Off-axis angle precision measurement method

Abstract

The invention provides an off-axis angle precision measurement method, relates to the technical field of optical measurement, and solves the problems that in the prior art, a mechanical contact type measurement method easily scratches a mirror surface and is low in resolution, an optical interference method depends on expensive elements and is easily interfered by the environment, and an autocollimator rotary table method is limited in measurement range and low in measurement cost. And a laser tracker multi-point calibration method is high in equipment cost and complex in data processing. According to the technical scheme, a photoelectric theodolite, an off-axis mirror and a laser interferometer are coaxially installed, a reference coordinate system is established through the photoelectric theodolite, it is ensured that light beams of the laser interferometer always enter the center of the off-axis mirror by means of a six-dimensional adjusting platform, a detection light path is established, and space coordinates of a calibration point are secondarily measured through the photoelectric theodolite; solving an off-axis angle value by combining a space coordinate transformation algorithm; the device and the method have the advantages of high measurement precision, avoidance of mirror surface damage, high environmental adaptability, large measurement range, high automation degree and the like, and are suitable for off-axis angle precision measurement requirements in optical system manufacturing and debugging.

Description

Technical Field

[0001] The present invention relates to the technical field of optical measurement, and particularly to a method for precisely measuring the off-axis angle. Background Art

[0002] During the manufacturing and debugging processes of optical systems, the precise measurement of the off-axis angle is a crucial link to ensure the performance and quality of optical components. The measurement accuracy of the off-axis angle directly affects the imaging quality of optical systems, the beam alignment accuracy, and the overall performance. Traditional methods for measuring the off-axis angle mainly include mechanical contact measurement methods, optical interference methods, autocollimator turntable methods, and laser tracker multi-point calibration methods, etc.

[0003] The mechanical contact measurement method directly contacts the mirror surface through a coordinate measuring machine or a probe, and calculates the off-axis angle by fitting the data. However, this method is limited by mechanical processing accuracy and assembly errors, and it is difficult to meet the high-precision measurement requirements at the micron or even sub-micron level. In addition, mechanical contact may damage the surface of the measured optical component, affecting its optical performance.

[0004] The optical interference method uses a laser interferometer and a reference mirror to analyze the phase change of interference fringes, thereby realizing high-precision measurement of the off-axis angle. Although this method has high precision, it relies on expensive customized compensation components (such as computer-generated holograms CGH), and is extremely sensitive to environmental disturbances, easily resulting in interference fringe aliasing and affecting the stability of the measurement results.

[0005] The autocollimator turntable method calculates the off-axis angle by measuring the offset of the optical axis, but its measurement range is usually small (usually less than 5°), and it relies on a high-precision turntable and the assumption of an ideal surface shape, which limits its wide application in practical situations.

[0006] The laser tracker multi-point calibration method calculates the off-axis angle through spatial coordinate transformation. Although it can achieve measurements over a large range, the equipment cost is high, the data processing is complex, and it requires adding marking points on the mirror surface, increasing the complexity of the operation and the risk of measurement errors.

[0007] In summary, the existing methods for measuring the off-axis angle have certain limitations in terms of accuracy, cost, operation complexity, and applicable range. Therefore, inventing a method for measuring the off-axis angle with high precision, low cost, simple operation, and wide applicable range is of great significance for improving the efficiency and quality of optical system manufacturing and debugging. Summary of the Invention

[0008] Therefore, the present invention solves the technical problems in the prior art that the traditional mechanical contact measurement method is prone to scratching the mirror and has low resolution, the optical interference method relies on expensive components and is easily affected by environmental interference, the autocollimator turntable method has a limited measurement range, and the laser tracker multi-point calibration method has high equipment cost and complex data processing; the present invention provides an off-axis angle precision measurement method, which realizes non-contact high-precision measurement; it has the advantages of high measurement accuracy, avoidance of mirror damage, strong environmental adaptability, large measurement range and high degree of automation, and is suitable for the off-axis angle precision measurement needs in optical system manufacturing and debugging.

