Optical System Imaging Distortion Measurement Device and Method Based on Local Focal Length

Abstract

The present invention discloses an imaging distortion measurement device and method for an optical system based on local focal length, belonging to the technical field of optical precision measurement. It includes a workbench, an optical unit, an optical adjustment mechanism, a resolution target, and a host computer. The optical adjustment mechanism is used to adjust the optical unit up and down in the vertical plane, adjust the optical unit back and forth, left and right in the horizontal plane, and rotate the optical unit in the horizontal plane. The optical unit includes a plane mirror, an imaging detector, and a light source, a collimating lens, a reticle, a beam splitter, and an objective lens arranged according to the optical path. The optical system to be measured is installed between the objective lens and the resolution target, and the plane mirror and the optical system to be measured are located on the same side of the objective lens, where: the half-field optical path passing through the objective lens points to the optical system to be measured, and the other half-field optical path passing through the objective lens points to the plane mirror. The present invention not only realizes the high-precision measurement of the imaging distortion of the optical system, but also has an accurate and efficient measurement method.

Description

Technical Field

[0001] The present invention relates to the field of optical precision measurement, and particularly relates to an optical system imaging distortion measurement device and method based on local focal length. Background Art

[0002] Distortion widely exists in various optical systems. The reason for the distortion of an optical system is that the vertical magnification changes with the change of the field angle of view and no longer remains constant, resulting in the loss of similarity between the image and the object. This imaging defect that deviates from the target mapping projection relationship is called distortion. Although distortion does not affect the clarity of imaging, it directly affects the geometric position accuracy of imaging.

[0003] By imaging a standard grid board, the measurement of the imaging distortion of an optical system can be achieved. However, for many application scenarios, it is difficult to construct the grid board. For example, for a wide-angle lens with a large field of view, the field angle exceeds 120°, and even up to 300°. The size of the required grid board is extremely large. There are also some more complex object-image projection mappings. For example, if the object surface is a curved surface, then a curved grid board needs to be constructed. If the mapping from the object surface to the image surface is a high-order function, then the grid pattern on the grid board is more complex. The difficulties in constructing the grid board in these scenarios all stem from the complex object side. Relatively speaking, the image side is always a planar detector, and the sizes of each pixel unit are the same and evenly distributed. Therefore, the image surface scenario is simpler than the object side scenario. Placing a target on the image surface and observing from the object side is a simple way to measure the imaging distortion of an optical system. At the same time, the "small image height method" as a description method for focal length measurement is highly consistent with the definition of the projection model and is a special case when the field angle of view in the projection model is extremely small. Summary of the Invention

[0004] Object of the Invention: In order to overcome the deficiencies existing in the prior art, the present invention provides an optical system imaging distortion measurement device and method based on local focal length to meet the high-precision measurement of the imaging distortion of various complex optical systems.

[0005] Technical Solution: To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] An imaging distortion measurement device for an optical system based on local focal length, comprising a workbench, an optical unit, an optical adjustment mechanism, a resolution target, and a host computer. The optical adjustment mechanism and the resolution target are installed on the workbench. The optical unit is installed on the optical adjustment mechanism. The optical adjustment mechanism is used to adjust the optical unit up and down in a vertical plane, adjust the optical unit back and forth, left and right in a horizontal plane, and rotate the optical unit in a horizontal plane. The optical unit includes a plane mirror, an imaging detector, and a light source, a collimating lens, a reticle, a beam splitter, and an objective lens arranged according to the optical path. Observation and aiming markings are provided on the reticle. The imaging detector is arranged on the side of the beam splitter away from the objective lens. The optical system to be measured is installed between the objective lens and the resolution target, and the plane mirror and the optical system to be measured are on the same side of the objective lens, where: the half-field optical path passing through the objective lens points to the optical system to be measured, and the other half-field optical path passing through the objective lens points to the plane mirror.

[0007] Preferably: The optical adjustment mechanism includes a first front-back adjustment component, a first left-right adjustment component, a first up-down adjustment component, and a first rotation component. The fixed end of the first front-back adjustment component is installed on the workbench. The fixed end of the first left-right adjustment component is installed on the movable end of the first front-back adjustment component. The fixed end of the first up-down adjustment component is installed on the movable end of the first left-right adjustment component. The fixed end of the first rotation component is installed on the movable end of the first up-down adjustment component. The optical unit is installed on the rotating end of the first rotation component.

[0008] Preferably: The plane mirror is installed on the workbench through a plane mirror adjustment mechanism. The plane mirror adjustment mechanism includes a second front-back adjustment component, a second left-right adjustment component, and a second up-down adjustment component. The fixed end of the second front-back adjustment component is installed on the workbench. The fixed end of the second left-right adjustment component is installed on the movable end of the second front-back adjustment component. The fixed end of the second up-down adjustment component is installed on the movable end of the second left-right adjustment component. The plane mirror is installed on the movable end of the first up-down adjustment component.

[0009] Preferably: The resolution target is installed on the workbench through a target adjustment mechanism. The target adjustment mechanism includes a horizontal movement component and a vertical movement component. The horizontal movement component is installed on the workbench. The vertical movement component is installed on the mobile head of the horizontal movement component. The resolution target is installed on the mobile head of the vertical movement component. The optical system to be measured is installed on the workbench through a third rotation component. The third rotation component is used to rotate the optical system to be measured 90 degrees along a vertical plane.

[0010] An imaging distortion measurement method for an optical system based on local focal length, using the imaging distortion measurement device for an optical system based on local focal length, comprising the following steps:

[0011] Step 1: Divide the sampling field of view according to the characteristics of the optical system to be measured. At the same time, determine the nominal image height value according to the characteristics of the optical system to be measured.

