Surface imaging system, imaging method and defect detection method for an object to be tested

Abstract

This invention discloses a surface imaging system, a surface imaging method, and a surface defect detection method for an object under test. The surface imaging system, used for imaging the surface of an object under test, includes a surface light source, a semi-transparent mirror, a line-structured light source, a first reflecting mirror, a reflecting component, and an imaging device. The photosensitive element of the imaging device has a first imaging area and a second imaging area. The first imaging area is adapted to receive the image formed by the surface light source illuminating the object under test, and the second imaging area is adapted to receive the image formed by the line-structured light source illuminating the object under test. Simultaneously, images containing surface information and images containing three-dimensional information are formed in different areas of the same image, and there is a correspondence between the two images, which helps to improve the detection accuracy and efficiency. Based on this imaging system, the surface imaging method and the surface defect detection method further improve the accuracy and efficiency of product defect detection.

Description

Technical Field

[0001] This invention relates to the field of product defect detection, specifically to an imaging system, imaging method, and defect detection method for the surface of an object under test. Background Technology

[0002] In the industrial product manufacturing process, the inspection of certain products includes the inspection of product surface information and product 3D information. The inspection of product surface information and product 3D information can already be achieved using machine vision to acquire relevant images and determine whether the product surface has defects and whether the product height meets standards. Current technology typically uses two or more imaging devices to separately acquire images containing surface information and images containing 3D information, in order to obtain surface defect information and product 3D information respectively. However, this approach has two drawbacks. First, because two or more imaging devices are used to acquire images containing surface information and images containing 3D information separately, the acquired images need to be correlated using a specific method to be integrated into the inspection information of the same product. Otherwise, it is very easy for the surface information and 3D information to not correspond to the same product, leading to disordered inspection information and low inspection efficiency. Second, using multiple imaging devices results in excessively high inspection equipment costs. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned defects or problems in the prior art and provide a surface imaging system for an object under test. This system acquires and images containing surface information and three-dimensional information from a single imaging device and images them onto different areas of the same image, thereby achieving a correspondence between the surface information and three-dimensional information of the same product. This is beneficial for improving detection accuracy, detection efficiency, and reducing detection costs.

[0004] To achieve the above objectives, the present invention and its preferred embodiments employ the following technical solutions, but the embodiments are not limited to the following solutions:

[0005] The first technical solution relates to a surface imaging system for an object under test, which is used to image the surface of the object under test, comprising: a surface light source that emits light toward the object under test; a semi-transparent mirror placed between the surface light source and the object under test, for allowing the light emitted by the surface light source to pass through and illuminate the object under test, and reflecting a first reflected light from the object under test onto the surface light source to form a second reflected light; a linear structure light source that emits linear structure light toward the object under test; a first reflector that reflects a third reflected light from the object under test onto the linear structure light to form a fourth reflected light; a reflective component having a first reflective part and a second reflective part, wherein the first reflective part reflects the second reflected light along a first direction to form a fifth reflected light; and the second reflective part reflects the fourth reflected light along the first direction to form a sixth reflected light; and an imaging device having an imaging optical axis extending along the first direction, wherein its photosensitive element has a first imaging area and a second imaging area, wherein the first imaging area is adapted to receive the fifth reflected light, and the second imaging area is adapted to receive the sixth reflected light.

[0006] The second technical solution is based on the first technical solution, wherein the light emitted by the surface light source is parallel light perpendicular to the first plane.

[0007] The third technical solution is based on the second technical solution, wherein the semi-transparent and semi-reflective mirror is tilted on the first plane.

[0008] The fourth technical solution is based on the third technical solution, wherein the line structured light is tilted towards the object to be tested along the first plane.

[0009] The fifth technical solution is based on the fourth technical solution, wherein at least one ray in the line structured light has its projection on the first plane intersecting with the normal plane of the object being tested.

[0010] The sixth technical solution is based on the fifth technical solution, wherein the reflective component is a prism, and the first reflective part and the second reflective part are formed on the two surfaces of the prism.

