JP2011174777A - Inspection device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inspection device capable of detecting accurately and automatically whether or not each position of paper heads (front edges) of piled sheets is aligned perfectly. <P>SOLUTION: This inspection device 200 includes an electronic camera 210 for imaging a side surface of the piled sheets S, and an image processing device 220 for detecting continuity of the sheets based on a difference between a gradation of each pixel in an image formed by the electronic camera 210 and a gradation of a pixel adjacent thereto. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an inspection apparatus that inspects the alignment of paper heads of stacked sheets.

  FIG. 1 is a schematic diagram illustrating an example of a configuration of a cutter machine according to a conventional technique. A continuously connected sheet S1 such as paper is guided to a slitter 20 by a lead-in reel 10 having two rotating rollers 11 and 12. The sheet S1 is continuously cut by the upper blade 21 and the lower blade 22 that rotate the slitter 20 so that the width thereof becomes a predetermined width. The sheet S1 that has passed through the slitter is sandwiched between the outer peripheral surfaces of the two rollers 31 and 32 that rotate the feed roll 30, and is pulled to the downstream side to the rotary knife 40 that has the rotating drums 41 and 42. It is conveyed toward. The blades 43 and 44 attached to the outer peripheral surfaces of the drums 41 and 42 press the sheet S1 at regular time intervals, whereby the sheet S1 is cut to a predetermined length. The sheet S2 cut to a predetermined length is placed on the conveyor 50 and conveyed downstream. After leaving the contact with the conveyor 50, the sheet S <b> 2 moves and drops toward the layboy 60 located downstream of the conveyor 50, and the paper head (front edge) a hits the stop wall 61 of the layboy 60. It touches and is aligned. Thus, a large number of sheets S2 having a predetermined width and a predetermined length are stacked on the layboy 60 in a stacked state.

  The large number of sheets S2 stacked in this way are then subjected to the next step (not shown) such as printing or cutting. For example, the paper tail (rear edge) b side is cut by guillotine on the entire stacked sheet S2, or printing is performed at a predetermined position determined based on the position of the paper head a. Is done. Therefore, it is necessary to accurately and automatically inspect whether or not all the positions of the paper heads a of the stacked sheets S2 are aligned before the next step.

  As a technique for performing such an inspection, one shown in FIG. 2 is known. FIG. 2 is a schematic diagram showing an inspection method according to the prior art. As shown in FIG. 2, a pair of transmission photoelectric sensors 71 and 72 are arranged so as to face each other in the vicinity of the paper tail b of a large number of sheets S2 that are stacked. When the sensors 71 and 72 detect the paper tail b ′ of the sheet S2 ′ protruding beyond the specified size, the presence of the sheet S2 ′ in which the paper head a ′ is not aligned is indirectly detected (estimated). The

However, in the inspection method according to the above-described prior art, in addition to the limit of the detection accuracy of the photoelectric sensor, there may be a dimensional variation among a large number of sheets that are stacked. It is difficult to detect head misalignment.
In this inspection method, the paper head of the sheet is not directly observed by the photoelectric sensor, but the presence or absence of the paper head of the sheet is indirectly detected by observing the paper tail of the sheet by the photoelectric sensor. It is only an estimate. Therefore, for example, when the distance from the paper head to the paper tail of a certain sheet is larger than that of other sheets (that is, when the dimension between the sheets varies), even if that sheet is Even if the position of the paper head of the sheet is aligned with the position of the paper head of the other sheet, the paper tail of the sheet pops out, so that it is estimated that the paper head of the sheet is not aligned.
As described above, the above-described inspection method according to the related art has a problem in that it is not possible to accurately detect whether or not all the positions of the paper heads of the stacked sheets are aligned.
The present invention has been made in view of such a problem, and provides an inspection apparatus that accurately and automatically detects whether or not all the positions of the paper heads (leading edges) of stacked sheets are aligned. It is intended.

  The inspection apparatus according to the present invention is based on an imaging unit that images the side surfaces of stacked sheets, and a difference between a gradation of each pixel in an image formed by the imaging unit and a gradation of an adjacent pixel adjacent to the pixel. And detecting means for detecting the continuity of the sheets.

  According to the present invention, the sheet head (front edge) of the sheet is detected by detecting the continuity of the sheet in the horizontal direction from the trailing edge of the sheet to the leading edge (along the vertical direction in which the sheets are stacked as necessary). ) Position can be detected. Thereby, it is possible to accurately detect whether or not the stacked sheets include sheets whose paper head positions are aligned and deviated from the surface.

