Method of measuring offset of optical axis

Abstract

A method for measuring optical axis offset is applicable to measuring the optical axis offset of an imaging system. The imaging system includes a lens and an image sensor, and is used to capture a central mark and two outer marks located on both sides of the central mark on a shooting surface. The method for measuring optical axis offset includes the following steps: (A) the imaging system is rotated by a target angle with an entrance pupil position as the rotation center, so that the central mark is imaged at the center of an image; (B) the computing device obtains the angle value of the target angle based on a visible angle of the imaging system and the distances of the two outer marks in the image from the center; and (C) the computing device obtains the optical axis offset of the imaging system based on the angle value of the target angle and an effective focal length of the imaging system.

Description

Technical Field

[0001] This invention relates to a method for measuring offset, and more particularly to a method for measuring the optical axis offset of an imaging system. Prior Technology

[0002] An ideal imaging system should have the optical axis of the imaging lens perpendicular to the optical center of the image sensor. However, due to design tolerances, manufacturing processes, and other factors, the optical axis of the lens does not pass through the center of the image sensor perfectly precisely after the product is assembled. There must be an offset between the optical axis of the lens and the center of the image sensor. In order to ensure image quality, this offset must be limited to an acceptable range.

[0003] Existing methods for measuring optical axis offset involve placing an imaging system under test inside a hemispherical shell with uniform luminous intensity to capture an image. Software is then used to analyze the image to locate the position of the brightest pixel. This position is the point where the lens optical axis passes, and the distance between this position and the center of the image is the offset between the lens optical axis and the center of the image sensor.

[0004] However, it is difficult to achieve completely uniform brightness on the object being photographed (i.e., the hemisphere), and the uniformity of the coating of the lens inside the lens of the imaging system under test will affect the overall relative illumination of the lens. In addition, the pixel response difference of the image sensor of the imaging system under test will affect the position of the pixel with the highest brightness, which may lead to inaccurate measurement results of the optical axis offset. Summary of the Invention

[0005] Therefore, one object of the present invention is to provide a highly accurate method for measuring optical axis offset.

[0006] Therefore, the optical axis offset measurement method of the present invention is applicable to measuring the optical axis offset of an imaging system, the imaging system including a lens and an image sensor for capturing images of a central mark disposed on an imaging surface and two outer marks located on both sides of the central mark. The imaging system is configured such that when capturing images of the central mark and the two outer marks, an entrance pupil position of the lens intersects a normal line passing through the central mark and perpendicular to the imaging surface. The optical axis offset measurement method includes the following steps:

[0007] (A) The imaging system is rotated by a target angle with the entrance pupil position as the rotation center, so that the center mark is imaged at the center of an image;

[0008] (B) The two outer marks are moved to the two edges of the image;

[0009] (C) A computing device obtains the angle value of the target angle based on a visible angle of the imaging system and the distances between the two outer markers and the central marker; and

[0010] (D) The computing device obtains the optical axis offset of the imaging system based on the angle value of the target angle and an effective focal length of the imaging system.

[0011] Another objective of this invention is to provide a highly accurate method for measuring optical axis offset.

[0012] Therefore, the optical axis offset measurement method of the present invention is applicable to measuring the optical axis offset of an imaging system, the imaging system including a lens and an image sensor for capturing images of a central mark disposed on an imaging surface and two outer marks located on both sides of the central mark and equidistant from the central mark. The imaging system is configured such that when capturing images of the central mark and the two outer marks, an entrance pupil position of the lens of the imaging system intersects a normal line passing through the central mark and perpendicular to the imaging surface. The optical axis offset measurement method includes the following steps:

[0013] (A) The imaging system is rotated by a target angle with the entrance pupil position as the rotation center, so that the center mark is imaged at the center of an image;

[0014] (B) The computing device obtains the angle value of the target angle based on a visible angle of the imaging system and the distances of the two outer marks in the image from the center; and

[0015] (C) The computing device obtains the optical axis offset of the imaging system based on the angle value of the target angle and an effective focal length of the imaging system.

[0016] Another objective of this invention is to provide a highly accurate method for measuring optical axis offset.