[0011] The seventh technical solution is based on the sixth technical solution, wherein the line structured light is a parallel strip of light that is incident at a 45° angle to the first plane, and the projection of the line structured light onto the first plane is perpendicular to the normal plane of the object to be tested; the first reflector is perpendicular to the normal plane of the object to be tested and forms an angle of 78.75° with the first plane; the lower surface of the semi-transparent mirror faces the first reflector and forms an angle of 33.75° with the first plane; the prism is placed between the semi-transparent mirror and the first reflector, the plane containing the first reflector is opposite to the semi-transparent mirror, the plane containing the second reflector is opposite to the first reflector, the angle between the two surfaces of the prism formed by the first and second reflectors is 90°, the angle between the plane containing the first and second reflectors and the bottom surface is 45°, the angle between the bottom surface and the first plane is 22.5°; and the first direction is perpendicular to the bottom surface.

[0012] The eighth technical solution is based on any one of the first to seventh technical solutions, and further includes a conveying device, which drives the object to be tested to move linearly in a direction parallel to the first plane, and the imaging device acquires an image of the surface of the object to be tested at a specific frequency.

[0013] The ninth technical solution relates to a method for imaging the surface of an object under test, which is based on the surface imaging system for an object under test described in the eighth technical solution to obtain an image of the surface of the object under test. The method for imaging the surface of an object under test includes obtaining an image of the surface of the object under test at a specific frequency using an imaging device when the conveying device drives the object under test to move in a straight line.

[0014] The tenth technical solution relates to a method for detecting surface defects of an object under test. It acquires an image of the object's surface using the surface imaging method described in the ninth technical solution, and then judges the image to obtain a detection result. Specifically, if a defect exists in the image domain located in the first imaging region of any image, the surface of the object under test is determined to have a two-dimensional defect; if a defect exists in the image domain located in the second imaging region of any image, the surface of the object under test is determined to have a three-dimensional defect.

[0015] As can be seen from the above description of the present invention and its preferred embodiments, compared with the prior art, the technical solution of the present invention and its preferred embodiments have the following beneficial effects due to the adoption of the following technical means:

[0016] Through continuous observation, experimentation, and research, the applicant has come to realize that the reason for the technical problems of "low product detection accuracy and efficiency, and easy error in detection results" in the existing technical solutions is that multiple imaging devices are required to acquire images containing product surface information and images containing product three-dimensional information, resulting in no accurate correspondence between the two images. It is impossible to combine the information from the two images to make a comprehensive judgment to obtain the detection result, and there is mutual influence between the optical paths.

[0017] In the first technical solution, the use of a surface light source facilitates better imaging of the surface of the object under test. Employing a line-structured light source to emit line-structured light onto the object under test is beneficial for obtaining the object's three-dimensional information through algorithms or judgment methods. Using a semi-transparent, semi-reflective mirror allows for the simultaneous introduction of light into the object and the reception of reflected light from the object. Since only one imaging device is used, its photosensitive element has a first imaging area and a second imaging area. The first imaging area is suitable for receiving a fifth reflected light, and the second imaging area is suitable for receiving a sixth reflected light. Therefore, the image formed by the surface light source illuminating the object under test in the first imaging area of ​​the photosensitive element contains information about the object's surface, while the image formed by the line-structured light source illuminating the object in the second imaging area of ​​the photosensitive element contains information about the object's three-dimensional information. This method simultaneously records images containing surface information and three-dimensional information of the same product under test in different regions of the same image. The two images are corresponding and correlated, allowing for a more comprehensive and accurate determination of whether two-dimensional and three-dimensional defects exist on the surface of the object under test. Furthermore, no additional correlation methods are required, and there is no miscorrelation between the surface and three-dimensional information images, thus improving detection accuracy. Simultaneously, imaging two types of information images in a single step increases detection efficiency. Moreover, using only a single imaging device also helps reduce detection costs.

[0018] In the second technical solution, since the light emitted by the surface light source is parallel light perpendicular to the first plane, it is beneficial to obtain a frontal image of the object under test and generate shadow contrast, which helps to improve image accuracy.

[0019] In the third technical solution, since a semi-transparent and semi-reflective mirror is tilted to the first plane, it is beneficial to introduce parallel light perpendicular to the first plane into the object under test, receive the positive reflection of the parallel light by the object under test and reflect it to other optical paths, avoid mutual interference between optical paths, and improve imaging quality.