FIG. 1 is a schematic diagram illustrating an example of a configuration of a cutter machine according to a conventional technique. FIG. 2 is a schematic diagram showing an inspection method according to the prior art. FIG. 3A is a diagram illustrating an example of an image obtained by imaging the front end portion C of the sheet S. FIG. 3B is an enlarged view of a region A in the vicinity of the paper head (front edge) a in the sheet S shown in FIG. FIG. 4 is a conceptual diagram showing a specific method for detecting the position of the paper head based on the change in the gradation of the pixel near the paper head. FIG. 5 is a schematic diagram showing a specific example of an inspection system using the inspection apparatus according to the embodiment of the present invention. FIG. 6 is a schematic diagram illustrating an example of an image (when there is no misalignment of the paper head) obtained by enlarging and imaging a certain portion of the front end portion 101 on the side surface of the sheet S. FIG. 7 is a schematic diagram illustrating an example of an image (when there is a paper head shift) obtained by enlarging and imaging a certain portion of the front end portion 101 on the side surface of the sheet S. FIG. 8 is a schematic diagram illustrating an example of an image (when there is a misalignment of the paper head) obtained by enlarging and imaging a certain portion of the front end portion 101 on the side surface of the sheet S. FIG. 9 is a schematic diagram showing the concept of the detection method along the horizontal direction executed by the image processing apparatus included in the inspection apparatus according to the embodiment of the present invention. FIG. 10 is a schematic diagram illustrating a concept of a detection method along the vertical direction executed by the image processing apparatus included in the inspection apparatus according to the embodiment of the present invention. FIG. 11 is a block diagram showing a configuration of an image processing apparatus included in the inspection apparatus according to the embodiment of the present invention.

1. Figure 3 outline of the present invention (a) is a diagram showing an example of an image obtained by imaging the front part C of the sheet S. FIG. 3B is an enlarged view of a region A in the vicinity of the paper head (front edge) a in the sheet S shown in FIG.
As shown in region B in FIG. 3B, the pixels corresponding to the sheet S have a high gradation (white), and the pixels corresponding to the region other than the sheet S have a low gradation (black). It can be seen that the gradation of the pixel decreases along the direction from the pixel corresponding to the sheet S to the pixel corresponding to the region other than the sheet. Therefore, the position of the paper head a can be detected based on the change in the gradation of the pixels in the vicinity of the paper head a of the sheet S.

FIG. 4 is a conceptual diagram showing a specific method for detecting the position of the paper head based on the change in the gradation of the pixel near the paper head.
A pixel group 80 having 1 pixel in the vertical direction and 10 pixels in the horizontal direction corresponds to the region B shown in FIG. The broken line 90 is a line obtained by continuously connecting the gradations of the respective pixels. The broken line 91 is a line obtained by continuously connecting the gradation difference (differential value) between adjacent pixels.

First, the case where the detection direction X (that is, the direction from the region other than the sheet S toward the sheet S) is used as the detection direction will be described. Since the differential value between the gradation of the pixel corresponding to the paper head (front edge) of the sheet S and the pixel corresponding to the area other than the sheet S is larger than the differential value between the other pixels, a large differential value is obtained. By detecting the existing position, the position of the paper head of the sheet S can be detected.
In order to detect a position where a large differential value exists, each differential value is compared with a threshold value along the detection direction X. When the threshold value 92 (its value is set to 30) is used, the differential value (its value is 40) between the pixel 80a and the pixel 80b is larger than the threshold value 92, so the pixel 80b Is to display the paper head of the sheet S, and it is detected that the pixel 80a is to display an area other than the sheet S. When the threshold value 93 (the value is set to 50) is used, the differential value between the pixel 80b and the pixel 80c (the value is set to 50) is used. The differential value between the pixel 80b and the pixel 80c (its value is 60) is larger than the threshold value 93, so that the pixel 80c displays the paper head of the sheet S, and the pixel 80b is an area other than the sheet S. Is detected.

  Next, when the detection direction Y is the detection direction (that is, the direction from the sheet S toward the area other than the sheet S), the differential value (the value is 60) between the pixel 80c and the pixel 80b is a threshold value. It is detected that the image 80c displays the paper head of the sheet S and the image 80b displays an area other than the sheet S because it is larger than both of the threshold 92 and the threshold 93.