[0009] The present invention provides a method for precise off-axis angle measurement, comprising the following steps:

[0010] Step 1: Strictly level the laser interferometer and the photoelectric theodolite and calibrate them coaxially to form a unified measurement reference axis; introduce the off-axis mirror to be measured between the optical paths of the two, and precisely adjust the spatial posture of the off-axis mirror through the six-dimensional adjustment platform to ensure that the mirror normal coincides with the reference axis;

[0011] Step 2: Arrange three high-reflectivity marking points on the off-axis mirror mounting platform, use the photoelectric theodolite to build a global coordinate system and measure the initial three-dimensional coordinates of the marking points, and simultaneously record the initial spatial posture parameters of the mirror body to provide geometric transformation benchmark data for subsequent off-axis angle calculation;

[0012] Step 3: Rotate the off-axis mirror to be measured around its central axis by a preset angle to introduce the off-axis value, and at the same time introduce a spherical reflector at the end of the optical path to construct a closed-loop interferometric detection optical path;

[0013] Step 4: Use laser interferometer to analyze the Zernike coefficients corresponding to the surface error in real time, focusing on the astigmatism terms Z5 / Z6 and the coma terms Z7 / Z8. In the off-axis angle precision measurement, the astigmatism terms Z5 / Z6 and the coma terms Z7 / Z8 are the "characteristic aberrations" of the off-axis mirror misalignment, and their amplitudes directly reflect the degree of deviation of the mirror attitude from the reference axis - Z5 / Z6 is caused by the tilt (pitch / yaw) of the mirror, and describes the astigmatism in the orthogonal direction; Z7 / Z8 is caused by the eccentricity (translation) of the mirror or the twisting around the optical axis, and manifests as asymmetric coma dispersion. Both are highly sensitive to the misalignment of the mirror such as tilt and eccentricity. During the measurement, the four low-order aberration coefficients are analyzed in real time by a laser interferometer. When the amplitude of Z5 / Z6 / Z7 / Z8 converges to near zero, it indicates that the mirror normal coincides with the reference axis. The six-dimensional adjustment stage is adjusted to perform multi-degree-of-freedom iterative adjustment. When the amplitude of the sensitive aberration coefficient converges to zero, the system reaches the optimal state of the wavefront, and the spatial position of the measured mirror is locked.

[0014] Step 5: Accurately align the optical axis of the photoelectric theodolite with the optimized mirror normal, remeasure the spatial coordinates of the preset marking point group on the tooling table, and solve the actual off-axis angle value through the rigid transformation matrix of the initial and final coordinate systems.

[0015] First, the optoelectronic theodolite, off-axis mirror and laser interferometer are coaxially installed, and a reference coordinate system is established through the optoelectronic theodolite to record the initial three-dimensional coordinates of the calibration points. When the off-axis mirror rotates around its central Z-axis, a six-degree-of-freedom adjustment platform is used to ensure that the beam of the laser interferometer is always incident on the center of the off-axis mirror, constructing a closed-loop interference detection optical path. The Zernike coefficients are calculated in real time by the laser interferometer and converted into geometric aberration parameters, and multi-degree-of-freedom adjustment is performed based on the misalignment-aberration sensitivity matrix until the system reaches the optimal wavefront quality. Finally, the spatial coordinates of the calibration points are measured twice by the optoelectronic theodolite, and the off-axis angle value is calculated by combining the spatial coordinate transformation algorithm.

[0016] Further, the angle measurement accuracy of the optoelectronic theodolite is better than 0.1 arcsecond, and the coordinate measurement accuracy is better than ±10 μm.

[0017] Further, the displacement measurement resolution of the laser interferometer is better than 0.1 nm, and the sampling frequency is not less than 100 Hz.

[0018] Further, the adjustment accuracy of the six-degree-of-freedom adjustment platform is at the nanometer level.

[0019] Further, when measuring the off-axis angle, the rotation angle is determined by calculating the change in the azimuth angle of the calibration points in the xy plane before and after rotation, and the final rotation angle is the average of the rotation angles of at least three calibration points.

[0020] Further, when performing coordinate transformation, if there is a position offset in the reinstallation of the optoelectronic theodolite, the translation vector is calculated by the centroid alignment method to correct the coordinates of the rotated calibration points for translation to the reference coordinate system.

[0021] The present invention has the following advantages compared with the prior art:

[0022] 1. The off-axis angle precision measurement method provided by the present invention realizes high-precision measurement of the off-axis angle through the high-precision spatial coordinate measurement of the optoelectronic theodolite and the high-sensitivity detection of the system misalignment by the laser interferometer. The measurement accuracy can reach the micron or even sub-micron level, meeting the strict requirements of modern optical systems for off-axis angle precision measurement.

