JP7358937B2 - Foreign object detection device and foreign object detection method - Google Patents

Description

The present disclosure relates to a foreign object detection device and a foreign object detection method.

Foreign matter detection devices that detect foreign matter on transparent plates are widely used. As a technique for detecting foreign matter, a foreign matter detection device including a first inspection device, an attached foreign matter removal device, and a second inspection device has been developed (for example, Patent Document 1).

In the technique of Patent Document 1, a transparent sheet is moved in the horizontal direction, and a first inspection device, an attached foreign matter removal device, and a second inspection device are provided in order from the upstream side in the moving direction. The first inspection device and the second inspection device are cameras. The first inspection device detects foreign matter on the transparent sheet. The attached foreign matter removing device blows air onto the transparent sheet to remove foreign matter attached to the surface of the transparent sheet. If the second inspection device detects a foreign object that has the same size and shape as the foreign object detected by the first inspection device, it is determined that the foreign object is within the transparent sheet. On the other hand, if the second inspection device does not detect a foreign object with the same size and shape as the foreign object detected by the first inspection device, it is determined that the foreign object is on the surface of the transparent sheet.

Japanese Patent Application Publication No. 2007-238261

The technique disclosed in Patent Document 1 requires two cameras and an attached foreign matter removal device, and there is a problem in that the cost required to measure the depth of the foreign matter increases.

Therefore, in view of such problems, an object of the present disclosure is to provide a foreign object detection device and a foreign object detection method that can measure the depth of a foreign object at low cost.

In order to solve the above problems, a foreign object detection device according to an embodiment of the present disclosure includes a mirror provided in contact with one surface of a transmission plate, a light emitting section provided on the other surface side of the transmission plate, and a light emitting section provided on the other surface of the transmission plate. The depth of the foreign object contained in the transparent plate is detected based on the imaging unit provided on the other side of the plate and the distance between the foreign object and the mirror image of the foreign object projected on the mirror in the image captured by the imaging unit. and a detection unit.

In order to solve the above problems, a foreign object detection device according to another aspect of the present disclosure includes a transmission plate and a mirror provided in contact with one surface of the transmission plate. A light emitting unit provided on the side of the object to be inspected, an imaging unit provided on the other side of the object to be inspected, and inspection based on the distance between the mirror image of the foreign object projected on the mirror and the foreign object in the image captured by the imaging unit. A detection unit that detects the depth of foreign matter contained in the target object.

In order to solve the above problems, a foreign object detection method according to one aspect of the present disclosure includes a step of installing a mirror in contact with one surface of a transmission plate, and a step of irradiating light from the other surface side of the transmission plate. The depth of the foreign object contained in the transparent plate is determined based on the distance between the foreign object and the mirror image of the foreign object projected on the mirror in the image taken in the imaging step. and a step of detecting.

According to the present disclosure, it is possible to measure the depth of a foreign object at low cost.

FIG. 1 is a diagram illustrating a foreign object detection device according to an embodiment. 3 is a flowchart showing the flow of a foreign object detection method according to an embodiment. It is a figure explaining the position (1st position) of the imaging part in the 1st imaging process. It is a figure explaining the position (2nd position) of the imaging part in the 2nd imaging process. FIG. 5A is a diagram illustrating the position P1 of the inspection object. FIG. 5B is a diagram illustrating an image when there is a foreign object at position P1. FIG. 5C is a diagram illustrating the position P2 within the object to be inspected. FIG. 5D is a diagram illustrating an image when there is a foreign object at position P2. FIG. 5E is a diagram illustrating the position P3 within the object to be inspected. FIG. 5F is a diagram illustrating an image when there is a foreign object at position P3. It is a figure explaining the foreign object detection apparatus of a modification.

Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. The dimensions, materials, and other specific numerical values shown in the embodiments are merely examples for easy understanding, and do not limit the present disclosure unless otherwise specified. Note that, in this specification and the drawings, elements having substantially the same functions and configurations are designated by the same reference numerals and redundant explanation will be omitted. Further, illustrations of elements not directly related to the present disclosure are omitted.

[Foreign object detection device 100]
FIG. 1 is a diagram illustrating a foreign object detection device 100 of this embodiment. In FIG. 1, dashed arrows indicate signal flows. In FIG. 1, a dashed line indicates light. Further, in the following figures including FIG. 1 of this embodiment, the X-axis, Y-axis, and Z-axis that intersect perpendicularly are defined as shown.

