Method and system for imaging upper surface and lower surface of transparent object based on structured light

Abstract

The invention provides a structured light-based transparent object upper and lower surface imaging method and system, and belongs to the field of optical imaging. The imaging method comprises the following steps: calculating the distance between the upper and lower surfaces of the transparent object caused by a projection angle; obtaining background light intensity and an initial phase; obtaining a mixed picture containing light intensity distribution of the upper and lower surfaces of the transparent object; solving light intensity modulation distribution of the upper and lower surfaces of the transparent object; the system comprises a projection device, an imaging system, a carrying platform and a data processing unit. Imaging is not affected by background environment illumination, and the stability and accuracy of imaging can be remarkably improved; the method and the system are suitable for detection of transparent objects with small thickness, can realize respective imaging of the upper surface and the lower surface, can quickly judge the surfaces to which flaws belong, and improve the detection efficiency and accuracy; the method can be widely used in the optical quantity detection field.

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 are widely used in electronic products, building materials, medical devices and other fields due to their transparent properties and visual beauty. Minor defects on the surface of transparent objects, such as scratches, dirt, and broken edges, may lead to a decline in product performance and have a crucial impact on safety and aesthetics. During the production process of transparent objects, defects may occur on both the upper and lower surfaces. In addition, some transparent products have different types of defects due to different processing techniques on the upper and lower surfaces.

[0003] As consumers' requirements for product quality continue to increase, surface defect detection of transparent objects is becoming increasingly important. Whether the defects of transparent objects are on the upper or lower surface has different effects on subsequent processing and performance. Distinguishing the upper and lower surfaces of transparent objects during defect detection can more accurately locate the location of defects, thereby achieving refined quality control, selecting different subsequent treatment methods for defects on different surfaces, and helping to analyze the causes of defects, and improve production processes in a targeted manner, improve production yields, and reduce production costs.

[0004] At present, the method of distinguishing the defects on the upper and lower surfaces of transparent objects includes a small depth of field microscope system imaging scheme, which uses the characteristics of the small depth of field of the microscope objective lens to image only one surface, exclude the imaging interference of the other surface, and judge the surface where the defect is located according to the Z-direction position of the defect imaging. In order to obtain a better distinction effect, this scheme needs to select a microscope objective lens with a larger numerical aperture and a higher magnification as much as possible. The imaging field of view is small, and a reference surface in the Z direction is required. In order to accurately determine the surface where the defect is located, the reference surface needs to be frequently focused and calibrated, which is inefficient. In addition, there is also a method of blocking the reflected light on the lower surface of the transparent object in the line scanning imaging system so that the defects on the lower surface cannot be imaged in the camera, and only the defects on the upper surface are detected. Then the transparent object is reversed and detected again, thereby realizing the separate detection of the two surfaces. This scheme is difficult to debug, and for thinner transparent objects (<1mm), the defects on another surface other than the surface to be detected will interfere with the imaging of the surface to be detected, which ultimately leads to the inclusion of defect information on both surfaces in the imaging, and the defects on the upper and lower surfaces cannot be well distinguished. Summary of the invention

[0005] In order to overcome the deficiencies of the prior art, an object of the present invention is 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: S1. Get the thickness of transparent objects 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-corrected fringe pattern obliquely onto a stage without a transparent object to be measured through a projection device, 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 the background light intensity With initial phase ; S3, obliquely projecting the pre-corrected fringe pattern onto the transparent object to be measured, collecting images through an imaging system, and obtaining a mixed image including the light intensity distribution of 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 , and obtain imaging images of the upper and lower surfaces respectively.

[0007] Preferably, the projection device is configured such that the angle θ between the projection device and the normal of the transparent object surface is: , the phase difference of the projected fringes on the upper and lower surfaces is .

[0008] Preferably, the pre-corrected fringe pattern is a sinusoidal periodic fringe pattern.

[0009] 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 .

[0010] Preferably, for the light intensity on the upper and lower surfaces of the mixed transparent object The distribution expression is: ; 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 value of the upper and lower surfaces.

[0011] 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.

[0012] 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 stage and a data processing unit; the projection device is a telecentric projection device, the projection direction of the projection device is arranged obliquely downward toward the stage, and is used to obliquely project a pre-corrected stripe pattern onto the stage or the transparent object to be measured; the imaging system adopts a telecentric imaging system, and the imaging system is arranged directly above the stage, and is used to capture pictures; the data processing unit is used to process pictures and obtain the light intensity modulation distribution on the upper and lower surfaces of the transparent object.

[0013] Compared with the prior art, the beneficial effects of the present invention are: the imaging of the present application is not affected by the background ambient light, and can significantly improve the stability and accuracy of imaging; the 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

[0014] Figure 1 Schematic diagram of structured light projection; Figure 2 This is a partial layout diagram of the system. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the 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 creative work are within the scope of protection of the present invention.

[0016] It should be understood that the "system", "device", "unit" and / or "module" used in this specification is a method for distinguishing different components, elements, parts, parts or assemblies at different levels. However, if other words can achieve the same purpose, the words can be replaced by other expressions.

[0017] Imaging methods A method for imaging the upper and lower surfaces of a transparent object based on structured light, the imaging method comprising the following steps.

[0018] S1. Get the thickness of transparent objects 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 ), .

[0019] S2, project the pre-corrected fringe pattern obliquely onto a stage without a transparent object to be measured through a projection device, 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 the background light intensity With initial phase .