[0020] In the fourth technical solution, since the line structured light is tilted towards the object under test on the first plane, it is beneficial to obtain the three-dimensional information of the object under test through the image formed by the reflection of the object under test.

[0021] In the fifth technical solution, since at least one ray of the line structured light used intersects the normal plane of the object being tested with the projection of the ray on the first plane, the reflected light formed by the ray illuminating the object being tested is not only formed on a normal plane, thereby forming an image containing three-dimensional information.

[0022] In the sixth technical solution, the use of a prism results in a simple structure, easy optical path propagation, and low cost.

[0023] In the seventh technical solution, the optimized setting of the incident angle of the line structured light and the positioning and angle settings of the semi-transparent mirror, the first reflecting mirror, and the prism result in a compact system structure. The surface light source path and the line structured light path do not interfere with each other, and the optical path is simple with low loss. Furthermore, the two light paths have similar optical paths, which is beneficial for clear imaging on the photosensitive element.

[0024] In the eighth technical solution, since the conveying device drives the object to be tested to move in a straight line parallel to the first plane, the imaging device acquires the surface image of the object to be tested at a specific frequency, which is beneficial to continuously obtain the image of the object to be tested. Furthermore, by comparing the images at each interval, the detection information can be obtained, which is beneficial to improve the detection efficiency and detection accuracy.

[0025] In the ninth technical solution, based on the imaging method adopted by the above-mentioned surface imaging system for the object under test, when the conveying device drives the object under test to move in a straight line, the imaging device obtains the surface image of the object under test at a specific frequency. It can continuously obtain surface images of multiple objects under test and / or multiple images of the same object under test, which is applicable to industrial production and helps to improve detection efficiency and detection accuracy.

[0026] In the tenth technical solution, the surface imaging method for the object under test acquires images of the object's surface based on the aforementioned surface imaging system and method. If a defect exists in the image domain of the first imaging region in any image, the surface of the object under test is determined to have a two-dimensional defect; if a defect exists in the image domain of the second imaging region in any image, the surface of the object under test is determined to have a three-dimensional defect. Thus, because the images of the first and second imaging regions have a corresponding relationship, the determination of two-dimensional and three-dimensional defects also has a corresponding relationship, which helps improve the accuracy of the determination and facilitates the classification of product defects for appropriate subsequent processing. For example, products with both two-dimensional and three-dimensional defects can be classified. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments are briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of the surface imaging system for the object under test, as shown in the embodiment.

[0029] Explanation of key figure labels:

[0030] 1. Includes a surface light source; 2. A line structure light source; 3. A first reflecting mirror; 4. A semi-transparent and semi-reflective mirror; 5. A reflecting component; 6. An imaging device; 7. The object to be measured; 8. A first plane; 9. A third reflected light; 10. A fourth reflected light; 11. A first reflected light; 12. A second reflected light; 13. A first surface; 14. A second surface; 15. A bottom surface; 16. A fifth reflected light; 17. A sixth reflected light. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are preferred embodiments of the present invention and should not be considered as excluding other embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0032] Unless otherwise expressly defined, the use of terms such as "first," "second," or "third" in the claims, description, and accompanying drawings of this invention is for distinguishing different objects and not for describing a specific order.

[0033] Unless otherwise expressly defined, in the claims, description, and accompanying drawings of this invention, the use of directional terms such as "center," "lateral," "longitudinal," "horizontal," "vertical," "top," "bottom," "inner," "outer," "upper," "lower," "front," "rear," "left," "right," "clockwise," and "counterclockwise" to indicate orientation or positional relationships is based on the orientation and positional relationships shown in the accompanying drawings and is only for the convenience of describing the invention and simplifying the description, and is not intended to indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the specific scope of protection of this invention.

[0034] Unless otherwise expressly defined, the terms "fixed connection" or "fixed connection" used in the claims, description and drawings of this invention should be interpreted broadly to refer to any connection in which there is no displacement or relative rotation relationship between the two parties, including non-removable fixed connection, detachable fixed connection, integral connection and fixed connection by other means or components.