[0023] 2. The off-axis angle precision measurement method provided by the present invention uses optical measurement and interference detection means throughout the measurement process, avoiding the contact operation of traditional mechanical measurement and not causing damage to the surfaces of optical components such as off-axis mirrors, protecting valuable optical components. During the rotation of the off-axis mirror, the laser interferometer calculates the system misalignment in real time, can timely detect and adjust the deviation of the system, and performs dynamic adjustment through the six-degree-of-freedom adjustment platform to ensure the accuracy and stability of the measurement process, improving the measurement efficiency and reliability.

[0024] 3. The off-axis angle precision measurement method provided by the present invention overcomes the problems of the traditional mechanical contact measurement method, such as easy scratching of the mirror surface, low resolution, the optical interference method relying on expensive components and being susceptible to environmental interference, the self-collimator turntable method having a limited measurement range, and the laser tracker multi-point calibration method having high equipment costs and complex data processing. This method has the advantages of high measurement accuracy, avoiding mirror surface damage, strong environmental adaptability, large measurement range, and high automation degree, and is suitable for the off-axis angle precision measurement requirements in the manufacturing and debugging of optical systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0026] Figure 1 It is a schematic diagram of the equipment installation and measurement system architecture of the present invention.

[0027] Figure 2 It is a schematic diagram of the optical path adjustment during the rotation of the off-axis mirror.

[0028] Figure 3 It is a diagram for verifying the coaxial relationship of the optoelectronic theodolite, off-axis mirror, and laser interferometer using a high-precision plane mirror.

[0029] Description of the reference numerals in the drawings:

[0030] 1. Laser interferometer; 2. Off-axis mirror; 3. Six-axis adjustment platform; 4. High-reflectivity marking point; 5. Optoelectronic theodolite; 6. High-precision spherical mirror; 7. High-precision plane mirror. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The following will clearly and completely describe the technical solutions of the present invention with reference to the drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0032] Embodiment 1

[0033] This embodiment relates to an off-axis angle precision measurement method, and the specific implementation is as follows:

[0034] First, the optoelectronic theodolite 5, off-axis mirror 2, laser interferometer 1, six-axis adjustment platform 3, and spherical mirror 6 are coaxially installed according to strict installation requirements, as Figure 1As shown, it shows the installation position relationship among the optoelectronic theodolite, off-axis mirror, laser interferometer, six-dimensional adjustment platform, and spherical mirror. Secondly, high-precision calibration tools and methods are used to ensure that the optical axes of each device are on the same axis, and the deviation is controlled within a very small range. Then, the off-axis mirror 2 is removed, and the parallel light emitted by the laser interferometer 1 is adjusted to be coaxial with the collimation axis of the optoelectronic theodolite 5. Finally, a high-precision plane mirror 7 is placed at the center position of the six-dimensional adjustment platform 3, and its surface shape is detected by the laser interferometer 1, and the translation of the adjustment table is adjusted to make the normal of the reflecting surface coincide with the optical axis.

[0035] Secondly, the establishment of the reference coordinate system and the initial calibration;

[0036] The optoelectronic theodolite 5 is used to establish the reference coordinate system, and the three-dimensional coordinates of the initial calibration points are accurately measured and the data is recorded. Three high-reflectivity marker points 4 are arranged on the mounting table of the off-axis mirror 2. These three marker points are non-collinearly distributed and far from the rotation center of the off-axis mirror 2 to improve the sensitivity of angle calculation.

[0037] Using the high-precision measurement function of the optoelectronic theodolite 5, the coordinates of these three marker points in the reference coordinate system are measured, and are respectively denoted as (x1, y1, z1), (x2, y2, z2), (x3, y3, z3), which serve as the basis for subsequent off-axis angle calculation.

[0038] Third, the rotation of the off-axis mirror and the adjustment of the optical path

[0039] The off-axis mirror 2 is precisely rotated around its central Z-axis, and at the same time, the six-dimensional adjustment platform 3 is adjusted in real time and dynamically according to the information fed back by the laser interferometer 1 to ensure that the parallel light beam emitted by the laser interferometer 1 always enters the geometric center of the off-axis mirror 2; as Figure 2 shown, it is a schematic diagram of the optical path adjustment during the rotation of the off-axis mirror, which details the optical path relationship between the beam emitted by the laser interferometer and the off-axis mirror and the spherical mirror.

