Automatic focusing method and device based on wavefront defocusing coefficient

Abstract

The invention discloses an automatic focusing method and device based on a wavefront defocusing coefficient, and relates to the technical field of automatic focusing, and the automatic focusing method comprises the following steps: S1, receiving a measurement light beam which is reflected by a target and passes through an objective lens; s2, measuring the wavefront of the measurement light beam by using a wavefront sensor; s3, calculating a wavefront defocusing coefficient c according to the measured wavefront; s4, determining the focusing error of the objective lens according to the wavefront defocusing coefficient c; and S5, controlling a motion mechanism to adjust the object distance between the objective lens and the target according to the focusing error so as to realize automatic focusing, wherein the focusing speed is high and the focusing consistency is good.

Description

Technical Field

[0001] This invention relates to the field of autofocus technology, and more specifically to an autofocus method and apparatus based on wavefront defocus coefficient. Background Technology

[0002] Objective lens focusing is a core component of modern optical systems and laser processing equipment. For optical imaging systems, image quality is optimal when the target is in focus; the image becomes blurry when it deviates from focus. For laser processing systems, the relative position of the workpiece and the laser focal point needs to be controlled to ensure processing quality. Therefore, frequent focusing operations are required to guarantee imaging or processing quality. Due to the inefficiency and inconsistent nature of manual focusing, various automatic focusing methods have been proposed.

[0003] Currently, the main methods for achieving autofocus are:

[0004] Method 1, the rangefinding method, works by first emitting a laser or ultrasonic wave to measure the object distance, then calculating the conjugate position of the object and image using optical imaging formulas, and driving the objective lens to the focusing position. This method is independent of ambient light and contrast, performing well at medium to long distances, in specific industrial applications, and as an auxiliary focusing system. However, this method relies on reflected signals, which are easily absorbed, scattered, or interfered with, making it unsuitable for all scenarios. Furthermore, the highest accuracy achieved by this method is only at the sub-mm level, making it unsuitable for high-precision applications.

[0005] To address the insufficient focusing accuracy in prior method 1, prior method 2, the image sharpness detection method (or contrast detection method), was proposed. This is currently the mainstream method with the most research and application. Its principle is to determine the position of the focal plane based on image sharpness, which is evaluated by an evaluation function. The advantage of this method is that it requires no additional auxiliary devices, and the focusing accuracy of high-magnification objectives can reach sub-μm accuracy. However, this method has the following technical challenges or shortcomings:

[0006] (1) Slow speed: Because the image sharpness at different positions needs to be checked multiple times, the lens needs to move repeatedly to test, and it cannot be used for dynamic focusing;

[0007] (2) Inconsistency: There are many evaluation functions used to quantify image sharpness, such as Tenengrad, Laplacian, Variance, EOG and Brenner. Using different evaluation functions will result in inconsistent results. Moreover, even if the same evaluation function is used, different regions of interest in the image will also result in inconsistent results.

[0008] (3) It is only applicable to targets with contrast, and cannot be used for transparent, solid color and other targets.

[0009] To address the problems in prior method 2, prior method 3, the phase detection method, was proposed. Its principle involves splitting the incident light into two beams, each projected onto a different area of ​​the photosensitive sensor. These two beams, due to their different transmission paths, generate a phase difference (i.e., an image position shift). In focused mode, the two beams perfectly overlap. When out of focus, the image position shift is proportional to the focal plane deviation, allowing for one-step focus adjustment. This results in fast focusing speeds without repeated attempts and can be used for dynamic focusing. However, in low-light illumination, the weak light signal makes focusing difficult. Furthermore, this method relies on a precisely calibrated optical path; even minor lens shifts, sensor displacements, or thermal expansion and contraction can cause optical path deviations, leading to significant focusing errors.

[0010] Method 4, the split-image focusing method, is a long-established and highly accurate manual focusing technique, but it also has many limitations. The principle of split-image focusing is to insert a pair of intersecting wedge-shaped prisms (split-image prisms) on the image-side focal plane of a finite conjugate lens. This splits the unfocused light rays into two misaligned images, which are then manually adjusted to overlap, achieving precise focusing. While this method offers high focusing accuracy, it is unusable in low-light conditions or with targets lacking contrast. Furthermore, it is difficult to achieve autofocus manually, making it unsuitable for dynamic focusing or scenarios requiring rapid response. Additionally, because split-image markers typically have a certain geometric size, it cannot be used for detecting small targets. Since the prisms need to be placed on the focal plane of the objective lens, it can only be used with finite conjugate lenses and is difficult to apply to infinite conjugate lenses.