[0012] Step 2: Select the line pairs on the resolution target as the observation targets, and adjust the line pairs to the image plane position of the optical system to be measured.

[0013] Step 3: Adjust the optical path of the optical unit through the optical adjustment mechanism, aim at the center scale line of the selected line pair, and make the image of the center scale line of the line pair located at the center of the imaging detector.

[0014] Step 4: Install a plane mirror to block the half-field optical path.

[0015] Step 5: Adjust the plane mirror through the plane mirror adjustment mechanism to image the aiming scale line on the reticle at the center of the imaging detector, so that the aiming scale line coincides with the center scale line of the line pair selected on the resolution target.

[0016] Step 6: Adjust the optical system to be measured through the third rotation assembly, rotate the optical path of the optical unit through the optical adjustment mechanism, aim at the adjacent scale lines of the line pair selected on the resolution target, obtain the local focal length value of a single field of view, and then obtain the local focal length values of all sampling field of view points.

[0017] Step 7: Obtain the discrete values of the actual image height according to the obtained local focal length values of all sampling field of view points, and then obtain the actual image height value through fitting. Compare the actual image height value with the nominal image height value to obtain the distortion value of the optical system to be measured.

[0018] Preferably, the calculation formula for the discrete values of the actual image height at each field of view position in Step 7 is:

[0019]

[0020] Wherein, represents the discrete value of the actual image height at each field of view position, represents the sampling field of view in the x direction, represents the sampling field of view in the y direction, represents that the full field of view is equally divided into m parts, represents the field angle.

[0021] Preferably, the method for selecting the line pairs on the resolution target as the observation targets in Step 2: First, according to the limiting resolution ability of the optical system to be measured, select the corresponding line pairs on the resolution target as the observation targets. The formula for selecting the corresponding line pairs on the resolution target as the observation targets is as follows:

[0022]

[0023] Wherein, It represents selecting the corresponding line pair width on the resolution target, and d represents the limit resolution of the optical system to be measured.

[0024] Then install the resolution target on the target adjustment mechanism, and adjust the horizontal and vertical displacements of the resolution target on the workbench through the target adjustment mechanism, so that the resolution target is adjusted to the image plane position of the optical system to be measured.

[0025] Preferably: The method in step 3 to make the imaging of the central ruling of the line pair located at the center of the imaging detector: Adjust the optical adjustment mechanism so that the rotation center of the optical adjustment mechanism coincides with the entrance pupil of the optical system to be measured. Through the target adjustment mechanism, adjust the central ruling of the selected line pair on the resolution target to the center field of view position of the image plane of the optical system to be measured, and adjust the optical adjustment mechanism to aim at the selected line pair, so that the imaging of the selected line pair is located at the center position of the imaging detector.

[0026] The method in step 5 to make the aiming ruling coincide with the central ruling of the selected line pair on the resolution target: Adjust the plane mirror through the plane mirror adjustment mechanism so that the aiming ruling on the graticule is reflected to the center position of the imaging detector after passing through the plane mirror, and the aiming ruling coincides with the central ruling of the selected line pair on the resolution target.

[0027] Preferably: The method in step 6 to obtain the local focal length value of a single field of view:

[0028] Adjust the optical system to be measured to be parallel to the horizontal plane through the third rotation assembly, divide the x-direction full field of view into m equal parts, denoted as , … , rotate the optical unit on the horizontal plane through the first rotation assembly, aim at the adjacent rulings of the selected line pair on the resolution target, read the rotation angle of the optical adjustment mechanism, and obtain the local focal length at the field of view position by dividing the line interval of the aiming ruling by the rotation angle. The local focal length of the field of view position.

[0029] The method in step 6 to obtain the local focal length value of the x-direction full sampling field of view points:

[0030] Rotate the optical unit on the horizontal plane through the first rotation assembly, aim at the adjacent rulings of the selected line pair on the resolution target, translate the aiming ruling to be observed on the resolution target to the next image plane sampling field of view point, and obtain the local focal length value of the new field of view point until the local focal length values of m sampling field of view points in the x-direction are obtained, denoted as , ... .

[0031] Method for obtaining the local focal length value of the full sampling field of view points in the y direction in Step 6: Rotate the optical system under test by 90 degrees through the third rotation component, that is, make the optical system under test perpendicular to the horizontal plane. Repeat the process of obtaining the local focal length values of the m sampling field of view points in the x direction to obtain the local focal length values of the m sampling field of view points in the y direction, denoted as , ... 。

[0032] Preferably: Method for dividing the sampling field of view in Step 1: When dividing the field of view interval measured by the optical system, if the change in the nominal local focal length within a certain position range is within the first focal length change threshold range, an equal interval sampling method is adopted. If the change in the nominal local focal length within a certain position range is within the second focal length change threshold range, the sampling density is increased at the corresponding position.

[0033] The present invention has the following beneficial effects compared with the prior art:

[0034] (1) The present invention is aimed at the high-precision measurement of the imaging distortion of the optical system, especially for the optical system with a complex object-image mapping relationship and the problem of difficult construction of the object-side grid plate. It changes the traditional measurement method of observing the object-side grid plate from the image side to the measurement method of observing the image-side target from the object side, and builds an optical system imaging distortion measurement device based on the local focal length. By obtaining the actual image height discrete values from the local focal length values that change with the field of view distribution, and then obtaining the actual image height expression function through polynomial fitting, the high-precision measurement of the optical system imaging distortion is realized.