[0017] Therefore, the optical axis offset measurement method of the present invention is applicable to measuring the optical axis offset of an imaging system, the imaging system including a lens and an image sensor for capturing a central mark and two outer marks located on both sides of the central mark on an imaging surface. The imaging system is configured such that when capturing the central mark and the two outer marks, an entrance pupil position of the lens intersects a normal line passing through the central mark and perpendicular to the imaging surface, and the imaging system is configured to rotate the target angle around the entrance pupil position to capture an image with the central mark imaged at a center. The optical axis offset measurement method includes the following steps:

[0018] (A) A computing device obtains the angle value of the target angle based on a visible angle of the imaging system and one of a first set of distances and a second set of distances, wherein the first set of distances are the distances between the two outer markers and the center marker, and the second set of distances are the distances between the two outer markers and the center in the image; and

[0019] (B) The computing device obtains the optical axis offset of the imaging system based on the angle value of the target angle and an effective focal length of the imaging system.

[0020] The advantages of this invention are: based on the visible angle of the imaging system and the distances between the two outer marks and the center mark, or the distances between the two outer marks and the center in the image, the angle value of the target angle is calculated, and the optical axis offset is obtained based on the angle value of the target angle and the effective focal length. The optical axis offset obtained in this way is not affected by the uniformity of the lens coating of the photographed object and the imaging system itself, or by the pixel response of the image sensor, thus having high accuracy. Simple Explanation of the Diagram

[0021] Other features and effects of the present invention will be clearly presented in the embodiments with reference to the drawings, wherein: Figure 1 is a schematic diagram illustrating that the imaging system is configured to capture a central mark and two outer marks placed on an imaging surface; Figure 2 is a flowchart illustrating a first embodiment of the optical axis offset measurement method of the present invention; Figure 3 is a schematic diagram illustrating the positional relationship between the central marker, the two outer markers, and the imaging system, and the target angle; and Figure 4 is a flowchart illustrating a second embodiment of the optical axis offset measurement method of the present invention. Implementation

[0022] Before the invention is described in detail, it should be noted that similar elements are represented by the same numbers in the following description.

[0023] Referring to Figure 1, a first embodiment of the optical axis offset measurement method of the present invention is applicable to measuring the optical axis offset of an imaging system 1. The imaging system 1 includes a lens 11 and an image sensor (not shown), which can be configured to be horizontal to capture a lateral image and vertical to capture a vertical image. The imaging system 1 is used to capture a central mark 31 disposed on an imaging surface 100, and two outer marks 32 located on both sides of the central mark 31 (direction not limited). In this embodiment, the two outer marks 32 are first positioned on a horizontal line passing through the central mark 31, and the imaging system 1 is erected vertically. The horizontal optical axis offset between the optical axis of the lens 11 of the imaging system 1 and the center of the image sensor of the imaging system 1 is obtained using the optical axis offset measurement method of the present invention. Then, the two outer marks 32 are positioned on a vertical line passing through the central mark 31 and perpendicular to the horizontal line, and the imaging system 1 is erected horizontally. The vertical optical axis offset between the optical axis of the lens 11 of the imaging system 1 and the center of the image sensor of the imaging system 1 is obtained using the optical axis offset measurement method of the present invention. The imaging system 1 is set on a rotating stage 2 and is configured such that when the central mark 31 and the two outer marks 32 are photographed, an entrance pupil position O of the lens 11 (see Figure 3) intersects a normal 101 passing through the central mark 31 and perpendicular to the photographing surface 100, and the optical axis of the lens of the imaging system 1 is coaxial with the normal 101. Both the imaging system 1 and the rotating stage 2 are signal-connected to a computing device 3. The entrance pupil position can be obtained from the lens specification drawing provided by the imaging system manufacturer.

[0024] Referring to Figures 1 to 3, a first embodiment of the optical axis offset measurement method of the present invention includes the following steps. Before the measurement begins, the imaging system 1 takes a picture of the center mark 31, which is offset from an original imaging point of an image to a center point of the image.