[0011] The prior method, intensity confocal focusing, achieves high-precision focusing by using a pinhole filter located at the conjugate position of the object and image, allowing only light signals emitted from the focal plane to pass through while blocking out-of-focus light. This method obtains images through scanning and 3D reconstruction, improving resolution and signal-to-noise ratio. However, this method requires extremely high calibration accuracy of optical components, resulting in high manufacturing, storage, and maintenance costs. Furthermore, it has a slow scanning speed, a small measurement range, and relatively low efficiency when used for autofocus.

[0012] Method 6, the spectral chromatic aberration confocal method, works by using a white light source with a specially designed high-dispersion lens to generate axial chromatic aberration. Light of different wavelengths forms a continuous monochromatic focal sequence along the optical axis, creating a linear chromatic aberration distribution. The surface of the object being measured only reflects wavelengths that match its distance (i.e., light of that wavelength is focused on the surface). The reflected light is filtered through a conjugate pinhole, while other defocused wavelengths are blocked due to spot dispersion. Only the focused light enters the spectrometer. The spectrometer detects the peak wavelength of the reflected light's spectral curve and calculates the distance to the object using a preset wavelength-distance mapping relationship. The advantages of this method are high resolution and accuracy, the ability to eliminate scanning when the target is within the focal sequence range, effectiveness for targets without contrast, and immunity to stray light interference. However, due to the use of a chromatic aberration lens, the image of the detection point cannot be observed. Furthermore, this method relies on strict calibration and stabilization of the entire optical path; the wavelength-distance mapping relationship is affected by factors such as temperature and minute lens displacements. Even a deviation at the μm level can lead to significant signal attenuation and malfunction. These factors increase the system's manufacturing difficulty and maintenance costs.

[0013] Method 7, the eccentric beam focusing method, works by irradiating the target with a laser beam of a certain shape, such as a semi-circular laser beam, and determining the focus state by observing the shape of the reflected beam spot. However, if the target is a diffuse reflective surface, the change in the shape of the reflected light makes it difficult to accurately determine the focus state, thus limiting the application of this method. Furthermore, the focal position of the laser beam must be strictly aligned with the focal position of the objective lens, requiring recalibration when changing objective lenses, or strict control over the objective lens's focus error. In addition, because the beam has a certain size, this method is not suitable for small targets or targets with fine structures.

[0014] Online Method 8, Astigmatic Focusing Method, has been successfully applied to focus detection of optical disc reading laser heads. Its principle is to detect the focal plane based on the different spot shapes of the astigmatic beam before and after the focal point. Since it also works by analyzing the changes in spot shape, its advantages and disadvantages are similar to those of the earlier Method 7.

[0015] In prior method 9, Chinese patent publication number CN119184613A, a method for measuring the defocus of a wavefront using a Hartmann wavefront sensor to achieve focal plane detection was disclosed. However, this method did not consider the applicability requirements of the Hartmann wavefront sensor. When the imaging conjugate beam is collimated into the wavefront sensor, it means that the light from all object points on the object surface enters the sensor. Each microlens receives mixed light from multiple object points, and the spot shape reflects blurred image points. The information is confused and cannot be accurately correlated with the original wavefront gradient. The wavefront slope information itself is distorted, and subsequent calculations are like "castles in the air," making it impossible to obtain the wavefront and defocus. Summary of the Invention

[0016] The purpose of this invention is to address the shortcomings of the prior methods by providing an autofocus method based on the wavefront defocus coefficient, comprising the following steps:

[0017] S1. Receives the measuring beam reflected from the target and passing through the objective lens;

[0018] S2. Measure the wavefront of the measurement beam using a wavefront sensor;

[0019] S3. Calculate the wavefront defocusing factor c based on the measured wavefront;

[0020] S4. Determine the focusing error of the objective lens based on the wavefront defocusing coefficient c;

[0021] S5. The focusing error control motion mechanism adjusts the object distance between the objective lens and the target to achieve autofocus.

