Optical system imaging distortion measuring device and method based on local focal length

Abstract

The invention discloses an optical system imaging distortion measuring device and method based on local focal length, and belongs to the technical field of optical precision measurement. Comprising a workbench, an optical unit, an optical adjusting mechanism, a resolution target and an upper computer, and the optical adjusting mechanism is used for adjusting the optical unit up and down on a vertical plane, adjusting the optical unit back and forth and left and right on a horizontal plane and rotationally adjusting the optical unit on the horizontal plane; the optical unit comprises a plane mirror, an imaging detector, a light source, a collimating lens, a reticle, a spectroscope and an objective lens, the light source, the collimating lens, the reticle, the spectroscope and the objective lens are arranged according to a light path, the optical system to be measured is installed between the objective lens and the resolution target, the plane mirror and the optical system to be measured are located on the same side of the objective lens, and a half-view-field light path passing through the objective lens points to the optical system to be measured. And the other half view field light path passing through the objective lens points to the plane mirror. According to the invention, high-precision measurement of imaging distortion of the optical system is realized, and the measurement method is accurate and efficient.

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 plate, the measurement of the imaging distortion of an optical system can be achieved. However, for many application scenarios, it is difficult to construct a grid plate. 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 plate 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 surface grid plate 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 plate is more complex. The difficulties in constructing the grid plate 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 plane scenario is simpler than the object side scenario. Placing a target on the image plane 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, highly coincides 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, which can meet the high-precision measurement of the imaging distortion of various types of 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, and the optical adjustment mechanism is used to adjust the optical unit up and down in a vertical plane, adjust the optical unit forward, backward, 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. Reticle lines for aiming 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, and 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, and 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 moving head of the horizontal movement component, and the resolution target is installed on the moving head of the vertical movement component. The optical system to be measured is installed on the workbench through a third rotation component, and 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 pair on the resolution target as the observation target, and adjust the line pair 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 optical path of the half field of view.

[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 line 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 value of the actual image height according to the 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 value of the actual image height at each field of view position in Step 7 is:

[0019]

[0020] where 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 pair on the resolution target as the observation target in Step 2: First, select the corresponding line pair on the resolution target as the observation target according to the limit resolution ability of the optical system to be measured. The formula for selecting the corresponding line pair on the resolution target as the observation target is as follows:

[0022]

[0023] where 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 for making the imaging of the central engraved line of the line pair in step 3 be 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. Adjust the central engraved line 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 through the target adjustment mechanism, 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 for making the aiming engraved line coincide with the central engraved line of the selected line pair on the resolution target in step 5: adjust the plane mirror through the plane mirror adjustment mechanism so that the aiming engraved line on the reticle is reflected to the center position of the imaging detector after passing through the plane mirror, and the aiming engraved line coincides with the central engraved line of the selected line pair on the resolution target.

[0027] Preferably: the method for obtaining the local focal length value of a single field of view in step 6:

[0028] Adjust the optical system to be measured to be parallel to the horizontal plane through the third rotation component, 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 component, aim at the adjacent engraved lines 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 engraved line by the rotation angle. The local focal length of the field of view position.

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

[0030] Rotate the optical unit on the horizontal plane through the first rotation component, aim at the adjacent engraved lines of the selected line pair on the resolution target, translate the aiming engraved line 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, high-precision measurement of the optical system imaging distortion is achieved.

