JP6040930B2 - Surface defect detection method and surface defect detection apparatus - Google Patents

Description

  The present invention relates to a surface defect detection method and a surface defect detection device for optically detecting a surface defect of a steel material.

  In recent years, in the manufacturing process of steel products, it has been required to detect surface defects of steel materials hot or cold from the viewpoint of improving yield by preventing mass nonconformity. The steel materials described here are steel products such as seamless steel pipes, welded steel pipes, hot-rolled steel sheets, cold-rolled steel sheets, thick steel plates, and other steel products, and are produced in the process of manufacturing these steel products. It means semi-finished products such as slabs. For this reason, as a method for detecting surface defects in steel materials, a method has been proposed in which a billet in the manufacturing process of a seamless steel pipe is irradiated with light to receive reflected light, and the presence or absence of surface defects is determined by the amount of reflected light. (See Patent Document 1). In addition, visible light in multiple wavelength ranges that do not affect each other and do not affect each other with the self-emission emitted from the hot steel material are obliquely symmetric with respect to the normal line of the hot steel surface. A method is also proposed in which an image by synthetic reflected light and an image by individual reflected light are obtained in the normal direction of the surface of the hot steel material, and surface defects of the hot steel material are detected from a combination of these images. (See Patent Document 2).

JP 11-37949 A JP 59-52735 A

  According to the method described in Patent Document 1, since the harmless pattern and the reflectance of the scale are different from the reflectance of the ground iron part, there is a possibility that a healthy harmless pattern or scale is erroneously detected as a surface defect. For this reason, in the method of patent document 1, a billet and a scale are discriminated using the fact that the shape of a billet is linear. However, the surface defects of the steel material have various shapes such as a circular shape as well as a straight shape. For this reason, it is difficult to apply the method of patent document 1 to the detection process of the surface defect of steel materials. On the other hand, in the method described in Patent Document 2, since there are a large number of types of defects, scales, harmless patterns, and the like, it is difficult to discriminate scales and harmless patterns from surface defects simply by combining images. Moreover, it is practically difficult to construct a detection logic corresponding to a large number of image combinations.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a surface defect detection method and a surface defect detection apparatus capable of accurately distinguishing scales and harmless patterns from surface defects.

  The surface defect detection method according to the present invention is a surface defect detection method for optically detecting a surface defect of a steel material, and illuminates the same inspection target site from different directions using two or more distinguishable light sources. An irradiation step of irradiating light, and a detection step of acquiring a surface defect in the inspection target site by acquiring an image of reflected light of each illumination light and performing a difference process between the acquired images And The difference process described here refers to a process of comparing not only a simple difference between images but also comparing images and extracting a difference or a degree of difference.

  The surface defect detection method according to the present invention is the surface defect detection method according to the invention described above, wherein the detection step removes the scale and harmless pattern image signals in the inspection target portion by performing the difference processing. And detecting a surface defect.

  In the surface defect detection method according to the present invention as set forth in the invention described above, the irradiating step includes a step of irradiating illumination light by causing two or more flash light sources to emit light repeatedly so that their light emission timings do not overlap each other. And

  In the surface defect detection method according to the present invention, in the above invention, the irradiation step includes a step of simultaneously irradiating two or more light sources of light sources whose wavelength regions do not overlap each other, and the detection step includes each of the mixed steps. The method includes a step of acquiring an image of the reflected light of each illumination light by separating the reflected light of the illumination light using a filter that transmits light having the same wavelength as the wavelength of the illumination light.

  In the surface defect detection method according to the present invention, in the above invention, the irradiation step includes a step of simultaneously irradiating illumination lights of two light sources having linear polarization characteristics orthogonal to each other, and the detection step includes each of the mixed steps. The method includes a step of acquiring an image of the reflected light of each illumination light by separating the reflected light of the illumination light using two polarizing plates having linear polarization characteristics orthogonal to each other.

  The surface defect detection method according to the present invention is characterized in that, in the above invention, an incident angle of illumination light of each light source with respect to the inspection target site is in a range of 25 ° to 55 °.

  The surface defect detection method according to the present invention is the surface defect detection method according to the above invention, wherein the detection step uses a half mirror, a beam splitter, or a prism to obtain an image by reflected light of each illumination light. Adjusting the optical axis of the device to be coaxial.