[0022] Further, step S5 includes:

[0023] When the wavefront defocusing coefficient c is greater than 0, the motion mechanism is controlled to reduce the object distance;

[0024] When the wavefront defocusing coefficient c is less than 0, the motion mechanism is controlled to increase the object distance.

[0025] Furthermore, it also includes:

[0026] The wavefront defocus coefficient c is monitored in real time, and step S5 is repeated when its absolute value is greater than a preset threshold ε, so as to maintain the absolute value of the wavefront defocus coefficient c less than the preset threshold ε.

[0027] Furthermore, the measuring beam is formed by the reflection of a laser beam used to process the target.

[0028] Furthermore, the measurement beam is formed by the reflection of a collimated laser beam generated by a point source onto the target.

[0029] Furthermore, it also includes:

[0030] Timing is controlled by a synchronous controller;

[0031] During the measurement phase, turn on the point light source and wavefront sensor, and turn off the imaging illumination source and camera;

[0032] During the image acquisition phase, the point light source and wavefront sensor are turned off, while the imaging illumination source and camera are turned on.

[0033] In addition, an autofocus device based on wavefront defocus coefficient is also provided, including:

[0034] Objective lens;

[0035] A wavefront sensor is used to measure the wavefront of a measurement beam reflected from a target and passing through an objective lens;

[0036] The processing unit, connected to the wavefront sensor, is used to calculate the wavefront defocus coefficient c and determine the focus error based on the measured wavefront.

[0037] The motion mechanism is used to adjust the distance between the objective lens and the target.

[0038] A motion controller, connected to the processing unit and the motion mechanism, is used to drive the motion mechanism according to the focus error;

[0039] A beam splitter, positioned in the optical path between the objective lens and the wavefront sensor, is used to guide the measurement beam to the wavefront sensor.

[0040] Furthermore, the measuring beam is formed by the reflection of a laser beam used to process the target.

[0041] Furthermore, it also includes:

[0042] A point light source is used to generate a collimated laser beam that forms the measurement beam.

[0043] Furthermore, it also includes:

[0044] Imaging unit;

[0045] The synchronization controller is connected to the processing unit, point light source, wavefront sensor, and imaging unit. It is used to control the operation of the point light source and wavefront sensor during the measurement phase and to control the operation of the imaging unit during the imaging phase.

[0046] Compared with the prior art, the present invention provides an autofocus method and apparatus based on wavefront defocus coefficient, which has the following beneficial effects:

[0047] 1. Fast focusing speed: The wavefront sensor and computer can complete the measurement of the wavefront defocus coefficient c within 10 milliseconds. The defocus coefficient c and the focusing error have a one-to-one correspondence, so there is no need for repeated attempts and the focus can be achieved in one go.

[0048] 2. Good focus consistency: The wavefront sensor is a phase detector, and the repeatability of the wavefront measurement is better than 20nm, which makes the measurement of the defocus coefficient also have high repeatability. Therefore, the consistency of the object distance after autofocus reaches the sub-μm level.

[0049] 3. The autofocus method provided by this invention does not require evaluating the focus state based on image sharpness, and is also applicable to targets with no contrast.

[0050] 4. This invention has a built-in probe light, which makes the autofocus of optical imaging independent of the illumination intensity, and it is also applicable to dark field imaging;

[0051] 5. This invention does not have position-sensitive structures such as conjugate pinholes, which reduces the accuracy and stability requirements of the optomechanical structure, thereby reducing the difficulty of calibration and maintenance;

[0052] 6. This invention uses an orthogonal Zernike polynomial function system. The adverse effects of aberrations such as tilt, astigmatism, coma, and spherical aberration in the optical system on the measurement of the wavefront defocusing coefficient c can be ignored. Therefore, the requirements for the wavefront quality of the optical system are reduced, which means the difficulty of optical path calibration is reduced.

