Point light source 4f optical angle calibration device

Abstract

This invention discloses an optical angle encoding device and method based on a pinhole point light source and a 4f system, belonging to the field of optical measurement and image processing technology. The device includes: a surface light source, a pinhole plate, a light-transmitting latitude and longitude grid film, a first convex lens, a second convex lens, and an image sensor. The surface light source is converted into a point light source through the pinhole, illuminating the light-transmitting latitude and longitude grid film. The light-transmitting grid is located on the front focal plane of the first convex lens, and its spatial position is encoded as the direction of parallel light by the first convex lens. The parallel light is decoded by the second convex lens to restore the grid image, which is recorded by the image sensor. This invention achieves precise encoding and decoding of spatial position and angle through a 4f system, establishing a pixel-angle mapping relationship to form a reusable optical angle encoding reference film. This mapping relationship is independent of the zoom mode of the lens under test: for lenses with a fixed focal center and only film movement, one calibration can be performed and reused at any focal length; for conventional zoom lenses, calibration can be performed separately at each focal length and permanently reused. This invention has the advantages of high angular resolution, geometrically faithful imaging, strong versatility, and mass production capability, and is widely used in lens distortion correction, spatial angle measurement, and optical system calibration.

Description

I. Technical Field

[0001] This invention relates to the field of optical measurement and image processing technology, specifically to an optical angle encoding device and method for lens distortion correction and spatial angle measurement, which is particularly suitable for high-precision calibration of small field-of-view optical systems such as standard lenses and telephoto lenses. II. Background Technology

[0002] 2.1 Current Technological Status Lens distortion correction is a fundamental problem in fields such as photogrammetry, machine vision, and augmented reality. Currently, mainstream distortion correction methods can be divided into two categories: Category 1: Algorithm calibration method Representative techniques: Zhang Zhengyou calibration method (planar chessboard grid), Tsai two-step method Principle: Photograph a calibration object with a known geometric shape, and use an algorithm to infer the distortion parameters. Limitations: Each calibration requires re-photographing the calibration board; calibration accuracy is affected by the manufacturing precision of the calibration object and the robustness of the algorithm; real-time angle reference cannot be provided. Category 2: Software Correction Method Representative technology: Post-processing correction based on lens profiles (such as Adobe Lens Profile) Principle: Pixel remapping of the image is performed using a pre-defined distortion model. Limitations: Relies on configuration files provided by lens manufacturers; cannot adapt to individual lens differences; cannot provide a physical angle reference. 2.2 Shortcomings of existing technologies Lack of physical angle coding methods: Existing methods all rely on software calculations and lack directly observable and traceable physical angle references; Low calibration efficiency: Recalibration is required every time the lens is changed or the focal length is altered, making it impossible to achieve "one-time calibration, permanent reuse"; Angle measurement is inconvenient: the true spatial angle between two points cannot be directly read from an image; Accuracy limitations: Algorithm calibration is affected by the accuracy of calibration board manufacturing, image processing algorithms, and camera noise. III. Summary of the Invention

[0003] 3.1 Purpose of the Invention This invention aims to provide an optical angle encoding device and method based on a pinhole point light source and a 4f optical system. By accurately encoding spatial angle information into the direction of parallel light, and then losslessly decoding it through a 4f system, high-precision, reusable lens calibration and angle measurement are achieved. This device is independent of the specific zoom mode of the lens under test and can be widely used in the calibration and correction of various optical systems.

[0004] 3.2 Technical Solution 3.2.1 Device Structure This invention provides an optical angle encoding device, comprising (see...) Figure 1 ): Surface light source (1): used to provide uniform illumination over a large area; Small hole plate (2): It is set on the light-emitting side of the surface light source and has a single small hole to convert the surface light source into a point light source; Transparent latitude and longitude grid sheet (3): a transparent substrate with a reflective layer on the surface. The reflective layer is removed at the latitude and longitude lines to form a light-transmitting window, which constitutes the latitude and longitude grid pattern. First convex lens (4): focal length is The light-transmitting warp and weft mesh sheet is located on its front focal plane; Second convex lens (5): exactly the same as the first convex lens, with the same focal length. It is coaxially positioned with the first convex lens, with a spacing of [missing information]. ; Image sensor (6): Located at the rear focal plane of the second convex lens, used to receive imaging light. 3.2.2 Optical path relationship The positional relationships of the components are as follows: The light emitted by the surface light source (1) becomes a single point light source after passing through the small hole on the perforated plate (2); The point light source illuminates the translucent latitude and longitude grid sheet (3), and each translucent window becomes a secondary point light source; The light-transmitting latitude and longitude grid sheet (3) is located at the front focal plane of the first convex lens (4); The distance between the first convex lens (4) and the second convex lens (5) is ; The image sensor (6) is located at the rear focal plane of the second convex lens (5). 3.2.3 Optical Principle The core optical principle of this invention is based on a 4f system (optical Fourier transform system) to achieve optical angle encoding and decoding: Point light source generation: A surface light source (1) forms a single point light source after passing through a small hole on a perforated plate (2). ; Secondary point light source array: point light source Irradiating a translucent lattice mesh sheet (3), each translucent window becomes a secondary point source, and the direction of the emitted light is determined by... The connection to the window determines the path; Angle coding (first lens): Since the light-transmitting window is located at the front focal plane of the first convex lens (4), the light emitted from each window becomes a parallel beam after passing through the first convex lens, and windows at different positions correspond to parallel light in different directions. Let the distance from the window to the optical axis be... Then the direction angle of the corresponding parallel light. satisfy: ; This is the encoding of spatial location to angle; Angle transmission: The parallel beam propagates between the first convex lens (4) and the second convex lens (5) while the direction information remains unchanged; Angle decoding (second lens): After the parallel beam passes through the second convex lens (5), it converges to a point on its rear focal plane. The position of this point is... satisfy: This is the decoding of angle into spatial position; Geometrical Fidelity Imaging: The real image of the grid recorded by the image sensor (6) is highly consistent with the original transparent grid sheet (3) in terms of geometrical relationship. The magnification is 1, and the residual distortion of the system is negligible. Key feature: The above encoding and decoding process relies solely on the focal length of the 4f system itself. This is independent of the internal structure and zoom method of the lens under test. The pixel-angle mapping relationship established by this device is a purely physical angular reference, applicable to any subsequent imaging system; 3.2.4 Light-transmitting mesh design The translucent grid uses an equal-angle interval design to cover the target's field of view. The position of a meridian on the grid is determined by the following formula: , in This represents the spatial angle corresponding to that meridian. The focal length is denoted by . Due to the nonlinearity of the tangent function, the equally spaced latitude and longitude lines are distributed on the grid with sparser edges and denser centers; this characteristic accurately encodes spatial angular information. 3.2.5 Working Method The present invention provides an optical angle encoding and calibration method based on the above-mentioned device, comprising the following steps.

[0005] Step S1: Obtain the reference coded image Turn on the surface light source (1), and the light becomes a point light source after passing through the small hole, illuminating the light-transmitting warp and weft mesh sheet (3). A geometrically accurate grid image is obtained on the image sensor (6) through a 4f system consisting of a first convex lens (4) and a second convex lens (5); Save this image as an optical angle-coded reference film.

[0006] Step S2: Establish pixel-angle mapping Extract the pixel coordinates of the grid intersections in the reference film. ; According to the lens focal length Calculate the spatial angle corresponding to each intersection point. : ;in The coordinates of the image center; Establish a pixel-angle mapping lookup table.

[0007] Step S3: Reuse the angle encoding reference (operation for different zoom methods) Depending on the zoom method of the lens under test, use any of the following operating modes: Mode 1 (Focus center fixed, only film moves): The lens under test is set to a working mode where the focus center is fixed and the focal length is changed only by moving the image sensor. In this case, the mapping relationship established in step S2 applies to target images captured at any focal length of the lens, eliminating the need for repeated calibration. Mode 2 (Focus on center movement, regular zoom): For conventional zoom lenses (which change focal length by moving the lens group), at each desired focal length, place the lens in the optical path of this device and repeat steps S1-S2 to obtain the dedicated mapping relationship corresponding to that focal length. This operation only needs to be performed once during calibration, and the mapping table for that focal length can be directly reused for subsequent shooting.

[0008] Step S4: Target image correction or measurement Remove or turn off the device; Capture images of typical targets; By utilizing the mapping relationship established at the corresponding focal length, distortion correction or angle measurement is performed on the target image.