  The surface defect detection apparatus according to the present invention is a surface defect detection apparatus that optically detects a surface defect of a steel material, and illuminates the same inspection target site from different directions using two or more distinguishable light sources. Irradiating means for irradiating light, and detecting means for acquiring an image of reflected light of each illumination light and detecting a surface defect in the inspection target part by performing a difference process between the acquired images And

  According to the surface defect detection method and the surface defect detection apparatus according to the present invention, it is possible to accurately discriminate between scales, harmless patterns, and surface defects.

FIG. 1 is a schematic diagram showing a configuration of a surface defect detection apparatus according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing a configuration of a modification of the area sensor shown in FIG. FIG. 3 is a timing chart showing drive timings of the light source and the area sensor shown in FIG. FIG. 4 is a diagram illustrating an example of two two-dimensional images obtained by photographing a surface defect, a scale, and a harmless pattern, and a difference image thereof. FIG. 5 is a schematic diagram showing a configuration of an apparatus used in an experiment for investigating the relationship between the incident angle of illumination light and the reflectance of a healthy part (ground iron part). FIG. 6 is a diagram showing the relationship between the incident angle of the laser and the amount of light received by the power meter. FIG. 7 is a schematic diagram for explaining the surface defect detection process according to the second embodiment of the present invention. FIG. 8 is a schematic diagram for explaining the surface defect detection process according to the third embodiment of the present invention. FIG. 9 is a schematic diagram illustrating the configuration of the apparatus used in the example. FIG. 10 is a diagram illustrating the surface defect detection processing result of the example. FIG. 11 is a diagram illustrating a result of the surface defect detection process for the portion where the scale is generated. FIG. 12 is a schematic diagram showing a configuration of a modification of the surface defect detection device according to the embodiment of the present invention. FIG. 13 is a schematic diagram showing the configuration of another modification of the surface defect detection device according to the embodiment of the present invention.

  Hereinafter, the configuration and operation of a surface defect detection apparatus according to an embodiment of the present invention will be described with reference to the drawings.

[Configuration of surface defect detection device]
First, with reference to FIG. 1, the structure of the surface defect detection apparatus which is one Embodiment of this invention is demonstrated.

  FIG. 1 is a schematic diagram showing a configuration of a surface defect detection apparatus according to an embodiment of the present invention. As shown in FIG. 1, a surface defect detection apparatus 1 according to an embodiment of the present invention is an apparatus that detects a surface defect of a cylindrical steel pipe P that is conveyed in the direction of an arrow in the figure, and includes light sources 2a and 2b, functions A generator 3, area sensors 4a and 4b, an image processing device 5, and a monitor 6 are provided as main components.

  The light sources 2a and 2b irradiate the illumination light L that can be discriminated to the same inspection target part on the surface of the steel pipe P in accordance with a trigger signal from the function generator 3. It is desirable that the light sources 2a and 2b be arranged symmetrically with respect to the inspection target part. Therefore, the light sources 2a and 2b are shifted by the same angle with respect to the normal vector of the surface of the steel pipe P, and are arranged so that the irradiation direction vector of the illumination light L and the normal vector of the surface of the steel pipe P are in the same plane. ing. The identity of the incident angle described here aims to make the optical conditions as equal as possible when discriminating light sources in different directions, and to greatly reduce the signal of a healthy part including a scale and a harmless pattern by differential processing. Further, the signal of the healthy part greatly depends on the surface property of the object, and it is difficult to guarantee the identity at a constant angle. Therefore, within the range of 25 to 55 °, even if the angles are somewhat different, the same angle is expressed as long as the signal of the healthy portion can be reduced by the difference processing. In the present embodiment, the number of light sources is two, but the number of light sources may be three or more as long as discrimination is possible. The discriminable light source described here refers to a light source capable of obtaining the amount of reflected light for each light source with respect to the reflected light obtained from the object.

  The area sensors 4 a and 4 b capture a two-dimensional image of the reflected light of the illumination light L emitted from the light sources 2 a and 2 b according to the trigger signal from the function generator 3. The area sensors 4 a and 4 b input captured two-dimensional image data to the image processing device 5. It is desirable that the area sensors 4a and 4b be installed on the normal vector of the region to be inspected as much as possible while ensuring the respective imaging fields of view.