2. Next, an embodiment that specifically realizes the above-described technical concept of the present invention will be described.
FIG. 5 is a schematic diagram showing a specific example of an inspection system using the inspection apparatus according to the embodiment of the present invention. The inspection system 100 includes a large number of stacked sheets S, a light 110 that irradiates the sheets S from the side surfaces, an electronic camera 210 that images the side surfaces of the large number of sheets S, and an image formed by the electronic camera 210. And an inspection device 200 including an image processing device 220 for processing.

  A weight 300 is placed on the uppermost part of the stacked sheets S, particularly near the paper head a. The reason why the weight 300 is placed on the sheet S is to suppress the occurrence of a shadow between the sheets S due to the bending of the sheet S in the image formed by the electronic camera 210.

  The electronic camera 210 is disposed at a position facing the front end portion 101 in order to take an image of the front end portion 101 on the side surface of the sheet S. In the present embodiment, the electronic camera 210 does not capture the entire front end portion 101 on the side surface of the sheet S at a time, but divides the front end portion 101 into a plurality of portions along the vertical direction. Are sequentially enlarged and imaged. Thereby, the resolution of the obtained image can be improved. In order to realize such imaging, a mechanism (not shown) for moving the electronic camera 210 up and down along the vertical direction can be used. Alternatively, the electronic camera 210 is fixed and the stacked sheets S themselves are placed vertically. A mechanism (not shown) that moves up and down along the direction can also be used. Alternatively, a configuration in which a necessary number of electronic cameras 210 are sequentially arranged along the vertical direction may be employed.

6 and 7 are schematic diagrams illustrating examples of images obtained by enlarging and imaging a certain portion of the front end portion 101 on the side surface of the sheet S. FIG. In the image shown in FIG. 6, the paper heads (front edges) a of all the sheets S are aligned along the butted surface 102. A region G other than the sheet S exists in front of the butted surface 102. In the image shown in FIG. 7, due to the position deviated from the matching surface 102 Kamiatama a ₁₂ sheet S ₁₂ is thrust, the gap formed in front of Kamiatama a ₁₂ sheet S ₁₂ becomes a shadow of the triangle Is displayed. Such an image captured by the electronic camera 210 is transmitted to the image processing apparatus 220 via the connection line 215.

The image processing apparatus 220 detects the continuity of the sheet S in the image formed by the electronic camera 210 based on the difference between the gradation of each pixel and the gradation of a pixel adjacent to this pixel. Taking the image 800 shown in FIG. 8 as an example, the image processing apparatus 220 first calculates the gradation of all the pixels constituting the image (here, pixels of M rows × N columns for convenience). Remember. Next, for the x-th row (1 ≦ x ≦ M) in the image 800, the image processing apparatus 220 determines whether the adjacent pixels are aligned along the horizontal direction from the pixel in the first column to the pixel in the N-th column. The calculated differential value is compared with a threshold value while calculating the gradation difference (differential value) between the two. As a result, as shown in FIG. 9, when a plurality of pixels P _{1 to} P ₁₀ continuously connected in the vertical direction displaying the paper head a ₁₂ of the sheet S ₁₂ are taken as an example, the gradation of the pixel P ₁ and this The difference (differential value) from the gray level of the horizontal adjacent pixel Q ₁ adjacent to the horizontal direction exceeds the threshold value. Similarly, for each of the remaining pixels P _{2 to} P ₁₀ , the difference (differential value) between the gradation of this pixel and the gradation of the corresponding horizontal adjacent pixels Q _{2 to} Q ₁₀ adjacent in the horizontal direction is The threshold will be exceeded. Therefore, the image processing apparatus 200 detects that the plurality of pixels P _{1 to} P ₁₀ are pixels that display the paper head of the sheet.

