CN112666172B - Differential confocal fixed surface interference target shot outer surface defect detection method and device - Google Patents
Differential confocal fixed surface interference target shot outer surface defect detection method and device Download PDFInfo
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Abstract
本发明涉及差动共焦定面干涉靶丸外表面缺陷检测方法及装置，属于光学成像与检测技术领域。该方法利用差动共焦光强响应曲线的过零点精确定位靶丸(被测面)和相机(探测面)的轴向位置，以此实现外表面准确、自动对焦成像；在干涉显微光路中采用短相干光源和球面参考镜产生外表面零位干涉图；然后利用移相干涉技术从外表面零位干涉图中测得外表面缺陷分布。本发明利用差动共焦技术的精确定位特性来为靶丸和相机的轴向位置装调提供了更为准确的调整判断依据，进而有效提高了传统干涉显微方法的外表面对焦、检测的精度和效率，并为自动化检测提供了必要技术保障。本发明为大批量靶丸的外表面缺陷高精度、高效率、自动化检测提供新的技术途径。
The invention relates to a method and a device for detecting the outer surface defect of a differential confocal fixed-plane interference target shot, belonging to the technical field of optical imaging and detection. This method uses the zero-crossing point of the differential confocal light intensity response curve to precisely locate the axial position of the target (measured surface) and the camera (detection surface), so as to achieve accurate and automatic focusing imaging of the outer surface; A short coherent light source and a spherical reference mirror produce an outer surface null interferogram; then the phase-shifting interferometry technique is used to measure the outer surface defect distribution from the outer surface null interferogram. The invention utilizes the precise positioning characteristics of the differential confocal technology to provide a more accurate adjustment and judgment basis for the axial position adjustment of the target pill and the camera, thereby effectively improving the outer surface focusing and detection accuracy of the traditional interference microscopy method and efficiency, and provide the necessary technical support for automatic detection. The invention provides a new technical approach for high-precision, high-efficiency and automatic detection of the outer surface defects of a large number of target pellets.
Description
Technical Field
The invention belongs to the technical field of optical imaging and detection, and is used for the precise detection of the outer surface defects of target pellets, which are the most central key devices in Inertial Confinement Fusion (ICF).
Background
Inertial confinement nuclear fusion (ICF) is not only an ideal technical means for human to obtain clean energy in the future, but also provides a powerful support for the development of advanced scientific researches on condensed state physics, celestial body physics and the like under the conditions of extremely high temperature and high pressure. ICF related research is vigorously carried out in China, America, Law, Russia, Japan and the like at present. ICF focuses multiple high-energy laser on a tiny hollow spherical target pellet filled with DT fuel, and the shell of the target pellet instantaneously implodes and uniformly compresses the DT fuel inside the target pellet to a high-temperature and high-pressure state, thereby realizing fusion ignition. The target pellet is the most central key part in the ICF device, small shape change on the outer surface of the target pellet is a key factor for causing ignition failure, and isolated defects on the outer surface are key factors for determining the shape quality of the outer surface, so that the symmetry and the stability of the implosion compression process of the target pellet are seriously reduced, and further ignition failure is caused. Therefore, how to realize the precise measurement of the defects on the outer surface of the target pellet is a key problem to be solved urgently in ICF research, and has important scientific significance and application value.
At present, the methods for detecting the defects on the outer surface of the target pill comprise an Atomic Force (AFM) scanning method, a microscopic method, an interference method and the like.
And the AFM scanning method is to contact the outer surface of the target pill by using an AFM probe, then rotate the target pill around an axis for a circle to obtain a stub line shape of the target pill, and further obtain the defect of the measured outer surface by scanning a plurality of stub lines. The AFM scanning method has the advantages that the AFM scanning method has extremely high axial resolution, however, the shape distribution on one trace can be obtained by single measurement, the transverse resolution is determined by the density of scanning tracks, the detection efficiency is low, the detection precision is seriously influenced by the motion error of a scanning mechanism, and isolated defect points among the tracks are easy to miss.
