Method and system for imaging upper and lower surfaces of transparent objects based on structured light

Abstract

The present invention provides a method and system for imaging the upper and lower surfaces of transparent objects based on structured light, belonging to the field of optical imaging. The imaging method includes calculating the distance between the upper and lower surfaces of the transparent object due to the projection angle, obtaining the background light intensity and initial phase, obtaining a mixed image containing the light intensity distribution of the upper and lower surfaces of the transparent object, and obtaining the light intensity modulation distribution of the upper and lower surfaces of the transparent object, thereby obtaining imaging images of the upper and lower surfaces respectively. The system includes a projection device, an imaging system, a stage, and a data processing unit. The imaging of the present invention is not affected by background ambient light and can significantly improve the stability and accuracy of imaging. The present method and system are suitable for detecting transparent objects with relatively small thicknesses, can realize the imaging of the upper and lower surfaces separately, quickly determine the surface to which the defect belongs, and improve detection efficiency and accuracy. The present invention can be widely used in the field of optical quantity detection.

Description

Technical Field

[0001] The present invention belongs to the field of optical imaging, and in particular relates to a method and system for imaging the upper and lower surfaces of a transparent object based on structured light. Background Art

[0002] Transparent objects, due to their transparency and visual aesthetic, are widely used in electronics, building materials, medical devices, and other fields. Minor surface defects on transparent objects, such as scratches, dirt, and chipped edges, can degrade product performance and significantly impact safety and aesthetics. During the production process, both the top and bottom surfaces of transparent objects may develop defects. Furthermore, some transparent products may exhibit different types of defects due to different processing techniques on the top and bottom surfaces.

[0003] As consumers' demands for product quality continue to rise, surface defect detection for transparent objects is becoming increasingly important. The impact of defects on the upper or lower surface of a transparent object on subsequent processing and performance varies. Distinguishing between the upper and lower surfaces of a transparent object during defect detection allows for more accurate location of defects, enabling refined quality control and selecting different treatment methods for defects on different surfaces. This also helps analyze the causes of defects and enable targeted improvements to production processes, increasing yield and reducing costs.

[0004] Currently, methods for distinguishing defects on the upper and lower surfaces of transparent objects include imaging with a small depth of field microscope system. This approach utilizes the small depth of field of a microscope objective lens to image only one surface, eliminating interference from the other surface. The defect's location is determined based on the Z-axis position of the defect image. To achieve effective differentiation, this approach requires a microscope objective lens with a large numerical aperture and high magnification. This results in a small imaging field and requires a Z-axis reference surface. Frequent focus calibration of the reference surface is required to accurately determine the defect's location, resulting in low efficiency. Alternatively, a line scan imaging system can block reflected light from the lower surface of the transparent object, preventing the lower surface from being imaged. Only the upper surface is detected, and the transparent object is then flipped over and inspected again, achieving separate inspection of both surfaces. This approach is difficult to debug, and for thinner transparent objects (less than 1 mm), defects on surfaces other than the surface being inspected can interfere with the image of the surface being inspected. Ultimately, the image contains defect information from both surfaces, making it difficult to distinguish between the upper and lower surfaces. Summary of the Invention

[0005] In order to overcome the deficiencies of the prior art, the present invention aims to provide a method and system for imaging the upper and lower surfaces of a transparent object based on structured light, which can solve the above-mentioned problems.

[0006] A method for imaging the upper and lower surfaces of a transparent object based on structured light, the imaging method comprising the following steps:

[0007] S1. Get the thickness of the transparent object and refractive index , and the angle between the projection device and the normal of the transparent object surface , calculate the distance between the upper and lower surfaces of the transparent object due to the projection angle ;

[0008] S2, project the pre-calibrated fringe pattern obliquely onto a stage where no transparent object to be measured is placed, and set the fringe period length , making the phase difference between the upper and lower surfaces , use the imaging system to collect pictures and obtain background light intensity With initial phase ;

[0009] S3, obliquely projecting the pre-calibrated fringe pattern onto the transparent object to be measured, capturing an image through an imaging system, and obtaining a mixed image including the light intensity distribution on the upper and lower surfaces of the transparent object;

[0010] S4. Construct the solution equation based on the acquired background light intensity , initial phase , mixed images, to obtain the light intensity modulation distribution on the upper and lower surfaces of transparent objects and , and obtain imaging images of the upper and lower surfaces respectively.