In the image shown in FIG. 9, even if were those pixels Q ₁ to Q ₁₀ displays the dust and dirt adhering to the sheet S ₁₂ rather than the area other than the sheet S _12, the image processing The apparatus 220 may detect that the pixels P _{1 to} P ₁₀ are pixels that display the paper head a ₁₂ of the sheet S ₁₂ .
If the pixels P _{1 to} P ₁₀ are actually pixels that display the paper head a ₁₂ of the sheet S ₁₂ , all of the plurality of pixels Q ₁ , R _1, and S ₁ continuously connected to the pixel P ₁ in the horizontal direction. There should be a pixel for displaying an area other than the sheet S _12. Therefore, in a preferred embodiment, the image processing apparatus 220 has a “first predetermined number” of horizontally adjacent pixels (for example, 3) continuously connected in the horizontal direction to the target pixel (for example, the pixel P ₁ ). For a horizontal target pixel composed of two horizontal adjacent pixels Q ₁ , R _1, and S ₁ , a gray level difference between adjacent horizontal target pixels (that is, the gray level of the pixel Q ₁ and the level of the pixel R ₁ ) The horizontal adjacent pixels Q ₁ , R _1, and S ₁ are sheets only when the difference between the tone and the difference between the gradation of the pixel R _{1 and} the gradation of the pixel S ₁ are all equal to or less than the threshold value. determines that a pixel displaying an area other than S _12. According to such a method, the pixels Q _{1 to} Q ₁₀ display dust or dirt adhering to the sheet S ₁₂ , and the pixels R _{1 to} R ₁₀ and the pixels S _{1 to} S ₁₀ display the sheet S _12. In this case, since the difference between the gradation of the pixel Q ₁ (dust etc.) and the gradation of the pixel R ₁ (sheet) exceeds the threshold value, the image processing apparatus 220 has the pixels P _{1 to} P ₁₀ in the sheet. It is no longer erroneously detected that this is a pixel displaying the paper head.

As described above, the image processing apparatus 220 can sufficiently detect pixels that display the sheet head of the sheet even when only the method of detecting the continuity of the sheet along the horizontal direction is used in the image. it can. However, as shown in FIG. 10, when there is a shadow SH ₁ extending in the horizontal direction between sheets in the captured image, a plurality of pixels (U ₉ , V ₉ , W ₉₎ continuously connected in the horizontal direction are present. ,..., C ₉ , D ₉ , E ₉ and U ₁₀ , V ₁₀ , W ₁₀ ,..., C ₁₀ , D ₁₀ , E ₁₀ ) display this shadow SH ₁ . At this time, the gradation and the pixel U ₉ for displaying the edge SH _1b after shadow SH ₁ (pixel U of a pixel T ₉ for displaying the sheet portion of the immediately preceding edge SH _1b after shadow SH ₁ (pixel T _{10) 10} ) The difference between the gradations exceeds the threshold. The difference between the gradation of the pixel adjacent thereto and the gradation of the pixel T ₁₀ also exceeds a threshold. Therefore, the image processing apparatus 220 erroneously determines that the pixels T ₉ and T ₁₀ that display the sheet portion immediately before the trailing edge SH _1b of the shadow SH ₁ are pixels that display the sheet head of the sheet. there is a possibility (in FIG. 10, in order to clearly show the number of pixels for displaying a shadow SH _1, the shadow SH ₁ has been displayed in white, is displayed actually black).

Therefore, in a preferred embodiment, the image processing apparatus 220 may use a method of detecting the continuity of the sheet along the vertical direction in addition to the method of detecting the continuity of the sheet along the horizontal direction. it can. Specifically, after determining that the pixels T ₉ and T ₁₀ are pixels for displaying the paper head of the sheet, the image processing apparatus 220 further performs the following processing. First, the image processing apparatus 220 regards a pixel U ₉ that is adjacent to the pixel T ₉ in the horizontal direction as a horizontal adjacent pixel, and is connected to the horizontal adjacent pixel U ₉ continuously in the vertical direction “second predetermined”. The “number” (for example, three) pixels U ₁₀ , J ₁ and J ₂ are regarded as vertically adjacent pixels. Furthermore, the image processing apparatus 220 determines the difference in gradation between adjacent vertical target pixels (that is, the vertical target pixels composed of the horizontal adjacent pixels U ₉ and the vertical adjacent pixels U ₁₀ , J ₁ and J ₂ (that is, , The difference in gradation between the pixels U ₉ and U ₁₀ , the difference in gradation between the pixels U ₁₀ and J ₁ , and the difference in gradation between the pixels J ₁ and J ₂ ) are all below the threshold value. only when the pixel T ₉ is finally determined when is pixels for displaying Kamiatama sheet.