The microscopy includes confocal microscopy, holographic microscopy, Scanning Electron Microscopy (SEM), and the like. The appearance of the outer surface of the target pill is directly detected by adopting the microscope, and then a defect distribution result is obtained. The advantages of the microscopy method are that the technology is mature, commercial instruments are provided, the integration of a measuring system is convenient, the measuring precision is high, and the like. However, confocal microscopes and holographic microscopes are not suitable for detecting the surface morphology of a spherical sample, and the effective measurement field of view is severely limited by the rise of the spherical surface, so that the measurement field of view is small and the defect detection efficiency is low. Scanning electron microscopes have very high magnification and large focal depth, but cannot achieve quantitative measurement of three-dimensional topography.
Compared with the two methods, the interference method has the advantages of large measurement field of view, high detection efficiency and difficulty in missing isolated defects, thereby being widely applied to the aspect of measuring the defects on the outer surface of the target pellet. The most critical step of the target pill outer surface defect detection based on the interference method is to accurately adjust the axial positions of the target pill and a camera, namely to accurately fix the surface, so that the detected surface is accurately imaged on a detection surface. The measured surface and the detection surface are positioned inaccurately, so that defocusing imaging is caused, further, the measured defect is fuzzy, and even the surface defect cannot be detected at all when the defocusing is serious.
However, the existing interference method is difficult to realize 'accurate surface fixing', and the axial positioning adjustment of the detection surface and the measured surface is finished by subjectively and visually judging whether the imaging is clear or not, so that an objective and accurate judgment basis is lacked. On one hand, the defect measurement precision is difficult to guarantee, on the other hand, the efficiency is low, manual participation is required, and efficient and automatic adjustment and measurement are difficult to realize. Therefore, the existing interference method is difficult to meet the requirements of high-precision, high-efficiency and large-batch automatic detection of the defects on the outer surface of the target pill.
Disclosure of Invention
The invention aims to solve the problem that the existing method is difficult to realize high-precision and large-batch automatic detection of the defects of the outer surface of a target pill, and provides a method for realizing precise, rapid and automatic positioning of the target pill and a camera in an interference light path by using a differential confocal positioning technology, thereby effectively ensuring the focusing precision and the defect measurement precision of the outer surface and providing a key technical basis and guarantee for rapid and automatic detection.
The purpose of the invention is realized by the following technical scheme.
The method for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pill comprises the following specific measurement steps:
(1-1) opening a point light source, converging a measuring beam emitted by the point light source at a focus of a microscope objective after passing through a collimating lens and the microscope objective, and accurately positioning a target pill to be measured and an interference camera respectively by utilizing a differential confocal high-precision positioning technology to accurately focus and image the outer surface of the target pill on the interference camera;
(1-2) adjusting an optical retardation of the reference arm from a near side to a far side, interfering with interference fringes which appear for the first time in the camera, i.e., outer surface interference fringes; the reference mirror is driven to perform mechanical phase shifting in an axial stepping mode, the interference camera collects N frames of phase shifting interferograms, and the light intensity expression of the phase shifting interferograms can be expressed as follows:
wherein IoIndicating the intensity of the outer surface interferogram, (x, y) indicating the pixel coordinates of the interferogram, N being 1,2 … N indicating the number of frames of the phase-shifted interferogram, a and B indicating the background intensity and the degree of modulation, respectively,
the phase of the external surface reflection wave surface is shown, and delta represents the phase shift quantity;
(1-3) extracting from the collected phase-shifted interferogram by using a phase-shifted interference algorithm
After unpacking, fitting and removing and high-pass filtering, the distribution h of the defects on the outer surface is obtained by using a formula (2)o
Where λ represents the center wavelength of the light source.