[0035] (2) For the optical system under test with different focal lengths, through the bright and dark stripes of different scales on the resolution target, it simulates the different scale image height increments corresponding to the extremely small field of view angle increments in the "small image height method", which is accurate and efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a schematic diagram of the principle optical path of the optical system imaging distortion measurement device based on the local focal length of the present invention.

[0037] Figure 2 is a schematic diagram of the rotation of the optical adjustment mechanism of the present invention.

[0038] Figure 3 is a schematic diagram of the resolution target of the present invention.

[0039] Figure 4 is Figure 3 a partial enlarged schematic diagram of part A in

[0040] Figure 5 is a schematic diagram of the flow of the optical system imaging distortion measurement method based on the local focal length of the present invention.

[0041] Figure 6 This is the curve graph of the measurement result of the imaging distortion of the optical system to be measured in the embodiment of the present invention.

[0042] In the figure, 1 - light source, 2 - collimating lens, 3 - reticle, 4 - beam splitter, 5 - objective lens, 6 - plane mirror, 7 - optical system to be measured, 8 - resolution target, 9 - imaging detector, 10 - host computer, 11 - optical axis, 12 - light ray, 13 - optical adjustment mechanism, 14 - rotation angle θ, 15 - image height increment Δy'. Specific embodiments

[0043] The present invention will be further clarified below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. After reading the present invention, various equivalent modifications made by those skilled in the art to the present invention all fall within the scope defined by the appended claims of this application.

[0044] Embodiment 1

[0045] An optical system imaging distortion measurement device based on local focal length, as Figures 1-4 shown, includes a workbench, an optical unit, an optical adjustment mechanism 13, a resolution target 8, and a host computer 10. The optical adjustment mechanism 13 and the resolution target 8 are installed on the workbench, the optical unit is installed on the optical adjustment mechanism 13, the optical adjustment mechanism 13 is a precision rotary table, and the precision of the precision rotary table is generally 5'. The optical adjustment mechanism 13 is used to adjust the optical unit up and down in the vertical plane, adjust the optical unit back and forth, left and right in the horizontal plane, and rotate the optical unit in the horizontal plane. The optical unit includes a plane mirror 6, an imaging detector 9, and a light source 1, a collimating lens 2, a reticle 3, a beam splitter 4, and an objective lens 5 arranged according to the optical path. A sighting scale is provided on the reticle 3. The imaging detector 9 is arranged on the side of the beam splitter 4 away from the objective lens 5. The optical system 7 to be measured is installed between the objective lens 5 and the resolution target 8, and the plane mirror 6 and the optical system 7 to be measured are on the same side of the objective lens 5. Among them: the half-field optical path passing through the objective lens 5 points to the optical system 7 to be measured, the other half-field optical path passing through the objective lens 5 points to the plane mirror 6, and the imaging detector 9 is a charge-coupled device, with the full English name of charge-coupled device, abbreviated as CCD.

[0046] The light generated by the light source 1 becomes parallel light after passing through the collimating lens 2. The parallel light irradiates on the reticle 3, and the light passing through the reticle 3 carries the aiming scale lines. The light with the aiming scale lines is reflected by the beam splitter 4 onto the objective lens 5. Under the action of the objective lens 5, the light with the aiming scale lines forms parallel light with the aiming scale lines. The parallel light with the aiming scale lines is divided into two half-field optical paths (dividing the field of view into two half-fields), denoted as half-field optical path one and half-field optical path two. In half-field optical path one, the parallel light with the aiming scale lines irradiates on the optical system to be measured 7 through half-field optical path one. After the action of the optical system to be measured 7, the light irradiates on the resolution target 8. The resolution target 8 reflects the light, and the reflected light sequentially passes through the optical system to be measured 7, the objective lens 5, and the beam splitter 4 and enters the center of the imaging detector 9, so that the line pairs of the resolution target 8 are imaged at the center of the field of view of the imaging detector. In half-field optical path two, the parallel light with the aiming scale lines irradiates on the plane mirror 6 through half-field optical path one. The plane mirror 6 reflects the light, and the reflected light sequentially passes through the objective lens 5 and the beam splitter 4 and enters the center of the imaging detector 9, so that the aiming scale lines on the reticle 3 are imaged at the center of the field of view of the imaging detector. Among them, as Figure 3 and 4 shown, the line pairs on the resolution target 8 are the key elements for quantifying the resolution of the imaging system and contain multiple groups of line pairs with different densities.

[0047] In another embodiment, as Figure 2As shown, the optical adjustment mechanism 13 includes a first front-back adjustment component, a first left-right adjustment component, a first up-down adjustment component, and a first rotation component. The fixed end of the first front-back adjustment component is installed on the workbench. The fixed end of the first left-right adjustment component is installed on the movable end of the first front-back adjustment component. The fixed end of the first up-down adjustment component is installed on the movable end of the first left-right adjustment component. The fixed end of the first rotation component is installed on the movable end of the first up-down adjustment component. The optical unit is installed on the rotating end of the first rotation component. In this embodiment, the first front-back adjustment component drives the first left-right adjustment component, the first up-down adjustment component, and the first rotation component to move back and forth on the horizontal plane, thereby driving the optical unit to move back and forth on the horizontal plane. The first left-right adjustment component drives the first up-down adjustment component and the first rotation component to move left and right on the horizontal plane, thereby driving the optical unit to move left and right on the horizontal plane. The first up-down adjustment component drives the first rotation component to move up and down in the vertical plane, thereby driving the optical unit to move up and down in the vertical plane. The first rotation component drives the optical unit to rotate on the horizontal plane, realizing the change of the field of view angle of the optical unit on the horizontal plane. The first front-back adjustment component, the first left-right adjustment component, and the first up-down adjustment component in this embodiment can be a screw adjustment mechanism, a slider adjustment mechanism, etc. The first rotation component can be a gear step rotation adjustment mechanism, a stepping motor rotation adjustment mechanism, etc. It only needs that the first front-back adjustment component can perform front-back movement adjustment under the condition of ensuring accuracy. Similarly, the first left-right adjustment component only needs to be able to perform left-right and front-back adjustment under the condition of ensuring accuracy. The first up-down adjustment component only needs to be able to perform up-down movement adjustment under the condition of ensuring accuracy. The first rotation component only needs to be able to perform rotation adjustment on the horizontal plane under the condition of ensuring accuracy. Therefore, the specific implementation structures of the first front-back adjustment component, the first left-right adjustment component, the first up-down adjustment component, and the first rotation component in this embodiment are common knowledge to those skilled in the art and will not be elaborated here.

[0048] In another embodiment, the planar mirror 6 is mounted on the workbench through a planar mirror adjusting mechanism. The planar mirror adjusting mechanism includes a second front-back adjusting component, a second left-right adjusting component, and a second up-down adjusting component. The fixed end of the second front-back adjusting component is mounted on the workbench. The fixed end of the second left-right adjusting component is mounted on the movable end of the second front-back adjusting component. The fixed end of the second up-down adjusting component is mounted on the movable end of the second left-right adjusting component. The planar mirror 6 is mounted on the movable end of the first up-down adjusting component. In this embodiment, the second front-back adjusting component drives the second left-right adjusting component, the second up-down adjusting component, and the second rotating component to move back and forth on the horizontal plane, thereby driving the planar mirror 6 to move back and forth on the horizontal plane. By driving the second up-down adjusting component and the second rotating component to move left and right on the horizontal plane through the second left-right adjusting component, the planar mirror 6 is driven to move left and right on the horizontal plane. By driving the planar mirror 6 to move up and down in the vertical plane through the second up-down adjusting component, the adjustment of the position of the planar mirror 6 is realized. The second front-back adjusting component, the second left-right adjusting component, and the second up-down adjusting component in this embodiment can be a lead screw adjusting mechanism or a slider adjusting mechanism, as long as the adjustment accuracy of the second front-back adjusting component, the second left-right adjusting component, and the second up-down adjusting component is ensured. Therefore, the specific implementation structures of the second front-back adjusting component, the second left-right adjusting component, and the second up-down adjusting component in this embodiment are common knowledge to those skilled in the art and will not be elaborated here.

[0049] In another embodiment, the resolution target 8 is mounted on the workbench through a target adjusting mechanism. The target adjusting mechanism includes a horizontal moving component and a vertical moving component. The horizontal moving component is mounted on the workbench. The vertical moving component is mounted on the moving head of the horizontal moving component. The resolution target 8 is mounted on the moving head of the vertical moving component. In this embodiment, the horizontal moving component drives the vertical moving component to move on the horizontal plane, thereby driving the resolution target 8 to move on the horizontal plane. The vertical moving component drives the resolution target 8 to move in the vertical plane. The optical system under test 7 is mounted on the workbench through a third rotating component. The third rotating component is used to rotate the optical system under test 7 by 90 degrees along the vertical plane. By the cooperation of the third rotating component and the optical adjusting mechanism 13, the change of the field of view angle is realized. The horizontal moving component and the vertical moving component in this embodiment can be a lead screw adjusting mechanism, a slider adjusting mechanism, etc. The third rotating component can be a gear step rotating adjusting mechanism, a stepping motor rotating adjusting mechanism, etc. As long as the horizontal moving component, the vertical moving component, and the third rotating component can be adjusted under the condition of ensuring accuracy. The specific implementation structures of the horizontal moving component, the vertical moving component, and the third rotating component in this embodiment are common knowledge to those skilled in the art and will not be elaborated here.

[0050] Embodiment 2

[0051] A method for measuring imaging distortion of an optical system based on local focal length divides multiple sampling fields of view within the full field of view, simulates the "small image height method" to calculate the local focal lengths of each field of view, obtains the discrete values of the actual image height through the local focal length values that vary with the distribution of the field of view, and then obtains the expression function of the actual image height through polynomial fitting to achieve high-precision measurement of the imaging distortion of the optical system. As Figure 5 shown, the specific steps are as follows:

[0052] Step 1: Divide the sampling fields of view according to the characteristics of the optical system 7 to be measured. At the same time, determine the nominal image height value according to the characteristics of the optical system 7 to be measured.

[0053] When dividing the field of view interval for optical system measurement, if the change of the nominal local focal length is gentle, an equal-interval sampling method is adopted. If the local distribution changes violently, the sampling density is appropriately increased at this position.

[0054] In another embodiment, the method for dividing the sampling fields of view: When dividing the field of view interval for optical system measurement, if the change of the nominal local focal length within a certain position range is within the set first focal length change threshold range, an equal-interval sampling method is adopted. If the change of the nominal local focal length within a certain position range is within the second focal length change threshold range, the sampling density is increased at the corresponding position.

[0055] The nominal image height value is the theoretical image height value obtained according to the projection models of different optical systems, and the following are respectively the expression formulas of the object-image mapping relationship projection models such as equilinear projection, equidistant projection, stereoscopic projection, equal solid angle projection, and orthogonal projection:

[0056]

[0057] Among them, represents the nominal image height value, represents the total focal length of the optical system to be measured, represents the field of view angle.

[0058] Step 2: Select the line pairs on the resolution target as the observation targets, and adjust the line pairs to the image plane position of the optical system 7 to be measured.

[0059] The ultimate resolution ability of an optical system refers to the minimum distance at which the "image" after the "object" passes through the optical system can be resolved in detail. According to the Rayleigh criterion, the ultimate resolution d of the optical system = 1.22λ × F, where λ is the wavelength and F is the F-number of the lens, that is, the reciprocal of the relative aperture. Therefore, the width of the line pairs selected on the resolution target should be slightly larger than and as close as possible to the ultimate resolution of the optical system to be measured.

[0060] In another embodiment, the method for selecting the line pairs on the resolution target as the observation targets: First, according to the ultimate resolution ability of the optical system 7 to be measured, select the corresponding line pairs on the resolution target 8 as the observation targets. The formula for selecting the corresponding line pairs on the resolution target 8 as the observation targets is as follows:

[0061]

[0062] Wherein, represents the width of the corresponding line pairs selected on the resolution target 8, and d represents the ultimate resolution of the optical system 7 to be measured.

[0063] Then install the resolution target 8 on the target adjustment mechanism, and adjust the horizontal and vertical displacements of the resolution target 8 on the workbench through the target adjustment mechanism, so that the resolution target 8 is adjusted to the image plane position of the optical system 7 to be measured.

[0064] Step 3: Adjust the optical path of the optical unit through the optical adjustment mechanism 13, aim at the center line of the selected line pairs, and make the image of the center line of the line pairs located at the center of the imaging detector 9.

[0065] In another embodiment, the method for making the image of the center line of the line pairs located at the center of the imaging detector 9: Fix the light source 1, collimating lens 2, reticle 3, beam splitter 4, objective lens 5, and imaging detector 9 on the optical adjustment mechanism 13, and adjust the optical adjustment mechanism 13 so that the rotation center of the optical adjustment mechanism 13 coincides with the entrance pupil of the optical system 7 to be measured. Adjust the center line of the selected line pairs on the resolution target 8 to the center field of view position of the image plane of the optical system 7 to be measured through the target adjustment mechanism, and adjust the optical adjustment mechanism 13 to aim at the selected line pairs, so that the image of the selected line pairs is located at the center position of the imaging detector 9.

[0066] Step 4: Install the plane mirror 6 to block the half-field optical path.

[0067] Install the plane mirror 6 between the objective lens 5 and the optical system 7 to be measured, so that the plane mirror 6 blocks the half-field optical path, that is, the half-field optical path passing through the objective lens 5 points to the optical system 7 to be measured, and the other half-field optical path passing through the objective lens 5 points to the plane mirror 6.

[0068] Step 5: Adjust the plane mirror 6 through the plane mirror adjustment mechanism to image the aiming line on the reticle 3 at the center of the imaging detector 9, so that the aiming line coincides with the center line of the selected line pairs on the resolution target 8.

[0069] In another embodiment, a method for aligning the aiming scale line with the center scale line of the selected line pair on the resolution target 8: Adjust the plane mirror 6 through the plane mirror adjusting mechanism so that the aiming scale line on the reticle 3 is reflected by the plane mirror 6 to the center position of the imaging detector 9, making the aiming scale line coincide with the center scale line of the selected line pair on the resolution target 8.

[0070] Step 6: Rotate the optical path of the optical unit through the optical adjusting mechanism 13 to aim at the adjacent scale lines of the selected line pair on the resolution target 8, obtain the local focal length value of a single field of view, and further obtain the local focal length values of all sampled field points.

[0071] The local focal length values of all sampled field points include the local focal length values of all sampled field points in the x direction and the local focal length values of all sampled field points in the y direction.

[0072] The original definition of the "small image height method" is to describe the experimental measurement method of the traditional focal length. By tracing the paraxial ray passing through the center of the entrance pupil and having a very small angle with the optical axis The height of the paraxial ray on the image plane after passing through the optical system Calculate the focal length, and the focal length f′ is expressed as .

[0073] The method for obtaining the local focal length value of a single field of view in Step 6:

[0074] Adjust the optical system 7 to be measured to be parallel to the horizontal plane through the third rotation component, divide the full field of view in the x direction into m equal parts, denoted as , … , rotate the optical unit on the horizontal plane through the first rotation component to aim at the adjacent scale lines of the selected line pair on the resolution target 8, read the rotation angle of the optical adjusting mechanism 13, and obtain the local focal length at the field of view position by dividing the line interval of the aiming scale line by the rotation angle The local focal length of the field of view position.

[0075] The method for obtaining the local focal length value of all sampled field points in the x direction in Step 6:

[0076] Rotate the optical unit on the horizontal plane through the first rotation component to aim at the adjacent scale lines of the selected line pair on the resolution target 8, translate the aiming scale line to be observed on the resolution target 8 to the next image plane sampling field point, obtain the local focal length value of the new field point, until the local focal length values of m sampling field points in the x direction are obtained, denoted as , ... .

[0077] Obtain the local focal length value of all sampled field points in the y direction in Step 6.

[0078] Rotate the optical system 7 to be measured by 90 degrees through the third rotation component, that is, make the optical system 7 to be measured perpendicular to the horizontal plane. Repeat the process of obtaining the local focal length values at m sampling field points in the x direction to obtain the local focal length values at m sampling field points in the y direction, denoted as , ... .

[0079] Step 7: Obtain the discrete values of the actual image height according to the local focal length values of the full-sampling field points in the x direction and the y direction, and then obtain the actual image height value through fitting. Compare the actual image height value with the nominal image height value to obtain the distortion value of the optical system 7 to be measured.