[0009] 3.3 Technical Effects Compared with the prior art, the present invention has the following beneficial effects: 1. Provides absolute physical optical angle encoding: Through a pinhole point light source and a 4f system, a strict mathematical relationship is established between the position of each light-transmitting window and its spatial angle. It achieves precise encoding and decoding from spatial position to angle and then from angle to spatial position, without the need for algorithm deduction, and has high calibration accuracy and good repeatability; 2. High versatility and adaptability to various zoom methods: The pixel-angle mapping established by this device is a purely physical angle reference. Regardless of whether the lens under test uses a fixed focus center (only the film moves) or a moving focus center (conventional zoom), calibration can be achieved through the corresponding operating mode. For the former, "one-time calibration, reuse at any focal length" can be achieved; for the latter, calibration can be performed separately at each focal length, and the mapping relationship for that focal length can be permanently reused after a single calibration. 3. High angular resolution: The size of the light-transmitting window can be controlled within 0.01-0.05mm, the diameter of the aperture can be controlled within 0.03-0.08mm, and the angular blurring is less than 0.03° (within...). (The system) meets the requirements for high-precision measurement. 4. Geometric Fidelity Imaging: The 4f system achieves 1:1 imaging, lossless transmission of angular information, minimal residual distortion, and the reference film can be directly used for pixel-angle mapping without additional correction, effectively avoiding the geometric distortion introduced by traditional single-lens imaging. 5. High manufacturability: Based on mature laser etching, optical coating and precision machining processes, it is suitable for mass production and the cost is controllable; 6. Angle measurement function: Based on high-fidelity geometric imaging relationships, the spatial angle between any two points can be read directly from the reference film or the calibrated image, providing a convenient measurement tool for fields such as photogrammetry and machine vision; 7. Full-range zoom lens support: Through discrete calibration and interpolation methods, distortion correction can be achieved throughout the entire zoom lens range, expanding the application scope of this invention. IV. Detailed Implementation

[0010] 4.1 Example 1: Standard 4F System This embodiment provides a complete pinhole point light source + 4f system angle reference device; Material selection: The position of the small holes: The distance between the small hole plate and the light-transmitting mesh sheet can be adjusted according to the uniformity of illumination, usually 10-50mm, with the mesh being evenly illuminated as the standard; Grid design parameters (covering a field of view of ±25°, allowing for denser grids): Production steps: Small perforated plate fabrication: Laser drilling is performed on stainless steel or glass substrates to create holes with a diameter of 0.05 mm. Clean and remove burrs; Fabrication of translucent mesh sheet: Vacuum depositing of aluminum film onto glass sheets; Using a laser direct writing device, a light-transmitting grid with a line width of 0.05mm is etched according to the CAD drawing; Clean the surface to remove residue; An anti-reflective coating can be applied to the light-transmitting window.

[0011] System assembly: The first convex lens is fixed on the optical support; The light-transmitting mesh sheet is installed at the front focal plane of the first convex lens (distance from the optical center of the lens). ); The second convex lens is mounted coaxially with the first convex lens, with a spacing of... ; The image sensor is mounted at the rear focal plane of the second convex lens (distance from the optical center of the lens). ); Install the perforated plate behind the light-transmitting mesh sheet (at a distance of 10-50mm). Install the surface light source behind the perforated plate.

[0012] System alignment: Use a laser pointer to adjust along the optical axis to ensure that the centers of all components are collinear; Use vernier calipers to accurately measure each spacing; The position of the focal plane was verified using the autocollimation method.

[0013] How to use: Turn on the surface light source and capture a reference image using an image sensor; Extract the pixel coordinates of grid intersections in the image; According to the formula Calculate the spatial angle corresponding to each intersection point and create a lookup table; For the lens to be calibrated, connect it to the optical path (replacing the image sensor position) and capture the target image; Distortion correction of the target image is performed using a lookup table.

[0014] Expected results: Reference image sharpness: Grid lines have sharp edges and no ghosting. Angle measurement accuracy: better than 0.1° Distortion correction residual: less than 0.5 pixels. V. Description of the attached drawings Figure 1 This is a schematic diagram of the overall structure of the device of the present invention; Figure 2 This is a schematic diagram of a grid design with equal-angle intervals for latitude and longitude lines; Figure 3 It is a flowchart of the work process.

Claims

1. A point light source 4f optical angle calibration device, characterized in that, include: A single light source is used to provide uniform illumination; A small perforated plate is disposed on the light-emitting side of the surface light source, and a single small hole is provided on it to convert the surface light source into a point light source; A light-transmitting latitude and longitude grid sheet is disposed on the light-emitting side of the perforated plate. A reflective layer is provided on its surface. The reflective layer is removed at the latitude and longitude lines to form a light-transmitting window, which constitutes the latitude and longitude grid. A first convex lens with a focal length of The light-transmitting warp and weft mesh sheet is located on its front focal plane; a A second convex lens, identical to the first convex lens, with the same focal length. It is coaxially positioned with the first convex lens, with a spacing of [missing information]. ; An image sensor is located at the rear focal plane of the second convex lens.

2. The apparatus according to claim 1, characterized in that, The diameter of the small hole is 0.03 mm to 0.08 mm.

3. The apparatus according to claim 1, characterized in that, The warp and weft lines of the light-transmitting mesh sheet are designed with equal-angle intervals, and the position of the light-transmitting window... Corresponding spatial angle satisfy .