The method for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pill utilizes the differential confocal technology to realize the precise focusing and imaging of the outer surface and comprises the following steps:
(2-1) accurately positioning the outer surface of the target pellet at the cat eye position: axially scanning the target pill at the cat eye position, and obtaining a differential confocal response curve as shown in a formula (3) by subtracting the light intensity differential detected by the first detector and the second detector,
wherein, IDRepresenting differential confocal light intensity, u representing normalized axial displacement, uMRepresenting the normalized defocus of the detector. Adjusting the axial position of the target pill to make the intensity of the differential confocal response curve zero;
(2-2) positioning the interference camera at a conjugate imaging position: and moving the target pill from the cat eye position to the direction close to the objective lens by a distance d to ensure that the distance between the focus point of the reflected light beam on the outer surface and the cat eye position is just the outer diameter of the target pill, and the reflected measuring light is converged on the target surface of the interference camera. Axially scanning the interference camera and recording the axial position thereof, arranging a virtual pinhole on the interference camera, detecting a confocal response curve by using the integral of all pixel gray values in the virtual pinhole as the detection light intensity,
wherein ICDenotes the confocal light intensity and u denotes the normalized axial displacement. Subjecting the curve to bilateral fitting differential processing or sinc2Fitting processing can accurately obtain the axial position coordinate of the vertex of the interference camera, and accurately adjusting the interference camera to the coordinate position;
(2-3) accurately positioning the outer surface of the target pellet at a confocal position: and continuously moving the target pill to the position near the confocal position in the direction close to the objective lens and axially scanning the target pill, wherein the differential confocal detection system can detect a differential confocal response curve, and the axial position of the target pill is adjusted to ensure that the intensity of the differential confocal response curve is zero.
The introduction of phase shift can be realized by a synchronous transient phase shift technology, namely, a quarter wave plate is added in an imaging light path without mechanically moving a reference arm, every 4 pixels of an interference camera are divided into a group, a polarizing film with the phase shift of 0 degree, 45 degrees, 90 degrees and 135 degrees is sequentially placed in front of each group of pixels, and then 4 frames of phase shift interference images with the phase shift of 90 degrees can be extracted from one frame of interference image.
The method for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pill comprises the following steps of bilateral fitting differential processing of a confocal curve: selecting data points with relative intensity between 0.45 and 0.65 at two sides of the confocal curve, and respectively fitting two straight lines lAAnd lBThe differential confocal straight line l can be obtained by subtracting the differential values of the two straight linesdcTo find ldcI.e. the axial coordinate of the vertex of the confocal curve.
Method for detecting defects on outer surface of differential confocal fixed-surface interference target pill and confocal curveSinc of wire2The fitting process steps are as follows: selecting data points near the vertex of the confocal curve according to sinc2And fitting the function to obtain the vertex axial coordinate of the fitting curve and the vertex axial coordinate of the confocal curve.
The device for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pill comprises the following devices: the device comprises a short coherent laser, an optical fiber, a collimating lens, a polarization beam splitter PBS, a first quarter wave plate, a measuring objective lens, a measured target pill, a second quarter wave plate, a reference objective lens, a reference ball, a first beam splitter BS, a first linear polarizer, a tube lens, a collimation imaging lens, an interference camera, a second linear polarizer, a narrow-band filter, a converging lens, a second BS, a first pinhole, a first detector, a second pinhole and a second detector.
Short coherent linear polarized light emitted by the short coherent laser enters one end of the optical fiber through coupling, light emitted by the other end of the optical fiber is collimated into parallel light by the collimating mirror, the parallel light is divided into two paths by the PBS, light beams penetrating through the PBS are converged at the sphere center of the target pill to be measured after passing through the first quarter-wave plate and the measuring objective lens, and reflected light beams on the outer surface of the target pill form measuring light after passing through the first quarter-wave plate and the measuring objective lens again. The light beam reflected by the PBS passes through the second quarter-wave plate and the reference object mirror and then is converged at the spherical center of the reference sphere, and the light beam reflected by the reference spherical original path passes through the second quarter-wave plate and the reference object mirror again to form reference light. The reference light and the measurement light are transmitted and reflected by the PBS, respectively, and then enter the first BS.