[0080] The method for obtaining the distortion value of the optical system 7 to be measured in Step 7:

[0081] First, regard the local focal length as the image height increment corresponding to a very small field angle increment at the sampling field. The object-image mapping relationship of the entire optical system is a distribution function of the local focal length varying with the field. Obtain the discrete values of the actual image height at each field position according to the local focal length values of the full-sampling field points in the x direction and the y direction.

[0082] Perform a numerical summation on the local focal length values of the full-sampling field points in the x direction and the y direction:

[0083]

[0084]

[0085] The calculation formula for the discrete value of the actual image height at each field position is:

[0086]

[0087] Among them, represents the discrete value of the actual image height at each field position, represents the sampling field in the x direction, represents the sampling field in the y direction, represents that the full field is equally divided into m parts, represents the field angle.

[0088] Secondly, in the object-image mapping relationships described by functions such as linear projection and equidistant projection, the field angle θ is the independent variable and the image height is the dependent variable. The image height of the optical system to be measured can be characterized by the fitting function of the local focal lengths of several sampling fields. y is the actual image height, which is obtained by performing a least-squares fit on the values of the local focal lengths varying with the field distribution. Obtain the actual image height expression function by polynomial fitting the discrete values of the actual image height at each field position, and then determine the actual image height value. Actual image height expression function:

[0089]

[0090] in, Indicates the actual image height value, 、 、 … represents the polynomial coefficients, Indicates the field of view angle.

[0091] Compare the actual image height value with the nominal image height value to obtain the distortion value of the optical system to be measured, that is, .in, represents the distortion of the optical system to be measured, Indicates the actual image height value, Indicates the nominal image high value.

[0092] Example 3

[0093] This embodiment is a simulation example. This embodiment is for a full field of view of 160°, a system focal length of 1.61 mm, an F number of 4.2, and a working wavelength of visible light. The device for the wide-angle lens with equidistant projection adopts the device of Example 1. The initial optical path is as follows: the light source emits light, which is converted into parallel light after passing through the collimating lens. The parallel light passes through the graticule to the beam splitter. The light of half the field of view passes through the objective lens and then is reflected back to the imaging detector by the plane reflector, so that the sighting line on the graticule is imaged in the center of the imaging detector's field of view. The reflected light of the other half of the field of view passes through the objective lens and the wide-angle lens to be measured, aiming at the target line pair on the resolution target, and is also imaged in the imaging detector's field of view. A method for measuring the imaging distortion of an optical system based on local focal length in this embodiment includes the following steps:

[0094] Step 1: Divide the sampling field of view according to the characteristics of the optical system to be measured.

[0095] According to the equidistant projection model of the wide-angle lens, an equal-interval sampling method is adopted when dividing the field of view interval, and the interval is selected as 8°.

[0096] Step 2: Select a reasonable line pair on the resolution target as the observation target and adjust it to the image plane position of the optical system to be measured.

[0097] The limiting resolution of this wide-angle lens is d = 1.22λ×F=1.22×0.587×10 -6×4.2≈3.01 µm. According to the standard of selecting reasonable line pairs on the resolution target as the observation object described in step 2, the line pairs selected on the resolution target should not be higher than and as close as possible to 1 / 3.01 µm≈332 lp / mm. In this embodiment, an Extreme Group 11 resolution target produced by Ready Optics is selected, and different resolution units are marked with corresponding groups and elements. The 3rd element mark of group 8 is selected as the observation object, with a resolution of 322.5 lp / mm. It is fixedly placed on a displacement stage with horizontal and vertical displacement functions and adjusted to the image plane position of the optical system to be measured.

[0098] Step 3: Adjust the optical path to aim at the center line of the selected line pair so that its image is located at the center of the imaging detector.

[0099] Fix the light source, collimating lens, reticle, beam splitter, objective lens, and imaging detector on a high-precision rotary workbench, and adjust its rotation center to coincide with the entrance pupil of the wide-angle lens to be measured. Adjust the center line of the 3rd element mark of group 8 of the resolution target to the center field of view position of the image plane of the wide-angle lens to be measured through the displacement stage, and adjust the optical adjustment mechanism to aim at this line, and the image of the line is located at the center position of the imaging detector.

[0100] Step 4: Insert a plane mirror to block the half-field optical path.

[0101] Insert a plane mirror between the objective lens and the wide-angle lens to be measured so that it blocks the half-field optical path.

[0102] Step 5: Adjust the plane mirror so that the aiming line on the reticle is imaged at the center of the imaging detector.

[0103] Adjust the plane mirror so that the aiming line on the reticle is reflected to the center position of the imaging detector after passing through the plane mirror, and the aiming line coincides with the center line of the 3rd element mark of group 8 of the resolution target.

[0104] Step 6: Rotate the optical adjustment mechanism to aim at the adjacent line and calculate the local focal length value of this field of view.

[0105] Rotate the optical adjustment mechanism to aim at the adjacent line of the 3rd element mark of group 8 of the resolution target, read the rotation angle of the optical adjustment mechanism, and the rotation angles of each sampling field of view are 0.1070°, 0.1103°, 0.1104°, 0.1106°, 0.1107°, 0.1108°, 0.1107°, 0.1104°, 0.1100°, 0.1097°, 0.1106° respectively. Calculate the local focal length of this field of view position by dividing the line interval of the aiming line pair by the rotation angle.

[0106] Translate the line pairs to be observed and aimed on the resolution target to the next image plane sampling field point. According to the above steps, obtain the local focal length values of the new field points. Repeat the above steps until the local focal length values of all sampling field points in the x direction are obtained, which are 1.6100 mm, 1.6094 mm, 1.6078 mm, 1.6056 mm, 1.6036 mm, 1.6027 mm, 1.6040 mm, 1.6084 mm, 1.6151 mm, 1.6194 mm, 1.6055 mm respectively.