  In order to solve the alignment problem, it is desirable that the area sensors 4a and 4b be as close as possible and the respective optical axes be as parallel as possible. Further, as shown in FIG. 2, adjustment may be made so that the optical axes of the area sensors 4a and 4b are coaxial using any of a half mirror, a beam splitter, and a prism. Thereby, the difference image mentioned later can be acquired accurately.

  The image processing device 5 is a device that detects a surface defect in a region to be inspected by performing difference processing described later between two two-dimensional images input from the area sensors 4a and 4b. The image processing device 5 outputs to the monitor 6 information related to the two-dimensional images and surface defect detection results input from the area sensors 4a and 4b.

  The surface defect detection apparatus 1 having such a configuration discriminates the scale and harmless pattern from the surface defect in the inspection target part by executing the following surface defect detection process. The surface defects described here are irregular defects. In addition, the scale and harmless pattern mean a surface film or a surface property having a different optical characteristic from a base metal portion having a thickness of several to several tens of μm, which becomes a noise factor in the surface defect detection processing. Part. Hereinafter, surface defect detection processing according to the first to third embodiments of the present invention will be described.

[First Embodiment]
First, the surface defect detection process according to the first embodiment of the present invention will be described with reference to FIGS.

  FIG. 3 is a timing chart showing drive timings of the light sources 2a and 2b and the area sensors 4a and 4b. In the figure, d represents the light emission time of the light sources 2a and 2b, and T represents the imaging period of the two-dimensional image by the area sensors 4a and 4b. In the surface defect detection processing according to the first embodiment of the present invention, the light sources 2a and 2b are discriminated by using the light sources 2a and 2b as flash light sources and repeatedly emitting light so that the flash light sources do not overlap with each other.

  That is, as shown in FIG. 3, in the present embodiment, first, the function generator 3 transmits a trigger signal to the light source 2a and the area sensor 4a, the light source 2a emits the illumination light L, and the area sensor within time d. 4a completes the photographing of the two-dimensional image. Then, after the two-dimensional image is captured by the area sensor 4a, the function generator 3 transmits a trigger signal to the light source 2b and the area sensor 4b, and similarly captures the two-dimensional image. According to the present embodiment, it is possible to take a two-dimensional image of individual reflected light with respect to the illumination light L emitted from each light source without causing a decrease in the amount of light at the time difference d.

  In addition, when the conveyance speed of the steel pipe P is fast, it is desirable that the flash light source has a short light emission time d. This is because the shorter the light emission time d, the smaller the shutter delay between the two two-dimensional images obtained by the area sensors 4a and 4b, and the smaller the positional deviation of the two-dimensional image due to the shutter delay. Further, when it is intended to detect a surface defect using a difference image of a two-dimensional image by individual reflected light, the light emission time d of the flash light source needs to satisfy the condition shown in the following formula (1). .

  Assuming that the size of the surface defect of the detection target is 20 mm, for example, it is necessary to have a resolution of 4 mm / pixel because a minimum five-pixel signal is required to detect the surface defect. Further, in this case, since the positional deviation due to the irradiation timing of the allowable illumination light L needs to be within 0.2 pixels from experience, the conveyance speed of the steel pipe P is 1, 3, 5 m / s. The light emission times of the light sources 2a and 2b must be 800, 270 and 160 μsec or less, respectively. In addition, when the conveyance speed and conveyance direction of the steel pipe P are constant, this positional deviation can be corrected after photographing a two-dimensional image.

  In the present embodiment, the image processing apparatus 5 performs image processing such as calibration, shading correction, and noise removal using camera parameters derived in advance for the two-dimensional images input from the area sensors 4a and 4b. After the application, a surface defect in the inspection target part is detected by performing a difference process between the two-dimensional images.

  Specifically, the luminance value of each pixel constituting the two-dimensional image Ia when the illumination light L is irradiated from the light source 2a is Ia (x, y) (where the number of pixels is X × Y and the x coordinate is 1 ≦ x ≦ X, y coordinate is 1 ≦ y ≦ Y), and when the luminance value of each pixel constituting the two-dimensional image Ib when the illumination light L is irradiated from the light source 2b is Ib (x, y), The luminance value I_diff (x, y) of each pixel of the difference image I_diff is expressed by the following formula (2).