[0025] In step 201, the imaging system 1 is rotated by a target angle around the entrance pupil position O as the rotation center, about a vertical axis passing through the entrance pupil position O. That is, the shooting direction is adjusted so that an adjusted imaging point of the center mark 31 coincides with the center point of the image. It is worth noting that after the imaging system 1 rotates by the target angle, even though the normal 101 passing through the center mark 31 still intersects the entrance pupil position O, an imaginary line extending from the entrance pupil position O (i.e., the lens optical axis, which is taken as a solid line in Figure 3) no longer intersects the center mark 31, but deviates from the normal 101 and intersects the shooting plane 100 at any point (see point E in Figure 3).

[0026] It is worth mentioning that, in this embodiment, the imaging system 1 is rotated by the rotating stage 2. The rotating stage 2 receives a series of instructions from a computing device 3 to rotate the imaging system 1 by the target angle. When the imaging system 1 is set on the rotating stage 2 and the entrance pupil position O of the lens 11 still intersects the normal 101, and whenever the imaging system 1 is rotated by the rotating stage 2, the imaging system 1 will capture the center mark 31 to obtain a current image of the center mark 31 (hereinafter referred to as the first image) and transmit it to the computing device 3. Whenever the computing device 3 receives the first image, it determines whether the original / adjusted imaging point of the center mark 31 (hereinafter referred to as the first center imaging point) coincides with the center point of the first image. When the computing device 3 determines that the first center imaging point does not coincide with the center point of the first image, the computing device 3 transmits the next instruction to the rotating stage 2 according to the positional relationship between the first center imaging point and the center point of the first image, until the first center imaging point coincides with the center point of the first image. However, in other embodiments, the imaging system 1 can also be rotated by a user's manual operation to make the center mark 31 imaged at the center of the image. The computing device can be a device with computing capabilities, such as a computer, microcontroller, or server.

[0027] In step 202, provided that the first central imaging point coincides with the center point of the first image, the two outer marks 32 are positioned at the two sides of the image at the imaging point of the imaging system 1.

[0028] It is worth mentioning that, in this embodiment, the central mark 31 and the two outer marks 32 are marks displayed on a screen (i.e., the imaging surface 100). Initially, the two outer marks 32 are displayed on both sides of the central mark 31 and at equal distances from it. The computing device 3 controls the position of the two outer marks 32 on the screen (i.e., the imaging surface 100) so that the imaging points (hereinafter referred to as outer imaging points) of the two outer marks 32 are located at the two side edges of the image. Initially, and whenever the display position of the two outer marks 32 is changed, the imaging system 1 captures images of the two outer marks 32 to obtain a current image (hereinafter referred to as a second image) of the outer marks and transmits it to the computing device 3. Whenever the computing device 3 receives the second image, it determines whether the outer imaging points of the two outer marks 32 coincide with the two side edges of the second image. When the computing device 3 determines that the two outer imaging points do not coincide with the two side edges of the second image, the computing device 3 changes the position of the two outer marks 32 displayed on the screen according to the positional relationship between the two outer imaging points and the two side edges of the second image, until the two outer imaging points coincide with the two side edges of the second image. However, in other embodiments, the center mark 31 and the two outer marks 32 are marks affixed to a screen or wall, and the two outer marks 32 can also be changed in position by a user until the two outer imaging points coincide with the two side edges of the image. After the two outer marks 32 are changed in position, the distance from the center mark 31 will be different, but the distance between the two outer imaging points and the first center imaging point will be the same.

[0029] In step 203, the imaging system 1 is rotated by a first angle around the entrance pupil position O as the center of rotation, about a vertical axis passing through the entrance pupil position O, so that the imaging point of the center mark 31 (hereinafter referred to as the second center imaging point) coincides with one of the two edges of the image. At this time, a first scale indicated by the rotating stage 2 is recorded. The imaging system 1 is rotated by the rotating stage 2, which receives instructions from the computing device 3 to rotate the imaging system 1 by the first angle. The computing device 3 performs control similar to that of the target angle; the details of the computing device 3 controlling the rotation of the imaging system 1 by the first angle will not be described here.

[0030] In step 204, the imaging system 1 is rotated by a second angle around the entrance pupil position O as the rotation center, about a vertical axis passing through the entrance pupil position O, so that the imaging point of the center mark 31 (hereinafter referred to as the third center imaging point) coincides with the other edge of the two side edges of the image. At this time, a second scale indicated by the rotating stage 2 is recorded, and the second angle is used as a field of view (FOV) angle of the imaging system 1. The second angle is the difference between the first scale and the second scale. The imaging system 1 is rotated by the rotating stage 2, which receives instructions from the computing device 3 to drive the imaging system 1 to rotate by the second angle. The computing device 3 performs control similar to that of the target angle, and the details of the computing device 3 controlling the rotation of the imaging system 1 by the second angle will not be described here.

[0031] It is worth mentioning that the rotating stage 2 is marked with an angle scale, and the second angle can be obtained through the angle scale marked on the rotating stage 2. Since the angle value of the second angle is relatively large, even if there is a slight error in the second angle measured through the angle scale marked on the rotating stage 2, it will not cause a serious deviation in the calculation of the optical axis offset. However, since the angle value of the target angle is relatively small, it cannot be measured through the angle scale marked on the rotating stage 2, and therefore must be obtained through the following steps 205-207. In addition, in other embodiments, the visible range angle can be provided by the manufacturer, for example, by using records from the production of the imaging system 1 or by using existing optical calculation formulas, without performing steps 203-204. However, since the visible range angle provided by the manufacturer is the average of the visible range angles of the same imaging system 1, its accuracy is lower than that of the visible range angle actually measured using steps 203-204.

[0032] In step 205, the computing device 3 calculates the viewing angle of the imaging system 1. The distance between the imaging system 1 and the imaging surface 100 and the distances between the two outer marks 32 and the center mark 31, respectively. The angle value of the target angle can be obtained using the following formula (1). . …(1)

[0033] The following explains the derivation of formula (1). Refer to Figure 3. Figure 3 illustrates the positional relationship between the central marker 31 (marked as C in the figure), the two outer markers 32 (marked as A and B in the figure), the entrance pupil position O of the imaging system 1, and any point (marked as E in the figure) where the imaginary line extending from the entrance pupil position O intersects. As can be seen from Figure 3, the distance between the imaging system 1 and the imaging surface 100 is... It can be expressed as the following formula (2). …(2)

[0034] From formula (2), we can see that, Based on the principle of similar triangles It is equal to Therefore, we can obtain ,Will Moving it to the left side of the equals sign yields... ,Will Move it to the left side of the equals sign, and... Moving the equation to the right side and simplifying, we get... Therefore, it can be known that ,because It is equal to Therefore ,again It can be represented as , can be obtained ,therefore , That is , That is .

[0035] In step 206, the computing device 3 calculates the angle value of the target angle. The effective focal length (EFL) of the imaging system 1 is obtained using the following formula (3), which is the horizontal optical axis offset of the imaging system 1. The EFL can be provided by the manufacturer, simulated by existing optical analysis software, or measured by existing experimental methods. …(3)

[0036] Referring to Figures 1 and 4, the second embodiment of the optical axis offset measurement method of the present invention is largely the same as the first embodiment, the main difference being that in the second embodiment, the distances between the two outer marks 32 and the center mark 31 are equal and do not need to be moved. Specifically, step 401 is similar to the aforementioned step 201, where the imaging system 1 is rotated by a target angle to adjust the shooting direction so that the first center imaging point coincides with the center point of the image. Step 402 is similar to the aforementioned step 203, where the imaging system 1 is rotated by a first angle so that the second center imaging point coincides with one side edge of the image. Step 403 is similar to the aforementioned step 204, where the imaging system 1 is rotated by a second angle so that the third center imaging point coincides with the other side edge of the image, and this second angle is taken as a field of view (FOV) angle of the imaging system 1. The main difference between this embodiment and the first embodiment lies in step 404. Under the premise that the first central imaging point coincides with the center point of the first image, the imaging system 1 captures the two outer marks 32 to obtain a second image and transmits it to the computing device 3. The computing device 3 calculates the image based on the viewing angle of the imaging system 1 and the distance between the imaging system 1 and the imaging surface 100. and the distances between the two outer imaging points in the second image and the first central imaging point. The angle value of the target angle can be obtained using the following formula (4). Finally, in step 405, the computing device 3 calculates the angle value of the target angle. The horizontal optical axis offset of the imaging system 1 is obtained by measuring the effective focal length of the imaging system 1. …(4)