[0107] The wide-angle lens to be measured in this embodiment is a rotationally symmetric system. Rotate the optical system to be measured 90° in the vertical plane and repeat the above process to obtain the local focal length values of all sampling field points in the y direction, which are 1.6111 mm, 1.6094 mm, 1.6076 mm, 1.6058 mm, 1.6037 mm, 1.6027 mm, 1.6041 mm, 1.6085 mm, 1.6150 mm, 1.6196 mm, 1.6059 mm respectively.

[0108] Step 7: Calculate the discrete values of the actual image height, and through polynomial fitting, obtain the expression function of the actual image height. Compare it with the nominal image height value to obtain the system distortion.

[0109] Perform numerical summation on the local focal length values that change with the field of view distribution in the x direction to obtain the discrete values of the actual image height at each field position, which are 0, 0.2248 mm, 0.4494 mm, 0.6737 mm, 0.8978 mm, 1.1216 mm, 1.3454 mm, 1.5696 mm, 1.7947 mm, 2.0206 mm, 2.2461 mm respectively. Perform numerical summation on the local focal length values that change with the field of view distribution in the y direction to obtain the discrete values of the actual image height at each field position, which are 0, 0.2249 mm, 0.4494 mm, 0.6735 mm, 0.8980 mm, 1.1217 mm, 1.3455 mm, 1.5697 mm, 1.7946 mm, 2.0207 mm, 2.2463 mm respectively. Calculate the root mean square values of the local image heights in the x direction and the y direction, which are

[0110] 0, 0.2249 mm, 0.4494 mm, 0.6736 mm, 0.8979 mm, 1.1217 mm, 1.3455 mm, 1.5697 mm, 1.7947 mm, 2.0207 mm, 2.2462 mm. The image height of the optical system to be measured can be characterized by the fitting function of the local focal lengths of several sampling fields. y is the actual image height, which is obtained by performing least squares fitting on the numerical values of the local focal length values that change with the field of view distribution. The actual image height y is: , wherein, , , … are parameters to be determined. After substituting the data, we get . As Figure 6 shown, the distortion value of the optical system to be measured is then obtained by comparing with the nominal image height, and the maximum distortion value of the optical system to be measured is 0.2498%.

[0111] The present invention is directed to the high-precision measurement of the imaging distortion of an optical system, especially for an optical system with a complex object-image mapping relationship. It breaks through the core problem of the difficulty in constructing the object-side grid plate, changes the traditional measurement method of observing the object-side grid plate from the image side to the measurement method of observing the image-side target from the object side, and builds an optical system imaging distortion measurement device based on the local focal length to achieve the high-precision measurement of the imaging distortion of the optical system. At the same time, for the measured objects of optical systems with different focal lengths, different scales of bright and dark stripes on the resolution target are used to simulate the different scales of image height increments corresponding to the extremely small field angle increments in the "small image height method", and the method is accurate and efficient.

[0112] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An imaging distortion measurement device for an optical system based on local focal length, characterized in that It includes a workbench, an optical unit, an optical adjustment mechanism (13), a resolution target (8), and a host computer (10). The optical adjustment mechanism (13) and the resolution target (8) are installed on the workbench. The optical unit is installed on the optical adjustment mechanism (13). The optical adjustment mechanism (13) is used to adjust the optical unit up and down in the vertical plane, adjust the optical unit forward, backward, left and right in the horizontal plane, and rotate the optical unit in the horizontal plane. The optical unit includes a plane mirror (6), an imaging detector (9), and a light source (1), a collimating lens (2), a reticle (3), a beam splitter (4), and an objective lens (5) arranged according to the optical path. The reticle (3) is provided with aiming graduations. The imaging detector (9) is arranged on the side of the beam splitter (4) away from the objective lens (5). The optical system to be measured (7) is installed between the objective lens (5) and the resolution target (8), and the plane mirror (6) and the optical system to be measured (7) are on the same side of the objective lens (5). Among them: The half-field optical path passing through the objective lens (5) points to the optical system to be measured (7), and the other half-field optical path passing through the objective lens (5) points to the plane mirror (6).

2. The imaging distortion measuring device of the optical system based on local focal length according to claim 1, wherein: The optical adjustment mechanism (13) includes a first front-back adjustment component, a first left-right adjustment component, a first up-down adjustment component, and a first rotation component. The fixed end of the first front-back adjustment component is installed on the workbench. The fixed end of the first left-right adjustment component is installed on the moving end of the first front-back adjustment component. The fixed end of the first up-down adjustment component is installed on the moving end of the first left-right adjustment component. The fixed end of the first rotation component is installed on the moving end of the first up-down adjustment component. The optical unit is installed on the rotating end of the first rotation component.

3. The imaging distortion measurement device of the optical system based on local focal length according to claim 2, wherein: The plane mirror (6) is installed on the workbench through a plane mirror adjustment mechanism. The plane mirror adjustment mechanism includes a second front-back adjustment component, a second left-right adjustment component, and a second up-down adjustment component. The fixed end of the second front-back adjustment component is installed on the workbench. The fixed end of the second left-right adjustment component is installed on the moving end of the second front-back adjustment component. The fixed end of the second up-down adjustment component is installed on the moving end of the second left-right adjustment component. The plane mirror (6) is installed on the moving end of the first up-down adjustment component.