[0040] The laser interferometer 1 calculates the Zernike coefficients in real time and obtains the geometric aberration parameters through the conversion relationship. Using the misalignment-aberration sensitivity matrix established by the optical simulation software, the misalignment component most sensitive to the residual aberration is analyzed.

[0041] Fourth, the calculation of aberration parameters and the adjustment optimization

[0042] During the adjustment process, focus on the Zernike coefficients corresponding to the astigmatism terms Z5 / Z6 and the coma terms Z7 / Z8. By continuously adjusting the six degrees-of-freedom parameters of the six-dimensional adjustment platform 3, the amplitudes of the sensitive aberration coefficients gradually converge.

[0043] When the interference fringe contrast is maximized and the residual RMS value is minimized, the system reaches the optimal wavefront state, and the spatial position of the measured mirror is locked. This process requires multiple iterative adjustments. After each adjustment, the laser interferometer 1 is required to collect new data and analyze it. According to the analysis results, the six-dimensional adjustment platform 3 is adjusted again until the conditions for optimal wavefront quality are met.

[0044] Fifth, off-axis angle measurement and coordinate transformation

[0045] The optical axis of the photoelectric theodolite 5 is precisely aligned with the normal line of the optimized mirror surface, and the spatial coordinates of the preset marking point group on the tooling table are remeasured.

[0046] If the photoelectric theodolite 5 is offset in position (ie, different from the origin) due to relocation, the coordinates of the rotated calibration points need to be converted to the reference coordinate system through translation.

[0047] Calculate the translation vector, assuming that the calibration point is the same physical point before and after the rotation (at least one common point is required in practical applications), calculate the translation through the center of mass alignment method, and then translate the corrected coordinates.

[0048] Since the object rotates only around the Z axis, the change in the azimuth in the horizontal plane (xy plane) is the rotation angle. The initial azimuth and the azimuth after rotation are calculated for each calibration point, and then the single-point rotation angle is obtained. The final rotation angle is the average of the rotation angles of the three calibration points.

[0049] After multiple measurements and verifications, the off-axis angle measurement accuracy in this embodiment can reach ±0.3μm.

[0050] The equipment parameters and performance in this embodiment are as follows:

[0051] The angle measurement accuracy of the photoelectric theodolite 5 is better than 0.1 arc second, and the coordinate measurement accuracy is better than ±10 μm.

[0052] The displacement measurement resolution of the laser interferometer 1 is better than 0.1 nm, and the sampling frequency is not less than 100 Hz.

[0053] The adjustment accuracy of the six-dimensional adjustment platform 3 is at nanometer level.

[0054] Example 2

[0055] This embodiment measures and verifies the results of the method provided in the above embodiment;

[0056] By designing comparative experiments, the accuracy, efficiency and applicability of the scheme of the present invention are compared with those of traditional off-axis angle measurement methods (laser tracker method, autocollimator turntable method, mechanical contact measurement method). The verification process is as follows:

[0057] Experimental design and test conditions

[0058] In an optical detection laboratory environment (temperature 20 ± 1 °C, vibration amplitude < 50 μm / s 2 ), a high-precision turntable (angle resolution 0.001°) is used as a standard device. The target off-axis angles of the off-axis mirror are set to 1°, 5°, and 10°. The measurement is carried out using the method of the present invention and the traditional method respectively. Each method is measured 10 times, and the mean error and repeatability (σ) are recorded.

[0059] Comparison of measured data

[0060] Comparison of measurement accuracy and repeatability:

[0061] Table 1: Data description table 1

[0062]

[0063]

[0064] As shown in Table 1 above, the errors of the solution of the present invention are all < 0.0021°, which is more than 10 times higher than that of the laser tracker method, and the repeatability (σ) reaches the sub-arcsecond level, verifying the high-precision characteristics.

[0065] The error of the autocollimator turntable method is 0.0045° at 5°, and the off-axis angle greater than 5° cannot be measured; the mechanical contact measurement error increases significantly with the increase of the angle, and there is a risk of scratching the mirror surface.

[0066] Comparison of measurement efficiency and equipment applicability:

[0067] Table 2: Data description table 2

[0068]

[0069]

[0070] As shown in Table 2 above, through the real-time aberration feedback of the laser interferometer and the automatic adjustment of the six-axis platform in the present invention, the measurement time is shortened by more than 50% compared with the traditional method, and there is no need to rely on a high-precision turntable or a harsh environment.