[0053] 7. The wavefront measurement beam in this invention can be equivalent to a parallel beam of a point source, and the size of the detection spot on the object side is on the order of μm, which can realize automatic focusing on targets on the μm scale;

[0054] 8. In this invention, wavefront detection and imaging are performed at different times, so there is no image pollution caused by focusing operations in the photographic images;

[0055] 9. This invention has the capability of real-time automatic closed-loop control, thus enabling dynamic tracking and focusing of targets in motion. Attached Figure Description

[0056] Figure 1 This is a schematic diagram of an autofocus device based on wavefront defocus coefficient. Figure 1 ;

[0057] Figure 2 This is a schematic diagram of an autofocus device based on wavefront defocus coefficient. Figure 2 ;

[0058] Figure 3 A flowchart of an autofocus method based on wavefront defocus coefficient;

[0059] In the diagram: 1-fixed substrate, 2-worktable, 3-workpiece, 4-objective lens, 5-beam splitter, 6-wavefront sensor, 7-computer, 8-motion controller, 9-secondary beam splitter, 10-point light source, 11-synchronization controller, 12-photographic eyepiece, 13-camera. Detailed Implementation

[0060] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms are interchangeable where appropriate; this is merely a way of distinguishing objects with the same attributes in the embodiments of this application. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion, so that a process, method, system, product, or apparatus that comprises a series of elements is not necessarily limited to those elements, but may include other elements not explicitly listed or inherent to those processes, methods, products, or apparatuses.

[0061] Example 1: In this embodiment of the invention, please refer to... Figure 1 and Figure 3 This paper presents an autofocusing method based on wavefront defocusing coefficient. The wavefront refers to the surface formed by connecting points of equal phase vibration during the propagation of a light wave; it is also called a wavefront or equiphase surface. The wavefront of a light beam can be measured using instruments such as a Shaker-Hartmann wavefront sensor or an interferometer. The value of the wavefront can be decomposed into a series of Zernike polynomials and linear combinations of their coefficients.

[0062] The wavefront defocusing coefficient refers to the coefficient of the term in an orthogonal Zernike polynomial function system where the radial component n=2 and the angular component m=0. Specifically, for a circular beam, the analytical expression for this term is:

[0063]

[0064] in( , () is a point inside the unit circle, that is Assume the wavefront value measured by the wavefront sensor is... The formula for calculating its defocus coefficient c is:

[0065]

[0066] In particular, for other non-circular beams, it is necessary to use the mathematical Gram-Schmidt orthogonalization operation to orthogonalize the Zernike polynomial function system, and calculate the defocus coefficient under the condition that the function system satisfies orthogonality.

[0067] The wavefront defocusing coefficient c calculated using the above formula has the following properties:

[0068] 1. When the object distance equals the focal length, the image-side beam is parallel, and the wavefront defocusing coefficient c = 0;

[0069] 2. When the object distance is less than the focal length, the image-side beam is divergent, and the wavefront defocusing coefficient c < 0;

[0070] 3. When the object distance is greater than the focal length, the image-side beam is converging, and the wavefront defocusing coefficient c > 0;

[0071] 4. The absolute value of the wavefront defocusing factor c increases monotonically with the increase of the objective lens focusing error;

[0072] 5. The wavefront defocusing coefficient c has a one-to-one mapping relationship with the focusing error of the objective lens.

[0073] Based on the properties of the wavefront defocus coefficient mentioned above, this invention provides an autofocus method based on the wavefront defocus coefficient, comprising the following steps:

[0074] S1. Receive the measurement beam reflected from the target and passing through objective lens 4;

[0075] S2. Measure the wavefront of the measurement beam using wavefront sensor 6;

[0076] S3. Calculate the wavefront defocusing factor c based on the measured wavefront;

[0077] S4. Determine the focusing error of objective lens 4 based on the wavefront defocusing coefficient c;

[0078] S5. The focusing error control motion mechanism adjusts the object distance between the objective lens 4 and the target to achieve autofocus.

[0079] Step S5 includes:

[0080] When the wavefront defocusing coefficient c is greater than 0, the motion mechanism is controlled to reduce the object distance;

[0081] When the wavefront defocusing coefficient c is less than 0, the motion mechanism is controlled to increase the object distance.

[0082] During implementation, the wavefront defocus coefficient c should be monitored in real time, and step S5 should be repeated when its absolute value is greater than the preset threshold ε, so as to maintain the absolute value of the wavefront defocus coefficient c less than the preset threshold ε.

[0083] When applied to laser processing, the measuring beam is formed by the reflection of the laser beam used to process the target.

[0084] In other words, the automatic focusing method for laser processing includes the following steps:

[0085] S1. After the laser is focused by the objective lens 4, it illuminates the workpiece 3. Due to the reflection from the surface of the workpiece 3, the beam is reflected back to the objective lens 4. After passing through the objective lens 4, it is reflected by the beam splitter 5 to the wavefront sensor 6.