[0011] Preferably, the projection device is configured such that the angle θ between the projection device and the normal to the surface of the transparent object is: , the phase difference of the projected fringes on the upper and lower surfaces is .

[0012] Preferably, the pre-corrected fringe pattern is a sinusoidal periodic fringe pattern.

[0013] Preferably, the image acquisition method of the imaging system is: using a four-step phase shift method to acquire the corresponding 4 pictures, each phase change is .

[0014] Preferably, for the light intensity on the upper and lower surfaces of the mixed transparent object The distribution expression is:

[0015] ;

[0016] in, For light intensity, is the background light intensity, 、 are the light intensity modulation distribution on the upper and lower surfaces of the transparent object, 、 Indicates the phase values ​​of the upper and lower surfaces.

[0017] Preferably, according to the initial phase The difference in phase between the upper and lower surfaces is used to determine and separate the imaging images of the upper and lower surfaces, and then determine the surface to which the defect belongs.

[0018] The present invention also provides a system for implementing the aforementioned structured light-based imaging method for the upper and lower surfaces of a transparent object, the system comprising a projection device, an imaging system, a carrier, and a data processing unit; the projection device is a telecentric projection device, and the projection direction of the projection device is arranged obliquely downward toward the carrier, and is used to obliquely project a pre-corrected stripe pattern onto the carrier or the transparent object to be measured; the imaging system adopts a telecentric imaging system, and the imaging system is arranged directly above the carrier, and is used to capture images; the data processing unit is used to process images and obtain the light intensity modulation distribution on the upper and lower surfaces of the transparent object.

[0019] Compared with the existing technology, the beneficial effects of the present invention are: the imaging of the present application is not affected by background ambient light, and can significantly improve the stability and accuracy of imaging; the present method and system are suitable for the detection of transparent objects with a smaller thickness, and can realize the imaging of the upper and lower surfaces separately, quickly determine the surface to which the defect belongs, and improve the detection efficiency and accuracy; it can be widely used in the field of optical quantity detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of structured light projection;

[0021] Figure 2 This is a partial layout diagram of the system. DETAILED DESCRIPTION

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0023] It should be understood that the terms "system," "device," "unit," and / or "module" used in this specification are a method for distinguishing different components, elements, parts, portions, or assemblies at different levels. However, if other terms can achieve the same purpose, the terms may be replaced by other expressions.

[0024] Imaging methods

[0025] A method for imaging the upper and lower surfaces of a transparent object based on structured light, the imaging method comprising the following steps.

[0026] S1. Get the thickness of the transparent object and refractive index , and the angle between the projection device and the normal of the transparent object surface , calculate the distance between the upper and lower surfaces of the transparent object due to the projection angle (See Figure 1 ), .

[0027] S2, project the pre-calibrated fringe pattern obliquely onto a stage where no transparent object to be measured is placed, and set the fringe period length , making the phase difference between the upper and lower surfaces , use the imaging system to collect pictures and obtain background light intensity With initial phase .

[0028] S3. Project the pre-calibrated fringe pattern obliquely onto the transparent object to be measured, collect images through the imaging system, and obtain a mixed image including the light intensity distribution on the upper and lower surfaces of the transparent object.

[0029] S4. Construct the solution equation based on the acquired background light intensity , initial phase , mixed images, to obtain the light intensity modulation distribution on the upper and lower surfaces of transparent objects and , and obtain imaging images of the upper and lower surfaces respectively.

[0030] In a preferred embodiment, the transparent object is a transparent flat plate. Of course, the upper and lower surfaces of a local portion of the transparent object may also be imaged, so it is not limited to a flat plate.

[0031] In the preferred embodiment, the projection device is set to: the angle between the projection device and the normal direction of the transparent object surface is The size is: , the phase difference of the projected fringes on the upper and lower surfaces is .

[0032] The pre-corrected fringe pattern is a sinusoidal periodic fringe pattern, and images with other distribution patterns may also be selected.

[0033] In steps S2 and S3, the image acquisition method of the imaging system is: four-step phase shift method is used to acquire the corresponding four pictures, each phase change is .

[0034] In step S4, the light intensity on the upper and lower surfaces of the mixed transparent object is The distribution expression is:

[0035] ;in, For light intensity, is the background light intensity, 、 They are the light intensity modulation distribution on the upper and lower surfaces of a transparent object (the example shown is a transparent flat plate). 、 Indicates the phase value of the upper and lower surfaces. The thickness change caused by the defects on the upper and lower surfaces of the transparent plate is much smaller than the thickness of the transparent plate, so the phase change of the upper and lower surfaces caused by the defects can be ignored, so the upper surface phase , lower surface phase .