4. The apparatus according to claim 1, characterized in that, The light-transmitting window is made by laser etching or photolithography, with a line width of 0.01 mm to 0.1 mm.

5. The apparatus according to claim 1, characterized in that, The first and second convex lenses are cemented doublet achromatic lenses used to reduce chromatic aberration and spherical aberration.

6. The apparatus according to claim 1, characterized in that, The surface light source is an LED surface light source, equipped with a light-diffusing sheet, so that the uniformity of the emitted light is greater than 80%.

7. A point light source 4f optical angle calibration device, characterized in that, Includes the following steps: Step S1: Turn on the surface light source. After passing through the small hole, the light becomes a point light source and illuminates the light-transmitting latitude and longitude grid sheet. Through the 4f system composed of the first convex lens and the second convex lens, a geometrically accurate grid image is obtained on the image sensor as an optical angle encoding reference film. Step S2: Extract the pixel coordinates of the grid intersection points in the reference base. According to the focal length of the lens Calculate the spatial angle corresponding to each intersection point. Establish pixel-angle mapping relationships; Step S3: Based on the zoom method of the lens under test, select any of the following modes to reuse the mapping relationship: Mode 1: When the lens under test adopts a fixed focus center and changes the focal length only by moving the image sensor, the mapping relationship is applicable to target images captured by the lens at any focal length; Mode 2: When the lens under test uses a conventional zoom method (changing the focal length by moving the lens group), repeat steps S1-S2 at each required focal length to obtain the dedicated mapping relationship corresponding to that focal length. Step S4: Using the mapping relationship obtained in step S3, perform distortion correction or angle measurement on the target image.

8. The method according to claim 7, characterized in that, The mapping relationship between pixel coordinates and spatial angles in step S2 is as follows: ， in The coordinates of the image center are... This is the focal length of the lens.

9. The method according to claim 7, characterized in that, The step S2, which establishes the pixel-angle mapping relationship, includes: Extract the pixel coordinates of the grid intersections in the reference film to form a feature point matrix; Based on the feature point matrix, an interpolation algorithm is used to obtain a pixel-angle mapping lookup table; The interpolation algorithm is bilinear interpolation, bicubic interpolation, or spline interpolation.

10. The method according to claim 7, characterized in that, The distortion correction in step S4 includes: Map the pixel coordinates of the target image to angle space to obtain the spatial angle of each pixel in the target image; The spatial angles are remapped back to the image plane using the ideal projection model to obtain the corrected image.

11. The method according to claim 7, characterized in that, The angle measurement in step S4 includes: Extract the pixel coordinates of the first and second points in the target image; By using the pixel-angle mapping relationship, they are converted into the first spatial angle and the second spatial angle respectively; Calculate the angle between the first spatial angle and the second spatial angle, and use it as the spatial angle between the two points.

12. The method according to claim 7, characterized in that, It also includes step S5: For zoom lenses, steps S1-S3 are executed at multiple typical focal lengths to obtain the distortion model parameters corresponding to each focal length; for the intermediate focal length between adjacent focal lengths, the distortion model parameters are obtained by interpolation.

13. A point light source 4f optical angle calibration device, characterized in that, include: A point light source generating unit is used to generate an approximate point light source; A light-transmitting grid unit is disposed on the light-emitting side of the point light source generating unit, and has multiple light-transmitting windows thereon, the light-transmitting windows forming a latitude and longitude grid pattern; A Fourier transform optical system is used to convert the spatial position information on the light-transmitting grid unit into the direction information of parallel light, and restore the direction information to high-fidelity spatial position information; An image sensor is used to receive the restored image.

14. The apparatus according to claim 13, characterized in that, The point light source generating unit includes a surface light source and a perforated plate. The perforated plate has a single small hole for converting the surface light source into a point light source.

15. The apparatus according to claim 13, characterized in that, The Fourier transform optical system includes: A first lens, wherein the light-transmitting grid unit is located on its front focal plane; A second lens, identical to the first lens, is coaxially positioned with a distance of twice the focal length from the first lens.

16. The apparatus according to claim 13, characterized in that, The Fourier transform optical system is a 4f optical system, a reflective Fourier transform system, or a Fourier transform system based on a Fresnel lens.

17. A calibration and standardization system for a point light source 4f optical angle calibration device, characterized in that, include: The acquisition module is used to obtain the pixel coordinates of the grid intersection points based on the reference film captured by the device according to any one of claims 1-6; The mapping module is used to calculate the spatial angle corresponding to each intersection point based on the lens focal length f and establish a pixel-angle mapping relationship. The correction module is used to perform distortion correction or angle measurement on subsequently captured target images based on the mapping relationship.

18. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the program performs the steps of the method described in any one of claims 7-12.

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