A method of measuring an ultra-large radius of curvature

Abstract

The application discloses a method for measuring super-large curvature radius, and relates to the field of optical measurement, which comprises the following steps: placing two autocollimators in parallel on one side of an optical element to be measured, calibrating the parallel state by using optical flat crystals; then placing the autocollimators near the curved surface to be measured, synchronously adjusting the pitch angle and position to make the light spots on the same CCD axis; calculating the centroid offset of the reflected light spots, and reducing the noise influence by combining with a denoising algorithm; finally, calculating the curvature radius according to the distance between the two autocollimators and the centroid offset distance of the light spots; the method is compatible with concave-convex mirror measurement, does not need to replace equipment or adjust the optical path, has a simple optical path structure, and the algorithm is easy to realize; the method does not depend on a high-precision optical platform, can be applied in an optical workshop, compared with the traditional autocollimation method, does not need a large-stroke displacement table, and can realize the measurement of the super-large curvature radius only by fine adjustment, so that the operation is simplified, the repeatability is good, and the precision is high.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, and more specifically, to a method for measuring ultra-large radii of curvature. Background Technology

[0002] Currently, methods for measuring the radius of curvature can be divided into contact measurement methods and non-contact measurement methods.

[0003] Common contact measurement methods include the spherical diameter meter method, surface profile measurement method, and coordinate measuring machine (CMM) method. The spherical diameter meter method measures the sag and diameter of a sphere and calculates the radius of curvature using geometric formulas. However, this method has significant measurement errors at large radii of curvature (with minimal sag), and its measurement range is limited by the equipment structure. The surface profile measurement method uses a stylus to scan along the surface profile, recording height changes and fitting a curve to calculate the local radius of curvature. However, this method can only measure one-dimensional profiles (in the generatrix direction), and stylus pressure may damage the surface of soft materials. The CMM method uses a contact probe to acquire the three-dimensional coordinates of discrete points on the surface, fits the surface equation, and calculates the radius of curvature. However, this method requires manual or programmed path planning, demands high operator skill, and has low measurement efficiency.

[0004] Non-contact measurement methods mainly include interferometry, laser differential confocal method, and autocollimation method. Interferometry calculates the radius of curvature by comparing the measured surface with a standard surface through interference fringe distribution. It is a high-precision measurement method, but it is highly sensitive to environmental vibrations and airflow, and its range is limited by the interferometer aperture. Laser differential confocal method utilizes the axial light intensity distribution characteristics of a confocal optical system to scan the measured surface and obtain contour information to calculate curvature. However, this method requires a cumbersome attitude adjustment process, and its efficiency needs improvement. The traditional autocollimation method uses the crosshairs of the autocollimator reflected from the vertex of the measured element to the center of curvature. The reflected light passes through a relay lens and the autocollimator optical system to form an image on a CCD camera. The distance between the real and virtual image points corresponds to the radius of curvature. This method is simple in principle, and many multifunctional optical measuring instruments based on this method have been developed both domestically and internationally. However, when using this method to measure the radius of curvature, it is usually limited by the travel of the displacement stage, making it difficult to measure ultra-large radii of curvature. Summary of the Invention

[0005] The purpose of this invention is to provide a method for measuring ultra-large curvature radii based on a dual autocollimator, so as to solve the problems of equipment travel limitations, environmental sensitivity and operational complexity of traditional measurement methods under ultra-large curvature radii, and to achieve simplified operation, good repeatability and high precision for the measurement of concave and convex mirrors.

[0006] The technical solution of this invention is: to provide a method for measuring ultra-large radius of curvature, the method comprising:

[0007] Construct an optical path containing two autocollimators: Place the first autocollimator and the second autocollimator in parallel, with their exit ports pointing to the same optical flat or the surface to be measured. Each of the first and second autocollimators includes a CCD and a light source. When the light source is working, the autocollimator emits parallel light rays from its exit port, which are reflected by the optical flat or the surface to be measured onto the CCD to form a light spot.