As shown in FIG. 1, the foreign object detection device 100 detects foreign objects attached to the object to be inspected 10 and foreign objects contained within the object to be inspected (hereinafter, both will be collectively referred to as "foreign objects included in the object to be inspected 10"). ”) is detected. In this embodiment, the inspection object 10 is moved in the X-axis direction and the Y-axis direction in FIG. 1 by a moving mechanism (not shown).

The inspection object 10 includes a transmission plate 12 and a protective film 14. The transmission plate 12 is a flat plate that can transmit light. The transmitting plate 12 and the protective film 14 are made of a material (for example, a transparent material) having a light transmittance of 10% or more in the wavelength range from infrared to ultraviolet (for example, 10 nm or more and 1000 μm or less) including visible light (360 nm or more and 830 nm or less). , or a translucent material). Further, the transmitting plate 12 and the protective film 14 are made of a material having a higher light transmittance than the foreign material C.

In FIG. 1, the length of the transmission plate 12 in the X-axis direction and the length in the Y-axis direction are greater than or equal to the thickness (length) in the Z-axis direction.

The protective film 14 is provided over the entire surface of one of the surfaces of the transmission plate 12 . The protective film 14 is in close contact with the transparent plate 12.

As shown in FIG. 1, the foreign object detection device 100 includes a mirror 110, a first light emitting section 120, an imaging section 130, a second light emitting section 140, and a central control section 150.

The mirror 110 is provided on one surface (back surface) of the transmission plate 12. In this embodiment, the mirror 110 is provided on the surface of the transmission plate 12 opposite to the surface on which the protective film 14 is provided. Further, the mirror 110 contacts one surface of the transmission plate 12.

In this embodiment, the mirror 110 is a beam splitter that reflects part of the incident light and transmits part of it. The beam splitter is, for example, a half mirror in which the intensity of reflected light and the intensity of transmitted light are substantially equal.

The first light emitting section 120 is provided on one surface side of the transmission plate 12. The first light emitting unit 120 is provided further away than the mirror 110 with respect to the inspection target 10. The first light emitting unit 120 irradiates the mirror 110 (one surface of the inspection object 10) with light in a wavelength range from infrared to ultraviolet light including visible light. The first light emitting unit 120 emits visible light, for example. The first light emitting section 120 is turned on and off by a central control section 150, which will be described later.

The imaging unit 130 is provided on the other surface (front surface) side of the transmission plate 12. That is, the imaging unit 130 is provided on the protective film 14 side of the inspection object 10. The imaging unit 130 is moved to a first position (indicated by a solid line in FIG. 1) and a second position (indicated by a broken line in FIG. 1) by a displacement mechanism (not shown). Note that in this embodiment, the first position and the second position are substantially on the same YZ plane.

The first position is a position where the optical axis K of the imaging unit 130 is perpendicular to the XY plane in FIG. That is, the first position is a position where the optical axis K of the imaging unit 130 is perpendicular to the surface of the inspection object 10 (the surface of the protective film 14). Further, the imaging section 130 faces the first light emitting section 120 at the first position. In this embodiment, the optical axis K of the imaging section 130 at the first position and the central axis M of the light emitted by the first light emitting section 120 are coaxially arranged.

In the second position, the angle formed by the optical axis K of the imaging unit 130 in the first position and the optical axis K of the imaging unit 130 in the second position is light emitted by a second light emitting unit 140, which will be described later, This is the position where the angle of reflection (that is, the angle of incidence θ) of the light reflected by the inspection object 10 (foreign object C) and the mirror 110 is reached. That is, the imaging unit 130 at the second position has an angle at which the light emitted by the second light emitting unit 140 and reflected by the foreign object C and the light reflected by the mirror image D of the foreign object projected onto the mirror 110 are incident. It is set up so that In the present embodiment, the imaging unit 130 is configured so that the optical axis K of the imaging unit 130 at the first position and the optical axis K of the imaging unit 130 at the second position are located on the surface of the mirror 110 (transmissive plate 12 are moved so that they intersect at the contact surface).

The second light emitting section 140 (light emitting section) is provided on the other surface side of the transmission plate 12. That is, the second light emitting section 140 is provided on the same side as the imaging section 130 with respect to the inspection object 10. Further, the second light emitting section 140 is provided on the opposite side to the imaging section 130 at the second position with respect to the imaging section 130 at the first position. In this embodiment, the second light emitting section 140 is provided on substantially the same YZ plane as the imaging section 130.

The second light emitting unit 140 irradiates the other surface (protective film 14) of the inspection object 10 with light in a wavelength range from infrared to ultraviolet, including visible light. The second light emitting unit 140 emits visible light, for example.

The second light emitting unit 140 irradiates the inspection object 10 with light at an incident angle θ (0 degrees < θ < 90 degrees). The second light emitting section 140 is turned on and off by the central control section 150.

The central control unit 150 is composed of a semiconductor integrated circuit including a CPU (central processing unit). The central control unit 150 reads programs, parameters, etc. for operating the CPU itself from the ROM. The central control unit 150 manages and controls the entire foreign object detection apparatus 100 in cooperation with the RAM as a work area and other electronic circuits.

In this embodiment, the central control section 150 drives the moving mechanism, drives the displacement mechanism, and turns on and off the first light emitting section 120 and the second light emitting section 140.

Further, in this embodiment, the central control unit 150 functions as a detection unit 152. The detection unit 152 acquires the image captured by the imaging unit 130. The detection unit 152 also detects the depth of the foreign object C included in the inspection object 10 based on the acquired image.

[Foreign object detection method]
Next, a foreign object detection method using the foreign object detection device 100 will be explained. FIG. 2 is a flowchart showing the flow of the foreign object detection method of this embodiment. As shown in FIG. 2, the foreign object detection method includes an installation step S110, an inspection completion determination step S120, a movement step S130, a first imaging step S140, a foreign object presence determination step S150, a displacement step S160, a second imaging step S170, and a foreign object depth The process includes a quality determination step S180, a failure determination step S190, and a pass determination step S200. Each step will be explained below.

[Installation process S110]
The installation step S110 is a step in which the central control unit 150 drives an installation mechanism (not shown) to install (bring in contact with) one surface of the inspection object 10 on the mirror 110.

[Inspection completion determination step S120]
The inspection completion determination step S120 is a step in which the central control unit 150 determines whether or not the foreign object inspection has been completed for the entire surface of the inspection target 10 (XY plane in FIG. 1). Specifically, the central control unit 150 determines whether the imaging unit 130 at the first position has finished imaging the entire surface of the inspection object 10. As a result, if it is determined that the foreign substance inspection has been completed (YES in S120), the central control unit 150 moves the process to pass determination step S200. On the other hand, if it is determined that the foreign object inspection has not been completed (NO in S120), the central control unit 150 moves the process to the moving step S130.

[Moving step S130]
In the moving step S130, first, the central control unit 150 drives the displacement mechanism to move the imaging unit 130 to the first position. Then, the central control unit 150 drives the moving mechanism to move the inspection object 10 along with the mirror 110 a predetermined distance in either or both of the X direction and the Y direction in FIG. The predetermined distance is the distance from the imaging range of the imaging unit 130 when the first imaging step S140 was executed last time to the imaging range of the adjacent imaging unit 130. The imaging range is the range of the inspection object 10 that can be imaged by the imaging unit 130.

[First imaging step S140]
FIG. 3 is a diagram illustrating the position (first position) of the imaging unit 130 in the first imaging step S140. As shown in FIG. 3, the first imaging step S140 is a step in which the central control unit 150 drives the imaging unit 130 at the first position to image the inspection object 10. At this time, the central control unit 150 turns on the first light emitting unit 120. Further, the central control unit 150 turns off the second light emitting unit 140.

[Foreign object presence/absence determination step S150]
The foreign object presence/absence determination step S150 is a step in which the detection unit 152 analyzes the image obtained in the first imaging step S140 to detect the presence or absence of a foreign object C in the imaging range. For example, the detection unit 152 binarizes the image and determines that the foreign object C is present when a predetermined number or more of consecutive black pixels are present. Then, when the central control unit 150 determines that there is a foreign object C (YES in S150), the process moves to the displacement step S160. On the other hand, if the central control unit 150 determines that there is no foreign object (NO in S150), the process returns to the inspection completion determination step S120.

[Displacement process S160]
The displacement step S160 is a step in which the central control section 150 drives the displacement mechanism to move the imaging section 130 from the first position to the second position.