[0086] S2. Wavefront sensor 6 detects the beam signal and transmits the data to computer 7, where wavefront sensor software calculates the wavefront defocus coefficient c.

[0087] S3. When the wavefront defocus coefficient c is greater than 0, the computer 7 sends a command to the motion controller 8 to shorten the object distance until the absolute value of the wavefront defocus coefficient c is less than a specified number ε. The value of ε can be set according to the focusing accuracy requirements. When the wavefront defocus coefficient c is less than 0, the computer sends a command to the motion controller 8 to increase the object distance until the absolute value of the wavefront defocus coefficient c is less than the specified number ε.

[0088] S4. Real-time detection of the wavefront defocus coefficient c. If the absolute value is less than the specified number ε, the current focus is considered to be good. If the absolute value is greater than the specified number ε, S3 is repeated to maintain the focus status.

[0089] S5. Repeating the above steps can achieve real-time closed-loop control of autofocus, thereby keeping the entire processing in focus.

[0090] An autofocus device based on wavefront defocus coefficient, matching this method, includes:

[0091] Objective lens 4;

[0092] Wavefront sensor 6 is used to measure the wavefront of the measurement beam reflected from the target and passing through objective lens 4;

[0093] The processing unit, connected to the wavefront sensor 6, is used to calculate the wavefront defocus coefficient c and determine the focus error based on the measured wavefront.

[0094] The motion mechanism is used to adjust the distance between the objective lens 4 and the target.

[0095] Motion controller 8, connected to the processing unit and motion mechanism, is used to drive the motion mechanism according to the focus error;

[0096] Beam splitter 5 is positioned in the optical path between objective lens 4 and wavefront sensor 6 to guide the measurement beam to wavefront sensor 6.

[0097] The measuring beam is formed by the reflection of a laser beam used to process the target.

[0098] The processing unit can be a computer 7, and the target is specifically the workpiece 3. The objective lens 4, beam splitter 5, and wavefront sensor 6 are mounted on the fixed substrate 1 as a whole. Therefore, the computer 7 calculates the wavefront defocus coefficient c and sends motion control commands to the motion controller 8 based on the wavefront defocus coefficient c. The motion controller 8 drives the fixed substrate 1 to move, so that the workpiece 3 is located on the focal plane of the laser.

[0099] In addition, this embodiment is an automatic focusing device for a 355nm ultraviolet laser processing equipment, which has an average beam power of 8W and a beam diameter of 16mm.

[0100] like Figure 1 As shown, the 355nm ultraviolet laser is a quasi-parallel beam. A beam splitter 5 is inserted in the optical path before the objective lens 4. After passing through the beam splitter 5, the laser beam is focused by the objective lens 4 and shines on the workpiece 3. Part of the laser beam is reflected from the workpiece 3 and collimated by the objective lens 4 before shining on the beam splitter 5. The laser beam reflected by the beam splitter 5 enters the wavefront sensor 6. The light signal collected by the wavefront sensor 6 is transmitted to the computer 7. The computer 7 calculates the wavefront defocus coefficient c and sends motion control commands to the motion controller 8 based on the value of c. The motion controller 8 drives the fixed base plate 1 to move, so that the workpiece 3 is located on the focal plane of the laser. Since the sensor 6 can continuously measure the defocus coefficient c of the beam, the above process can realize closed-loop control, so that the device is in an autofocus state.

[0101] In particular, in this embodiment, the fixed base plate 1 is equipped with a one-dimensional moving mechanism controlled by the motion controller 8 to realize the adjustment of the object distance.

[0102] Specifically, the reflectivity design of the beam splitter 5 needs to ensure that the power injected into the wavefront sensor 6 is in the 100 microwatt range. In practical implementation, an optical attenuator can also be inserted between the beam splitter 5 and the wavefront sensor 6, depending on convenience, to ensure that the power injected into the wavefront sensor 6 is in the 100 microwatt range. In this embodiment, the beam splitter 5 is made of ultraviolet fused silica material, with a diameter of 30 mm and a thickness of 3 mm. Both sides are coated with a 355 nm antireflection film, and the reflectivity ratio of the side near the sensor to the other side is 5:1 to avoid interference between the beams from the two reflecting surfaces affecting the wavefront measurement.