[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 process 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 imaging distortion measurement result 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 with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments 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 of the present invention by those skilled in the art 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 viewing and aiming 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, and the other half-field optical path passing through the objective lens 5 points to the plane mirror 6. The imaging detector 9 is a charge-coupled device, with the full English name of charge-coupled device and 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 passes through the optical system to be measured 7, the objective lens 5, and the beam splitter 4 in sequence 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 passes through the objective lens 5 and the beam splitter 4 in sequence 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 stepping 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 well-known 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 adjustment mechanism. The planar 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 mounted on the workbench. The fixed end of the second left-right adjustment component is mounted on the movable end of the second front-back adjustment component. The fixed end of the second up-down adjustment component is mounted on the movable end of the second left-right adjustment component. The planar mirror 6 is mounted on the movable end of the first up-down adjustment component. In this embodiment, the second front-back adjustment component drives the second left-right adjustment component, the second up-down adjustment component, and the second rotation 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 the second left-right adjustment component driving the second up-down adjustment component and the second rotation component to move left and right on the horizontal plane, thereby driving the planar mirror 6 to move left and right on the horizontal plane. By the second up-down adjustment component driving the planar mirror 6 to move up and down in the vertical plane, the adjustment of the position of the planar mirror 6 is realized. The second front-back adjustment component, the second left-right adjustment component, and the second up-down adjustment component in this embodiment can be a lead screw adjustment mechanism or a slider adjustment mechanism, as long as the adjustment accuracy of the second front-back adjustment component, the second left-right adjustment component, and the second up-down adjustment component is ensured. Therefore, the specific implementation structures of the second front-back adjustment component, the second left-right adjustment component, and the second up-down adjustment component in this embodiment are well-known 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 adjustment mechanism. The target adjustment mechanism includes a horizontal movement component and a vertical movement component. The horizontal movement component is mounted on the workbench. The vertical movement component is mounted on the moving head of the horizontal movement component. The resolution target 8 is mounted on the moving head of the vertical movement component. In this embodiment, the horizontal movement component drives the vertical movement component to move on the horizontal plane, thereby driving the resolution target 8 to move on the horizontal plane. The vertical movement component drives the resolution target 8 to move in the vertical plane. The optical system 7 to be measured is mounted on the workbench through a third rotation component. The third rotation component is used to rotate the optical system 7 to be measured 90 degrees along the vertical plane. By the cooperation of the third rotation component and the optical adjustment mechanism 13, the change of the field of view angle is realized. The horizontal movement component and the vertical movement component in this embodiment can be a lead screw adjustment mechanism, a slider adjustment mechanism, etc. The third rotation component can be a gear step rotation adjustment mechanism, a stepping motor rotation adjustment mechanism, etc. As long as the horizontal movement component, the vertical movement component, and the third rotation component can be adjusted under the condition of ensuring accuracy. The specific implementation structures of the horizontal movement component, the vertical movement component, and the third rotation component in this embodiment are well-known 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 length of each field of view, obtains the discrete values of the actual image height through the local focal length values that change with the distribution of the field of view, and then obtains the expression function of the actual image height through polynomial fitting, realizing high-precision measurement of the imaging distortion of the optical system. As Figure 5 shown, it specifically includes the following steps:

[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 of the 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 of the 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. The following are the expression formulas of the projection models of the object-image mapping relationships of optical systems such as isometric projection, equidistant projection, stereoscopic projection, equal solid angle projection, and orthogonal projection respectively:

[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 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 limit resolution ability of the 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 limit 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 limit 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 limiting 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 limiting 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 central scale line of the selected line pairs, and make the image of the central scale 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 central scale 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 central scale 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 scale line on the reticle 3 at the center of the imaging detector 9, so that the aiming scale line coincides with the central scale line of the selected line pairs on the resolution target 8.

[0069] In another embodiment, a method for making the aiming line coincide with the center line of the line pair selected on the resolution target 8 is as follows: adjusting the plane mirror 6 by a plane mirror adjustment mechanism so that the aiming line on the graticule plate 3 is reflected to the center position of the imaging detector 9 after passing through the plane mirror 6, so that the aiming line coincides with the center line of the line pair selected on the resolution target 8.

[0070] Step 6: Rotate the optical path of the optical unit through the optical adjustment mechanism 13, aim at the adjacent scribed lines of the selected line pair on the resolution target 8, obtain the local focal length value of the single field of view, and then obtain the local focal length value of the full sampling field of view point.

[0071] The local focal length value of the full sampling field of view point includes the local focal length value of the full sampling field of view point in the x direction and the local focal length value of the full sampling field of view point in the y direction.

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

[0073] Method for obtaining the local focal length value of the single field of view in step 6:

[0074] The third rotating assembly is used to adjust the optical system 7 to be parallel to the horizontal plane, and the full field of view in the x direction is divided into m equal parts, which are recorded as , … The optical unit is rotated in the horizontal plane by the first rotating assembly, and the adjacent scribed lines of the selected line pair on the resolution target 8 are aimed at, and the rotation angle of the optical adjustment mechanism 13 is read, and the line interval of the sighted scribed lines is divided by the rotation angle to obtain The local focal length of the field of view.

[0075] The method for obtaining the local focal length value of the full sampling field of view point in the x direction in step 6 is:

[0076] The optical unit is rotated on the horizontal plane by the first rotating assembly, aiming at the adjacent scribed lines of the selected line pair on the resolution target 8, and the scribed lines to be observed on the resolution target 8 are translated to the next image plane sampling field point, and the local focal length value of the new field point is obtained, until the local focal length values ​​of the m sampling field points in the x direction are obtained, which are recorded as , ... .