[0035] In the claims, description and accompanying drawings of this invention, the terms "comprising," "having," and variations thereof are used to mean "including but not limited to."

[0036] See Figure 1 , Figure 1 An imaging system for the surface of an object under test is shown, which is used to image the surface of the object under test, including a surface light source 1, a line structure light source 2, a first reflector 3, a semi-transparent and semi-reflective mirror 4, a reflective component 5, an imaging device 6, and a conveying device.

[0037] A surface light source 1 is placed on the object to be measured 7, which is placed on a first plane 8. Figure 1 As shown, the first plane 8 is a plane perpendicular to the vertical direction. The surface light source 1 emits light towards the object 7 under test, and the emitted light is parallel light perpendicular to the first plane 8.

[0038] The line structure light source 2, employing a line laser emitter, is positioned to the left of the object under test 7, tilted relative to the first plane 8, and emits line structure light towards the object under test 7. In this embodiment, the line structure light is a beam of parallel stripes of light, with each strip extending perpendicular to the plane 8. Figure 1 The line structured light, positioned on the plane (i.e., in the front-to-back direction) of the plane 8, is incident on the object 7 under test at a 45° angle to the first plane 8. After reflection from the object 7, a third reflected light 9 is emitted. The projection of each strip of light onto the first plane 8 is in the front-to-back direction and is perpendicular to the normal plane of the reflected light formed by the incident object 7. In other embodiments, the line structured light can be other forms of strip light or grid light, and at least one ray of the line structured light has its projection onto the first plane 8 intersecting the normal plane formed by its incident object 7.

[0039] The first reflecting mirror 3, which is a plane mirror, is positioned above and to the right of the object under test 7. Its mirror surface faces the third reflected light 9, is perpendicular to the normal plane formed by the linear structured light incident on the object under test 7, and makes an angle of 78.75° with the first plane. The third reflected light 9 enters the first reflecting mirror 3 and exits as the fourth reflected light 10.

[0040] A semi-transparent and semi-reflective mirror 4, which can both transmit and reflect light, is placed between the surface light source 1 and the object to be measured 7. Its lower mirror surface faces the first reflecting mirror 3 and is tilted at an angle of 33.75° to the first plane 8. The semi-transparent and semi-reflective mirror 4 allows light emitted from the surface light source 1 to pass through and illuminate the object to be measured 7, and reflects the first reflected light 11 from the surface light source 7 to form a second reflected light 12.

[0041] The reflective component 5 employs a prism; in other embodiments, a double-sided mirror may also be used. The prism has a first face 13, a second face 14, and a bottom face 15. The first face 13 has a first reflective portion, and the second face 14 has a second reflective portion. The prism is positioned between the semi-transparent mirror 4 and the first reflector 3. The prism extends parallel to the first reflector 3. The first face 13 faces the semi-transparent mirror 4, and the second face 15 faces the first reflector 3. The angle between the first face 13 and the second face 14 is 90°, and the angles between the first face 13, the second face 14, and the bottom face 15 are both 45°. The angle between the bottom face and the first plane is 22.5°. The second reflected light 12 enters the first reflective portion and exits as a fifth reflected light 16. The fourth reflected light 10 enters the second reflective portion and exits as a sixth reflected light 17. The exit direction of the fifth reflected light 16 is the first direction. In this embodiment, based on the angles and positions of the surface light source 1, the line structure light source 2, the first reflector 3, the semi-transparent and semi-reflective mirror 4, and the reflective component 5, it can be calculated that the fifth reflected light 16 and the sixth reflected light 17 are parallel to each other and have an angle of 67.5° with the first plane.

[0042] Imaging device 6 has an imaging optical axis extending along a first direction. Its photosensitive element has a first imaging region and a second imaging region. The first imaging region receives a fifth reflected light 16, and the second imaging region receives a sixth reflected light 17. In this embodiment, its imaging optical axis is perpendicular to the second reflected light and makes an angle of 67.5° with the first plane. It also intersects the line of intersection of the first surface 13 and the second surface 14. Its photosensitive element is equally divided into the first imaging region and the second imaging region.