[0036] For image acquisition, after each phase change The four-step phase shift of the collected 4 images numbered 1 to 4 is expressed as follows:

[0037] ;

[0038] ;

[0039] ;

[0040] .

[0041] The above formula can be sorted out:

[0042] (1.1);

[0043] (1.2);

[0044] (1.3);

[0045] (1.4);

[0046] The difference between formula (1.1) and formula (1.3) is:

[0047] (1.5);

[0048] The difference between formula (1.2) and formula (1.4) is:

[0049] (1.6);

[0050] Will Substituting into equation (1.5) and equation (1.6), we can get:

[0051] (1.7);

[0052] In the imperative formula (1.7) , , ,but:

[0053] (1.8);

[0054] The average of formulas (1.1) to (1.4) yields:

[0055] (1.9);

[0056] Similar to formula (1.9), the four pictures taken when the transparent plate to be detected is not placed are averaged. 、 The values ​​of are all 0, so the background light intensity can be solved .

[0057] In the imperative (1.9) ,

[0058] but (1.10);

[0059] The difference between formula (1.10) and formula (1.8) is:

[0060] (1.11);

[0061] because 、 It is known that, therefore, we can solve equations (1.9) and (1.11) to obtain and , that is, the light intensity modulation distribution on the upper and lower surfaces of the transparent plate.

[0062] when and Phase difference between When , we can get from equations (1.5) and (1.6):

[0063] , so we can get .

[0064] Finally determine the initial phase of the projected fringes and The area with a larger deviation from the initial phase is regarded as the same surface, and a mask for separating the upper and lower surface imaging is generated according to the degree of phase deviation, thereby obtaining imaging pictures of the upper and lower surfaces of the transparent plate respectively.

[0065] Furthermore, based on the image, the quality of the object to be tested can be measured or detected. Taking defect detection as an example, based on the initial phase The difference in phase between the upper and lower surfaces is used to determine and separate the imaging images of the upper and lower surfaces, and then determine the surface to which the defect belongs.

[0066] Imaging system

[0067] A system for implementing the aforementioned structured light-based imaging method for upper and lower surfaces of a transparent object includes a projection device, an imaging system, a stage (not shown), and a data processing unit (not shown).

[0068] The projection device is a telecentric projection device, and the projection direction of the projection device is arranged obliquely downward toward the stage, and is used to obliquely project the pre-corrected fringe pattern onto the stage or the transparent object to be measured.

[0069] Among them, the imaging system adopts a telecentric imaging system, which is arranged directly above the stage and is used to collect pictures.

[0070] The data processing unit is used to process the image and obtain the light intensity modulation distribution on the upper and lower surfaces of the transparent object.

[0071] In a specific example, a transparent plate with a thickness of h < 1 mm is used for detection. It is preferred that the angle between the optical axis of the telecentric projection device and the surface to be measured is ≥ 45°, and the phase difference of the projection fringes on the upper and lower surfaces is In this embodiment, it is preferred to use the optical axis of the image acquisition system to be perpendicular to the surface of the transparent plate for image acquisition. Figure 2 As shown, the angle between the projection device and the surface to be measured on the transparent flat plate is The angle of view is 45°, the projection chip resolution is 1920×1080 (other resolutions are also considered in this application. Different resolutions correspond to different application scenarios. 1920×1080 is used as an example here). The micromirror pitch is 5.4μm (Pitch refers to the distance between the lenses; other pitch ranges are also considered in this application). The projection lens uses a telecentric lens (such as a telecentric Sham lens). The optical axis of the image acquisition system is perpendicular to the surface to be measured. The camera uses a C-mount 5M monochrome camera with a resolution of 2448×2048 (other resolutions are also considered in this application). The lens uses a C-mount telecentric lens with a magnification of 0.5X and a working distance of 110mm. The aforementioned magnification, resolution, working distance, chip model, micromirror pitch, camera type, etc. can be adjusted according to the actual application scenario and are not specifically limited.

[0072] The specifications of the fringe projection system (telecentric projection device) and the image acquisition system (telecentric imaging system) can be adjusted on the premise of meeting the layered imaging conditions proposed in the present invention, and can also be combined according to different lighting requirements, such as dark field lighting, bright field lighting and transmitted lighting.