4. The imaging distortion measurement device for an optical system based on local focal length according to claim 3, wherein: The resolution target (8) is installed on the workbench through a target adjustment mechanism. The target adjustment mechanism includes a horizontal movement component and a vertical movement component. The horizontal movement component is installed on the workbench. The vertical movement component is installed on the moving head of the horizontal movement component. The resolution target (8) is installed on the moving head of the vertical movement component. The optical system to be measured (7) is installed on the workbench through a third rotation component. The third rotation component is used to rotate the optical system to be measured (7) 90 degrees along the vertical plane.

5. A measurement method using the imaging distortion measurement device of the optical system based on local focal length described in claim 1, characterized in that, It includes the following steps: Step 1: Divide the sampling field of view according to the characteristics of the optical system to be measured (7); at the same time, determine the nominal image height value according to the characteristics of the optical system to be measured (7). Step 2: Select the line pair on the resolution target as the observation target, and adjust the line pair to the image plane position of the optical system (7) to be measured; Step 3: Adjust the optical path of the optical unit through the optical adjustment mechanism (13), aim at the center scale line of the selected line pair, and make the image of the center scale line of the line pair located at the center of the imaging detector (9); Step 4: Install the plane mirror (6) to block the half-field optical path; Step 5: Adjust the plane mirror (6) through the plane mirror adjustment mechanism, image the aiming scale line on the reticle (3) at the center of the imaging detector (9), so that the aiming scale line coincides with the center scale line of the line pair selected on the resolution target (8); Step 6: Adjust the optical system (7) to be measured through the third rotation assembly, rotate the optical path of the optical unit through the optical adjustment mechanism (13), aim at the adjacent scale line of the line pair selected on the resolution target (8), obtain the local focal length value of a single field of view, and then obtain the local focal length value of the full sampling field of view points; Step 7: Obtain the discrete value of the actual image height according to the obtained local focal length value of the full sampling field of view points, and then obtain the actual image height value through fitting. Compare the actual image height value with the nominal image height value to obtain the distortion value of the optical system (7) to be measured.

6. The measurement method according to claim 5, wherein: The calculation formula for the discrete value of the actual image height at each field of view position in Step 7 is: Among them, represents the discrete value of the actual image height at each field position, represents the sampling field of view in the x direction, represents the sampling field of view in the y direction, represents that the full field of view is equally divided into m parts, represents the field angle.

7. The measuring method according to claim 6, characterized in that: The method for selecting the line pair on the resolution target as the observation target in Step 2: First, according to the limit resolution ability of the optical system (7) to be measured, select the corresponding line pair on the resolution target (8) as the observation target. The formula for selecting the corresponding line pair on the resolution target (8) as the observation target is as follows: Among them, represents the corresponding line pair width on the resolution target (8), and d represents the limit resolution of the optical system to be measured; Then install the resolution target (8) on the target adjustment mechanism, and adjust the horizontal and vertical displacements of the resolution target (8) on the workbench through the target adjustment mechanism, so that the resolution target (8) is adjusted to the image plane position of the optical system (7) to be measured.

8. The measurement method according to claim 7, characterized in that: The method for making the image of the center scale line of the line pair located at the center of the imaging detector (9) in Step 3: Adjust the optical adjustment mechanism (13) so that the rotation center of the optical adjustment mechanism (13) coincides with the entrance pupil of the optical system (7) to be measured; Adjust the center scale line of the line pair selected on the resolution target (8) to the center field of view position of the image plane of the optical system (7) to be measured through the target adjustment mechanism, and adjust the optical adjustment mechanism (13) to aim at the selected line pair, so that the image of the selected line pair is located at the center position of the imaging detector; The method for making the aiming scale line coincide with the center scale line of the line pair selected on the resolution target (8) in Step 5: Adjust the plane mirror (6) through the plane mirror adjustment mechanism, so that the aiming scale line on the reticle (3) is reflected to the center position of the imaging detector (9) after passing through the plane mirror (6), and the aiming scale line coincides with the center scale line of the line pair selected on the resolution target (8).

9. The measurement method according to claim 8, characterized in that: The method for obtaining the local focal length value of a single field of view in Step 6: Adjust the optical system to be measured (7) to be parallel to the horizontal plane through the third rotation component, divide the full field of view in the x direction into m equal parts, denoted as , … , rotate the optical unit on the horizontal plane through the first rotation component, aim at the adjacent engraved lines of the selected line pairs on the resolution target (8), read the rotation angle of the optical adjustment mechanism (13), and obtain the local focal length at the field of view position by dividing the line interval of the observation and aiming engraved lines by the rotation angle ; The method for obtaining the local focal length value of the full sampling field of view points in the x direction in Step 6: Rotate the optical unit on the horizontal plane through the first rotating component to aim at the adjacent engraved lines of the selected line pair on the resolution target (8), translate the aiming engraved line to be observed and aimed at on the resolution target (8) to the next image plane sampling field point, and obtain the local focal length value of the new field point until the local focal length values of m sampling field points in the x direction are obtained, denoted as , ... ; Method for obtaining the local focal length values of all sampling field points in the y direction in step 6: Rotate the optical system to be measured (7) by 90 degrees through the third rotating component, that is, make the optical system to be measured (7) perpendicular to the horizontal plane; Repeat the process of obtaining the local focal length values of m sampling field points in the x direction to obtain the local focal length values of m sampling field points in the y direction, denoted as , ... .

10. The measurement method according to claim 9, characterized in that: Method for dividing sampling field of view in Step 1: When dividing the field of view interval measured by the optical system, if the change in the nominal local focal length within a certain position range is within the first focal length change threshold range, an equal-interval sampling method is adopted; if the change in the nominal local focal length within a certain position range is within the second focal length change threshold range, the sampling density is increased at the corresponding position.

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