[0103] Specifically, wavefront sensor 6 is a Hartmann wavefront sensor with a detection aperture of 11.2×11.2 mm², an effective number of microlenses of 56×56, a wavefront measurement peak-valley repeatability of 10 nm, a fastest exposure time of 12 microseconds, and a wavefront measurement frequency of 60 Hz.

[0104] Specifically, computer 7 is a ThinkPad 15P Gen3, with Windows 11, Hartmann wavefront sensor software, and motion control software installed.

[0105] Specifically, this embodiment includes software for calculating the wavefront defocus coefficient. This software is capable of interacting with Hartmann wavefront sensor software and motion control software. It first receives wavefront data from the Hartmann wavefront sensor software. Then, according to the formula:

[0106]

[0107] The defocus coefficient c is calculated, and then a command is sent to the motion control software to move the fixed base plate 1 a distance kc so that the workpiece 3 is in the focus position. Here, k is a proportional coefficient, which can be set and optimized according to the experimental conditions.

[0108] In this embodiment, the Hartmann wavefront sensor can continuously measure the wavefront of the beam at a frequency of 60Hz and dynamically measure the change in object distance. Therefore, the object distance can be adjusted in real time to achieve closed-loop control autofocus.

[0109] Example 2: Please refer to Figure 2 and Figure 3 The difference from Embodiment 1 is that, when applied to optical imaging, the measurement beam is formed by the reflection of a collimated laser beam generated by the point light source 10 from the target. More specifically, the timing is controlled by the synchronization controller 11:

[0110] During the measurement phase, the point light source 10 and wavefront sensor 6 are turned on, while the imaging illumination source and camera 13 are turned off.

[0111] During the image acquisition phase, the point light source 10 and wavefront sensor 6 are turned off, while the imaging illumination source and camera 13 are turned on.

[0112] In other words, the autofocus method for optical imaging includes the following steps:

[0113] S1. Computer 7 sends a command to synchronous controller 11 to turn on point light source 10 and wavefront sensor 6;

[0114] S2. The collimated laser beam generated by the point light source 10 is reflected by the beam splitter 5 and the secondary beam splitter 9, and then shines on the workpiece 3 through the objective lens 4.

[0115] S3. The light reflected from workpiece 3 returns to objective lens 4, passes through objective lens 4, then through beam splitter 5 and secondary beam splitter 9 to reach wavefront sensor 6;

[0116] S4. Wavefront sensor 6 detects the beam signal and transmits the data to the computer, where the wavefront sensor software calculates the wavefront defocusing coefficient c.

[0117] S5. When the wavefront defocus coefficient c is greater than 0, the computer 7 sends a command to the motion controller 8 to shorten the object distance until the absolute value of the wavefront defocus coefficient c is less than a specified number ε. The value of ε can be set according to the focusing accuracy requirements. When the wavefront defocus coefficient c is less than 0, the computer 7 sends a command to the motion controller 8 to increase the object distance until the absolute value of the wavefront defocus coefficient c is less than the specified number ε.

[0118] S6. Real-time detection of wavefront defocus coefficient c. If the absolute value is less than the specified number ε, the current focus is determined to be good. If the absolute value is greater than the specified number ε, S5 is repeated to maintain the focus.

[0119] S7. Computer 7 sends a command to synchronous controller 11 to turn off point light source 10 and wavefront sensor 6, and turn on illumination source and camera 13. Camera 13 completes image acquisition.

[0120] S8. Repeating the above steps can achieve real-time closed-loop control of autofocus, thereby keeping the entire optical imaging process in focus.

[0121] An autofocusing device based on wavefront defocusing coefficient, which is compatible with this method, includes: objective lens 4;

[0122] Wavefront sensor 6 is used to measure the wavefront of the measurement beam reflected from the target and passing through objective lens 4;

[0123] The processing unit, connected to the wavefront sensor 6, is used to calculate the wavefront defocus coefficient c and determine the focus error based on the measured wavefront.

[0124] The motion mechanism is used to adjust the distance between the objective lens 4 and the target.

[0125] Motion controller 8, connected to the processing unit and motion mechanism, is used to drive the motion mechanism according to the focus error;

[0126] Beam splitter 5 is positioned in the optical path between objective lens 4 and wavefront sensor 6 to guide the measurement beam to wavefront sensor 6.

[0127] Point light source 10 is used to generate a collimated laser beam that forms the measurement beam.

