Normal tracking post-split pupil differential confocal freeform surface measurement method and device

Abstract

The normal tracking post-split pupil differential confocal freeform surface measurement method and device disclosed by the application belong to the field of optical precision detection.The application comprises a sensor probe, a three-dimensional motion control system and a computer software processing module, and adopts nanometer-level driving to realize positioning of the object table and the sensor probe.The sensor probe is a post-split pupil differential confocal sensor probe.Under the premise of optimizing the structure of the laser differential confocal measurement system, the normal tracking technology based on the position sensitive detector (PSD) feedback is combined with the post-split pupil differential confocal technology, the normal tracking system based on the position sensitive detector (PSD) feedback is used to track the measured freeform surface sample in a large-angle range, the post-split pupil differential confocal sensor is used to axially focus the measured freeform surface sample with high precision and high stability, and large-inclination high-precision freeform surface measurement is realized.The application can reduce the difficulty and cost of optical path adjustment, and has the advantages of anti-inclination, high precision and high stability.

Description

TECHNICAL FIELD

[0001] The application relates to a method and device for high-precision post-split pupil differential confocal large-inclination high-precision measurement of a free-form surface by normal tracking technology, and belongs to the field of optical precision detection. BACKGROUND

[0002] A free-form optical element has a large surface topography freedom, can easily eliminate aberration in an imaging system, and has the advantages of improving imaging quality of an optical system, improving resolution, increasing an action distance, simplifying an instrument structure, reducing an instrument volume and weight, and improving reliability, and has been more and more widely applied. However, the free-form surface increases the design freedom, and puts forward higher requirements for optical design, processing and detection. With successful application of optical CAD and numerical control diamond point processing technology in optical design and manufacturing, the design and processing of the free-form surface are no longer the main technical obstacles, and a high-precision measurement method becomes a key research direction. The free-form optical surface has the characteristics of poor roughness, sharp change of an inclination angle and irregular normal direction, and therefore, it is of great significance to study a high-precision detection method for the free-form optical surface with large inclination and anti-reflectivity and anti-roughness.

[0003] At present, free-form optical surface measurement methods mainly include a probe method, a laser confocal / differential confocal detection method, an interference measurement method, a phase measurement method and a Hartmann wavefront detection method.

[0004] The probe method can be divided into a contact probe method and a non-contact optical probe method. The contact probe method directly contacts a mechanical probe and a sample surface, has the characteristics of high scanning precision and fast scanning speed, but the mechanical probe has contact force on the sample, and the optical element surface is easily scratched, which seriously affects the precision of the optical element. The non-contact optical probe method has high measurement precision and will not damage the optical element surface, but when the surface slope of the free-form surface is large, the dense interference fringes increase the detection difficulty, and the non-contact optical probe method is easily affected by the surface inclination and has low measurement precision.

[0005] The interference method mainly includes zero-position interference measurement and non-zero-position interference measurement. The interference method has high sensitivity, and the theoretical limit of axial positioning can reach 1nm, but the measurement environment is harsh, and the interference method is easily affected by the differences in characteristics such as the surface inclination and roughness of the sample, and the practical engineering application is greatly limited.

[0006] The phase measurement method mainly uses a phase measurement deflection method. The system parameter calibration process of the method is complex, and the calibration precision greatly limits the measurement precision of the method. In addition, when the sample surface slope changes greatly, the method has high requirements for the performance of a CCD.

[0007] The Shack-Hartmann wavefront sensor splices the slope information of the whole aperture by collecting the information of each sub-aperture, has the advantages of large sampling density, short detection time, high measurement accuracy and high sensitivity, but has a small measurement dynamic range and is not suitable for detecting a freeform surface sample with large surface slope change.

[0008] The confocal method has the ability to resist the measurement of the tilt angle, but the detection of the focal point position is not sensitive, which limits the measurement resolution. The laser differential confocal detection technology has high measurement accuracy and high resistance to tilt angle measurement, and has high detection accuracy in a large range of surface tilt angle, but the method needs to use two detectors with consistent defocusing amount, has large measurement error and difficult system adjustment.