According to this method, even if a shadow SH ₁ exists in the image, the gradation difference between the vertical target pixels U ₉ and U ₁₀ is equal to or less than the threshold value, but between the vertical target pixels U ₁₀ and J _1. since the difference in gray level is greater than the threshold value in, ultimately, the pixels T ₉ is not determined from the pixels displaying Kamiatama sheet.
Regarding the second predetermined number, the second predetermined number can be set so that the overall length of the vertical target pixel in the vertical direction is greater than the length of the shadow between the sheets in the vertical direction. By setting the second set number in this way, it is possible to prevent a situation in which a pixel located immediately before the shadow generated between the sheets is erroneously detected as a pixel displaying the paper head of the sheet.

Returning to FIG. 8 again, no matter which of the various embodiments described above is used, pixels representing the paper head a ₁₂ of the sheet S ₁₂ (these pixels are continuously connected in the vertical direction). Can be reliably detected. Incidentally, on the basis of the respective coordinates of the detected pixel as to express these paper head, determine whether the matching surface 102 position of the paper head sheet S ₁₂ is butted how (e.g. what mm) deviated and the You can also.

  Next, a specific configuration example of the image processing apparatus 220 will be described with reference to FIG. FIG. 11 is a block diagram showing a configuration of an image processing apparatus included in the inspection apparatus according to the embodiment of the present invention.

The communication processing unit 201 performs wired or wireless communication with the electronic camera 200 and receives information regarding an image captured and formed by the electronic camera 200.
The storage unit 202 receives information about the image from the communication processing unit 201 and stores it. In addition, the storage unit 202 stores various information including processing results in each unit.
The gradation calculation unit 203 receives information about the image stored in the storage unit 202 and calculates the gradation of each pixel in this image. Further, the gradation calculation unit 203 causes the storage unit 202 to store information regarding the calculated gradation.

  The horizontal direction inspection unit 204 receives information about the image and information about each pixel in the image from the storage unit 202, and calculates a gradation difference (differential value) between pixels adjacent in the horizontal direction. That is, the horizontal direction inspection unit 204 (from the assumption that the image is composed of pixels of M rows × N columns) for the x-th row (1 ≦ x ≦ M) in the image starts from the pixels in the first column to the N-th row. A gradation difference (differential value) between pixels adjacent in the horizontal direction is calculated along the horizontal direction toward the pixels in the column. The horizontal direction inspection unit 204 causes the storage unit 202 to store information related to the calculated differential value in the horizontal direction.

  The vertical direction inspection unit 205 receives information about the image and information about each pixel in the image from the storage unit 202, and calculates a gradation difference (differential value) between pixels adjacent in the vertical direction. That is, the vertical direction inspection unit 205 (from the assumption that the image is composed of pixels of M rows × N columns) for the y th column (1 ≦ x ≦ N) in the image from the first row pixels to the M th row. A gradation difference (differential value) between pixels adjacent in the vertical direction is calculated along the vertical direction toward the pixels in the row. The vertical direction inspection unit 205 causes the storage unit 202 to store information regarding the calculated differential value in the vertical direction.

The detection unit 206 receives information related to the image and a differential value in the horizontal direction of the image (if necessary, a differential value in the vertical direction of the image) from the storage unit 202, and according to the above-described method, These pixels display the paper head of the sheet, which pixels display the sheet area, and which pixels display the area other than the sheet.
The control unit 207 controls the operation of each unit via a line (not shown).

  Note that the communication processing unit 201 can also transmit information about an image stored in the storage unit 202 to an external monitor or the like. Thus, the operator can check an image captured and formed by the electronic camera 210 and stored and processed in the image processing apparatus 202 with an external monitor or the like.

Further, the image processing apparatus 220 can additionally include an image composition unit 208 that synthesizes various displays with the image stored in the storage unit 202.
For example, as illustrated in FIG. 8, the image composition unit 208 synthesizes an edge line 801 that vertically cuts pixels that are continuously connected in the vertical direction, which are determined as pixels for displaying the paper head a <b> 12 of the sheet S <b> 12. be able to. Similarly, as shown in FIG. 8, when the difference in gradation between two pixels (upper pixel and lower pixel) adjacent in the vertical direction exceeds a threshold value, such an upper image is horizontal. In the case of continuous connection in the direction, an edge line 802 that traverses the pixels continuously connected in the horizontal direction can be combined with this image. The synthesized image is stored in the storage unit 202 and further transmitted to the external monitor or the like by the communication processing unit 201.

