect calibration data got with the calibration module  1400 . 
         [0221]    2-4. Defects are inspected at high magnification to guarantee the micron-scale inspection precision. First, the imaging magnification of the MS-DFI unit  400  is zoomed to high magnification; then, according to the positions obtained by Step 2-3, surface defects are moved to the center of the object field of view to acquire images at high magnification; Finally, feature extraction at high magnification is conducted and micron-scale evaluation results of defects are obtained taking advantages of the defect calibration data got with the calibration module  1400 . 
         [0222]    2-5. Evaluation results are output in the form of 3D panoramic preview of the spherical surface, electronic report and defect location map.