Optical system wave aberration in-situ measurement and adjustment method

Abstract

The invention provides an optical system wavefront aberration measuring and adjusting method according to an inverse Hartmann wavefront detection principle. According to the method, a wave aberration detection system is formed by a measurement camera and a display, a sixteen-step phase shift fringe pattern is displayed on the display as a structured light source, the camera collects a deflection fringe pattern emitted by a to-be-detected optical system, phase information is obtained through solving based on sixteen-step phase shift and a three-frequency heterodyne algorithm, and the structured light source is obtained. The slope of the actual emergent light of the optical system can be obtained by changing the posture of the display for multiple times and combining with a sight line calibration method, then the wavefront aberration is obtained, and in-situ adjustment is completed according to aberration expression. The device has the advantages of simple structure, low detection cost, high precision and high resolution.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for in-situ measurement and adjustment of wavefront aberration of an optical system based on vision ray calibration, in particular for a coaxial optical system. BACKGROUND

[0002] Aberration in an optical system is one of the key factors affecting image quality and optical performance. With the continuous development of optical technology, precise optical systems are increasingly widely used in various fields. For high-precision optical systems such as satellite imaging, medical imaging, and laser radar, the control of aberration is particularly important. Reflective devices in optical systems, such as large astronomical telescopes, are designed with large apertures to achieve higher resolution. The primary mirror is usually composed of multiple sub-mirrors, and manufacturing and assembly errors can affect the co-phase state of the sub-mirrors, making the detection and correction of aberration in reflective systems an important task in the field of optical engineering. Traditional detection methods mainly include Foucault knife-edge detection, Ronchi detection, star point detection, interference detection, and Shack-Hartmann detection. Among them, Foucault knife-edge detection, Ronchi detection, and star point detection are still mainly used for qualitative analysis. Interference is a common high-precision detection method that can calculate the aberration in the system by observing the shape and number of interference fringes. However, interference detection has a small dynamic range, is easily affected by the environment, and is expensive, making it difficult to apply in the assembly process of large mirrors. Shack-Hartmann detection obtains the slope information of the wavefront by sampling the wavefront with a lens array, and the wavefront is recovered by integrating the slope to achieve aberration measurement. This method is simple to implement and can achieve high precision, but its measurement accuracy is affected by the machining precision of the lens array and is expensive. Lai and Gao et al. implemented a lens wavefront aberration detection method based on deflection, but this method requires prior knowledge of the system structure parameters for ray tracing and cannot perform parameter-free detection. Li et al. proposed a method for measuring the position and orientation of a single plane mirror based on structured light reflection. Wang et al. proposed a computer-aided calibration method to measure the wavefront of a transmissive element and the aberration introduced by system adjustment. However, the iterative optimization process is very complex and time-consuming, making it difficult to implement in online assembly detection. SUMMARY

[0003] In order to overcome the problems existing in the above-mentioned technologies, the present application proposes an optical system wavefront aberration measurement and adjustment method based on line-of-sight calibration. The method uses a measurement camera and a display to form a wave aberration detection system, displays a phase shift fringe pattern on the display as a structured light source, the camera collects the deflected fringe pattern emitted by the optical system to be measured, and then the phase information is obtained by solving based on the sixteen-step phase shift and three-frequency heterodyne algorithm, and then the display coordinates are obtained. By changing the posture of the display multiple times and combining the line-of-sight calibration method, the slope of the actual emitted light of the optical system can be obtained, and then the wavefront aberration is obtained, and the in-situ adjustment is completed according to the aberration performance. It has the advantages of simple device, low detection cost, high precision and high resolution.

[0004] The technical scheme adopted by the present application is an optical system wavefront aberration measurement and adjustment method based on line-of-sight calibration. According to the inverse Hartmann wavefront detection principle, the present application establishes an optical system wavefront aberration measurement and adjustment system based on line-of-sight calibration. Starting from the principle of inverse Hartmann wavefront detection, the camera is regarded as a point light source, the light is considered to be emitted from the center of the entrance pupil of the camera lens, and after refraction or reflection by the optical system to be measured, it reaches the display. In the case that the camera and the optical system to be measured as a whole remain stationary, the direction of the emitted light of the system will not change, and by changing the posture of the display multiple times, the line-of-sight calibration method can be used to calibrate the slope of the emitted light of the exit pupil of the optical system. The slope is integrated using Noll Zernike polynomials to obtain the wave aberration of the optical system to be measured, and then the adjustment is completed according to the specific performance of the wave aberration. The specific steps of the method are as follows:

[0005] Step one: Coaxial adjustment of camera and display

[0006] A high-precision electrically controlled displacement table equipped with a point light source microscope is used as a coaxial adjustment device. First, the posture of the display is adjusted so that it coincides with the world coordinate system Secondly, a 0.8mm diameter aperture stop is installed in front of the camera, and the position of the aperture stop and the posture of the camera are adjusted using the point light source microscope to determine the position of the aperture stop center in the world coordinate system as , so as to ensure that the optical axis of the camera is coaxial with the z-axis of the world coordinate system.

[0007] Step two: Measurement of optical system wave aberration

[0008] Display a phase shift fringe pattern on the display. In the case that the camera and the optical system as a whole remain stationary, the camera collects multiple display fringe images under different postures, and the direction of the emitted light of the optical system is solved by using line-of-sight calibration:

[0009] r ( z ) = [ x 0 y 0 0 ] + [ k x k y 1 ] z (1)

[0010] wherein is the intersection coordinate of the exiting light ray on the display plane in the first posture, wherein is the slope of the light ray in the world coordinate system, then the wave aberration of the optical system can be expressed as:

[0011] (2)

[0012] wherein is the wave aberration of the optical system, wherein is the refractive index on the image side, is the tangent value of the angle between the actual light ray and the theoretical light ray of the optical system in the case of no aberration.

[0013] Step three: posture adjustment of the optical system

[0014] Noll Zernike polynomials are used to integrate the slope data obtained by the line-of-sight calibration in the mode method, wherein the Z5~Z8 terms of the polynomials represent 45° primary astigmatism, 0° primary astigmatism, y-axis primary coma, and x-axis primary coma; the adjustment direction of the low-order off-axis aberration can be determined according to the obtained Zernike coefficients, and the eccentricity and tilt of the optical system are gradually adjusted by using a five-dimensional adjustment base. After adjustment, step two is repeated to measure the wave aberration, and then the Zernike coefficients are adjusted again, and the low-order off-axis aberration is repeatedly adjusted to the minimum, and the adjustment is completed at this time. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is the schematic diagram of the system device principle of the present application;

[0016] Figure 2 is the sixteen-step phase shift fringe displayed by the display in the present application. DETAILED DESCRIPTION

[0017] In order to make the purpose and scheme of the present application clearer, the present application will be described in detail below by examples in combination with the drawings. It is necessary to point out here that the following examples are only used for further description of the present application and cannot be understood as a limitation on the protection scope of the present application. The skilled person in the art can make some non-essential improvements and adjustments to the present application according to the above content of the present application, which still belongs to the protection scope of the present application.

[0018] The present application provides a wave aberration in-situ measurement and adjustment method of an optical system based on line-of-sight calibration, which has a CCD camera 1, an optical system to be measured 2, a display 3, and a computer for control and data processing; the method uses one of the measurement cameras 1 and the display 3 to form a wave aberration detection system, and displays a fringe pattern such as Figure 2The sixteen-step phase-shift fringe pattern is shown as a structured light source. Aberration of the optical system 2 to be measured will cause the fringe pattern collected by the measuring camera 1 to be deformed. The phase information is obtained by using the sixteen-step phase-shift algorithm combined with the three-frequency heterodyne algorithm, and then the display coordinates are obtained. The slope of the actual outgoing light of the optical system 2 can be obtained by changing the posture of the display multiple times and combining the line-of-sight calibration method, and then the wavefront aberration is obtained. According to the aberration performance, the in-situ adjustment is completed. The specific implementation process is as follows:

[0019] Step one: Coaxial adjustment of the camera and the display

[0020] A high-precision electrically controlled displacement table of a point light source microscope is used as a coaxial adjustment device. First, the posture of the display 3 is adjusted so that it coincides with the world coordinate system. Second, a 0.8mm-diameter aperture stop is installed in front of the camera 1, and the point light source microscope is used to adjust the position of the aperture stop and the posture of the camera 1, so that the position of the aperture stop center in the world coordinate system is determined as , and the optical axis of the camera 1 is coaxial with the z-axis of the world coordinate system.