[0128] Imaging unit;

[0129] The synchronization controller 11 is connected to the processing unit, the point light source 10, the wavefront sensor 6, and the imaging unit. It is used to control the operation of the point light source 10 and the wavefront sensor 6 during the measurement phase and to control the operation of the imaging unit during the imaging phase.

[0130] The processing unit may be a computer 7, and the target is specifically the workpiece 3.

[0131] More specifically, regarding the autofocus mechanism of the microscope, this device employs a structure of an infinite conjugate objective lens and an imaging unit (photographic eyepiece 12 and camera 13) to achieve microscopic imaging, such as... Figure 2 As shown.

[0132] exist Figure 2In the process, computer 7 sends a command to synchronization controller 11, which triggers point light source 10 and wavefront sensor 6. The collimated laser generated by point light source 10 is reflected by secondary beam splitter 9, focused by objective lens 4, and then shines on workpiece 3. Part of the laser is reflected from workpiece 3, collimated by objective lens 4, and then shines on secondary beam splitter 9, and then transmitted to beam splitter 5. The laser reflected by beam splitter 5 enters wavefront sensor 6. The light signal collected by wavefront sensor 6 is transmitted to computer 7, which calculates the wavefront defocus coefficient c and sends motion control command to motion controller 8 based on the value of c. Motion controller 8 drives fixed base plate 1 to move, so that workpiece 3 is located on the focal plane of the laser. When the absolute value of wavefront defocus coefficient c is less than the set value, it is considered to be in focus. At this time, computer sends a command to synchronization controller 11 to turn off point light source 10 and wavefront sensor 6, and turn on microscope illumination light and camera 13. At this time, camera 13 can capture a clear image. Since sensor 6 can continuously measure the defocus coefficient c of the light beam, the above process can achieve closed-loop control, so that the device is in autofocus mode.

[0133] Specifically, the fixed substrate is equipped with a precision screw-driven motion mechanism with a stroke of 150 mm in the object distance direction and a minimum step accuracy of 0.1 μm.

[0134] Specifically, the worktable 2 is equipped with a motion mechanism that can move in two dimensions in a plane parallel to the focal plane, with a stroke of 300mm and a maximum movement speed of 100mm / s.

[0135] Specifically, the beam splitter 5 and the secondary beam splitter 9 are made of K9 glass, with a diameter of 25.4 mm and a thickness of 3 mm. The side near the wavefront sensor is coated with a beam splitting film with a visible light transmittance-to-reflection ratio of 1:1, and the other side is coated with a visible light anti-reflection film.

[0136] Specifically, the synchronization controller 11 is a control board developed with a microcontroller that can interact with the computer 7 to send high and low levels to the point light source 10, wavefront sensor 6, camera 13 and microscope illumination light to achieve switching control.

[0137] Specifically, the point light source 10 uses a 630nm wavelength LD point light source laser, which includes a collimating lens to collimate the point light source into parallel light with a diameter of 8mm.

[0138] Specifically, wavefront sensor 6 is a Hartmann wavefront sensor with a detection aperture of 11.2×11.2 mm², an effective number of microlenses of 56×56, a wavefront measurement peak-valley repeatability of 10 nm, a fastest exposure time of 12 microseconds, and a wavefront measurement frequency of 60 Hz.

[0139] Specifically, computer 7 is a ThinkPad 15P Gen3, with Windows 11, Hartmann wavefront sensor software, and motion control software installed.

[0140] Specifically, this embodiment includes software for calculating the wavefront defocus coefficient. This software is capable of interacting with Hartmann wavefront sensor software and motion control software. It first receives wavefront data from the Hartmann wavefront sensor software. Then according to the formula

[0141]

[0142] The defocus coefficient c is calculated, and then a command is sent to the motion control software to move the fixed base plate 1 a distance kc so that the workpiece 3 is in the focus position. Here, k is a proportional coefficient, which can be set and optimized according to the experimental conditions.

[0143] In particular, this embodiment also includes a synchronous controller host computer software, which can interact with the wavefront defocus coefficient calculation software to realize the switching control of the illumination light of the point light source 10, the wavefront sensor 6, the camera 13, and the microscope.