The reference light and the measuring light reflected by the first BS interfere with each other after passing through a first linear polarizer with a transmission direction of 45 degrees, interference fringes sequentially pass through a tube lens and a collimation imaging lens and then are imaged on an interference camera, and the interference camera collects and records interference patterns.
The reference light and the measuring light transmitted by the first BS enter the differential confocal detection system, and after passing through the second linear polarizer in the direction of the transmission vibration direction S, the reference light is filtered out, and only the measuring light is reserved. The measuring light is converged by the converging lens after passing through the narrow-band optical filter, then is transmitted and reflected by the second BS, and the transmitted and reflected measuring light is collected by the first detector and the second detector after respectively passing through the first pinhole and the second pinhole. The first pinhole and the second pinhole are placed out of focus, the out-of-focus amount is equal in size, and the directions are opposite.
According to the device for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pill, reference light and measuring light reflected by an original path are collimated into parallel light and then are emitted onto an interference camera, the interference camera can move axially to be adjusted to achieve accurate focusing imaging of the outer surface, and the size of an interference image is kept unchanged in the axial movement adjustment process of the interference camera.
Advantageous effects
Compared with the prior art, the invention has the following innovation points:
1) the absolute zero point of a differential confocal curve is utilized to realize the accurate positioning of a detected surface and a detection surface in an interference light path for the first time, so that the measurement precision of the outer surface defect and the measurement focusing adjustment efficiency are effectively ensured;
2) the short coherent light source is used as a measuring light source, so that the influence of parasitic stripes on the inner surface on the measurement of the defects on the outer surface is effectively avoided;
3) the standard spherical surface is used as a reference surface, zero interference is generated between the standard spherical surface and the measured outer surface, and the effective measurement field of view is effectively enlarged.
Compared with the prior art, the invention has the following remarkable advantages:
1) compared with the traditional interference microscopy method, the method provides accurate judgment basis for focusing adjustment by using a differential confocal technology, and obviously improves the measurement precision of the outer surface defects;
2) compared with a focusing adjustment method based on subjective visual observation in the traditional interference microscopy method, the method provides objective judgment basis for focusing adjustment by using a differential confocal technology, obviously improves the efficiency of measurement adjustment, and provides necessary technical guarantee for automatic measurement;
3) compared with the traditional interference microscopy method, the method adopts the short coherent light source and the spherical reference surface, effectively avoids the influence of the parasitic fringes on the inner surface on the measurement precision and enlarges the effective measurement field of view.
Drawings
FIG. 1 is a schematic diagram of the differential confocal fixed-plane interference detection method of the present invention;
FIG. 2 is a schematic view of the outer surface focusing imaging of the present invention;
FIG. 3 is a flow chart of the outer surface defect measurement of the present invention;
FIG. 4 is a differential confocal graph of the present invention;
FIG. 5 is a schematic view of confocal curves and bilateral fitting differential processing according to the present invention;
FIG. 6 is a schematic diagram of a differential confocal fixed-plane interference detection apparatus according to the present invention;
FIG. 7 shows the outer surface defects measured in the examples of the present invention.
Wherein: 1-short coherent laser, 2-optical fiber, 3-collimating lens, 4-PBS, 5-first quarter wave plate, 6-measuring objective lens, 7-measured target pill, 8-second quarter wave plate, 9-reference objective lens, 10-reference sphere, 11-first BS, 12-first linear polaroid, 13-tube lens, 14-collimating imaging lens, 15-interference camera, 16-second linear polaroid, 17-narrow band filter, 18-converging lens, 19-second BS, 20-first pinhole, 21-first detector, 22-second pinhole, 23-second detector, 24-rotary table, 25-target pill translation table, 26-length measuring interferometer, 27-piezoelectric ceramic and 28-reference arm translation table, 29-camera translation stage, 30-master control computer.
Detailed Description
The invention is further illustrated by the following figures and examples.