[0043] Conveying device, Figure 1 Not shown, a conveyor belt or robotic arm can be used to move the object 7 to be tested linearly in a direction parallel to the first plane 8. In this embodiment, the conveying device moves the object 7 to be tested on the first plane 8 and perpendicular to the extension direction of the strip light (i.e., Figure 1 (As shown in the left-right direction) linear motion. During this motion, the imaging device acquires images of the surface of the object under test at a specific frequency. The specific structure of the robot or conveyor belt is prior art in this field and will not be described in detail here.

[0044] After the surface imaging system of the object under test is assembled according to the preferred embodiment described above, the surface imaging system of the object under test is debugged and the imaging device 6 is calibrated. In the actual product testing process, the conveying device drives the object under test 7 along... Figure 1The light source moves from left to right. During this movement, the surface light source 1 emits parallel light perpendicularly downwards and passes through the semi-transparent mirror 4. When the surface light source 1 faces the object under test 7, the surface light emitted from the surface light source 1 towards the object under test 7 through the semi-transparent mirror 4 is reflected by the object under test 7 to form a first reflected light 11. The first reflected light 11 is reflected back to the semi-transparent mirror 4 and emitted as a second reflected light 12. The structured light source 2 emits a linear structured light at an angle of 45° to the first plane 8. The linear structured light is reflected by the object under test 7 to form a third reflected light 9. The third reflected light 9 faces the first reflecting mirror 3, and the first reflecting mirror 3 reflects the third reflected light 9 to form a fourth reflected light 10. The second reflected light 12 and the fourth reflected light 10 are incident on the reflecting assembly 5 (prism) in opposite directions. The second reflected light 12 is reflected by the first reflecting part of the reflecting assembly 5 to form a fifth reflected light 16 extending along the first direction, and the fourth reflected light 10 is reflected by the second reflecting part of the reflecting assembly 5 to form a sixth reflected light 17 extending along the first direction. Imaging device 6 exposes the image at a specific frequency so that the photosensitive element receives the fifth reflected light 16 and the sixth reflected light 17. The fifth reflected light 16 and the sixth reflected light 17 are received by the first imaging area and the second imaging area of ​​the photosensitive element, respectively. This forms an image on the first imaging area of ​​the photosensitive element, where the surface light source 1 illuminates the object under test 7, thus creating an image containing surface information of the object under test. Conversely, it forms an image on the second imaging area of ​​the photosensitive element, where the line structure light source 2 illuminates the object under test 7, thus creating an image containing three-dimensional information of the object under test. Algorithms or methods for obtaining three-dimensional information of a product by capturing an image from an object using line structure light are existing technologies and are not the focus of this application; therefore, they will not be elaborated upon here. In this way, images containing surface information and images containing three-dimensional information are simultaneously formed in different areas of the same image. These two images have a corresponding relationship and are interconnected, allowing for a more comprehensive and accurate determination of whether the surface of the object under test has two-dimensional defects and whether the object under test has three-dimensional defects. Furthermore, no special correlation methods are required, and there is no miscorrelation between the surface information image and the three-dimensional information image, which helps improve detection accuracy. At the same time, imaging two types of information images in a single process improves detection efficiency. Furthermore, using only one imaging device also helps reduce detection costs.

[0045] The above describes one embodiment of the imaging system for the surface of the object under test. For ease of explanation and understanding, this embodiment sets specific angles and positions. Obviously, those skilled in the art can also use other positions or angles, simply by adjusting the angles and positions of the light source, reflector, etc., according to the basic principle of light reflection, to achieve the technical solution of this application.

[0046] A surface imaging method for an object under test is disclosed, which utilizes the aforementioned surface imaging system. A conveying device drives one or more objects 7 under test in linear motion, and an imaging device 6 captures images of the object surface at a specific frequency. This allows for the continuous acquisition of surface images of multiple objects under test and / or multiple images of the same object, making it suitable for industrial production and improving detection efficiency and accuracy.

[0047] A method for detecting surface defects of an object under test involves judging images acquired using an imaging method of the object's surface. If a defect exists in the image domain of a first imaging region in any image, the object's surface is determined to have a two-dimensional defect; if a defect exists in the image domain of a second imaging region in any image, the object's surface is determined to have a three-dimensional defect, i.e., a height defect. Algorithms or judgment methods for obtaining three-dimensional information of a product by using line structured light incident on an object to capture images already exist and will not be detailed here. Within the same image, the images of the first and second imaging regions have a corresponding relationship, enabling a correspondence between the judgment of two-dimensional and three-dimensional defects. This improves the accuracy of the judgment and facilitates the classification of product defects for appropriate subsequent processing.