[0144] In this embodiment, the Hartmann wavefront sensor can continuously measure the wavefront of the beam at a frequency of 60Hz and dynamically measure the change in object distance. Therefore, the object distance can be adjusted in real time to achieve closed-loop control autofocus.

[0145] Experimental results show that with a 2x magnification objective lens and a working distance of 94mm, the focusing repeatability is better than 0.5μm, achieving autofocus over the entire 150mm travel of the substrate, meaning a focusing range exceeding 150mm. With a 10x magnification objective lens and a working distance of 17.7mm, the focusing repeatability is better than 0.1μm, and the focusing range reaches 10mm. When a tilted workpiece is placed on the worktable and moved at 100mm / s, no visible defocus blur appears in the captured images, demonstrating high-speed automatic tracking and focusing on dynamic targets. Autofocus is also smoothly achieved when switching between various materials such as transparent glass, silver-plated high-reflectivity mirrors, paper, and anodized black aluminum plates. Autofocus is also smooth when the target is a bonding wire of an integrated circuit with a diameter of several μm. Furthermore, even when switching from a low-magnification objective lens with a working distance of 100mm to a high-magnification objective lens with a working distance of 1mm, there is no need to recalibrate the optical path, and autofocus can be performed directly. Additionally, since the point light source 10 is off when the camera 13 is taking pictures, the images captured by the camera 13 are free from the contamination of additional markings.

[0146] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An autofocusing method based on wavefront defocus coefficient, characterized in that, Includes the following steps: S1. Receive the measurement beam reflected from the target and passing through the objective lens (4); S2. Measure the wavefront of the measurement beam using a wavefront sensor (6); S3. Calculate the wavefront defocusing factor c based on the measured wavefront; S4. Determine the focusing error of the objective lens (4) based on the wavefront defocusing coefficient c; S5. Adjust the object distance between the objective lens (4) and the target according to the focusing error control motion mechanism to achieve autofocus.

2. The autofocus method according to claim 1, characterized in that, Step S5 includes: When the wavefront defocusing coefficient c is greater than 0, the motion mechanism is controlled to reduce the object distance; When the wavefront defocusing coefficient c is less than 0, the motion mechanism is controlled to increase the object distance.

3. The autofocus method according to claim 2, characterized in that, Also includes: The wavefront defocus coefficient c is monitored in real time, and step S5 is repeated when its absolute value is greater than a preset threshold ε, so as to maintain the absolute value of the wavefront defocus coefficient c less than the preset threshold ε.

4. The autofocus method according to claim 1, characterized in that: The measuring beam is formed by the reflection of a laser beam used to process the target.

5. The autofocus method according to claim 1, characterized in that: The measurement beam is formed by the reflection of a collimated laser beam generated by a point light source (10) onto the target.

6. The autofocus method according to claim 5, characterized in that, Also includes: Timing is controlled by a synchronous controller (11); During the measurement phase, the point light source (10) and wavefront sensor (6) are turned on, and the imaging illumination source and camera (13) are turned off. During the image acquisition phase, the point light source (10) and wavefront sensor (6) are turned off, and the imaging illumination source and camera (13) are turned on.

7. An autofocusing device based on wavefront defocusing coefficient, characterized in that, include: Objective lens (4); A wavefront sensor (6) is used to measure the wavefront of the measurement beam reflected from the target and passing through the objective lens (4); The processing unit is connected to the wavefront sensor (6) and is used to calculate the wavefront defocus coefficient c and determine the focus error based on the measured wavefront. The motion mechanism is used to adjust the distance between the objective lens (4) and the target; A motion controller (8) is connected to the processing unit and the motion mechanism and is used to drive the motion mechanism according to the focus error. A beam splitter (5) is positioned in the optical path between the objective lens (4) and the wavefront sensor (6) to guide the measurement beam to the wavefront sensor (6).

8. The autofocus device according to claim 7, characterized in that: The measuring beam is formed by the reflection of a laser beam used to process the target.

9. The autofocus device according to claim 7, characterized in that, Also includes: A point light source (10) is used to generate a collimated laser beam that forms the measurement beam.

10. The autofocus device according to claim 9, characterized in that, Also includes: Imaging unit; The synchronization controller (11) is connected to the processing unit, the point light source (10), the wavefront sensor (6) and the imaging unit. It is used to control the operation of the point light source (10) and the wavefront sensor (6) during the measurement phase and to control the operation of the imaging unit during the imaging phase.

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