With reference to fig. 1-6, a method and a device for detecting defects on the outer surface of a differential confocal fixed-surface interference target pellet are provided, wherein short coherent linear polarized light emitted by a short coherent laser 1 enters one end of an optical fiber 2 through coupling, light emitted from the other end of the optical fiber 2 is collimated into parallel light by a collimating mirror 3, the parallel light is divided into two paths by a PBS4, a light beam transmitted through the PBS4 passes through a first quarter-wave plate 5 and a measurement objective 6 and then is converged at the center of a sphere of a target pellet 7 to be measured, and a light beam reflected by the outer surface passes through the first quarter-wave plate 5 and the measurement objective 6 again to form measurement light. The light beam reflected by the PBS4 passes through the second quarter-wave plate 8 and the reference objective lens 9 and then converges at the center of the reference spherical surface 10, and the light beam reflected by the reference spherical surface 10 passes through the second quarter-wave plate 8 and the reference objective lens 9 again to form the reference light. The reference light and the measurement light are transmitted and reflected by the PBS4, respectively, and then enter the first BS 11.
The reference light and the measuring light reflected by the first BS11 interfere with each other after passing through the first linear polarizer 12 with the transmission direction of 45 degrees, the interference fringes sequentially pass through the tube lens 13 and the collimating imaging lens 14 and then are imaged on the interference camera 15, and the interference camera 15 collects and records an interference pattern.
The reference light and the measurement light transmitted by the first BS11 enter the differential confocal detection system, and after passing through the second linear polarizer 16 in the direction of the transmission direction S, the reference light is filtered out, and only the measurement light is retained. The measuring light is converged by the converging lens 18 after passing through the narrow band filter 17, and then transmitted and reflected by the second BS19, and the transmitted and reflected measuring light is collected by the first detector 21 and the second detector 23 after passing through the first pinhole 20 and the second pinhole 22, respectively. The first pinhole 20 and the second pinhole 22 are arranged out of focus, the out-of-focus amount is equal, and the directions are opposite.
According to the device for detecting the outer surface defects of the differential confocal fixed-surface interference target pill, reference light and measuring light reflected by an original path are collimated into parallel light and then are emitted onto the interference camera 15, the interference camera 15 can move axially to be adjusted to achieve accurate focusing imaging of the outer surface, and the size of an interference pattern is kept unchanged in the axial movement adjusting process of the interference camera 15.
The target pellet 7 to be measured is fixed to a rotation table 24, which is fixed to a translation table 25. The length measuring interferometer 26 is used to monitor the axial position of the target pellet. The reference objective 9 and the reference sphere 10 are jointly fixed on a piezo-ceramic 27, the piezo-ceramic 27 being used to drive the phase shift of the reference arm. The piezo-ceramic 27 is then fixed to a reference arm translation stage 28, and the reference arm translation stage 28 is used to adjust the optical retardation of the reference light. The interference camera 15 is fixed on a camera translation stage 29, and the camera translation stage 29 is used to axially adjust the position of the camera and is provided with a grating scale to monitor its axial displacement. The host computer 30 is used for motion control and data acquisition in the device.
With reference to the basic principle of fig. 1, the process of measuring the defects on the outer surface of the target pellet in this embodiment is as follows:
(1) opening the short coherent laser 1, converging the emitted measuring beam at a focal point after passing through a collimating lens 3 and a measuring objective 6, and accurately positioning a target pill 7 to be measured and an interference camera 15 respectively by utilizing a differential confocal high-precision positioning technology to accurately focus and image the outer surface on the interference camera 15;
(2) the optical delay of the reference arm is adjusted in the near-far direction, interference fringes appearing for the first time in the interference camera 15, namely outer surface interference fringes, drive the reference mirror in an axial stepping mode to perform mechanical phase shifting, the interference camera 15 collects N frames of phase-shifting interferograms, and the light intensity expression of the phase-shifting interferograms can be expressed as:
wherein IoIndicating the intensity of the outer surface interferogram, (x, y) indicating the pixel coordinates of the interferogram, N being 1,2 … N indicating the number of frames of the phase-shifted interferogram, a and B indicating the background intensity and the degree of modulation, respectively,
the phase of the external surface reflection wave surface is shown, and delta represents the phase shift quantity;
(3) extraction from collected phase-shifted interferograms using a phase-shifting interferometry algorithm
After unpacking, fitting and removing and high-pass filtering, the distribution h of the defects on the outer surface is obtained by using a formula (2)o
Where λ represents the center wavelength of the light source.