  Here, FIGS. 4A, 4B, and 4C show examples of the two-dimensional images Ia and Ib and the difference image I_diff obtained by imaging the surface defect, the scale that is not a defect, and a harmless pattern, respectively. As shown in FIGS. 4A, 4B, and 4C, in the healthy portion, the normal vector and the angle formed by the light source 2a and the normal vector and the angle formed by the light source 2b are independent of the scale and the harmless pattern. Therefore, the luminance value Ia (x, y) = the luminance value Ib (x, y), that is, the luminance value I_diff (x, y) = 0. However, since the surface has an uneven shape in the surface defect portion, there is always a portion where the normal vector and the angle formed by the light source 2a are not equal to the angle formed by the normal vector and the light source 2b, and the luminance value Ia (x, y) ≠ luminance value Ib (x, y), that is, luminance value I_diff (x, y) ≠ 0.

  Therefore, by generating a difference image of two two-dimensional images by the differentiator 11, scales and harmless patterns that are not defects can be removed, and only surface defects can be detected. Then, only the surface defect is detected in this way, and final evaluation is performed on whether or not the surface defect is harmful by various feature amounts, and the evaluation result is displayed on the monitor 6.

  When there is a positional shift between two two-dimensional images and affects the difference image, it is desirable to apply a two-dimensional low-pass filter to reduce the influence of the positional shift between the two-dimensional images. In this case, assuming that the two-dimensional low-pass filter is H, the luminance value I′_diff (x, y) of the difference image is expressed by the following formula (3).

  The light sources 2a and 2b should be the same, and each light source should be irradiated so as to be as uniform parallel light as possible. However, surface defects can be detected by general shading correction even when the surface is somewhat uneven or when applied to a gently curved surface such as a steel pipe P.

  Further, it is desirable that the incident angle of the illumination light L be within a range in which a specular reflection component does not enter the reflected light of the healthy part and a sufficient amount of light can be secured. The inventors of the present invention conducted an experiment for investigating the relationship between the incident angle of the illumination light L and the reflectance of the healthy part (ground iron part). The configuration of the apparatus used for the experiment is shown in FIG. As shown in FIG. 5, in this experiment, the power meter 12 was fixed at a position directly above the slab sample 14, and the power meter 12 when the incident angle θ of the laser 13 was changed from 0 ° to 90 °. The amount of light received was measured. The experimental results are shown in FIG. As shown in FIG. 6, when the incident angle θ is in the range of 0 ° to 20 °, the amount of light received by the power meter 12 is large because the specular reflection component is included, but when the incident angle θ is 60 ° or more. The amount of light received by the power meter 12 is greatly reduced. Therefore, it is desirable that the incident angle of the illumination light L be in the range of 25 ° to 55 ° with respect to the normal vector of the inspection target part.

  The resolution in the depth direction of the inspection target part depends on the inclination angle of the defect and the resolution of the area sensors 4a and 4b. Here, the inclination angle of the defect means that the “normal vector of the defect part” is orthogonally projected onto the “plane formed by the normal vector of the surface of the healthy part of the inspection target site and the light source direction vector”, and the orthogonally projected vector And the normal vector of the surface of the healthy part. Depending on the surface properties of the part to be inspected, for example, when incident light is irradiated at an incident angle of 45 °, the defect signal is detected by differential processing if the inclination angle of the defect is about 10 ° or more relative to the light source direction. It has been confirmed that it can be done. Therefore, assuming that the resolution of one pixel is 0.5 mm, the resolution in the depth direction is theoretically about 0.5 × tan 10 ° = 0.09 mm.

[Second Embodiment]
Next, with reference to FIG. 7, the surface defect detection process which is the 2nd Embodiment of this invention is demonstrated.

  In the surface defect detection processing according to the second embodiment of the present invention, the light sources 2a and 2b are discriminated by using the light sources 2a and 2b as light sources whose wavelength regions do not overlap each other. Specifically, as shown in FIG. 7, two types of wavelength selection filters 20a and 20b whose wavelength regions do not overlap with the light sources 2a and 2b are installed, and the wavelength region of the illumination light L is selected. In addition, wavelength selection filters 21a and 21b having the same wavelength selection characteristics are installed in the area sensors 4a and 4b.