[Second imaging step S170]
FIG. 4 is a diagram illustrating the position (second position) of the imaging section 130 in the second imaging step S170. As shown in FIG. 4, the second imaging step S170 is a step in which the central control unit 150 drives the imaging unit 130 at the second position to image the inspection object 10. At this time, the central control unit 150 turns on the second light emitting unit 140. Further, the central control unit 150 turns off the first light emitting unit 120.

[Foreign object depth determination step S180]
The foreign object depth determination step S180 is a step in which the detection unit 152 analyzes the image obtained in the second imaging step S170 to detect the depth of the foreign object C included in the inspection object 10. In this embodiment, the detection unit 152 binarizes the image and calculates the distance between the foreign object C and the mirror image D on the image.

FIG. 5 is a diagram illustrating the difference in distance between the foreign object C on the image and the mirror image D depending on the depth of the foreign object C within the inspection object 10. FIG. 5A is a diagram illustrating the position P1 of the inspection object 10. FIG. 5B is a diagram illustrating an image when a foreign object C is present at position P1. FIG. 5C is a diagram illustrating the position P2 within the inspection object 10. FIG. 5D is a diagram illustrating an image when there is a foreign object C at position P2. FIG. 5E is a diagram illustrating the position P3 within the inspection object 10. FIG. 5F is a diagram illustrating an image when there is a foreign object C at position P3.

As shown in FIG. 5A, position P1 is a predetermined position on the other surface of the inspection object 10, that is, the surface of the protective film 14 (XY plane in FIG. 5A). The position P1 is separated from the surface of the mirror 110 in the Z-axis direction in FIG. Therefore, the position P1a on the mirror 110 where the mirror image D is formed is separated from the position P1 of the foreign object C in the Y-axis direction in FIG. Therefore, an optical path difference L1 occurs between the light Rc reflected by the foreign object C and the light Rd reflected by the mirror image D. As a result, as shown in FIG. 5B, in the image 132, the foreign object C and the mirror image D are separated by a distance L1a. Note that the distance between the foreign object C and the mirror image D in the image 132 is the distance between the end Ec of the foreign object C and the end Ed corresponding to the end Ec of the foreign object C in the mirror image D.

As shown in FIG. 5C, position P2 is a predetermined position on one surface of the inspection object 10, that is, the contact surface with the mirror 110. The position P2 is substantially the same as the surface of the mirror 110 in the Z-axis direction in FIG. Therefore, the position P2a on the mirror 110 where the mirror image D is formed is substantially equal to the position P2 of the foreign object C. Therefore, the optical path difference between the light Rc reflected by the foreign object C and the light Rd reflected by the mirror image D becomes substantially zero. As a result, the foreign object C and the mirror image D are superimposed in the image 132, as shown in FIG. 5D. That is, when the foreign object C is present at the position P2, the distance between the foreign object C and the mirror image D in the image 132 is substantially zero.

As shown in FIG. 5E, position P3 is a predetermined position within the inspection object 10. The position P3 is separated from the surface of the mirror 110 in the Z-axis direction in FIG. Therefore, the position P3a on the mirror 110 where the mirror image D is formed is separated from the position P3 of the foreign object C in the Y-axis direction in FIG. Therefore, an optical path difference L3 occurs between the light Rc reflected by the foreign object C and the light Rd reflected by the mirror image D. As a result, as shown in FIG. 5F, in the image 132, the foreign object C and the mirror image D are separated by a distance L3a.

Therefore, in the foreign object depth determination step S180, the detection unit 152 first calculates the distance La (for example, the number of pixels) between the foreign object C and the mirror image D on the image 132 obtained in the second imaging step S170. Then, the detection unit 152 detects the thickness T (the length in the Z-axis direction in FIG. 5E) of the inspection object 10, the distance L1a (for example, the number of pixels), and the second light emitting unit 140, which are held in a memory (not shown). Based on the incident angle θ of Derive F.
W = La × cosθ...Formula (1)
F = T - W...Formula (2)
In the above formula (1), W indicates the distance from one surface of the inspection object 10 (the contact surface with the mirror 110) to the foreign object C (the length in the Z-axis direction in FIG. 5E). In the above formula (2), F indicates the distance from the other surface of the inspection object 10 (the surface of the protective film 14) to the foreign object C (the length in the Z-axis direction in FIG. 5E).