[0008] The parallelism of the beams of the two autocollimators is calibrated: the exit ports of the two autocollimators are aligned with the same optical flat, and the rotation angles of the two autocollimators are adjusted so that the centroid of the light spot formed by the reflection on the CCD coincides with the center of the CCD. This indicates that the two autocollimators are perpendicular to the optical flat, that is, the two autocollimators are parallel to each other. The relative positions of the two autocollimators are fixed to form a double autocollimator.

[0009] Orient the exit port of the dual autocollimator toward the surface to be measured, and calibrate the attitude of the dual autocollimator: finely adjust the overall pitch angle and placement of the two autocollimators so that the light spots on the two CCDs are located on the same axis, and the two light spots can be on the same side or opposite sides.

[0010] Calculate the coordinates of the spot centroid and derive the offset angle: Calculate the coordinates of the spot centroids on the two CCD axes respectively, obtain the distance of the spot centroid from the CCD center, and then calculate the offset angle of the light emitted from the collimator relative to the tangent plane of the surface to be measured based on geometric principles.

[0011] Calculating the radius of curvature of the surface under test: When the two autocollimators are located on opposite sides of the vertex of the surface under test, the reflected light spots of the two autocollimators are located on opposite sides of their respective CCD axes. At this time, the radius of curvature is... for:

[0012] ;

[0013] When two autocollimators are located on the same side of the vertex of the surface under test, the reflected light spots of the two autocollimators are located on the same side of their respective CCD axes. At this time, the radius of curvature... for:

[0014] ;

[0015] in, The distance between the two autocollimators. and These represent the centroid offset distances of the two autocollimators. This is the focal length of the collimating objective lens.

[0016] In any of the above technical solutions, the first autocollimator and the second autocollimator each further include: a semi-transparent and semi-reflective prism and a collimating objective lens; the light source is set on the side of the autocollimator, and the light source emits light from the side toward the semi-transparent and semi-reflective prism. The semi-transparent and semi-reflective prism reflects the light along the optical axis of the autocollimator to the collimating objective lens, and the light is collimated into parallel light by the collimating objective lens and emitted from the outlet of the autocollimator; the emitted light is reflected by the optical flat or the surface to be measured and the reflected light is focused by the collimating objective lens, passes through the semi-transparent and semi-reflective prism, and finally forms an image on the CCD set behind the semi-transparent and semi-reflective prism.

[0017] In any of the above technical solutions, the first autocollimator and the second autocollimator further employ different reticles to distinguish the reflected light spots.

[0018] In any of the above technical solutions, the further step of calculating the centroid coordinates of the light spot and deriving the offset angle includes: when the exit port of the autocollimator is not perpendicular to the tangent plane of the mirror to be measured, the reflected light is focused at a non-central point O′ on the CCD axis, i.e., the centroid position of the reflected light spot, and the centroid coordinates are calculated using a first-order moment centroid algorithm. , )for:

[0019] ;

[0020] in, , For pixel coordinates, For pixel grayscale values, Given the pixel resolution of the CCD, the distance between the CCD center point O and point O′ can then be calculated. ;

[0021] Based on geometric relationships, the offset angle of the autocollimator for: .

[0022] In any of the above technical solutions, the radius of curvature of the surface to be measured must further satisfy a critical condition. Where R is the radius of curvature, The objective lens aperture, This refers to the CCD pixel size.

[0023] In any of the above technical solutions, a further step is to use a denoising algorithm to reduce the influence of noise and background dark level on the CCD before calculating the centroid spacing of the light spot: First, the light spot image is thresholded, and the threshold is determined according to the quality and brightness of the light spot on the CCD; then the image is convolved with a Gaussian filter to reduce the influence of Gaussian noise while retaining low-frequency edge and contour information.

[0024] In any of the above technical solutions, further, the calculated radius of curvature or If it is positive, it indicates that the surface to be measured is a concave mirror; if the radius of curvature is positive... or A negative value indicates that the surface to be measured is a convex mirror.