  According to such a configuration, the reflected light of the illumination light L from the light source 2a is received only by the area sensor 4a by the wavelength selection filters 20a and 21a, and the reflected light of the illumination light L from the light source 2b is aread by 20b and 21b. Light is received only by the sensor 4b. Therefore, by matching the photographing timings of the area sensors 4a and 4b, it is possible to photograph a two-dimensional image by the reflected light of the illumination light L from the light sources 2a and 2b without misalignment. The processing after taking a two-dimensional image is the same as in the first embodiment.

  When the movement speed of the examination target part is high, the light sources 2a and 2b are used as flash light sources in order to prevent displacement due to the movement of the examination target part, and the two-dimensional operation is performed without changing the irradiation timing of the light sources 2a and 2b. You may shorten the imaging time of an image. In addition, when the wavelength selection filter 20a is a blue transmission filter and the wavelength selection filter 20b is a green transmission filter, a two-dimensional image is taken using a single color camera, so that the illumination light L from the light source 2a is captured in the blue channel. Only the reflected light may be received, and only the reflected light of the illumination light L from the light source 2b may be received by the green channel.

[Third Embodiment]
Next, with reference to FIG. 8, the surface defect detection process which is the 3rd Embodiment of this invention is demonstrated.

  In the surface defect detection processing according to the third embodiment of the present invention, the light sources 2a and 2b are discriminated by using the light sources 2a and 2b as light sources having linear polarization characteristics orthogonal to each other. Specifically, as shown in FIG. 8, linearly polarizing plates 30a and 30b are installed in light sources 2a and 2b at α ° and (α + 90) ° (α is an arbitrary angle), and light components of polarization components orthogonal to each other. Permeate only. Here, the linear polarizing plate means a filter that transmits only a linearly polarized light component in a certain direction with respect to incident light. Further, linear polarizing plates 31a and 31b having the same linear polarization characteristics as the linear polarizing plates 30a and 30b are installed in the area sensors 4a and 4b at α ° and (α + 90) °.

  According to such a configuration, the reflected light of the illumination light L from the light source 2a is received only by the area sensor 4a, and the reflected light of the illumination light L from the light source 2b is received only by the area sensor 4b. Therefore, by matching the photographing timings of the area sensors 4a and 4b, it is possible to photograph a two-dimensional image by the reflected light of the illumination light from each light source without misalignment.

  If the moving speed of the measurement target region is high, the light sources 2a and 2b may be flash light sources, and the imaging time of the two-dimensional image may be shortened without changing the irradiation timing of the light sources 2a and 2b. Hereinafter, the processing after alignment and two-dimensional image capturing is the same as in the first and second embodiments.

〔Example〕
In this embodiment, as shown in FIG. 9, a flash light source is used as the light sources 2a and 2b, and a surface defect of the steel pipe P is detected by using a method of changing the light emission timing of the light sources 2a and 2b. The area sensors 4a and 4b were juxtaposed to take a two-dimensional image and aligned by image processing. FIG. 10 shows the detection result of the surface defect. 10A is a two-dimensional image when the illumination light is irradiated from the light source 2a, FIG. 10B is a two-dimensional image when the illumination light L is irradiated from the light source 2b, and FIG. It is a difference image between the two-dimensional image shown in a) and the two-dimensional image shown in FIG. The S / N ratios of the images shown in FIGS. 10A to 10C are 3.5, 3.5, and 6.0 in this order, and the SN ratio of the difference image is more than that when the illumination light is simply irradiated from one direction. Improved.

  FIG. 11 is a diagram illustrating a surface defect detection processing result for a steel pipe portion where a scale has occurred. FIG. 11A is a two-dimensional image when the illumination light is irradiated from the light source 2a, FIG. 11B is a two-dimensional image when the illumination light L is irradiated from the light source 2b, and FIG. It is a difference image between the two-dimensional image shown in a) and the two-dimensional image shown in FIG. A black spot spreading over the entire two-dimensional image shown in FIGS. 11A and 11B is a scale that causes noise. Since the scale shape was flat, the scale image was removed by acquiring the difference image. Further, in the difference image, the signal of the scale that becomes noise is reduced to about ¼ as compared with the case where the illumination light is simply irradiated from one direction.