[0048] The foregoing description of the specifications and embodiments is intended to explain the scope of protection of this invention, but does not constitute a limitation on the scope of protection of this invention. Modifications, equivalent substitutions, or other improvements to the embodiments of this invention or a portion thereof that can be obtained by those skilled in the art through logical analysis, reasoning, or limited experimentation, based on the teachings of this invention or the foregoing embodiments, in conjunction with common knowledge, general technical knowledge, and / or existing technology, should all be included within the scope of protection of this invention.

Claims

1. A surface imaging system for an object under test, used to image the surface of the object under test on a first plane, characterized in that, include: A surface light source emits light toward the object to be measured, and the light emitted by the surface light source is parallel light perpendicular to the first plane; A semi-transparent and semi-reflective mirror is placed between a surface light source and the object to be tested. The semi-transparent and semi-reflective mirror is tilted on the first plane to allow light emitted from the surface light source to pass through and illuminate the object to be tested, and to reflect the first reflected light from the surface light source to form a second reflected light. A line structure light source emits line structure light toward the object under test, and the line structure light is inclined to the first plane and directed toward the object under test. The first reflecting mirror reflects the third reflected light of the line structured light from the object under test to form the fourth reflected light; A reflective component is provided with a first reflective part and a second reflective part, wherein the first reflective part reflects the second reflected light along a first direction to form a fifth reflected light; and the second reflective part reflects the fourth reflected light along the first direction to form a sixth reflected light. An imaging device having an imaging optical axis extending along the first direction, and a photosensitive element having a first imaging region and a second imaging region, the first imaging region being adapted to receive the fifth reflected light, and the second imaging region being adapted to receive the sixth reflected light.

2. The surface imaging system for an object under test as described in claim 1, characterized in that, In the line structured light, at least one ray's projection onto the first plane intersects with the normal plane of the object being measured.

3. The surface imaging system for an object under test as described in claim 2, characterized in that, The reflective component is a prism, and the first reflective part and the second reflective part are formed on the two surfaces of the prism.

4. The surface imaging system for an object under test as described in claim 3, characterized in that, The line structured light is a parallel strip of light incident at a 45° angle to the first plane. The projection of the line structured light onto the first plane is perpendicular to the normal plane of the object under test. The first reflector is perpendicular to the normal plane of the object under test and forms an angle of 78.75° with the first plane. The lower surface of the semi-transparent mirror faces the first reflector and forms an angle of 33.75° with the first plane. The prism is positioned between the semi-transparent mirror and the first reflector. The plane containing the first reflector is opposite to the semi-transparent mirror, and the plane containing the second reflector is opposite to the first reflector. The angle between the two surfaces of the prism formed by the first and second reflectors is 90°. The angle between the plane containing the first and second reflectors and the bottom surface is 45°. The angle between the bottom surface and the first plane is 22.5°. The first direction is perpendicular to the bottom surface.

5. A surface imaging system for an object under test as described in any one of claims 1 to 4, characterized in that, It also includes a conveying device that drives the object under test to move linearly in a direction parallel to the first plane, and the imaging device acquires an image of the surface of the object under test at a specific frequency.

6. A method for imaging the surface of an object under test, which obtains an image of the surface of the object under test based on the surface imaging system of the object under test as described in claim 5, characterized in that, This includes using an imaging device to obtain an image of the object's surface at a specific frequency while the conveying device is moving the object in a straight line.

7. A method for detecting surface defects of an object under test, wherein the detection result is obtained by judging the surface image of the object under test acquired by the surface imaging method of the object under test as described in claim 6, characterized in that, If a defect exists in the image domain of the first imaging region in any image, it is determined that there is a two-dimensional defect on the surface of the object under test; if a defect exists in the image domain of the second imaging region in any image, it is determined that there is a three-dimensional defect on the surface of the object under test.

Images