Then, the central control unit 150 determines whether the derived depth F of the foreign object C is on the surface of the inspection object 10 (protective film 14), within the protective film 14, or between the protective film 14 and the transmission plate 12. Determine. As a result, the central control unit 150 determines that when the depth F of the foreign object C is on the surface of the inspection object 10 (protective film 14), within the protective film 14, or between the protective film 14 and the transmission plate 12 ( (YES in S180), the process moves to inspection completion determination step S120. On the other hand, if the depth F of the foreign object C is not on the surface of the inspection object 10 (protective film 14), inside the protective film 14, or between the protective film 14 and the transmission plate 12 (S180 (NO), the process moves to rejection determination step S190.

[Rejection determination step S190]
Returning to FIG. 2, the rejection determination step S190 is a step in which the central control unit 150 determines that the inspection object 10 is rejected. The inspection object 10 that is determined to be rejected is, for example, discarded.

[Pass determination step S200]
The acceptance determination step S200 is a step in which the central control unit 150 determines that the inspection object 10 is acceptable. The inspection object 10 that has been determined to pass is transported to the next process or shipped.

As described above, the foreign object detection device 100 and the foreign object detection method using the same according to the present embodiment include the mirror 110, the imaging section 130, and the second light emitting section 140. Thereby, the foreign object detection device 100 can measure the depth F of the foreign object C with a simple configuration that only takes an image with the imaging unit 130 at the second position.

Further, the foreign object detection device 100 can share the imaging section 130 that detects the foreign object C (the imaging section 130 at the first position) and the imaging section 130 that measures the depth F of the foreign object C. Therefore, the foreign object detection device 100 can detect the foreign object C at low cost and measure the depth F of the foreign object C.

Further, as described above, the mirror 110 is provided in contact with the inspection object 10. Thereby, the imaging unit 130 can eliminate disturbances. Therefore, the imaging unit 130 can accurately image the foreign object C and the mirror image D.

Further, as described above, the mirror 110 is a beam splitter. Thereby, the foreign object detection device 100 can guide the light that has passed through the inspection object 10 to the imaging section 130 at the first position. Therefore, the boundary of the foreign object C can be made clear by the first light emitting section 120, and the accuracy of determining the presence or absence of the foreign object C can be improved in the foreign object presence/absence determination step S150. Therefore, the foreign object detection device 100 can efficiently determine the presence or absence of the foreign object C and measure the depth F of the foreign object C.

Further, as described above, the detection unit 152 sets the distance between the end Ec of the foreign object C in the image 132 and the end Ed of the mirror image D as the distance between the foreign object C and the mirror image D in the image 132. Thereby, the detection unit 152 can accurately measure the distance between the foreign object C and the mirror image D in the image 132, regardless of the shape of the foreign object C.

[Modified example]
In the above embodiment, the configuration in which the foreign object detection device 100 includes the mirror 110 has been exemplified. However, depending on the configuration of the inspection object 20, the mirror 110 can be omitted. FIG. 6 is a diagram illustrating a foreign object detection device 200 of a modified example. In FIG. 6, dashed arrows indicate the flow of signals. In FIG. 6, a dashed line indicates light. As shown in FIG. 6, the foreign object detection device 200 includes a first light emitting section 120, an imaging section 130, a second light emitting section 140, and a central control section 150. Note that components that are substantially the same as those of the foreign object detection device 100 described above are given the same reference numerals and descriptions thereof will be omitted.

In the modified example, the inspection object 20 includes a transmission plate 12 and a protective film 24. The protective film 24 is provided in contact with one surface of the transmission plate 12 . In a variant, the protective film 24 is a beam splitter. That is, in the foreign object detection device 200 of the modified example, the protective film 24 that is peeled off when the inspection object 20 (transmission plate 12) is used functions as the mirror 110 of the foreign object detection device 100.

As a result, the foreign object detection device 200 can measure the depth F of the foreign object C with a simple configuration in which the inspection object 20 is only imaged by the imaging unit 130 at the second position, without the need for the mirror 110. be able to.

Although the embodiments have been described above with reference to the accompanying drawings, it goes without saying that the present disclosure is not limited to the above embodiments. It is clear that those skilled in the art can come up with various changes and modifications within the scope of the claims, and it is understood that these naturally fall within the technical scope of the present disclosure. be done.