[0037] It is worth mentioning that the rotation process in step 401 changes the original forward-facing image of the imaging surface 100 to an oblique image. When the imaging system 1 takes an oblique image, the image will be distorted due to the asymmetry of magnification. Physically, the distances from the two outer marks 32 (A, B) to the center are equal. However, the distances from the two outer marks 32 to the center in the image will be unequal due to distortion. Therefore, we can use the distances from the two outer marks 32 to the center in the image... Calculate the angle value of the target angle. Similarly, in the first embodiment, by imaging the two outer marks 32 onto the two side edges of the image, the distances between the two outer marks 32 and the center mark 31 in the image are equal, while the distances between the two outer marks 32 and the center mark 31 on the imaging surface 100 are not equal. Due to the characteristics of optical conjugation, the distances between the two outer marks 32 and the center mark 31 on the imaging surface 100 are... The ratio of the distances between the two outer markers 32 and the center in the image of the second embodiment. The ratios will be exactly the same.

[0038] The two outer marks 32 are positioned above and below the central mark 31, and the imaging system 1 is set up horizontally. The vertical optical axis offset between the optical axis of the lens 11 of the imaging system 1 and the center of the image sensor of the imaging system 1 is obtained by using the method steps of any of the above embodiments.

[0039] In summary, the optical axis offset measurement method of the present invention involves rotating the imaging system 1 by the target angle so that the center mark 31 is imaged at the center of the image, causing distortion in the captured image. Then, the angle value of the target angle is calculated based on the visible angle of the imaging system 1, the distance between the two outer marks 32 and the center mark 31, or the distance between the two outer imaging points 2 and the first center imaging point in the image. The optical axis offset is obtained based on the angle value of the target angle and the effective focal length. The optical axis offset obtained in this way is not affected by the uniformity of the lens coating of the photographed object and the imaging system 1 itself, or the pixel response of the image sensor, thus having high accuracy and effectively achieving the purpose of the present invention.

[0040] However, the above description is merely an embodiment of the present invention and should not be construed as limiting the scope of the present invention. Any simple equivalent changes and modifications made in accordance with the scope of the patent application and the contents of the patent specification of the present invention shall still fall within the scope of the patent of the present invention.

[0041] 1: Imaging System 11: Lens 100: Shooting angle 101: Normal O: Entrance pupil position D: Distance 31. C: Central marker 32. A, B: Outer markings E: Intersection point 2: Rotary table 3: Computing device Steps 201-206 :distance :Viewable range angle : Target angle value Steps 401-405

Claims

1. A method for measuring optical axis offset, applicable to measuring the optical axis offset of an imaging system, the imaging system including a lens and an image sensor for capturing images of a central mark disposed on a shooting surface and two outer marks located on either side of the central mark, the imaging system being configured such that, when capturing the central mark and the two outer marks, an entrance pupil position of the lens intersects a normal line passing through the central mark and perpendicular to the shooting surface, and the imaging system being configured to rotate around the entrance pupil position as a rotation center about a vertical axis passing through the entrance pupil position by a target angle to capture an image of the imaging point of the central mark at its center point, the optical axis offset measurement method comprising the following steps: (A) obtaining the angle value of the target angle based on a visible range angle of the imaging system and one of a first distance and a second distance, wherein... The first set of distances is the distance between the two outer marks and the center mark respectively, the second set of distances is the distance between the two outer marks and the center in the image respectively; and (B) the optical axis offset of the imaging system is obtained based on the angle value of the target angle and an effective focal length of the imaging system.

2. The optical axis offset measurement method as described in claim 1, wherein, In step (A), when the two outer marks are imaged on the two sides of the image, the angle value of the target angle is obtained according to the visible range angle of the imaging system and the first set of distances; when the distance between the two outer marks and the center mark is equal, the angle value of the target angle is obtained according to the visible range angle of the imaging system and the second set of distances.