[0077] In step 6, the local focal length value of the full sampling field of view point in the y direction is obtained.

[0078] Rotate the optical system 7 to be measured by 90 degrees through the third rotating 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 based on the local focal length values of the full-sampling field points in the x and y directions, 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 based on the local focal length values of the full-sampling field points in the x and y directions.

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

[0083]

[0084]

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

[0086]

[0087] where, represents the discrete values 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 least squares fitting 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] Among them, represents the actual image height value, , , … represents the polynomial coefficients, represents the field of view angle.

[0091] The distortion value of the optical system to be measured is obtained by comparing the actual image height value with the nominal image height value, that is . Among them, represents the distortion of the optical system to be measured, represents the actual image height value, represents the nominal image height value.

[0092] Example 3

[0093] This example is a simulation example. This example is for a wide-angle lens with 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, satisfying equidistant projection. The device uses the device in Example 1. The initial optical path is as follows: The light source emits light, which becomes parallel light after passing through the collimating lens. The parallel light passes through the reticle to the beam splitter. The light in the half field of view passes through the objective lens and is reflected back to the imaging detector by the plane mirror, so that the aiming scale on the reticle is imaged at the center of the field of view of the imaging detector. The reflected light in the other half 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 within the field of view of the imaging detector. A method for measuring the imaging distortion of an optical system based on local focal length in this example 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 this wide-angle lens, when dividing the field of view interval, an equally spaced sampling method is adopted, 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 limit resolution d of this wide-angle lens = 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, the 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 the 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 the high-precision rotating workbench, and adjust its rotation center to coincide with the entrance pupil of the wide-angle lens to be measured. Through the displacement stage, 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, and adjust the optical adjustment mechanism to aim at this line. 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 by the plane mirror to the center position of the imaging detector, 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. 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. Place the optical system to be measured rotated 90° in the vertical plane. 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 with the nominal image height value to obtain the system distortion.

[0109] Numerically sum 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. Numerically sum 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 respectively. 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 the least squares fitting of the numerical values of the local focal length values that change with the field of view distribution. The actual image height y is: , where , , … are the parameters to be determined. After substituting the data, we get . As Figure 6 shown, and then compared with the nominal image height to obtain the distortion value of the optical system to be measured. 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 optical system under test with different focal lengths, by the bright and dark stripes with different scales on the resolution target, it simulates the different scale 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 optical system imaging distortion measurement device based on local focal length, characterized in that: The optical unit comprises 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 mounted on the workbench; the optical unit is mounted on the optical adjustment mechanism (13); the optical adjustment mechanism (13) is used to adjust the optical unit up and down on a vertical plane, to adjust the optical unit forward and backward and left and right on a horizontal plane, and to rotate the optical unit on a horizontal plane; the optical unit comprises a plane reflector (6), an imaging detector (9), and a light source arranged according to an optical path. (1), a collimating lens (2), a graticule (3), a beam splitter (4), and an objective lens (5), wherein the graticule (3) is provided with an observation and aiming scribed line; the imaging detector (9) is arranged on a 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 reflector (6) and the optical system to be measured (7) are located on the same side of the objective lens (5), wherein: a half-field light path passing through the objective lens (5) points to the optical system to be measured (7), and the other half-field light path passing through the objective lens (5) points to the plane reflector (6).

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

3. The optical system imaging distortion measurement device based on local focal length according to claim 2, characterized in that: The plane reflector (6) is mounted on a workbench via a plane mirror adjustment mechanism; the plane mirror adjustment mechanism comprises a second front-to-back adjustment component, a second left-to-right adjustment component, and a second up-to-down adjustment component; the fixed end of the second front-to-back adjustment component is mounted on the workbench, the fixed end of the second left-to-right adjustment component is mounted on the movable end of the second front-to-back adjustment component, the fixed end of the second up-to-down adjustment component is mounted on the movable end of the second left-to-right adjustment component, and the plane reflector (6) is mounted on the movable end of the first up-to-down adjustment component.

4. The optical system imaging distortion measurement device based on local focal length according to claim 3, characterized in that: The resolution target (8) is mounted on a workbench via a target adjustment mechanism, the target adjustment mechanism comprising 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 a moving end of the horizontal moving component, and the resolution target (8) is mounted on a moving end of the vertical moving component; the optical system to be measured (7) is mounted on the workbench via a third rotating component, the third rotating component being used to rotate the optical system to be measured (7) by 90 degrees along a vertical plane.