For example, in the above-described embodiments and modified examples, the case where the mirror 110 and the protective film 24 are constituted by a beam splitter has been exemplified. However, the mirror 110 or the protective film 24 may completely reflect the incident light, or may reflect a portion of the incident light and absorb a portion of the incident light. In this case, the first light emitting section 120 is provided on the other side of the inspection objects 10 and 20, that is, on the same side as the imaging section 130.

Further, in the above embodiment, the case where the inspection target object 10 (transmission plate 12) is a flat plate is exemplified. However, the transmission plate 12 may be curved. In this case, the mirror 110 is preferably made of a flexible material. Further, at this time, the mirror 110 may be pressed against the transmission plate 12 by air or the like.

Further, in the above embodiment, the case where the mirror 110 is provided in contact with the inspection target object 10 has been exemplified. However, the mirror 110 and the inspection target 10 may be separated. In this case, T is a value obtained by adding the separation distance between the mirror 110 and the inspection object 10 to the thickness of the inspection object 10.

In the above embodiment, the distance between the end Ec of the foreign object C in the image 132 and the end Ed of the mirror image D is defined as the distance between the foreign object C and the mirror image D in the image 132. However, the distance between the center of gravity of the foreign object C in the image 132 and the center of gravity of the mirror image D may be the distance between the foreign object C and the mirror image D in the image 132.

Furthermore, in the above embodiments, the configuration is exemplified in which the imaging unit 130 is fixed at the first position and the second position, and the inspection object 10 and the mirror 110 are moved. However, the inspection object 10 and the mirror 110 may be fixed, and the imaging unit 130 may be moved (scanned) at the first position and the second position.

Furthermore, in the above embodiments, the case where the first light emitting section 120 and the second light emitting section 140 emit visible light has been exemplified. However, the first light emitting section 120 and the second light emitting section 140 may emit light in a wavelength range from infrared to ultraviolet. In this case, the imaging section 130 only needs to be able to receive the light emitted by the first light emitting section 120 and the second light emitting section 140 and the reflected light of this light. Further, the first light emitting section 120 and the second light emitting section 140 may emit electromagnetic waves. In this case, the imaging unit 130 only needs to be able to receive electromagnetic waves.

Moreover, in the embodiment described above, the case where the inspection object 10 includes the protective film 14 was exemplified. However, the inspection object 10 only needs to include at least the transmission plate 12 . In this case, the foreign object detection device 100 detects the depth F of the foreign object C attached to the transparent plate 12 and the foreign object C included in the transparent plate 12 (the foreign object C included in the transparent plate 12).

Note that each step of the foreign object detection method of this specification does not necessarily need to be processed in chronological order according to the order described as a flowchart, and may include processing in parallel or by a subroutine.

The present disclosure can be used in a foreign object detection device and a foreign object detection method.

C Foreign object D Mirror image 12 Transmission plate 20 Inspection object 24 Protective film 100 Foreign object detection device 110 Mirror 130 Imaging section 140 Second light emitting section (light emitting section)
152 Detection unit 200 Foreign object detection device S110 Installation step S170 Second imaging step S180 Foreign object depth determination step

Claims (3)

a mirror provided in contact with one surface of the transmission plate;
a light emitting section provided on the other surface side of the transmission plate;
an imaging section provided on the other side of the transmission plate;
a detection unit that detects the depth of the foreign object contained in the transmission plate based on a distance between the foreign object and a mirror image of the foreign object projected on the mirror in the image captured by the imaging unit;
A foreign object detection device comprising: a light emitting unit provided on the other surface of the object to be inspected, including a transmission plate and a mirror provided in contact with one surface of the transmission plate;
an imaging unit provided on the other surface side of the inspection target;
a detection unit that detects the depth of the foreign object contained in the inspection object based on a distance between the foreign object and a mirror image of the foreign object projected on the mirror in the image captured by the imaging unit;
A foreign object detection device comprising: a step of installing a mirror in contact with one surface of the transparent plate;
irradiating light from the other side of the transmission plate;
a step of capturing an image from the other side of the transmission plate;
detecting the depth of the foreign object contained in the transmission plate based on the distance between the foreign object and the mirror image of the foreign object projected on the mirror in the image taken in the imaging step;
Foreign object detection method including.

Images