[0025] The beneficial effects of this invention are:

[0026] The technical solution in this invention adopts a configuration of parallel placement of dual autocollimators, which eliminates the need for a complex optical system and makes setup and operation convenient; the measurement process only requires fine adjustment of the pitch angle and position, eliminating the need for a precision displacement stage or a stable environment, thus reducing equipment costs;

[0027] The centroid calculation uses a first-order moment algorithm combined with noise reduction processing, which is simple to calculate and easy to implement in software.

[0028] Compared to the traditional autocollimation method, this invention can extend the measurement range through initial calibration and attitude adjustment, thus solving the travel limitation under ultra-large curvature radius. Attached Figure Description

[0029] The advantages of the above and additional aspects of the present invention will become apparent and readily understood in the description of the embodiments in conjunction with the following drawings, wherein:

[0030] Figure 1 This is a schematic flowchart of a method for measuring ultra-large radius of curvature according to an embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram of the optical path for measuring ultra-large radius of curvature using an optical flat to calibrate the initial parallel state of a self-collimator according to an embodiment of the present invention.

[0032] Figure 3 This is a schematic diagram of the internal optical path of an autocollimator where the optical surface under test is approximately planar, according to an embodiment of the present invention, for measuring ultra-large radius of curvature.

[0033] Figure 4 This is a schematic diagram of the imaging optical path when the optical surface under test has a finite radius of curvature, according to an embodiment of the present invention, for measuring ultra-large radius of curvature.

[0034] Figure 5 This is a schematic diagram of the optical path for measuring the ultra-large radius of curvature of a mirror using the double autocollimation method, according to an embodiment of the present invention.

[0035] Figure 6 This is a schematic diagram of the CCD spot position during the measurement process of a method for measuring ultra-large curvature radius according to an embodiment of the present invention, using the dual autocollimation method. Detailed Implementation

[0036] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0037] In the following description, many specific details are set forth in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0038] like Figure 1 As shown, this embodiment provides a method for measuring ultra-large radius of curvature, the method comprising:

[0039] like Figure 2 As shown, firstly, an optical path containing two autocollimators is constructed: the first autocollimator 6 and the second autocollimator 7 are placed parallel to each other on one side of the optical element to be tested. The two use different reticles to distinguish the reflected light spots. The exit ports of the first autocollimator 6 and the second autocollimator 7 point to the optical flat 8 or the surface to be tested 9.

[0040] like Figure 3 As shown, the first autocollimator 6 and the second autocollimator 7 each include a CCD 1, a semi-transparent and semi-reflective prism 2, a collimating objective lens 3, and a light source 5. The light source 5 is located on the side of the autocollimator. The light source 5 emits light from the side toward the semi-transparent and semi-reflective prism 2. The semi-transparent and semi-reflective prism 2 reflects the light along the optical axis of the autocollimator to the collimating objective lens 3. The light is collimated into parallel light by the collimating objective lens 3 and emitted from the exit port of the autocollimator. The emitted light is reflected by the optical flat 8 or the surface to be measured 9 and focused by the collimating objective lens 3. The light passes through the semi-transparent and semi-reflective prism 2 and is finally imaged on the CCD 1 located behind the semi-transparent and semi-reflective prism 2.

[0041] The beam parallelism of the two autocollimators is calibrated by adjusting their rotation angles so that the parallel light emitted by them illuminates the same optical flat 8. The reflected light spots then return to the center of their respective autocollimator CCDs. At this point, due to the autocollimation effect, when the centroid of the reflected light spot formed on the CCD coincides with the center of the CCD, it indicates that both autocollimators are perpendicular to the optical flat 8, meaning that the two autocollimators are parallel to each other. The relative positions of the two autocollimators are fixed to form a dual autocollimator. This calibration method requires no additional optical system and is quick and easy to operate.

[0042] Orient the exit port of the dual autocollimator toward the surface to be measured 9, and adjust the attitude of the dual autocollimator: After the dual autocollimator is placed near the surface to be measured 9, its emitted parallel light may not be aligned with the generatrix of the surface to be measured, causing the light spot to deviate from the CCD axis, such as... Figure 6The first picture shows ( Figure 6 (This is for illustrative purposes only, showing the combined light spots from two CCDs on a single image.) At this point, by fine-tuning the overall pitch angle and placement of the two autocollimators, ensuring the light spots on both CCDs are aligned on the same axis, measurement can begin. Figure 6 As shown in the second or third image, the two light spots can be on the same side or opposite sides.

[0043] Calculate the centroid coordinates of the light spot and derive the offset angle: (e.g.) Figure 3 As shown, when the optical surface under test is approximately planar, light emitted from the internal light source 5 of the collimator is reflected by the semi-transparent prism 2 and then exits as parallel light through the collimating objective lens 3. If the outgoing light is perpendicular to the tangent plane of the mirror surface 4 under test, the reflected light is focused at the center point O of the CCD according to the self-collimation principle; if the outgoing light is not perpendicular to the tangent plane of the mirror surface 4 under test, the reflected light is focused at point O′ on the CCD axis, i.e., the centroid position of the reflected light spot. The centroid coordinates are calculated using the first-order moment centroid algorithm. , )for:

[0044] ;

[0045] in, , For pixel coordinates, For pixel grayscale values, Given the pixel resolution of the CCD, the distance between the CCD center point O and point O′ can then be calculated. .

[0046] Based on geometric relationships, the offset angle of the autocollimator for: ,in It is the focal length of collimating objective lens 3. This formula converts the centroid offset distance into an angular quantity.

[0047] Calculate the radius of curvature of the surface to be measured 9: First, analyze the actual measurement process. When the surface of the optical element to be measured has a finite radius of curvature R (R > 0 for concave mirrors and R < 0 for convex mirrors), the reflected light is a spherical wave rather than a plane wave, which causes the image to deviate from the CCD image plane.

[0048] like Figure 4 As shown, the center of curvature of the reflected spherical wave is located at At that point, after being imaged by the collimating objective lens, the amount of displacement of the image point along the optical axis is... for: For a concave mirror, the image point is located on the side of the collimating objective lens's focal plane closer to the objective lens; for a convex mirror, the image point is located on the side of the collimating objective lens's focal plane farther from the objective lens.

[0049] In ultra-large radius of curvature ( Under the approximation, The distance between collimating objective lens 3 and the surface to be measured is denoted as . This offset causes a diffuse spot to form on the CCD image plane, with a diameter of [missing information]. for: ,in To ensure that the incident light can be approximated as a plane wave, the objective lens aperture must meet a critical condition. ,in The size is the CCD pixel size; if this approximation condition is not met, forcibly approximating it as a plane will introduce centroid detection error.

[0050] To reduce the impact of noise and dark background levels on the CCD, a denoising algorithm is used before centroid calculation: First, the spot image is thresholded, and the threshold is determined based on the spot quality and brightness on the CCD; then, the image is convolved with a Gaussian filter to reduce the influence of Gaussian noise while preserving low-frequency edge and contour information.

[0051] like Figure 5 As shown in the two figures above, when the two autocollimators are located on opposite sides of the vertex 9 of the surface under test, the reflected light spots of the two autocollimators are located on opposite sides of their respective CCD axes. At this time, the radius of curvature... for:

[0052] ;

[0053] like Figure 5 As shown in the two figures below, when the two autocollimators are located on the same side of vertex 9 of the surface under test, the reflected light spots of the two autocollimators are located on the same side of their respective CCD axes. At this time, the radius of curvature is... for:

[0054] ;

[0055] in, The distance between the two autocollimators. and The radius of curvature is calculated from the centroid offset distances of the two autocollimators. or If the value is positive, it indicates that the surface 9 to be measured is a concave mirror, and if the radius of curvature is... or A negative value indicates that the surface to be measured 9 is a convex mirror. This invention achieves compatible measurement of concave and convex mirrors without the need to change equipment or adjust the optical path.

[0056] To verify this method, focal length was used. ,spacing Taking a dual autocollimator with a CCD pixel size of 2.4μm, a horizontal dimension of 13.19mm, and a vertical dimension of 8.81mm as an example: when the collimating objective lens is at a distance of 1 / 3 mm from the surface to be measured... At this time, it can detect curved surfaces with a radius of curvature of 3m, and the diameter of the dispersion spot is... ≈3.135mm, location of the centroid of the light spot The blur spot falls completely on the CCD image plane, allowing for centroid calculation. This dual autocollimator can also detect curved surfaces with a radius of curvature greater than 3m. Furthermore, as the curvature increases, the defocusing amount decreases, the blur spot shrinks, and the measurement accuracy increases.