[0071] Although the laser tracker method can measure large angles, the equipment cost is high and the data processing is complex; due to the risk of contact damage, the mechanical contact measurement is not suitable for precision optical components.

[0072] Verification conclusion

[0073] The experimental results show that the method of the present invention is significantly superior to the traditional method in key indicators such as measurement accuracy (error < 0.0021°), repeatability (σ = 0.0001°), measurement range (0 - 15°), and environmental adaptability. Especially in the measurement of large off-axis angles of 10°, the problems of insufficient range of the autocollimator turntable method and deterioration of measurement accuracy of the mechanical contact method are solved, and the high cost and complex operation of the laser tracker are avoided. The measured data verify the high precision, high efficiency, and engineering practicability of the technical solution of the present invention, which is applicable to the precise measurement requirements of off-axis angles in optical system manufacturing.

[0074] Through the above steps, the finally calculated off-axis angle is the average of the rotation angles of the three calibration points, ensuring the reliability and accuracy of the measurement results.

[0075] The measurement method of the present invention has the characteristics of high precision, high stability, and simple operation, and is applicable to the precise measurement requirements of off-axis angles in optical system manufacturing and debugging.

[0076] Obviously, the above embodiments are merely examples for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for precise measurement of off-axis angle, characterized in that, The following steps are involved: S1: strictly level and coaxially calibrate the laser interferometer (1) and the photoelectric theodolite (5) to form a unified measurement reference axis; introduce the off-axis mirror (2) to be measured between the optical paths of the two, and precisely adjust the spatial posture of the off-axis mirror (2) through the six-dimensional adjustment platform (3) to ensure that the mirror surface normal coincides with the reference axis; S2: Arrange three high-reflectivity marking points (4) on the off-axis mirror (2) mounting platform, use the photoelectric theodolite (5) to construct a global coordinate system and measure the initial three-dimensional coordinates of the marking points, and simultaneously record the initial spatial posture parameters of the mirror body to provide geometric transformation benchmark data for subsequent off-axis angle calculation; S3: rotating the off-axis mirror (2) to be measured around its central axis by a preset angle to introduce an off-axis value, and at the same time introducing a spherical reflector at the end of the optical path to construct a closed-loop interferometric detection optical path; S4: The Zernike coefficients corresponding to the surface error are analyzed in real time by the laser interferometer (1), with a focus on the astigmatism term Z5 / Z6 and the coma term Z7 / Z8. The six-dimensional adjustment platform (3) is adjusted to perform multi-degree-of-freedom iterative adjustment. When the amplitude of the sensitive aberration coefficient converges to zero, the system reaches the optimal wavefront state, and the spatial position of the measured mirror is locked at this time. S5: accurately align the optical axis of the photoelectric theodolite (5) with the normal line of the optimized mirror surface, re-measure the spatial coordinates of the marking point group arranged on the mounting table, and calculate the actual off-axis angle value through the rigid transformation matrix of the initial and final coordinate systems.

2. The off-axis angle precision measurement method according to claim 1, wherein The angle measurement accuracy of the photoelectric theodolite (5) is better than 0.1 arc second, and the coordinate measurement accuracy is better than ±10 μm.

3. The off-axis angle precision measurement method according to claim 1, characterized in that The displacement measurement resolution of the laser interferometer (1) is better than 0.1 nm, and the sampling frequency is not less than 100 Hz.

4. The off-axis angle precision measurement method according to claim 1, characterized in that, The adjustment accuracy of the six-dimensional adjustment platform (3) is at nanometer level.

5. The off-axis angle precise measurement method according to claim 1, characterized in that In step S3, when measuring the off-axis angle, the rotation angle is determined by calculating the azimuth change of the calibration point in the xy plane before and after the rotation, and the final rotation angle is the average of the rotation angles of at least three calibration points.

6. The off-axis angle precision measurement method according to claim 1, wherein In step S5, when re-measuring the spatial coordinates of the marking point group arranged on the mounting platform, when performing coordinate transformation, if there is a positional offset when the photoelectric theodolite (5) is relocated, a translation vector is calculated to perform translation correction on the coordinates of the rotated calibration points so as to convert them into the reference coordinate system.

7. The off-axis angle precision measurement method according to claim 6, wherein The translation vector is calculated by the centroid alignment method to perform translation correction on the coordinates of the rotated calibration points.

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