[0020] S3. Obliquely project the pre-corrected fringe pattern onto the transparent object to be measured, collect images through an imaging system, and obtain a mixed image including the light intensity distribution of the upper and lower surfaces of the transparent object.

[0021] 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.

[0022] In a preferred embodiment, the transparent object is a transparent flat plate. Of course, the upper and lower surfaces of a local position of the transparent object may also be imaged, so it is not limited to a flat plate.

[0023] 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 .

[0024] The pre-corrected fringe pattern is a sinusoidal periodic fringe pattern, and images with other distribution patterns may of course be selected.

[0025] In steps S2 and S3, the image acquisition method of the imaging system is: a four-step phase shift method is used to acquire the corresponding four pictures, and each phase change is .

[0026] In step S4, the light intensity on the upper and lower surfaces of the mixed transparent object The distribution expression is: ;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 in the figure is a transparent 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 .

[0027] For image acquisition, after each phase change is The four-step phase shift, the light intensity distribution expression of the four pictures numbered 1 to 4 is: ; ; ; .

[0028] The above formula can be rearranged to obtain: (1.1); (1.2); (1.3); (1.4); The difference between formula (1.1) and formula (1.3) is: (1.5); The difference between formula (1.2) and formula (1.4) is: (1.6); Will Substituting into equation (1.5) and equation (1.6), we can get: (1.7); In the imperative formula (1.7) , , ,but: (1.8); The average of formulas (1.1) to (1.4) is: (1.9); 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 .

[0029] In the imperative (1.9) , but (1.10); The difference between formula (1.10) and formula (1.8) is: (1.11); because , It is known that, therefore, we can solve equation (1.9) and equation (1.11) to obtain and , that is, the light intensity modulation distribution on the upper and lower surfaces of the transparent plate.

[0030] when and The phase difference between When , we can get from equation (1.5) and equation (1.6): , so we can get .

[0031] Finally, the initial phase of the projected fringes is determined and The difference between the initial phase and the initial phase is taken as the same surface, and a mask for separating the upper and lower surface imaging is generated according to the phase deviation degree, so as to obtain the imaging pictures of the upper and lower surfaces of the transparent plate respectively.

[0032] 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.

[0033] Imaging System A system for implementing the above-mentioned method for imaging the upper and lower surfaces of a transparent object based on structured light, the system includes a projection device, an imaging system, a stage (not shown) and a data processing unit (not shown).

[0034] 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.

[0035] Among them, the imaging system adopts a telecentric imaging system, and the imaging system is arranged directly above the stage for collecting pictures.

[0036] 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.

[0037] In a specific example, a transparent plate with a thickness of h < 1 mm is used for detection. Preferably, 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 flat 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 resolution of the projection chip is 1920×1080 (other resolutions are also within the scope of consideration of this application, different resolutions correspond to different application scenarios, 1920×1080 is taken as an example here), the micromirror pitch is 5.4um (Pitch is the lens spacing, other Pitch ranges are also within the scope of consideration of this application); the projection lens adopts 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 adopts a C-mount 5M black and white camera with a resolution of 2448×2048 (other resolutions are also within the scope of consideration of this application); the lens adopts 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 without specific limitation.

[0038] 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.

[0039] According to the known parameters of the transparent plate, set the fringe projection period , so that it satisfies The images with and without the transparent plate are obtained by four-step phase shift acquisition. , and solve to obtain 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 according to 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.

[0040] Storage Media The present invention also provides a computer-readable storage medium on which computer instructions are stored, and when the computer instructions are executed, the steps of the aforementioned imaging method are executed. The aforementioned method is described in detail in the aforementioned part, and will not be described again here.

[0041] Those skilled in the art will appreciate that all or part of the steps in the various methods of the above embodiments can be completed by instructing the relevant hardware through a program, and the program can be stored in a computer-readable storage medium, and the computer-readable medium includes permanent and non-permanent, removable and non-removable media, and information storage can be implemented by any method or technology. Information can be a computer-readable instruction, a data structure, a module of a program, 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 technology, read-only compact disk read-only memory (CD-ROM), digital versatile disk (DVD) or other optical storage, magnetic cassettes, magnetic tape magnetic disk storage or other magnetic storage devices or any other non-transmission 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 temporary computer-readable media (transitory media), such as modulated data signals and carriers.

[0042] 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 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 comprises the following steps: S1. Get the thickness of transparent objects 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-corrected fringe pattern obliquely onto a stage without a transparent object to be measured through a projection device, 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 the background light intensity With initial phase ; S3, obliquely projecting the pre-corrected fringe pattern onto the transparent object to be measured, collecting images through an imaging system, and obtaining a mixed image including the light intensity distribution of 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 , and obtain imaging images of the upper and lower surfaces respectively.

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

3. The imaging method according to claim 1, characterized in that: The spacing The calculation formula is: .

4. The imaging method according to claim 1, characterized in that: 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. The imaging method according to claim 1, characterized in that: The pre-corrected fringe pattern is a sinusoidal periodic fringe pattern.

6. The imaging method according to claim 1, characterized in that: In steps S2 and S3, the image acquisition method of the imaging system is: a four-step phase shift method is used to acquire the corresponding four pictures, and each phase change is .

7. The imaging method according to claim 1, characterized in that: For the light intensity of the upper and lower surfaces of a mixed transparent object The distribution expression is: ； 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 value of the upper and lower surfaces.

8. The imaging method according to claim 1, characterized in that: 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.

9. 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 8, 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-corrected 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 pictures; 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.

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