[0057] This method features a simple optical path structure, an easy-to-implement algorithm, and mature autocollimator technology. It does not rely on a high-precision optical platform and can be applied in optical workshops. Compared to traditional autocollimation methods, this invention eliminates the need for a large-stroke displacement stage, allowing for the measurement of ultra-large radii of curvature simply by fine-tuning the pitch angle. This simplifies operation, improves measurement repeatability, and enhances accuracy.

[0058] In summary, this invention proposes a method for measuring ultra-large radius of curvature, comprising:

[0059] Construct an optical path containing two autocollimators: Place the first autocollimator and the second autocollimator in parallel, with their exit ports pointing to the same optical flat or the surface to be measured. Each of the first and second autocollimators includes a CCD and a light source. Parallel light rays are emitted from the light source at the exit port and reflected onto the CCD by the optical flat or the surface to be measured to form a light spot.

[0060] The parallelism of the beams of the two autocollimators is calibrated: the exit ports of the two autocollimators are aligned with the same optical flat, and the rotation angles of the two autocollimators are adjusted so that the centroid of the light spot formed by the reflection on the CCD coincides with the center of the CCD. This indicates that the two autocollimators are perpendicular to the optical flat, that is, the two autocollimators are parallel to each other. The relative positions of the two autocollimators are fixed to form a double autocollimator.

[0061] Orient the exit port of the dual autocollimator toward the surface to be measured, and calibrate the attitude of the dual autocollimator: finely adjust the overall pitch angle and placement of the two autocollimators so that the light spots on the two CCDs are located on the same axis, and the two light spots can be on the same side or opposite sides.

[0062] Calculate the coordinates of the spot centroid and derive the offset angle: Calculate the coordinates of the spot centroids on the two CCD axes respectively, obtain the distance of the spot centroid from the CCD center, and then calculate the offset angle of the light emitted from the collimator relative to the tangent plane of the surface to be measured based on geometric principles.

[0063] Calculating the radius of curvature of the surface under test: When the two autocollimators are located on opposite sides of the vertex of the surface under test, the reflected light spots of the two autocollimators are located on opposite sides of their respective CCD axes. At this time, the radius of curvature is... for:

[0064] ;

[0065] When two autocollimators are located on the same side of the vertex of the surface under test, the reflected light spots of the two autocollimators are located on the same side of their respective CCD axes. At this time, the radius of curvature... for:

[0066] ;

[0067] in, The distance between the two autocollimators. and These represent the centroid offset distances of the two autocollimators. This is the focal length of the collimating objective lens.

[0068] The steps in this invention can be adjusted, combined, or deleted according to actual needs.

[0069] The units in the device of the present invention can be merged, divided, or reduced according to actual needs.

[0070] In this invention, the terms "installation," "connection," "linking," and "fixing" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; "linking" can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of these terms in this invention according to the specific circumstances.

[0071] The shapes of the components in the accompanying drawings are schematic and may differ from their actual shapes. The drawings are only used to illustrate the principles of the present invention and are not intended to limit the present invention.

[0072] Although the invention has been disclosed in detail with reference to the accompanying drawings, it should be understood that these descriptions are merely exemplary and not intended to limit the application of the invention. The scope of protection of the invention is defined by the appended claims and may include various modifications, alterations, and equivalents made to the invention without departing from the scope and spirit of the invention.