Thus, according to the embodiment of the present invention, by detecting the continuity of the sheet in the horizontal direction from the trailing edge to the leading edge of the sheet (and along the vertical direction in which the sheets are stacked as necessary), The position of the paper head (leading edge) of the sheet can be detected. Thereby, it is possible to accurately detect whether or not the stacked sheets include sheets whose paper head positions are aligned and deviated from the surface.
Furthermore, since the resolution of the obtained image can be increased by increasing the enlargement magnification by the electronic camera, not only the slip of the paper head of about 5 mm, which was impossible with the method using the photoelectric sensor, It is possible to detect even smaller paper head deviations as required.
Furthermore, since the continuity of the sheet is detected toward the paper head of the sheet, that is, the paper head of the sheet is actually observed, regardless of whether or not there is a dimensional variation between the stacked sheets, It is possible to more reliably detect whether or not a sheet in which the position of the paper head is out of alignment is included.

In the above-described embodiment, in order to divide and image the side surface of the seat into a plurality of portions along the vertical direction, the configuration in which the electronic camera 210 is moved up and down along the vertical direction, the electronic camera 210 is fixed. The configuration in which the seat itself is moved up and down along the vertical direction and the configuration in which the plurality of electronic cameras 210 are arranged along the vertical direction are employed.
In addition to this, in order to divide the side surface of the sheet into a plurality of parts along the horizontal direction and take an image, the electronic camera 210 is moved along the horizontal direction, the electronic camera 210 is fixed, and the sheet itself A configuration in which the camera is moved along the horizontal direction and a configuration in which a plurality of electronic cameras 210 are arranged along the horizontal direction can also be used. According to such a configuration, even when the paper head of the sheet is largely deviated from the aligned surface of the paper heads of other sheets, there is a sheet in which the position of the paper head is shifted among these sheets. This can be reliably detected.

S sheet a sheet sheet head 100 inspection system 101 front side edge of sheet 102 abutting surface 200 inspection device 201 communication processing unit 202 storage unit 203 gradation calculation unit 204 horizontal direction inspection unit 205 vertical direction inspection unit 206 detection unit 208 Image composition unit 210 Electronic camera 220 Image processing device

Claims (7)

Imaging means for imaging the side surfaces of the stacked sheets;
Detecting means for detecting the continuity of the sheet based on the difference between the gradation of each pixel in the image formed by the imaging means and the gradation of an adjacent pixel adjacent to the pixel;
An inspection apparatus comprising: The detection means includes
First detecting means for detecting a difference between a gray level of each pixel along a horizontal direction from a rear edge to a front edge of the sheet and a gray level of a horizontal adjacent pixel adjacent to the pixel in the horizontal direction; ,
When the difference between the gradation of a certain pixel and the gradation of a horizontal adjacent pixel adjacent to the pixel in the horizontal direction exceeds a threshold value, the certain pixel displays the leading edge of the sheet The inspection apparatus according to claim 1, wherein the inspection apparatus is a pixel and determines that the horizontal adjacent pixel is a pixel that displays an area other than the sheet.   The detection means is configured to detect a gradation between adjacent horizontal target pixels for a horizontal target pixel including a first predetermined number of horizontal adjacent pixels continuously connected in the horizontal direction to the certain pixel. Only when all the differences are less than or equal to a threshold value, the certain pixel is a pixel that displays the leading edge of the sheet, and the horizontal adjacent pixel is a pixel that displays an area other than the sheet. The inspection apparatus according to claim 2. The detection means includes
Second detection means for detecting a difference between a gradation of each pixel along a vertical direction in which the sheets are stacked and a gradation of a vertical adjacent pixel adjacent to the pixel in the vertical direction;
A gradation between adjacent vertical target pixels with respect to a vertical target pixel composed of the horizontal adjacent pixels and a second predetermined number of vertical adjacent pixels continuously connected in the vertical direction to the horizontal adjacent pixels. It is determined that the certain pixel is a pixel that displays the leading edge of the sheet and the horizontal adjacent pixel is a pixel that displays an area other than the sheet only when all the differences are equal to or less than the threshold value. The inspection apparatus according to claim 2 or 3.   The detection means sets the second predetermined number so that an overall length of the vertical target pixel in the vertical direction is larger than a length of a shadow generated between the sheets in the vertical direction. 4. The inspection apparatus according to 4.   The inspection apparatus according to any one of claims 1 to 5, wherein the imaging unit divides a side surface of the stacked sheets into a plurality of parts along a vertical direction.   The inspection apparatus according to claim 1, wherein the imaging unit divides a side surface of the stacked sheets into a plurality of parts along a horizontal direction.