[0009] In summary, the measurement accuracy of the existing measurement method is greatly affected by the differences in sample surface roughness, fluctuation, tilt angle and other characteristics, which is the main technical bottleneck for improving the freeform surface profile measurement accuracy. SUMMARY

[0010] In order to overcome the shortcomings of the prior art, the main purpose of the present application is to provide a normal tracking post-split pupil differential confocal freeform surface measurement method and device, which realizes high-precision measurement of a freeform surface sample with large tilt angle by combining a normal tracking system based on a PSD with a post-split pupil differential confocal measurement system, reduces the difficulty and cost of optical path adjustment, and has the advantage of high test efficiency.

[0011] The purpose of the present application is achieved by the following technical solutions.

[0012] The normal tracking post-split pupil differential confocal freeform surface measurement method disclosed in the present application combines the normal tracking technology based on a position sensitive detector (PSD) feedback with the post-split pupil differential confocal technology, uses the normal tracking system based on the PSD feedback to track the normal direction of the measured freeform surface sample in a large angle range, uses the post-split pupil differential confocal sensor to focus on the measured freeform surface sample in the axial direction with high precision and high stability, and thus realizes high-precision measurement of the freeform surface with large tilt angle.

[0013] The normal tracking post-split pupil differential confocal freeform surface measurement method comprises the following steps:

[0014] Step one, the measured freeform surface sample is placed on a stage, the pose adjustment of the measured freeform surface sample is performed to eliminate the tilt and eccentricity, and the three-dimensional motion control system is used to realize the nanometer-level accurate displacement scanning of the measured freeform surface sample;

[0015] Step two, the light beam generated by the laser light source is converted into a parallel light beam by a beam expander, the reflected light of the parallel light beam passes through a 1 / 4 wave plate and an objective lens to irradiate on the measured freeform surface sample;

[0016] Step three, the reflected light of the measured free-form surface sample successively passes through the objective lens, the 1 / 4 wave plate, and then enters the light splitter through the transmission beam of the polarizing beam splitter, and the reflected beam of the light splitter enters the photosensitive area for spot centroid position detection, and the transmission beam of the light splitter converges through the collection lens, half of the light beam converged by the collection lens is blocked by the D-shaped diaphragm, and the other half of the light beam passes through the D-shaped diaphragm, enters the relay lens for magnification, and is received by the two-quadrant photodetector after passing through the physical double pinhole;

[0017] Step four, the computer calculates the distance ΔL of the PSD target surface light spot centroid deviating from the target surface center according to the PSD signal strength detected by the data acquisition module, and the geometric relationship ΔL = f*tan 2β (where f is the focal length of the objective lens) can be calculated according to the optical principle, so as to calculate the angle of the sensor probe deviating from the normal direction of the measured point of the free-form surface sample Then the controller controls the sensor probe to rotate by a corresponding angle β, so that the PSD light spot centroid is always at the target surface center, the sensor probe measurement beam axis direction is consistent with the normal direction of the local inclination angle of the surface of the free-form surface sample to be detected, and the normal tracking of the sensor probe is realized.

[0018] Step five, the computer controls the sensor probe to move up and down through the controller, so that the focal point of the measurement objective lens moves up and down on the profile surface of the measured free-form surface sample, the computer reads the signal of the photodetector in the sensor probe through the data acquisition module, and then the front focus signal and the rear focus signal of the detected differential confocal signal are normalized and subtracted to obtain the rear split pupil differential confocal axial intensity curve, and the objective lens axial position corresponding to the zero point of the measured differential confocal axial intensity curve is the axial coordinate of the measured free-form surface sample, and the axial position measurement of the measurement point M is completed by using the rear split pupil differential confocal measurement method on the premise of realizing the normal tracking in step four;

[0019] Step six, after completing the axial position measurement of the measurement point M, steps one to five are repeated to measure the axial position of the next coordinate point of the measured free-form surface sample, and scanning measurement is performed according to the preset scanning trajectory, until all scanning points are completed, and large-angle high-precision measurement of the three-dimensional profile of the free-form surface is realized.

[0020] The application also discloses a normal tracking rear split pupil differential confocal free-form surface measurement device for realizing the normal tracking rear split pupil differential confocal free-form surface measurement method.

[0021] The normal tracking rear split pupil differential confocal free-form surface measurement device adopts nanometer-level driving technology to realize nanometer resolution positioning of the objective table and the sensor probe.

[0022] The sensor probe is a rear split pupil differential confocal sensor probe, comprising a laser light source, a beam expander, a polarization beam splitter, a 1 / 4 wave plate, an objective lens, a beam splitter, a PSD, a collection lens, a D-shaped diaphragm, a relay lens, double pinholes, and a two-quadrant photodetector.

[0023] The laser light source generates a laser beam, and the beam expander and the polarization beam splitter are placed in sequence along the direction of the laser beam. The 1 / 4 wave plate and the objective lens are placed in sequence along the direction of the reflected light beam of the polarization beam splitter. The control system makes the focal point of the objective lens on the surface of the sample. The measurement light beam reflected by the measured free-form surface sample is detected by the objective lens, and the 1 / 4 wave plate, the PSD, the beam splitter, and the collection lens are placed in sequence along the direction of the measurement light beam. The measurement light beam enters the PSD photosensitive area through the reflected light beam of the beam splitter to detect the position of the spot centroid, and the normal tracking of the sensor probe is realized through computer software algorithm processing. The transmitted light of the measurement light beam through the beam splitter enters the collection lens for convergence. The D-shaped diaphragm is placed between the collection lens and the focal point of the collection lens. The relay lens is placed behind the collection lens for beam amplification. The focal points of the collection lens and the relay lens coincide. Double physical pinholes and a photodetector are placed in sequence behind the relay lens. The double pinholes are placed on the focal plane of the relay lens, and the position of the photodetector must be able to collect all the light intensity through the double pinholes to detect the front focal signal and the back focal signal of the transverse differential confocal signal.

[0024] The three-dimensional motion control system comprises an R-direction motion control module, a Z-direction motion control module, a sensor shaft motion control module, a gas floating rotary spindle motion control module, and a voice coil motor motion control module.

[0025] The R-direction motion control module carries the Z-direction motion control module, the sensor shaft motion control module, and the sensor probe motion control module. The Z-direction motion control module carries the sensor shaft and the sensor probe motion control module. The rear split pupil differential confocal sensor probe is carried on the end face of the sensor shaft. The R-direction motion control module, the Z-direction motion control module, and the sensor shaft maintain a pairwise orthogonal relationship. The R-direction motion control module drives the sensor probe to move transversely in the normal measurement plane, and the Z-direction motion control module drives the sensor probe to move axially in the normal measurement plane, so as to realize large-stroke two-dimensional displacement movement of the rear split pupil differential confocal probe in the measurement plane.

[0026] The gas floating rotary spindle is fixed on the marble reference table, the object table is fixed on the end face of the gas floating rotary spindle, and the rotary motion of the gas floating rotary spindle drives the measured free-form surface sample carried by the object table to move accurately.

[0027] The sensor probe is fixed on the end face of the sensor shaft, and the angular rotation of the sensor shaft realizes the normal position tracking of the sensor probe on the measured point of the measured free-form surface sample.

[0028] The objective lens is fixed on the voice coil motor motion control module of the sensor probe. The controller controls the movement of the voice coil motor motion control module to achieve the axial micro-displacement of the objective lens.

[0029] The computer software processing module includes a data acquisition module and control software. The computer, through a controller, achieves three-dimensional nanometer-level motion displacement control of the system. Based on the acquired PSD signal, it processes the algorithm to control the sensor shaft to achieve normal tracking of the sensor probe. Based on the acquired rear-positioned pupil differential confocal signal, it controls the sensor probe to perform axial fixed-focus tracking. The computer processes the acquired signals using algorithms, thereby achieving large-angle, high-precision measurement of the surface profile of the freeform sample.

[0030] Beneficial effects:

[0031] 1. The present invention discloses a method and apparatus for measuring freeform surfaces using a normal-tracking rear-pupillary differential confocal method. The rear-pupillary differential confocal method utilizes a D-type aperture to convert the defocusing change of the sample in the freeform surface measurement system into a lateral movement of the light spot on the focal plane. Then, it uses off-axis symmetrically distributed dual physical pinholes to detect the intensity of the Ellie spot on the focal plane, enabling rapid detection of the pre-focus and post-focus intensity signals. Normalized differential subtraction processing is then performed to obtain a scatter-resistant, highly sensitive rear-pupillary lateral differential confocal curve. The zero point of the rear-pupillary lateral differential confocal curve is used to precisely focus the position of the measured freeform surface sample, achieving high-precision measurement of the freeform surface profile. Furthermore, it reduces the complexity of the system's optical path assembly and adjustment, lowers system cost, and facilitates system miniaturization and efficiency improvement.

[0032] 2. The normal tracking rear-positioned split pupil differential confocal freeform surface measurement method and device disclosed in this invention utilizes a normal tracking system based on PSD feedback to ensure that the measurement system beam is always perpendicularly focused on the surface of the freeform sample being measured, and that the reflected beam and the measurement beam are coaxial, which is beneficial for maintaining high sensitivity and focusing capability of differential confocal detection over a large angle range.

[0033] 3. The normal tracking rear-position split pupil differential confocal freeform surface measurement method and device disclosed in this invention utilizes the R-axis motion control module and the Z-axis motion control module to realize the large-stroke two-dimensional linear motion of the sensor probe, the sensor shaft motion control module to drive the sensor probe to rotate and perform normal position tracking, the air-bearing rotary spindle to realize the rotational position movement of the freeform surface sample under test, and the voice coil motor motion control module to realize the axial micro-displacement movement of the sensor probe, ultimately realizing the three-dimensional contour scanning of the freeform surface.

[0034] 4. The normal tracking rear-position split pupil differential confocal freeform surface measurement method and device disclosed in this invention utilizes a computer to realize normal tracking and axial focus tracking of the freeform surface under test through a three-dimensional motion control system device, which has high testing efficiency. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the overall measurement device for a normal tracking rear-positioned beam splitting pupil differential confocal freeform surface disclosed in this invention;

[0036] Figure 2 This is a schematic diagram of the optical path of the present invention;

[0037] Figure 3 This is a schematic diagram of the PSD optical path of the present invention;

[0038] Figure 4 This is a schematic diagram of the front focal length, rear focal length, and differential confocal signal curves of the present invention;

[0039] Figure 5 This is a control block diagram of the present invention;

[0040] Figure 6 This is a schematic diagram of the scanning process of the present invention;

[0041] In the diagram: 1-R-axis motion control module, 2-Z-axis motion control module, 3-sensor shaft motion control module, 4-sensor probe, 5-stage, 6-air-bearing rotary spindle motion control module, 7-marble reference platform, 8-free-form surface sample under test, 9-laser source, 10-beam expander, 11-polarizing beam splitter, 12-1 / 4 wave plate, 13-voice coil motor motion control module, 14-objective lens, 15-beam splitter, 16-PSD, 17-collecting lens, 18-D-type aperture, 19-relay lens, 20-double pinhole, 21-two-quadrant photodetector, 22-computer, 23-controller, 24-data acquisition module, 25-incident laser, 26-front focal signal, 27-back focal signal, 28-differential confocal axial intensity curve. Detailed Implementation

[0042] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0043] As shown in Figure 1, the specific implementation steps of the normal tracking post-split pupil differential confocal freeform surface measurement method and apparatus disclosed in this embodiment are as follows:

[0044] Step 1: The freeform surface sample 8 to be tested is placed on the stage 5 driven by the air-bearing rotary spindle 6. The sample 8 is adjusted to remove tilt and eccentricity. The computer 22 controls the air-bearing rotary spindle 6 through the controller 23 to realize the rotational motion of the sample 8. The computer 22 controls the motion system to perform R-axis and Z-axis motion through the controller to realize the large-stroke two-dimensional linear nanometer-level precise displacement movement of the sensor probe.

[0045] Step 2: The objective lens 14 of the sensor probe 4 detects the freeform surface sample 8 under test. The linearly polarized light emitted by the laser source 9 is emitted through the beam expander 10 and reflected by the polarizing beam splitter 11. It then passes through the quarter wave plate 12 and the objective lens 14 in sequence to illuminate the freeform surface sample 8 under test. The reflected light from the sample 8 then passes through the quarter wave plate 12, the polarizing beam splitter 11 in sequence, and enters the beam splitter 15 for beam splitting. The reflected beam after beam splitting by the beam splitter 15 enters the photosensitive area of ​​the PSD 16 to detect the centroid position of the light spot. The transmitted beam after beam splitting by the beam splitter 15 is converged by the collecting lens 17. Half of the beam is blocked by the D-type aperture 18, and the other half passes through the aperture 18 to enter the relay lens 19 for amplification. Then, the intensity of the Ellie spot on the focal plane is detected by the off-axis symmetrically distributed physical double pinholes 20 and the two-quadrant photodetector 21. The focal point of the collecting lens coincides with the focal point of the relay mirror. The double pinhole 20 is placed on the focal plane of the relay mirror 19. The position of the photodetector 21 must be able to collect all the light intensity transmitted through the double pinhole 20 in order to detect the front focal signal 26 and the back focal signal 27 curves of the differential confocal signal. By performing normalized differential subtraction, the differential confocal axial intensity curve 28 can be obtained. The axial position of the zero point of the differential confocal axial intensity curve 28 is the surface profile of the freeform surface sample 8 under test.

[0046] When the freeform surface sample 8 is not tilted, the light beam reflected from the surface of the sample 8 is collected by the objective lens 14, and the light spot reaching the target surface of PSD16 is a uniform circular spot with its centroid located at the center of the target surface, meaning the distance between the centroid and the target surface center is zero. When the freeform surface sample 8 is tilted, the light beam reflected from the tilted sample is collected by the objective lens 14, and the light spot reaching the target surface of PSD16 is an elliptical spot with uniform intensity distribution, but the centroid of the spot will deviate from the target surface center by a distance ΔL, where PSD is a position-sensitive detector.

[0047] Step 3: The computer software 22 calculates the distance ΔL between the centroid of the PSD16 target spot and the center of the target surface, based on the signal intensity detected by the data acquisition module 24. This allows the computer to calculate the angle at which the sensor probe 4 deviates from the normal direction of the measured point. The computer then controls the sensor shaft 3 to rotate the sensor probe 4 by the corresponding angle, ensuring that the optical axis of the measuring beam of the sensor probe 4 is aligned with the normal direction of the local tilt angle of the surface of the freeform sample 8 being measured. The centroid of the PSD16 spot remains at the center of the target surface, thus achieving normal tracking of the sensor probe 4.

[0048] Step 4: The computer 22 reads the output signal of the photodetector 21 in the rear-mounted pupil differential confocal module through the data acquisition module 24, and controls the rear-mounted pupil sensor probe 4 to make nanometer-level axial movement through the controller 23 so that the measured point of the freeform surface sample 8 is at the focal position of the objective lens 14. The signal intensity detected by the two photodetectors 21 is the maximum and similar, thereby realizing axial fixed-focus tracking of the measured freeform surface sample 8. The signal processing obtains the differential confocal axial response curve 28. Under the premise of normal tracking achieved in step 3, the axial position measurement of the measurement point M of the measured freeform surface sample 8 is realized according to the zero point of the differential confocal axial curve 28.

[0049] Step 5: After completing the axial position measurement of measurement point M, repeat steps one to five to measure the axial position of the next coordinate point of the freeform surface sample 8, according to... Figure 6 The preset scanning trajectory is used for scanning and measurement until all scanning points are completed, thereby achieving high-precision measurement of the large tilt angle of the three-dimensional surface profile of the freeform surface.

[0050] like Figure 1 As shown, this embodiment also discloses a normal tracking rear-pupillary differential confocal freeform surface measurement device, which is used to realize the above-mentioned normal tracking rear-pupillary differential confocal freeform surface measurement method. The normal tracking rear-pupillary differential confocal freeform surface measurement device adopts nanoscale driving technology to realize nanoscale resolution positioning of the stage 5 and the sensor probe 4.

[0051] The normal tracking rear-positioned split pupil differential confocal freeform surface measurement device includes a sensor probe 4, a three-dimensional motion control system, and a computer software processing module.

[0052] The sensor probe 4 is a rear-mounted beam splitter differential confocal sensor probe, including a laser source 9, a beam expander 10, a polarizing beam splitter 11, a quarter-wave plate 12, an objective lens 14, a beam splitter 15, a PSD 16, a collecting lens 17, a D-type aperture 18, a relay lens 19, a double pinhole 20, and a two-quadrant photodetector 21.

[0053] A laser light source 9 generates a laser beam. A beam expander 10 and a polarizing beam splitter 11 are placed sequentially along the laser beam direction. A quarter-wave plate 12 and an objective lens 14 are placed sequentially along the beam reflection direction of the polarizing beam splitter 11. The control system 23 focuses the objective lens 14 onto the surface of the sample 8. The measurement beam reflected from the freeform surface sample 8 is detected by the objective lens 14. A quarter-wave plate 12, a PSD 16, a beam splitter 15, and a collecting lens 17 are placed sequentially along the measurement beam direction. The measurement beam, reflected by the beam splitter 15, enters the photosensitive area of ​​the PSD 16 for spot centroid position detection. The computer 22 uses software algorithms to achieve normal tracking of the sensor probe 4. The measurement beam is transmitted through the beam splitter 15 and then converged into the collecting lens 17. A D-type aperture 18 is placed between the collecting lens 17 and its focal point. A relay lens 19 is placed behind the collecting lens 17 to amplify the beam. The focal point of the collecting lens 17 coincides with the focal point of the relay lens 19. Behind the relay lens 19, a dual physical pinhole 20 and a photodetector 21 are placed in sequence. The dual pinhole 20 is placed on the focal plane of the relay lens 19. The photodetector 21 must be positioned to collect all the light intensity transmitted through the dual pinhole 20 in order to detect the front focal signal 26 and the back focal signal 27 of the lateral differential confocal signal.

[0054] The three-dimensional motion control system includes an R-axis motion control module 1, a Z-axis motion control module 2, a sensor shaft motion control module 3, an air-bearing rotary spindle motion control module 6, and a voice coil motor motion control module 13.

[0055] The R-axis motion control module 1 carries the Z-axis motion control module 2, the sensor shaft motion control module 3, and the sensor probe motion control module 4; the Z-axis motion control module 2 carries the sensor shaft 3 and the sensor probe motion control module 4; the rear-positioned differential confocal sensor probe 4 is mounted on the end face of the sensor shaft 3; the R-axis motion control module 1, the Z-axis motion control module 2, and the sensor shaft 3 maintain a pairwise orthogonal relationship; the R-axis motion control module 1 drives the sensor probe 4 to perform lateral movement in the normal measurement plane, and the Z-axis motion control module 2 drives the sensor probe 4 to perform axial movement in the normal measurement plane, so as to realize the large-stroke two-dimensional displacement movement of the rear-positioned differential confocal sensor probe in the measurement plane.

[0056] The air-bearing rotary spindle 6 is fixed on the marble reference platform 7, and the stage 5 is fixed on the end face of the air-bearing rotary spindle 6. The rotational motion of the air-bearing rotary spindle 6 will cause the stage 5 to carry the tested free-form surface sample 8 to move together in a precise rotational displacement.

[0057] The objective lens 14 is fixed on the voice coil motor motion control module 13 of the sensor probe 4. The controller 23 controls the movement of the voice coil motor motion control module 13 to realize the axial micro-displacement movement of the objective lens 14.

[0058] The computer 22 software processing module includes signal acquisition and processing 24 and software control. The computer 22 realizes three-dimensional nanometer-level motion displacement control of the system through the controller 23. Based on the acquired PSD signal, it controls the sensor shaft 3 to achieve normal tracking of the sensor probe 4 through algorithm processing. Based on the acquired rear-positioned pupil differential confocal signal, it controls the sensor probe 4 to perform axial fixed-focus tracking. The computer 22 processes the acquired signals through algorithms, thereby realizing large-angle high-precision measurement of the surface contour of the freeform surface sample 8.

[0059] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for measuring a free-form surface with a normal tracking rear-positioned split pupil and differential confocal surface, characterized in that: The freeform surface sample (8) under test is tracked over a large angle range using a normal tracking system with PSD feedback. The freeform surface sample (8) under test is then axially focused with high precision and high stability using a rear-mounted split-pupil differential confocal sensor. This achieves high-precision measurement of the freeform surface profile at large tilt angles. The process includes the following steps: Step 1: Place the freeform surface sample (8) to be tested on the stage (5), and adjust the pose of the freeform surface sample (8) to remove tilt and eccentricity. The three-dimensional motion control system realizes the nanometer-level precise displacement scanning of the freeform surface sample (8). Step 2: The beam generated by the laser source (9) becomes a parallel beam through the beam expander (10). The parallel beam is reflected by the polarizing beam splitter (11) and then passes through the quarter wave plate (12) and the objective lens (14) to illuminate the freeform surface sample (8) to be tested. Step 3: The reflected light from the freeform surface sample (8) passes through the objective lens (14) and the quarter-wave plate (12) in sequence, and then the transmitted beam from the polarizing beam splitter (11) enters the beam splitter (15) for beam splitting. The reflected beam from the beam splitter (15) enters the photosensitive area (16) for spot centroid position detection. The transmitted beam from the beam splitter (15) is converged by the collecting lens (17). Half of the beam converged by the collecting lens (17) is blocked by the D-type aperture (18), and the other half of the beam enters the relay mirror (19) through the D-type aperture (18) for amplification. Then, it is received by the two-quadrant photodetector (21) after passing through the physical double pinhole (20). Step 4: The computer (22) calculates the distance ΔL between the centroid of the PSD target spot and the target center by detecting the PSD signal intensity through the data acquisition module (24). Based on optical principles, the existing geometric relationship ΔL=f*tan 2β is calculated, where f is the focal length of the objective lens. Thus, the angle between the sensor probe and the normal direction of the measured point is calculated. Then the control system (23) controls the sensor probe (4) to rotate by the corresponding angle β, so that the centroid of the PSD spot is always in the center of the target surface, so that the direction of the optical axis of the measuring beam of the sensor probe (4) is consistent with the direction of the local tilt angle normal of the surface of the freeform sample being detected, and the normal tracking of the sensor probe (4) is realized. Step 5: The computer (22) controls the sensor probe (4) to move up and down through the control system (23), so that the focal point of the measuring objective (14) moves up and down on the contour surface of the freeform sample (8) to be measured. The computer (22) reads the signal of the photodetector (21) in the sensor probe (4) through the data acquisition module (24), and then normalizes and subtracts the front focal signal (26) and the back focal signal (27) of the detected differential confocal signal to obtain the back pupil differential confocal axial intensity curve (28). The objective axial position corresponding to the zero point of the measured differential confocal axial intensity curve (28) is the axial coordinate of the freeform sample to be measured. Under the premise of normal tracking in step 4, the axial position measurement of the measurement point M is completed by using the back pupil differential confocal measurement method. Step 6: After completing the axial position measurement of measurement point M, repeat steps 1 to 5 to measure the axial position of the next coordinate point of the freeform surface sample. Perform scanning measurement according to the preset scanning trajectory until all scanning points are completed, so as to achieve high-precision measurement of the large tilt angle of the three-dimensional surface profile of the freeform surface.

2. A device for measuring a free-form surface with a normal-tracking rear-positioned beam splitting pupil and differential confocal focus, used to implement the above-mentioned method for measuring a free-form surface with a normal-tracking rear-positioned beam splitting pupil, characterized in that: Includes sensor probe (4), three-dimensional motion control system, and computer software processing module; The sensor probe includes a laser source (9), a beam expander (10), a polarizing beam splitter (11), a quarter wave plate (12), an objective lens (14), a beam splitter (15), a PSD (16), a collecting lens (17), a D-type aperture (18), a relay lens (19), a double pinhole (20), and a two-quadrant photodetector (21). A laser light source (9) generates a laser beam. A beam expander (10) and a polarizing beam splitter (11) are placed sequentially along the direction of the laser beam. A quarter-wave plate (12) and an objective lens (14) are placed sequentially along the direction of the reflected beam of the polarizing beam splitter (11). The control system (23) focuses the objective lens (14) on the surface of the freeform surface sample (8) to be measured. The measurement beam reflected by the freeform surface sample (8) to be measured is detected by the objective lens (14). A quarter-wave plate (12), a PSD (16), a beam splitter (15), and a collecting lens (17) are placed sequentially along the direction of the measurement beam. The measurement beam enters the photosensitive area of ​​the PSD (16) through the reflected beam of the beam splitter (15) to detect the centroid position of the spot. The computer (22) uses software algorithms to process and realize the sensor. Normal tracking of probe (4); the transmitted light of the measuring beam through beam splitter (15) enters the collecting lens (17) for convergence, D-type aperture (18) is placed between the collecting lens (17) and the focal point of the collecting lens, and a relay mirror (19) is placed behind the collecting lens (17) to amplify the beam. The focal point of the collecting lens (17) coincides with the focal point of the relay mirror (19). A double physical pinhole (20) and a photodetector (21) are placed behind the relay mirror (19) in sequence. The double pinhole (20) is placed on the focal plane of the relay mirror (19). The position of the photodetector (21) must be able to collect all the light intensity transmitted through the double pinhole (20) in order to detect the front focal signal (26) and the back focal signal (27) of the lateral differential confocal signal. A three-dimensional motion control system is characterized by comprising an R-axis motion control module (1), a Z-axis motion control module (2), a sensor shaft (3) motion control module, an air-bearing rotary spindle (6) motion control module, and a voice coil motor motion control module (13). The R-axis motion control module (1) carries the Z-axis motion control module (2), the sensor shaft (3) motion control module, and the sensor probe (4); the Z-axis motion control module (2) carries the sensor shaft (3) and the sensor probe (4); the sensor shaft (3) has the sensor probe (4) on its end face; the R-axis motion control module (1), the Z-axis motion control module (2), and the sensor shaft (3) maintain a pairwise orthogonal relationship; the R-axis motion control module (1) drives the sensor probe (4) to perform lateral movement in the normal measurement plane, and the Z-axis motion control module (2) drives the sensor probe (4) to perform axial movement in the normal measurement plane, so as to realize the large-stroke two-dimensional displacement movement of the rear-mounted beam splitting pupil differential confocal probe in the measurement plane; The air-bearing rotary spindle (6) is fixed on the marble reference platform 7, and the stage (5) is fixed on the end face of the air-bearing rotary spindle (6). The rotational motion of the air-bearing rotary spindle (6) will cause the stage (5) to carry the tested free-form surface sample (8) to move together in a precise rotational displacement. The sensor probe (4) is fixed on the end face of the motion control module of the sensor shaft (3). The angle rotation of the motion control module of the sensor shaft (3) will enable the sensor probe (4) to track the normal position of the measured point of the freeform surface sample (8). The objective lens (14) is fixed on the voice coil motor motion control module (13) of the sensor probe (4). The control system (23) controls the voice coil motor motion control module (13) to move, thereby realizing the axial micro-displacement movement of the objective lens (14). The computer software processing module includes a data acquisition module (24) and control software. The computer (22) realizes the three-dimensional nanoscale motion displacement control of the system through the control system (23). According to the acquired PSD signal, the sensor shaft (3) is controlled by the algorithm to realize the normal tracking of the sensor probe (4). According to the acquired rear-positioned pupil differential confocal axial intensity curve (28), the sensor probe (4) is controlled to perform axial fixed-focus tracking. The computer (22) performs algorithm processing on the acquired signal, thereby realizing the large-angle high-precision measurement of the surface contour of the free-form surface sample (8) under test.

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