[Modification 1]
FIG. 12 is a schematic diagram showing a configuration of a modification of the surface defect detection device according to the embodiment of the present invention. As shown in FIG. 12, in this modification, the illumination light irradiated from one light source 2a is divided by a plurality of mirrors 40a, 40b, 40c, and 40d, and finally the inspection target site of the steel pipe P1 is illuminated from two directions. Irradiate light. In this case, the same effects as those of the second and third embodiments can be obtained by installing the wavelength selection filters 20a and 20b and the linear polarizing plates 30a and 30b in the optical paths of the illumination light. In addition, although this modification irradiates illumination light from two directions, it is the same when irradiating illumination light from three or more directions.

[Modification 2]
FIG. 13 is a schematic diagram showing the configuration of another modification of the surface defect detection device according to the embodiment of the present invention. As shown in FIG. 13, this modification is a surface defect detection device according to the second embodiment of the present invention, and does not limit the wavelength of the light source by the wavelength selection filters 20a and 20b, but a pulse laser 51a. , 51b and diffuser plates 50a, 50b are used to limit the wavelength of the light source. In this modification, the light sources are discriminated by irradiating laser light from two pulse lasers 51a and 51b having different wavelength regions from the left and right directions of the inspection target part. At this time, in order to irradiate the entire region to be inspected with the laser light emitted from the pulse lasers 51a and 51b, the diffusion plates 50a and 50b are inserted into the optical path of the laser light. In addition, although this modification irradiates illumination light from two directions, it is the same when irradiating illumination light from three or more directions.

[Modification 3]
This modification is a surface defect detection apparatus according to the second embodiment of the present invention shown in FIG. 7, and uses a dichroic mirror instead of the wavelength selection filters 21a and 21b installed in the area sensors 4a and 4b. It is. The dichroic mirror is a mirror that reflects light of a specific wavelength component and transmits light of other wavelength components. Using a dichroic mirror eliminates the need for a wavelength selection filter. In addition, although this modification irradiates illumination light from two directions, it is the same when irradiating illumination light from three or more directions.

  The embodiment to which the invention made by the present inventors is applied has been described above, but the present invention is not limited by the description and the drawings that form part of the disclosure of the present invention according to this embodiment. That is, other embodiments, examples, operational techniques, and the like made by those skilled in the art based on this embodiment are all included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Surface defect detection apparatus 2a, 2b Light source 3 Function generator 4a, 4b Area sensor 5 Image processing apparatus 6 Monitor L Illumination light P Steel pipe

Claims (8)

A surface defect detection method for optically detecting surface defects of a moving steel material,
By causing two or more distinguishable light sources, whose light emission times are set based on at least an allowable amount of image misalignment, to repeatedly emit light so that the light emission timings do not overlap each other, the same site to be inspected from different directions An irradiation step of irradiating illumination light;
A detection step of acquiring an image by reflected light of each illumination light and detecting a surface defect in the inspection target part by performing a difference process between the acquired images;
A method for detecting surface defects, comprising: 2. The surface defect detection method according to claim 1, wherein the light emission time is set based on a minimum image resolution and a moving speed of the steel material in addition to an allowable amount of image displacement. 2. The surface according to claim 1, wherein a relationship among the light emission time, the allowable amount of positional deviation of the image, the minimum resolution of the image, and the moving speed of the steel material satisfies the following formula (1). Defect detection method.
The image by the reflected light of each said illumination light is acquired using the some imaging device adjusted so that an optical axis might be coaxial, Any one of Claims 1-3 characterized by the above-mentioned. The surface defect detection method according to item. The detection step, a half mirror, beam splitter, and using any of the prisms, in claim 4, wherein the optical axes of the plurality of imaging devices, characterized in that it comprises adjusting so as to be coaxial The surface defect detection method as described. The detecting step includes a step of detecting uneven surface defects in the inspection target portion by removing the scale and harmless pattern image signals in the inspection target portion by performing the difference processing. The surface defect detection method according to any one of claims 1 to 5 . The surface defect detection method according to any one of claims 1 to 6 , wherein an incident angle of illumination light of each light source with respect to the inspection target site is in a range of 25 ° or more and 55 ° or less. A surface defect detection device for optically detecting a surface defect of a moving steel material,
By causing two or more distinguishable light sources, whose light emission times are set based on at least an allowable amount of image misalignment, to repeatedly emit light so that the light emission timings do not overlap each other, the same site to be inspected from different directions Irradiating means for irradiating illumination light;
Detection means for detecting a surface defect in the inspection target part by acquiring an image by reflected light of each illumination light and performing difference processing between the acquired images;
A surface defect detection apparatus comprising: