High-precision finite distance optical system distortion test method based on projection backstepping

Abstract

The application belongs to the technical field of optical precision detection, and particularly relates to a high-precision limited-range optical system distortion testing method based on projection backstepping. A scale plate is placed on the image plane of a measured lens, a laser homogenization illumination device performs double-wavelength laser homogenization illumination on the scale plate, the scale lines of the scale plate are projected to the object plane far field measurement plane of the measured lens with a limited conjugate distance through the measured lens to form a projection fringe sequence, a CCD slides along a vertical guide rail to scan the projection fringe sequence, the actual interval of adjacent projection fringes is obtained based on the centroid positions of each projection fringe contained in the projection fringe sequence, and finally, the absolute distortion and relative distortion of the measured lens under the current field of view angle are inversely calculated based on the actual interval of adjacent projection fringes. The application breaks through the traditional image plane measurement precision bottleneck and realizes sub-micron level distortion measurement.

Description

Technical Field

[0001] This invention belongs to the field of optical precision testing technology, and in particular relates to a high-precision finite-distance optical system distortion testing method based on projection back-calculation. Background Technology

[0002] In the research, development, assembly, and quality inspection of optical systems, lens distortion, as a core defect affecting imaging accuracy, leads to geometrical distortion of the image, thereby reducing the accuracy of subsequent measurement, recognition, and positioning processes. Its measurement accuracy directly determines the overall performance of the optical system. Distortion is one of the core aberrations of optical systems. Due to differences in imaging magnification at different fields of view, it causes geometrical distortion of the image. Lenses often operate in imaging scenarios with limited distances. In such cases, distortion errors are further amplified, severely impacting the reliability and measurement accuracy of the equipment. Therefore, achieving high-precision distortion measurement under limited distance conditions has urgent practical needs and significant engineering value.

[0003] Currently, the common method for distortion detection in existing finite conjugate distance optical systems is to place a distortion-free standard grid plate on the object side, image the system under test, and then measure the positional deviation of the grid at the image plane to calculate the distortion. This method is suitable for conventional precision testing, but it is difficult to meet the requirements of ultra-high precision measurements at the micrometer and sub-micrometer levels, and has significant limitations.

[0004] When distortion measurement accuracy requirements reach the micrometer or sub-micrometer level, the direct image plane measurement method is highly sensitive to equipment resolution, assembly accuracy, environmental vibration, and temperature drift. Errors caused by minute disturbances can mask the effective signal, leading to distorted measurement results and making ultra-high precision detection difficult. Furthermore, measurement errors are difficult to suppress; image plane measurement errors directly translate into final distortion errors, which cannot be eliminated by optimizing the structure or improving the detection method, thus failing to meet the requirements of high-end optical systems. In long-distance measurement scenarios of 0.5m to 4m, traditional equipment is limited by insufficient mechanical rigidity and poor guide rail straightness and platform flatness, making it impossible to simultaneously achieve large stroke and high-precision detection. Moreover, the measurement accuracy of existing equipment cannot be directly calibrated and traced. Since there are currently no standard distortion lenses in the industry and metrology system, traditional detection devices cannot provide measurement uncertainty, lack quantitative evidence for detection accuracy, and cannot carry out traceability work. The equipment has a low level of automation and poor detection repeatability. The entire process relies on manual adjustment, aiming, and reading, which not only easily introduces large human errors, but also has problems such as low work efficiency and poor data consistency. At the same time, the uniformity of traditional lighting systems is insufficient, which easily causes uneven stripe dispersion and attenuation of edge light intensity, and is superimposed with various noise interferences, further reducing the positioning accuracy of the target center. Summary of the Invention

[0005] In view of this, the present invention aims to provide a high-precision finite-distance optical system distortion testing method based on projection back-inference, in order to solve the technical problems of low distortion detection accuracy, inability to balance range and accuracy, lack of traceability of measurement values, and insufficient automation level in traditional methods. The present invention breaks through the bottleneck of traditional image plane measurement accuracy and realizes sub-micron level distortion measurement.

[0006] To achieve the above objectives, the technical solution created by this invention is implemented as follows: A high-precision distortion testing method for a finite-distance optical system based on projection back-calculation specifically includes the following steps: S1: Set up the test platform, which includes a laser homogenization illumination device, a reticle, the lens under test, a vertical guide rail, and a CCD. S2: Place the reticle on the image plane of the lens under test, and use a laser homogenization illumination device to perform dual-wavelength laser homogenization illumination on the reticle. The reticle's lines are projected and magnified to the far field of the object plane of the lens under test, forming the projected fringe sequence of the lens under test at the current field of view. S3: The CCD slides along the vertical guide rail to scan the sequence of projected fringes. Based on the centroid position of each projected fringe contained in the sequence, the actual interval between adjacent projected fringes is obtained. S4: Based on the measured position and theoretical position of the lens under test at the current field of view, calculate the deviation of the object projection position of the lens under test at the current field of view. S5: Based on the deviation of the object plane projection position of the lens under test at the current field of view, and combined with the actual interval of adjacent projection fringes, obtain the absolute distortion and relative distortion of the image plane of the lens under test at the current field of view. S6: Adjust the field of view of the lens under test, replace the current field of view with the next field of view of the lens under test, and repeat steps S2-S5 to obtain the absolute distortion and relative distortion of the image plane of the lens under test under different field of view.

[0007] Furthermore, in step S5, the calculation formulas used to obtain the absolute distortion and relative distortion of the image plane of the lens under test at the current field of view are as follows: ; Y= ; ； in, For absolute distortion of the image plane, This refers to the lateral magnification. This represents the difference between the actual and theoretical intervals of adjacent projected fringes. The actual interval between adjacent projected fringes. This represents the theoretical interval between adjacent projected fringes. For image plane relative distortion, This represents the theoretical image height of the lens under test at the current field of view.

[0008] Furthermore, the lateral magnification is the ratio of the reticle line spacing to the actual interval between adjacent projected stripes.

[0009] Furthermore, the testing platform also includes a grating ruler, which is rigidly connected to the CCD and acquires the spatial position of the CCD in real time.

[0010] Furthermore, the CCD is a linear CCD.

[0011] Furthermore, the measured position of each line on the reticle deviates from the theoretical position by less than 0.1 μm.

[0012] Furthermore, the center of the reticle is offset from the center of the lens being tested by less than 0.1 mm.

[0013] Compared with the prior art, the present invention can achieve the following beneficial effects: (1) The high-precision finite-distance optical system distortion testing method based on projection back-reasoning described in this invention breaks through the bottleneck of traditional image plane measurement accuracy and realizes sub-micron level distortion measurement. This invention proposes the projection back-reasoning measurement principle, which reduces the error by tens to hundreds of times using the magnification of the object image; this invention provides an automated testing device with a large stroke of 0.5m–4m, high stability, and high rigidity, and realizes the calculability and traceability of uncertainty by establishing a precision calibration method under a standard lens. This invention can realize automatic scanning, acquisition, centroid extraction, and distortion calculation throughout the entire process.

[0014] (2) The high-precision finite-distance optical system distortion testing method based on projection back-reasoning described in this invention is a high-precision, highly stable, and widely applicable distortion testing method for finite-distance optical systems. It achieves a proportional reduction in measurement error with magnification, significantly improving resolution and accuracy. By constructing a long-stroke, high-rigidity, and low-drift mechanical structure, this invention ensures stability in long-distance measurements, enabling fully automatic, non-contact, rapid, repeatable, and high-precision distortion measurement, as well as sub-micron distortion testing that is traceable to national metrology standards. Attached Figure Description

[0015] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 A schematic flowchart of the high-precision finite-distance optical system distortion testing method based on projection back-reasoning as described in the embodiments of the present invention; Figure 2 A schematic diagram of the structure of the test platform described in the embodiments of the present invention; Figure 3 A schematic diagram of the structure of the reticle described in the embodiment of the present invention; Figure 4 The CCD-collected stripe energy distribution map is described in the embodiment of the present invention.

[0016] Explanation of reference numerals in the attached figures: 1. Laser homogenization illumination device; 2. Reticle; 3. Lens under test; 4. Projected stripes; 5. Vertical guide rail; 6. CCD. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0018] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0019] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0020] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0022] like Figures 1-3 As shown, this invention provides a high-precision finite-distance optical system distortion testing method based on projection back-calculation, specifically including the following steps: S1: Set up the test platform, which includes a laser homogenization illumination device 1, a reticle 2, the lens under test 3, a vertical guide rail 5, and a CCD 6; S2: Place the reticle 2 on the image plane of the lens under test 3. The laser homogenization illumination device 1 performs dual-wavelength laser homogenization illumination on the reticle 2. The scribe lines of the reticle 2 are projected and magnified to the far field of the object plane of the lens under test 3, forming the sequence of projected stripes 4 corresponding to the lens under test 3 at the current field of view. S3: CCD6 slides along vertical guide rail 5 to scan the sequence of projected stripes 4. Based on the centroid position of each projected stripe 4 contained in the sequence of projected stripes 4, the actual interval between adjacent projected stripes 4 is obtained. S4: Based on the measured position and theoretical position of the lens under test 3 at the current field of view, calculate the deviation of the object projection position of the lens under test 3 at the current field of view; S5: Based on the deviation of the object plane projection position of the lens under test 3 at the current field of view, and combined with the actual interval of adjacent projection stripes 4, the absolute distortion and relative distortion of the image plane of the lens under test 3 at the current field of view are obtained. S6: Adjust the field of view of the lens under test 3, replace the current field of view with the next field of view of the lens under test 3, and repeat steps S2-S5 to obtain the absolute distortion and relative distortion of the image plane of the lens under test 3 under different field of view.

[0023] It should be noted that the reticle 2 is placed on the image plane of the lens under test 3, and the laser homogenization illumination device 1 performs dual-wavelength laser homogenization illumination on the reticle 2. The scribe lines of the reticle 2 are projected onto the far-field measurement plane of the object surface of the lens under test 3 with a finite conjugate distance, forming a sequence of projected fringes 4. The CCD 6 slides along the vertical guide rail 5 to scan the sequence of projected fringes 4. Based on the centroid position of each projected fringe 4 contained in the sequence of projected fringes 4, the actual interval of adjacent projected fringes 4 is obtained. Finally, based on the actual interval of adjacent projected fringes 4, the absolute distortion and relative distortion of the lens under test 3 under the current field of view are calculated.

[0024] This invention completely overturns the traditional measurement path: an ultra-precise reticle 2 is placed on the image plane of the lens under test 3, and illuminated by dual-wavelength laser homogenization; the lines of the reticle 2 are projected and magnified to the far field of the object plane by the lens under test 3; the object plane of the lens under test 3 is scanned using a linear CCD 6 + high-precision grating ruler, with the grating ruler acquiring the absolute spatial position of the linear CCD 6 to obtain the centroid of each projected fringe 4; then, the difference between the measured position and the theoretical position of the lens under test 3 is used to obtain the object plane projection position deviation; the object plane projection position deviation is divided by the lateral magnification to inversely calculate the true distortion of the image plane. The object image magnification is typically 25–200 times, and the measurement error is simultaneously reduced by 25–200 times, achieving sub-micron level measurement.

[0025] Example 1 This invention provides a high-precision finite-distance optical system distortion testing method based on projection back-calculation, which specifically includes the following steps: Step S1: Level the vibration isolation table to a flatness ≤10μm, and ensure the perpendicularity of the vertical guide rail 5 to the horizontal plane of the vibration isolation table (double zero-grade marble platform) is ≤10μm. The grating ruler is rigidly connected to the CCD6, and the grating ruler is calibrated using a laser interferometer. The marble platform containing the vertical guide rail 5 is double zero-grade (flatness 5μm), and the vertical guide rail 5 is a precision ball bearing guide. The CCD6 probe moves on the vertical guide rail 5. The grating ruler reading head is rigidly connected to the linear CCD6, ensuring that the position change of the CCD6 is synchronized with the grating ruler counting. The grating ruler is used to detect the displacement of the CCD6 slide. The CCD6 probe is connected to a ball screw fixed on the platform. A stepper motor drives the ball screw to rotate, which in turn moves the CCD6 probe up and down, acquiring different stripes from the reticle 2. The CCD6 probe consists of a linear CCD6 sensor, a CCD6 drive acquisition circuit, and a mechanical fixing device. The CCD6 is 35mm long, has 7400 pixels, with a pixel size of 4.7μm×4.7μm, and a spectral response band of 400nm~1300nm.

[0026] Step S2: The reticle 2 is mounted on the image plane of the lens under test, and a coaxial stop structure is used for positioning. After assembly, the deviation of the center of the reticle 2 relative to the optical axis of the lens is ≤0.1mm. The three-dimensional spatial position and basic optical axis orientation of the lens under test 3 are determined by a coordinate measuring machine (CMM), achieving initial positioning of the lens under test 3. Then, a six-dimensional lens adjustment frame is used to precisely adjust the optical axis orientation, center height, and left and right offset of the lens under test 3. The adjustment of the lens under test 3 is assisted by the CMM for position and height positioning, and then fine-tuned by monitoring the self-collimated image of the plane mirror in front of the lens under test 3 using an autocollimating theodolite. At the same time, the CMM is used to monitor the placement position and optical axis direction of the illumination system corresponding to the reticle 2, ensuring that the illumination system corresponding to the reticle 2 illuminates the reticle 2 perpendicularly and concentrically, ensuring the accuracy of the stripes projected by the lens under test 3.

[0027] Step S3: The dual-wavelength laser includes two types of lasers, 808nm and 850nm, and uses a micromirror array homogenization system (i.e., laser homogenization illumination device 1) to form uniform illumination at the reticle 2.

[0028] Step S4: The fringe image of the reticle 2 is projected onto the photosensitive surface of the CCD 6 through the lens 3 under test. The CCD 6 probe acquires data for each fringe in stages, and the centroid position of each fringe is automatically calculated using centroid extraction software. To improve measurement accuracy, the same fringe is sampled and averaged dozens of times to eliminate the influence of fluctuations in the light source intensity distribution. Because the fringes have a certain width of energy distribution, the fringe position is obtained using centroid extraction software. The data from the grating ruler is also acquired by computer and combined with the fringe centroid data from the CCD 6 to form the position data of the fringe projected by the lens. The measurement accuracy of the grating ruler throughout its entire range is calibrated using a laser interferometer, with an accuracy better than 3μm.

[0029] Step S5: Based on the measurement accuracy of the actual interval of the projected fringes, calculate the lens distortion measurement accuracy (divided by the object and image magnification). Calculate the object plane projection position deviation at different field of view angles, and calculate the absolute and relative image plane distortion at the corresponding field of view angles. Object plane projection position deviation Y= ; Actual measured projection position; Y: Theoretical projection position Lateral magnification β = paraxial image plane reticle spacing 2 / paraxial object plane projected fringe spacing 4; absolute image plane distortion Relative distortion .

[0030] It is important to note that the object distance range is between 0.5m and 4m. Since the selected linear CCD 6 has a photosensitive length of 33mm, calculations show that at an object distance of 0.5m (i.e., the spacing between two adjacent fringes is 14.34mm), a blurred image of two adjacent lines will appear simultaneously on the CCD 6. Therefore, the photosensitive surface of the CCD 6 must be partially blocked during measurement. When the test distance is greater than 1m, the CCD 6 will not simultaneously capture two fringes. At a maximum field of view of 16° at 4m, the fringe height is 1147mm, while the grating ruler's height travel is 1200mm, which meets the measurement range requirements.

[0031] Figure 4To acquire the stripe energy distribution for CCD6, since the imaging position of each line of the reticle 2 on the object surface through the lens 3 is fixed, the scanning position of the grating ruler probe is also fixed. That is, for each line scanned, the grating ruler probe moves to that position under computer control. At this time, the linear CCD6 exposes and images the bright lines with a certain diffuse distribution, and the centroid position is determined by image processing. The centroid position combined with the grating ruler position data is the actual imaging position of the line. The process of CCD6 acquiring stripes is shown in the figure.

[0032] The vertical guide rail 5 consists of a double-zero grade marble platform and a high-precision ball bearing guide fixed on the double-zero grade marble platform. The flatness of the 4m long double-zero grade platform is 10μm (00 grade = 2×(1+d / 1000)μm, where d is the diagonal length of the double-zero grade platform). According to the task requirements, the horizontal travel of this equipment can meet the moving measurement range of 0.5m to 4m.

[0033] Lens distortion is measured by projecting the image of the precisely engraved reticle 2 onto the vertical surface of the CCD 6 probe, where the CCD 6 acquires and calculates the image. The precisely engraved reticle 2 is crucial for lens distortion measurement. In this device, the reticle 2 is engraved using a laser direct-writing method with an engraving accuracy of 0.1 μm. This reticle 2 was tested by the National Institute of Metrology (NIM), and the test results show that the deviation between the measured position and the theoretical value of each engraving line is <0.1 μm. The NIM provided the inspection results and test report. The relative positional relationship between the reticle 2 and the lens requires high precision, with a center deviation of <0.1 mm. The reticle 2 is installed in the lens barrel, and the lens is fixed to the lens via a coaxial stop. This precision is ensured by precision machining, which guarantees an accuracy of less than 0.05 mm, meeting the 0.1 mm requirement of this device. The measurement accuracy of this device mainly comes from: The reticle 2's engraving accuracy was calibrated by the National Institute of Metrology, with a positional error of 0.1 μm. The CCD 6 pixels are made using photolithography, and their pixel size and spacing errors are less than 0.2 μm. The CCD 6's measurement accuracy for the fringe centroid is better than 0.5 μm. The calibrated grating ruler has an accuracy of 2.6 μm. These three random error terms are divided by the magnification M = 25~200. When M = 25, the measurement random accuracy is the worst, so the measurement accuracy is 3σ = 0.18 μm. Therefore, the system measurement uncertainty = 0.1 + 0.18 = 0.28 μm, achieving sub-micron level measurement. The method proposed in this invention has a large measurement range, strong applicability, a horizontal measurable length of 0.5–4 m, a vertical travel of 1200 mm, and a maximum field of view of 16°, covering mainstream finite-distance lenses. The accuracy of this invention is traceable and quantifiable. The reticle 2 is calibrated by the National Institute of Metrology, and the error terms can be independently calculated, satisfying metrological traceability. Highly automated and with excellent repeatability, the entire process operates automatically, with a repeatability of ≤0.1μm after averaging 30 measurements. The entire instrument is equipped with a dual zero-grade marble platform, which is shock-resistant, temperature-drift-resistant, and exhibits excellent mechanical stability. Relying on error synthesis and magnification inverse calculation algorithms, accuracy calibration can be completed without a standard lens. It is also compatible with the 808nm and 850nm wavelength bands, adaptable to visible light and near-infrared systems, and adopts a non-contact detection method, which will not damage the lens under test.

[0034] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0035] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A high-precision finite-distance optical system distortion testing method based on projection back-calculation, characterized in that: Specifically, the steps include the following: S1: Set up the test platform, which includes a laser homogenization illumination device, a reticle, the lens under test, a vertical guide rail, and a CCD. S2: Place the reticle on the image plane of the lens under test, and use a laser homogenization illumination device to perform dual-wavelength laser homogenization illumination on the reticle. The reticle's lines are projected and magnified to the far field of the object plane of the lens under test, forming the projected fringe sequence of the lens under test at the current field of view. S3: The CCD slides along the vertical guide rail to scan the sequence of projected fringes. Based on the centroid position of each projected fringe contained in the sequence, the actual interval between adjacent projected fringes is obtained. S4: Based on the measured position and theoretical position of the lens under test at the current field of view, calculate the deviation of the object projection position of the lens under test at the current field of view. S5: Based on the deviation of the object plane projection position of the lens under test at the current field of view, and combined with the actual interval of adjacent projection fringes, obtain the absolute distortion and relative distortion of the image plane of the lens under test at the current field of view. S6: Adjust the field of view of the lens under test, replace the current field of view with the next field of view of the lens under test, and repeat steps S2-S5 to obtain the absolute distortion and relative distortion of the image plane of the lens under test under different field of view.

2. The high-precision finite-distance optical system distortion testing method based on projection back-calculation according to claim 1, characterized in that: In step S5, the calculation formulas used to obtain the absolute distortion and relative distortion of the image plane of the lens under test at the current field of view are as follows: ； Y= ； ； in, For absolute distortion of the image plane, This refers to the lateral magnification. This represents the difference between the actual and theoretical intervals of adjacent projected fringes. The actual interval between adjacent projected fringes. This represents the theoretical interval between adjacent projected fringes. For image plane relative distortion, This represents the theoretical image height of the lens under test at the current field of view.

3. The high-precision finite-distance optical system distortion testing method based on projection back-calculation according to claim 1, characterized in that: The lateral magnification is the ratio of the spacing between the reticle lines to the actual spacing between adjacent projected stripes.

4. The high-precision finite-distance optical system distortion testing method based on projection back-calculation according to claim 1, characterized in that: The testing platform also includes a grating ruler, which is rigidly connected to the CCD and acquires the spatial position of the CCD in real time.

5. The high-precision finite-distance optical system distortion testing method based on projection back-calculation according to claim 1, characterized in that: CCD stands for Linear CCD.

6. The high-precision finite-distance optical system distortion testing method based on projection back-calculation according to claim 1, characterized in that: The measured position of each line on the reticle deviates from the theoretical position by less than 0.1 μm.

7. The high-precision finite-distance optical system distortion testing method based on projection back-calculation according to claim 1, characterized in that: The center of the reticle is offset from the center of the lens being tested by less than 0.1 mm.

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