3. The optical axis offset measurement method as described in claim 1, wherein, In step (A), the angle value of the target angle is obtained using the following formula based on the visible range angle of the imaging system, the distance between the imaging system and the shooting surface, and one of the first distance and the second distance: , where, when the angle value of the target angle is obtained based on the first distance, and, when the angle value of the target angle is obtained based on the second distance, and.

4. The optical axis offset measurement method as described in claim 1, wherein, In step (B), the optical axis offset is obtained using the following formula based on the angle value of the target angle and the effective focal length: .

5. A method for measuring optical axis offset, applicable to measuring the optical axis offset of an imaging system, the imaging system including a lens and an image sensor for capturing a central mark disposed on a shooting surface and two outer marks located on both sides of the central mark, the imaging system being configured such that when capturing the central mark and the two outer marks, an entrance pupil position of the lens intersects a normal line passing through the central mark and perpendicular to the shooting surface, the optical axis offset measurement method comprising the following steps: (A) the imaging system is rotated by a target angle about the entrance pupil position as the rotation center around a vertical axis passing through the entrance pupil position, so that the imaging point of the central mark coincides with a center point of an image; (B) the imaging points of the two outer marks in the imaging system are respectively located at the two side edges of the image; (C) the angle value of the target angle is obtained according to a visible range angle of the imaging system and the distances of the two outer marks from the central mark; and (D) the optical axis offset of the imaging system is obtained according to the angle value of the target angle and an effective focal length of the imaging system.

6. The optical axis offset measurement method as described in claim 5, wherein, In step (C), the angle value of the target angle is obtained using the following formula based on the visible angle of the imaging system, the distance between the imaging system and the shooting surface, and the distances between the two outer marks and the center mark: .

7. The optical axis offset measurement method as described in claim 5, wherein, In step (D), the optical axis offset is obtained using the following formula based on the angle value of the target angle and the effective focal length: .

8. The optical axis offset measurement method as described in claim 5, wherein, Between steps (B) and (C), the following steps are also included: (E) the imaging system is rotated about the vertical axis by a first angle with the entrance pupil position as the center of rotation, so that the imaging point of the center mark coincides with one of the two edges of the image; and (F) the imaging system is rotated about the vertical axis by a second angle with the entrance pupil position as the center of rotation, so that the imaging point of the center mark coincides with the other edge of the two edges of the image, and the second angle is used as the visible range angle.

9. A method for measuring optical axis offset, applicable to measuring the optical axis offset of an imaging system, the imaging system including a lens and an image sensor for capturing images of a central mark disposed on a shooting surface and two outer marks located on both sides of the central mark and equidistant from the central mark, the imaging system being configured such that when capturing the central mark and the two outer marks, an entrance pupil position of the lens of the imaging system intersects a normal line passing through the central mark and perpendicular to the shooting surface, the optical axis offset measurement method comprising the following steps: (A) the imaging system is rotated by a target angle about the entrance pupil position as the rotation center about a vertical axis passing through the entrance pupil position, so that the imaging point of the central mark coincides with a center point of an image; (B) the angle value of the target angle is obtained according to a visible range angle of the imaging system and the distances of the two outer marks in the image from the center; and (C) the optical axis offset of the imaging system is obtained according to the angle value of the target angle and an effective focal length of the imaging system.

10. The optical axis offset measurement method as described in claim 9, wherein, In step (B), the angle value of the target angle is obtained using the following formula based on the visible angle of the imaging system, the distance between the imaging system and the shooting surface, and the distances between the two outer marks in the image and the center: .

11. The optical axis offset measurement method as described in claim 9, wherein, In step (C), the optical axis offset is obtained using the following formula based on the angle value of the target angle and the effective focal length: .

12. The optical axis offset measurement method as described in claim 9, wherein, Between steps (A) and (B), the following steps are also included: (D) the imaging system is rotated about the vertical axis by a first angle with the entrance pupil position as the center of rotation, so that the imaging point of the center mark coincides with one of the two edges of the image; and (E) the imaging system is rotated about the vertical axis by a second angle with the entrance pupil position as the center of rotation, so that the imaging point of the center mark coincides with the other edge of the two edges of the image, and the second angle is used as the viewing angle.

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