With reference to fig. 2, fig. 4 and fig. 5, the steps of using differential confocal technology to achieve the external surface precise focusing imaging are as follows:
(1) accurately positioning the outer surface of the target pellet at the cat eye position: the target pellet is scanned axially at the cat eye position, the differential differences of the light intensity detected by the first detector 21 and the second detector 23 are subtracted to obtain the differential confocal response curve shown in the formula (3) and shown in fig. 4,
wherein, IDRepresenting differential confocal light intensity, u representing normalized axial displacement, uMRepresenting the normalized defocus of the detector. Adjusting the axial position of the target pill to make the intensity of the differential confocal response curve zero;
(2) positioning the interference camera 15 in the outer surface conjugate imaging position: and moving the target pill from the cat eye position to the direction close to the objective lens by a distance d to ensure that the distance between the focus point of the reflected light beam on the outer surface and the cat eye position is just the outer diameter of the target pill, and the reflected measuring light is converged on the target surface of the interference camera 15. The interference camera 15 is axially scanned and the axial position thereof is recorded, a virtual pinhole is arranged on the interference camera 15, the confocal response curve as shown in figure 5 can be detected by taking the integral of all the gray values of the pixels in the virtual pinhole as the detection light intensity,
wherein ICDenotes the confocal light intensity and u denotes the normalized axial displacement. Bilateral fitting differential processing is carried out on the curve, so that the axial position coordinate of the vertex of the curve can be accurately obtained, and the interference camera 15 is accurately adjusted to the coordinate position;
(3) the outer surface of the target pellet was accurately positioned in a confocal position: and continuously moving the target pill to the position near the confocal position in the direction close to the objective lens and axially scanning the target pill, wherein the differential confocal detection system can detect a differential confocal response curve, and the axial position of the target pill is adjusted to ensure that the intensity of the differential confocal response curve is zero.
As shown in fig. 5, the bilateral fitting differential processing steps of the confocal curves are as follows: selecting data points with relative intensity between 0.45 and 0.65 at two sides of the confocal curve, and respectively fitting two straight lines lAAnd lBThe differential confocal straight line l can be obtained by subtracting the differential values of the two straight linesdcTo find ldcI.e. the axial coordinate of the vertex of the confocal curve.
On the measuring device shown in fig. 6, the detection flow shown in fig. 3 is adopted, the finally detected outer surface defects are shown in fig. 7, and a plurality of clear outer surface convex defects can be seen in fig. 7, so that the feasibility and the good effect of the invention are proved.
While the invention has been described in connection with specific embodiments thereof, it will be understood that these should not be construed as limiting the scope of the invention, which is defined in the following claims, and any variations which fall within the scope of the claims are intended to be embraced thereby.
Claims (5)
1. The method for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pill is characterized by comprising the following steps of: the method comprises the following detection steps:
(1-1) opening a short coherent laser, converging a measuring beam at a focus after the measuring beam passes through a collimating lens and a measuring objective lens, and accurately positioning a target pill to be measured and an interference camera respectively by utilizing a differential confocal high-precision positioning technology to accurately focus and image the outer surface of the target pill on the interference camera;
(1-2) adjusting an optical retardation of the reference arm from a near side to a far side, interfering with interference fringes which appear for the first time in the camera, i.e., outer surface interference fringes; the reference mirror is driven to mechanically shift the phase in an axial stepping mode, the interference camera collects N frames of phase-shifting interferograms, and the light intensity expression of the phase-shifting interferograms is as follows:
wherein IoIndicating the intensity of the outer surface interferogram, (x, y) indicating the pixel coordinates of the interferogram, N being 1,2 … N indicating the number of frames of the phase-shifted interferogram, a and B indicating the background intensity and the degree of modulation, respectively,
representing wave surfaces reflected from the outer surfacePhase, δ represents the amount of phase shift;
(1-3) extracting from the collected phase-shifted interferogram by using a phase-shifted interference algorithm
After unpacking, fitting and removing and high-pass filtering, the distribution h of the defects on the outer surface is obtained by using a formula (2)o
Where λ represents the center wavelength of the light source.