[0021] Step two: Measurement of the wave aberration of the optical system

[0022] The phase-shift fringe pattern is displayed on the display 3. In the case that the camera 1 and the optical system 2 as a whole remain stationary, the camera 1 collects multiple fringe images of the display 3 in different postures. The direction of the outgoing light of the optical system 2 can be solved by using line-of-sight calibration:

[0023] r ( z ) = [ x 0 y 0 0 ] + [ k x k y 1 ] z (1)

[0024] In the formula, is the intersection coordinate of the outgoing light on the display 3 in the first posture, wherein is the slope of the light in the world coordinate system. Then the wave aberration of the optical system 2 can be expressed as:

[0025] (2)

[0026] In the formula, is the wave aberration of the optical system 2, wherein is the refractive index on the image side, is the tangent value of the angle between the actual light and the theoretical light of the optical system in the case of no aberration.

[0027] Step three: Adjustment of the posture of the optical system

[0028] ​Noll Zernike polynomials are used to integrate the slope data obtained by the line-of-sight calibration, wherein the Z5~Z8 terms of the polynomials represent 45° primary astigmatism, 0° primary astigmatism, y-axis primary coma, and x-axis primary coma; the adjustment direction of the low-order off-axis aberrations can be determined according to the obtained Zernike coefficients; and the eccentricity and tilt of the optical system 2 are gradually adjusted by using a five-dimensional adjustment base. After the adjustment, the wavefront aberration measurement of step two is repeated, and the adjustment is re-performed according to the Zernike coefficients; the adjustment is repeated for multiple times until the low-order off-axis aberrations are minimized, and the adjustment is completed.

Claims

1. A method for in-situ measurement and adjustment of wavefront aberration in an optical system, comprising a camera, a display, an optical system under test, a five-dimensional adjustment base, an electrically controlled displacement stage with a microscope equipped with a point light source, an optical platform, and a computer for control and data processing. The method utilizes a measuring camera and a display to form a wavefront aberration detection system. A phase-shifted fringe pattern is displayed on the display as a structured light source. The camera acquires the deflection fringe pattern emitted from the optical system under test. The phase information is then obtained by solving a sixteen-step phase-shift and three-frequency heterodyne algorithm, thereby obtaining the display coordinates. By repeatedly changing the display orientation and combining it with a line-of-sight calibration method, the slope of the actual emitted light from the optical system can be obtained, thus yielding the wavefront aberration. In-situ adjustment is then completed based on the aberration characteristics. The specific steps are as follows: Step 1: Coaxial adjustment of camera and monitor Using a high-precision electrically controlled displacement stage equipped with a point light source microscope as a coaxial adjustment device, the monitor's orientation was first adjusted to align with the world coordinate system. The surfaces coincide; secondly, an aperture stop with a diameter of 0.8 mm is installed in front of the camera, and the position of the aperture stop is adjusted using a point light source microscope to determine its position in the world coordinate system. Ensure that the camera's optical axis is coaxial with the z-axis of the world coordinate system; Step 2: Measurement of wavelet aberration of the optical system The display shows a phase-shifted fringe pattern. With the camera and optical system remaining stationary as a whole, the camera captures multiple images of the display fringe in different orientations. The direction of the outgoing light rays from the optical system can be determined using line-of-sight calibration. （1） In the formula Let be the coordinates of the intersection point of the emitted light rays on the display plane in the first posture, where Let be the slope of the light ray in the world coordinate system, then the wave aberration of the optical system can be expressed as: （2） In the formula Let be the wave aberration of the optical system, where For image-side refractive index, It is the tangent of the angle between the actual ray and the theoretical ray of the optical system under aberration-free conditions; Step 3: Attitude adjustment of the optical system The Noll Zernike polynomial is used to integrate the slope data obtained from line-of-sight calibration using the mode method. The Z5 to Z8 terms of the polynomial represent the 45° primary astigmatism, 0° primary astigmatism, y-axis primary coma, and x-axis primary coma. The adjustment direction of low-order off-axis aberrations can be determined based on the fitted Zernike coefficients. The eccentricity and tilt of the optical system are gradually adjusted using a five-dimensional adjustment base. After adjustment, step two is repeated to measure wave aberrations, and then readjustment is performed based on the Zernike coefficients. This adjustment is repeated multiple times until the low-order off-axis aberrations are minimized, at which point the adjustment is complete.

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