5. A measurement method using the optical system imaging distortion measurement device based on local focal length according to claim 1, characterized in that: The following steps are involved: Step 1: dividing the sampling field of view according to the characteristics of the optical system (7) to be measured; and determining the nominal image height value according to the characteristics of the optical system (7) to be measured; Step 2, selecting a line pair on the resolution target as an observation target, and adjusting the line pair to the image plane position of the optical system to be measured (7); Step 3, adjusting the optical path of the optical unit by means of the optical adjustment mechanism (13), aiming at the center scribed line of the selected line pair, so that the image of the center scribed line of the line pair is located at the center of the imaging detector (9); Step 4, install a plane reflector (6) to block the half field of view light path; Step 5, adjusting the plane reflector (6) by means of the plane mirror adjustment mechanism, imaging the sighting line on the reticle (3) at the center of the imaging detector (9), so that the sighting line coincides with the center line of the selected line pair on the resolution target (8); Step 6, adjusting the optical system to be measured (7) through the third rotating component, rotating the optical path of the optical unit through the optical adjustment mechanism (13), aiming at the adjacent scribed lines of the selected line pair on the resolution target (8), obtaining the local focal length value of the single field of view, and then obtaining the local focal length value of the full sampling field of view point; Step 7: Obtain a discrete value of the actual image height according to the local focal length value of the full sampling field of view point, and then obtain the actual image height value by fitting, and 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 measuring method according to claim 5, characterized in that: The calculation formula for the actual image height discrete value at each field of view position in step 7 is: in, Indicates the actual image height discrete value 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, It means that the whole field of view is divided into m equal parts. Indicates the field of view.

7. The measuring method according to claim 6, characterized in that: The method for selecting the line pairs on the resolution target as the observation target in step 2 is as follows: first, according to the limiting resolution capability of the optical system (7) to be measured, the corresponding line pairs on the resolution target (8) are selected as the observation targets. The formula for selecting the corresponding line pairs on the resolution target (8) as the observation targets is as follows: in, represents the corresponding line pair width on the selected resolution target (8), and d represents the limiting resolution of the optical system to be measured; The resolution target (8) is then mounted on a target adjustment mechanism, and the horizontal and vertical displacements of the resolution target (8) on the workbench are adjusted by 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 measuring method according to claim 7, characterized in that: The method for making the image of the central scribed line of the line pair in step 3 be located at the center of the imaging detector (9): adjusting 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; adjusting the central scribed line of the selected line pair on the resolution target (8) to the central field of view position of the image plane of the optical system (7) to be measured by the target adjustment mechanism, and adjusting 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 line coincide with the center line of the line pair selected on the resolution target (8) in step 5 is as follows: the plane mirror (6) is adjusted by the plane mirror adjustment mechanism so that the aiming line on the graticule plate (3) is reflected to the center position of the imaging detector (9) after passing through the plane mirror (6), and the aiming line coincides with the center line of the line pair selected on the resolution target (8).

9. The measuring method according to claim 8, characterized in that: Method for obtaining the local focal length value of the single field of view in step 6: The third rotating assembly is used to adjust the optical system (7) to be parallel to the horizontal plane, and the full field of view in the x direction is divided into m equal parts, which are recorded as , … The optical unit is rotated in a horizontal plane by a first rotating assembly, the adjacent scribed lines of the selected line pair on the resolution target (8) are aimed at, and the rotation angle of the optical adjustment mechanism (13) is read. The rotation angle is obtained by dividing the line interval of the sighting scribed lines by the rotation angle. The local focal length of the field of view position; The method for obtaining the local focal length value of the full sampling field of view point in the x direction in step 6 is: The optical unit is rotated in a horizontal plane by a first rotating assembly, aiming at adjacent scribed lines of a selected line pair on the resolution target (8), and the scribed line to be observed on the resolution target (8) is translated to the next image plane sampling field of view point, and the local focal length value of the new field of view point is obtained, until the local focal length values ​​of m sampling field of view points in the x direction are obtained, which are recorded as , ... ; A method for obtaining the local focal length values ​​of all sampling viewpoints in the y direction in step 6: the optical system (7) to be tested is rotated 90 degrees by a third rotation component, that is, the optical system (7) to be tested is made perpendicular to the horizontal plane; the process of obtaining the local focal length values ​​of the m sampling viewpoints in the x direction is repeated to obtain the local focal length values ​​of the m sampling viewpoints in the y direction, which are recorded as , ... .

10. The measuring method according to claim 9, characterized in that: The method of dividing the sampling field of view in step 1: when dividing the field of view interval measured by the optical system, if the change of 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 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.

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