[0073] According to the known parameters of the transparent plate, set the fringe projection period , so that it satisfies The images with and without transparent plate are acquired through four-step phase shifting. , solve to get the initial phase on the fringe projection stage , background light intensity , Light intensity modulation distribution on upper and lower surfaces 、 and the corresponding phase 、 Finally, the imaging images of the upper and lower surfaces are judged and separated based on the difference between the initial phase and the phase of the upper and lower surfaces, and then the surface to which the defect belongs is determined.

[0074] Storage media

[0075] The present invention also provides a computer-readable storage medium having computer instructions stored thereon, which, when executed, execute the steps of the aforementioned imaging method. The aforementioned method is described in detail in the preceding section and will not be further described here.

[0076] Those skilled in the art will appreciate that all or part of the steps in the various methods of the above embodiments can be performed by instructing the relevant hardware through a program, and the program can be stored in a computer-readable storage medium. Computer-readable media include permanent and non-permanent, removable and non-removable media, and can implement information storage by any method or technology. Information can be computer-readable instructions, data structures, program modules, or other data. Examples of computer storage media include, but are not limited to, phase change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, compact disc read-only memory (CD-ROM), digital versatile disc (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transmitting media that can be used to store information that can be accessed by a computing device. As defined herein, computer-readable media does not include transitory computer-readable media, such as modulated data signals and carrier waves.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for imaging the upper and lower surfaces of a transparent object based on structured light, characterized in that ,The imaging method includes the following steps: S1. Get the thickness of the transparent object and refractive index , and the angle between the projection device and the normal of the transparent object surface , calculate the distance between the upper and lower surfaces of the transparent object due to the projection angle ; S2, project the pre-calibrated fringe pattern obliquely onto a stage where no transparent object to be measured is placed, and set the fringe period length , making the phase difference between the upper and lower surfaces , use the imaging system to collect pictures and obtain background light intensity With initial phase ; S3, obliquely projecting the pre-calibrated fringe pattern onto the transparent object to be measured, capturing an image through an imaging system, and obtaining a mixed image including the light intensity distribution on the upper and lower surfaces of the transparent object; S4. Construct the solution equation based on the acquired background light intensity , initial phase , mixed images, to obtain the light intensity modulation distribution on the upper and lower surfaces of transparent objects and , respectively obtain the imaging pictures of the upper and lower surfaces; Mixed light intensity on the upper and lower surfaces of a transparent object The distribution expression is: ; For light intensity, is the background light intensity, 、 are the light intensity modulation distribution on the upper and lower surfaces of the transparent object, 、 Indicates the phase values ​​of the upper and lower surfaces; upper surface phase , lower surface phase ; After four phase shifts, the light intensity distribution expression of the four images numbered 1 to 4 is: ； ； ； ； make , , ,but: ； ； The four pictures taken when the transparent plate to be detected is not placed are averaged. 、 The values ​​of are all 0, so the background light intensity is obtained ; make ,but ; Sakusa: ; because 、 Known, solve and , that is, the light intensity modulation distribution on the upper and lower surfaces of the transparent plate; when and Phase difference between hour, , to obtain ; Finally determine the initial phase of the projected fringes and The area with a larger deviation from the initial phase is regarded as the same surface, and a mask for separating the upper and lower surface imaging is generated according to the degree of phase deviation, thereby obtaining imaging pictures of the upper and lower surfaces of the transparent plate respectively.

2. The imaging method according to claim 1, wherein: The transparent object is a transparent flat plate.

3. The imaging method according to claim 1, wherein: The spacing The calculation formula is: 。 4. The imaging method according to claim 1, wherein: The projection device is set to: the angle between the projection device and the normal of the transparent object surface The size is: , the phase difference of the projected fringes on the upper and lower surfaces is .

5. A system for implementing the method for imaging the upper and lower surfaces of a transparent object based on structured light according to any one of claims 1 to 4, characterized in that: The system includes a projection device, an imaging system, a stage and a data processing unit. The projection device is a telecentric projection device, and the projection direction of the projection device is arranged obliquely downward toward the stage, and is used to obliquely project the pre-calibrated fringe pattern onto the stage or the transparent object to be measured; The imaging system adopts a telecentric imaging system, which is arranged directly above the stage and is used to collect images; The data processing unit is used to process the image and obtain the light intensity modulation distribution on the upper and lower surfaces of the transparent object.

Images