2. The method for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pellet according to claim 1, which is characterized in that: in the step (1-1), a differential confocal high-precision positioning technology is utilized to accurately position the target pill to be detected and the interference camera respectively, so that the outer surface of the target pill is focused and imaged on the interference camera in an accurate manner, and the steps are as follows:
(2-1) accurately positioning the outer surface of the target pellet at the cat eye position: axially scanning the target pill at the cat eye position, and obtaining a differential confocal response curve as shown in formula (3) by subtracting the light intensity differential detected by the first detector and the second detector,
wherein, IDRepresenting differential confocal light intensity, u representing normalized axial displacement, uMRepresenting the normalized defocus amount of the detector; adjusting the axial position of the target pill to make the intensity of the differential confocal response curve zero;
(2-2) positioning the interference camera at the outer surface conjugate imaging position: moving the target pill from the cat eye position to the direction close to the measuring objective lens by a distance d to ensure that the distance between the focus point of the reflected light beam on the outer surface and the cat eye position is just the outer diameter of the target pill, and the reflected measuring light is converged on the target surface of the interference camera; the interference camera is axially scanned and the axial position of the interference camera is recorded, a virtual pinhole is arranged on the interference camera, the confocal response curve can be detected by taking the integral of all pixel gray values in the virtual pinhole as the detection light intensity,
wherein ICRepresents the confocal light intensity, u represents the normalized axial displacement; bilateral fitting differential processing or sinc on confocal response curve2Fitting processing can accurately obtain the axial position coordinates of the vertex of the interference camera, and accurately adjusting the interference camera to the coordinate position;
(2-3) accurately positioning the outer surface of the target pellet at a confocal position: and continuously moving the target pill to the direction close to the measuring objective lens to the position close to the confocal position and axially scanning the target pill, wherein the differential confocal detection system can detect a differential confocal response curve, and the axial position of the target pill is adjusted to ensure that the intensity of the differential confocal response curve is zero.
3. The method for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pellet according to claim 1, which is characterized in that: the phase shift in the step (1-2) can also be obtained by a synchronous transient phase shift technology, namely, the reference mirror is not required to be driven in an axial stepping manner to carry out mechanical phase shift;
the synchronous transient phase shifting method comprises the following steps: a quarter-wave plate is added in an imaging light path, every 4 pixels of the interference camera are divided into a group, polarizing plates of 0 degree, 45 degrees, 90 degrees and 135 degrees are sequentially placed in front of each group of pixels, and then 4 frames of phase-shifting interference images with phase-shifting quantity of 90 degrees can be extracted from one frame of interference image.
4. The method for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pellet as claimed in claim 2, wherein the method comprises the following steps: the bilateral fitting differential processing of the confocal response curve in the step (2-2) comprises the following steps: selecting data points with relative intensity between 0.45 and 0.65 at two sides of the confocal curve, and respectively fitting two straight lines lAAnd lBTo make two straightObtaining a differential confocal straight line l by subtracting line differential differencesdcTo find ldcI.e. the vertex axial position coordinate of the confocal curve.
5. The method for detecting the defects on the outer surface of the differential confocal fixed-surface interference target pellet as claimed in claim 2, wherein the method comprises the following steps: carrying out sinc on the confocal response curve in the step (2-2)2The fitting process steps are as follows: selecting data points near the vertex of the confocal curve according to sinc2And fitting the function to obtain the vertex axial coordinate of the fitting curve, namely the vertex axial position coordinate of the confocal curve.
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