Claims

1. A method for measuring ultra-large radius of curvature, characterized in that, The method includes: Construct an optical path containing two autocollimators: Place the first autocollimator and the second autocollimator in parallel, with their exit ports pointing to the same optical flat or the surface to be measured. Each of the first and second autocollimators includes a CCD and a light source. When the light source is working, the autocollimator emits parallel light rays from its exit port, which are reflected by the optical flat or the surface to be measured onto the CCD to form a light spot. The parallelism of the beams of the two autocollimators is calibrated: the exit ports of the two autocollimators are aligned with the same optical flat, and the rotation angles of the two autocollimators are adjusted so that the centroid of the light spot formed by the reflection on the CCD coincides with the center of the CCD. This indicates that the two autocollimators are perpendicular to the optical flat, that is, the two autocollimators are parallel to each other. The relative positions of the two autocollimators are fixed to form a double autocollimator. Orient the exit port of the dual autocollimator toward the surface to be measured, and calibrate the attitude of the dual autocollimator: finely adjust the overall pitch angle and placement of the two autocollimators so that the light spots on the two CCDs are located on the same axis, and the two light spots can be on the same side or opposite sides. Calculate the coordinates of the spot centroid and derive the offset angle: Calculate the coordinates of the spot centroids on the two CCD axes respectively, obtain the distance of the spot centroid from the CCD center, and then calculate the offset angle of the light emitted from the collimator relative to the tangent plane of the surface to be measured based on geometric principles. Calculating the radius of curvature of the surface under test: When the two autocollimators are located on opposite sides of the vertex of the surface under test, the reflected light spots of the two autocollimators are located on opposite sides of their respective CCD axes. At this time, the radius of curvature is... for: ； When two autocollimators are located on the same side of the vertex of the surface under test, the reflected light spots of the two autocollimators are located on the same side of their respective CCD axes. At this time, the radius of curvature... for: ； in, The distance between the two autocollimators. and These represent the centroid offset distances of the two autocollimators. This is the focal length of the collimating objective lens.

2. The method for measuring ultra-large radius of curvature as described in claim 1, characterized in that, The first and second autocollimators each further include: a semi-transparent and semi-reflective prism and a collimating objective lens; the light source is located on the side of the autocollimator, and the light source emits light from the side toward the semi-transparent and semi-reflective prism. The semi-transparent and semi-reflective prism reflects the light along the optical axis of the autocollimator to the collimating objective lens, and the collimating objective lens collimates the light into parallel light, which is then emitted from the exit port of the autocollimator; the emitted light is reflected by the optical flat or the surface under test, and the reflected light is focused by the collimating objective lens, passes through the semi-transparent and semi-reflective prism, and finally forms an image on the CCD set behind the semi-transparent and semi-reflective prism.

3. The method for measuring ultra-large radius of curvature as described in claim 1, characterized in that, The first and second autocollimators use different reticles to distinguish the reflected light spots.

4. The method for measuring ultra-large radius of curvature as described in claim 1, characterized in that, The steps for calculating the centroid coordinates of the light spot and deriving the offset angle include: when the exit of the collimator is not perpendicular to the tangent plane of the mirror to be measured, the reflected light is focused at a non-central point O′ on the CCD axis, i.e., the centroid position of the reflected light spot. The centroid coordinates are calculated using the first-order moment centroid algorithm. , )for: ； in, , For pixel coordinates, For pixel grayscale values, Given the pixel resolution of the CCD, the distance between the CCD center point O and point O′ can then be calculated. ; Based on geometric relationships, the offset angle of the autocollimator for: .

5. The method for measuring ultra-large radius of curvature as described in claim 1, characterized in that, The radius of curvature of the surface to be measured must satisfy the critical condition. Where R is the radius of curvature, The objective lens aperture, This refers to the CCD pixel size.

6. The method for measuring ultra-large radius of curvature as described in claim 1, characterized in that, Before calculating the centroid spacing of the light spot, a denoising algorithm is used to reduce the influence of noise and background dark level on the CCD: First, the light spot image is thresholded, and the threshold is determined according to the quality and brightness of the light spot on the CCD; then the image is convolved with a Gaussian filter to reduce the influence of Gaussian noise while preserving low-frequency edge and contour information.

7. The method for measuring ultra-large radius of curvature as described in claim 1, characterized in that, Calculated radius of curvature or If it is positive, it indicates that the surface to be measured is a concave mirror; if the radius of curvature is positive... or A negative value indicates that the surface to be measured is a convex mirror.

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