JP2003018374A - Method of specifying defect position of shading correction plate and method of removing influence in image reading device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To specify a shading correction plate in an image reading device and remove the influence of the shading correction plate. SOLUTION: In order to specify a defect position of a shading correction plate, shading correction data is created from a plurality of positions in the sub-scanning direction, and a result of performing the shading correction at the plurality of positions in the sub-scanning direction again. A defect position on the shading correction plate is specified by comparing the results of the shading correction performed at a plurality of positions in the sub-scanning direction. The comparing means also detects only a defect having a size equal to or larger than the matching / mismatching size. After the defect position is specified, a portion having no defect is set as a shading correction data creation position. If there is no defect-free portion, a position with the least number of defects is selected, or a position where all the defects are within the designated area is selected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dust detection method included in a shading correction plate used for correcting variations in an illumination optical system and the like in an image reading apparatus, and an effect removal method associated therewith.

[0002]

2. Description of the Related Art FIG. 1 and FIG. 2 are schematic block diagrams of a general image reading apparatus. A document cover holding a document 12 is placed on the document table 10. Light is emitted from the lower part of the original 12 by the light source 15. The light reflected on the original 12 is given to the line sensor 20 via the lens 14. The line sensor 20 has a CCD which is a photoelectric conversion element which is one-dimensionally arranged along the main scanning direction,
Each CCD converts the received light into an electric signal. As a result, an electric signal corresponding to the image of the original 12 (one scanning line) is obtained.

When the reading of one scanning line is completed, the reading unit 13 is moved by the motor 62 in the direction of the arrow X (sub-scanning direction), and the next scanning line is read in the same manner. By repeating this, the entire document 12 is read.

By the way, the image signal read by the line sensor 20 is affected by variations in the sensitivity of each CCD, dark current fluctuations, and unevenness in the amount of light due to the optical system. That is, a graph showing the relationship between the light amount and the output of the light source 15 for x CCDs forming a line sensor is shown in FIG.
As in A, there are variations. Therefore, in order to eliminate these influences and match the output characteristics of all x CCDs as shown in FIG. 3B, CCD output correction (called shading correction) is performed.

This shading correction is performed as follows. The reading unit 13 is driven by the motor 62.
Is moved to a position where the shading correction plate 11 provided at the end of the document table 10 can be read (hereinafter referred to as a home position). In this state, each CC of the line sensor 20 obtained when light is emitted from the light source 15
The output of D is stored as the correction data W. Further, the output of each CCD of the line sensor 20 obtained when the light source 15 is turned off (or the light is shut off) is stored as the correction data B.

Next, when reading the image of the original 12,
This correction data is read out and the output data is corrected for each CCD as shown in the following equation.

SD = k.multidot. (SB) / (WB) where S is CCD output data, B is correction data B, W is correction data W, k is a coefficient, and SD is shading correction. This is the completed data.

FIG. 4 shows a detailed shading correction circuit. First, the analog signal output from the line sensor is converted into a digital value S by an A / D converter via an amplifier.
Convert to (n). Next, in order to correct the black level B (n), B (n) is subtracted from S (n), and the shading-corrected data SD () is multiplied by ShC (n) that is shading correction data. n) is obtained. Since this correction is performed for each pixel of the line sensor,
The black level B, the white level W, the shading correction data ShC, and the like differ depending on each pixel. Therefore (n)
Is attached.

In this way, uneven output between the CCDs is corrected, and more faithful reading of the original 12 is realized.

However, the conventional shading correction method as described above has the following problems.

If dust or dirt is attached to the shading correction plate 11, erroneous data will be taken in as the correction data W. That is, in a portion having a defect such as dust or dust, the output of the CCD is reduced due to these defects. Therefore, if the shading correction is performed based on the correction data W, the output of the CCD corresponding to the portion where dust or dust is present is excessively corrected and appears as streaky unevenness in the read image.

For example, it is assumed that dust 21 as shown in FIG. 5A has adhered to the shading correction plate 11. The correction data W obtained by reading the shading correction plate 11 has a reduced output at β due to the influence of dust 21, as shown in FIG. 5B. In the shading correction, the correction data W is used to perform correction so that the corrected data becomes flat for all CCDs as shown in FIG. 5D. That is, it can be said that, conceptually, when the data shown in FIG. 5B is read, it is multiplied by the data shown in FIG. 5C to obtain the flat data shown in FIG. 5D.

If the original 12 having a uniform density is read, output data as shown in FIG. 6A is obtained from each CCD. Applying shading correction to this means multiplication of the data of FIG. 6B (same as FIG. 5C), so corrected data as shown in FIG. 6C is obtained. That is, the uneven portion δ occurs in the corrected data due to the influence of the excessively corrected portion (γ in FIG. 6B) due to the dust 12. There is a problem that this appears as streaky unevenness in the read image.

Therefore, a proposal has been made to eliminate the influence of defects in the shading correction plate. In Japanese Patent No. 2736536, a shading correction plate is read before image reading to create shading correction data, the shading correction plate is read at another position displaced in the sub-scanning direction, shading correction is performed, and the result is obtained. The defect of the shading correction plate at the position where the shading correction plate is read is detected. When a defect is found, the shading correction data is created by displacing in the sub-scanning direction to find a position where no defect is found.

[0015]

However, the correction of the data for shading correction described in the above publication has a problem that it takes a long time to read the image because such work is performed every time before the image is read. .

In order to solve the above problems, it is an object of the present invention to detect a defect such as dust on the shading correction plate and eliminate the influence thereof.

[0017]

According to the defect position specifying method of the shading correction plate and the accompanying effect removal method described in the claims of the present invention, the shading correction plate at different positions in the sub-scanning direction is read. As a result, the defect on the shading correction plate is identified, and the position in the sub-scanning direction where there is no defect is set as the position to be read by the shading correction plate, thereby eliminating the influence of the defect. Further, when there is no defect-free position, the influence is reduced by selecting a portion where the influence is not noticeable in the read image.

As a result, the defect position of the shading correction plate is specified and the reading position of the shading correction plate is determined in advance with respect to the above problem, so that the image reading time is not affected.

Further, in more detail, the above-mentioned problems can be solved by the following constitution.

(1) By a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally in the main scanning direction,
The correction plate (shading correction plate) that has a uniform reference density and has a length that can cover the entire area in the main scanning direction is read, and correction data is stored based on the output of each photoelectric conversion element. In reading an image, in an image reading apparatus having a shading function for correcting the output of each photoelectric conversion element based on the correction data, the shading correction plate is read by the line sensor at a plurality of positions in the sub-scanning direction. A means for creating and storing data for shading correction from the minimum value of the output of the photoelectric conversion element, and using the data for shading correction, the shading correction plate is read at a plurality of positions in the sub-scanning direction to perform shading correction. Means for storing the data, reading the shading correction plate at a plurality of positions in the sub-scanning direction, and reading the shading correction plate. Means for comparing the loading correction execution result, the defect position specifying how to present the shading correction plate made.

(2) By a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally in the main scanning direction,
The correction plate (shading correction plate) that has a uniform reference density and has a length that can cover the entire area in the main scanning direction is read, and correction data is stored based on the output of each photoelectric conversion element. In the image reading apparatus having a shading function for correcting the output of each photoelectric conversion element based on the correction data when reading, the shading correction plate is read by the line sensor at a plurality of positions in the sub-scanning direction, and shading is performed. A means for creating correction data and creating and storing shading correction data by using the maximum value of the correction data corresponding to each photoelectric conversion element, a plurality of means in the sub-scanning direction using the shading correction data. Means for reading the shading correction plate at the position to perform shading correction and storing the shading correction plate, the sub-scanning method Read shading correction plate at a plurality of locations, means for comparing the shading correction execution result, the defect position specifying how to present the shading correction plate made.

(3) The means for comparing is for examining the disagreement between the two, and if there is a disagreement, it is considered that the shading correction plate is defective, and the means (1) or (2) is characterized. A method for locating defects existing on the shading correction plate.

(4) The means for comparing is to check by the magnitude of the difference between the two values, and if it is larger than a certain value, it is considered that the shading correction plate is defective. Alternatively, the defect position identifying method existing in the shading correction plate according to (2).

(5) When there are a predetermined number or more of consecutively judged defective spots, these are judged again as defective ones, and if they do not reach the predetermined number, they are not judged to be defective. The method for identifying a defect existing in the shading correction plate according to any one of (1) to (4) above.

(6) By a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally in the main scanning direction,
The correction plate (shading correction plate) that has a uniform reference density and has a length that can cover the entire area in the main scanning direction is read, and correction data is stored based on the output of each photoelectric conversion element. In the image reading apparatus equipped with a shading function for correcting the output of each photoelectric conversion element based on the correction data when reading, the main scanning line having no defect based on the defect position information existing on the shading correction plate. The method for removing the influence of a defect existing in the shading correction plate, wherein the center of the maximum area among the areas continuously existing in the sub-scanning direction is set as the creation position of the shading correction data.

(7) By a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally in the main scanning direction,
The correction plate (shading correction plate) that has a uniform reference density and has a length that can cover the entire area in the main scanning direction is read, and correction data is stored based on the output of each photoelectric conversion element. In the image reading apparatus having a shading function for correcting the output of each photoelectric conversion element based on the correction data when reading, the defect on all the main scanning lines is based on the defect position information existing in the shading correction plate. When there is a defect, the method for removing the influence of the defect existing in the shading correction plate is characterized in that the position having the smallest number of defects is set as the shading correction data creation position.

(8) In the method of removing the influence of defects existing in the shading correction plate, if there are a large number of positions where the number of defects is the smallest, the position where the defects are most at the edge of the shading correction plate is used for shading correction. The method for removing the influence of a defect existing in the shading correction plate according to (7), wherein the data creation position is set.

(9) In the method of removing the influence of defects existing on the shading correction plate, if there are many positions where the number of defects is the smallest, shading correction is performed on the positions where the defects are only within a predetermined area. The method for removing the influence of a defect existing in the shading correction plate according to (7) above, wherein the effect data creation position is set.

(10) A line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally in the main scanning direction has a length capable of covering the entire main scanning direction with a uniform reference density. Plate (Shading correction plate)
Image reading with a shading function that stores the correction data based on the output of each photoelectric conversion element and corrects the output of each photoelectric conversion element based on the correction data when reading the image of the document. In the device, based on the defect position information existing in the shading correction plate,
When defects are present on all main scanning lines, the effect of defects existing on the shading correction plate is characterized in that the position where the defects exist only within a predetermined area is used as the shading correction data creation position. Removal method.

[0030]

FIG. 7 shows a shading correction plate 11.
It is the figure which looked at from the line sensor 20 side. In the figure, 22 indicates a home position for reading the white plate to obtain the shading correction data, and 21 indicates a defect such as dust. Conventionally, a predetermined home position 2
The shading correction data was obtained at the position 2. In the present invention, by reading the shading correction plate at a plurality of locations, the position of the defect on the shading correction plate is specified, and the data for shading correction is obtained at the position where there is no defect, thereby affecting the defects included in the shading correction plate. Have been removed. Hereinafter, the method of the present invention will be described in detail.

First, a method of identifying a defect position will be described.

First, the shading correction plate is read at a position in the sub-scanning direction, and the read data and the position in the sub-scanning direction are stored in the memory in the shading correction circuit. Next, by driving the reading unit with a motor, the shading correction plate is read at a position different from the position in the sub-scanning direction that was read earlier, and compared with the reading data of the shading correction plate stored in the memory. The smaller one is stored in the memory in the shading correction circuit as the read data of the shading correction plate. At this time, the position in the sub-scanning direction is also stored in the memory. Then, the shading correction plate is displaced to a position in the sub-scanning direction which has not been read yet, and the shading correction plate 1
The main scanning line segment is read, the obtained read data of the shading correction plate is compared with the read data of the shading correction plate stored in the memory, and the smaller value is used as the read data of the shading correction plate in the shading correction circuit. Stored in memory. Similarly,
Scan the shading correction plate at different positions in the sub-scanning direction, compare the read data of the obtained shading correction plate with the read data of the shading correction plate stored in memory one after another, and perform shading correction for the smaller value. It is stored in the memory in the circuit. Then, all the positions in the sub-scanning direction where the shading correction plate is read are stored in the memory. After reading the shading correction plate, shading correction data is created based on the read data of the shading correction plate stored in the memory. FIG. 8 shows a state of actually read data of the correction plate and a state of finally obtained shading correction data. In the example of FIG. 8, A, B,
I am reading the shading correction plate at three points, C. If there is no defect on the reading line like A, read data that draws a continuous curve can be obtained, but if there is a defect on the reading line like B or C, the read data corresponding to the position where the defect exists deteriorates. However, it becomes discontinuous. Then, since the read data of the shading correction plate is configured to take the minimum value of the read data, the read data of the shading correction plate stored in the memory is as shown in FIG. 8-D. The shading correction data created based on FIG. 8-D is as shown in FIG. 8-E.

In the above, the shading correction data is obtained from the reading data of the shading correction plate at a plurality of positions in the sub-scanning direction. Each time the shading correction plate is read, the shading correction data is created and stored in the memory. The larger one than the existing shading correction data may be sequentially stored in the memory as new shading correction data.

Next, the shading correction data stored in the memory is used to perform the shading correction at all positions in the sub-scanning direction stored in the memory. Less than,
Details will be given. First, it moves to one of the positions in the sub-scanning direction stored in the memory. Read the shading correction plate at that position, perform shading correction using the shading correction data stored in the memory,
The result is stored in memory. Next, it moves to another position in the sub-scanning direction stored in the memory. The shading correction plate is read again at that position, and the shading correction is performed using the shading correction data stored in the memory. The result and the shading correction result stored in the memory are compared for each pixel of the CCD, and the larger one is stored in the memory as a result of newly performing the shading correction. Then, the shading correction plate is read by moving to a position in the sub-scanning direction stored in the memory where shading correction has not yet been performed, and correction is performed using the shading correction data stored in the memory. Then, the obtained result and the result stored in the memory are compared for each pixel of the CCD,
The larger one is newly stored in the memory as the result of shading correction. This is performed for all the positions in the sub-scanning direction stored in the memory. FIG. 9 shows the result of the actual shading correction and the final result of the shading correction. Figure 9
In the above example, the shading correction is performed at three points A, B, and C, which are the same as in FIG. When there is no defect on the read line as in A, the output is larger than the output that should be originally output at the location in the main scanning direction where the defect exists due to the influence of the defect. If there is a defect on the reading line such as B or C, the output at the position where the defect exists is the data for shading correction affected by the defect, so the original output is obtained and the output is higher than others. There is nothing to do. Then, since the shading correction result is configured to take the maximum value of the shading correction data, the shading correction result stored in the memory is as shown in FIG. 9-D.

Finally, by comparing the result obtained by performing the shading correction again using the shading correction data stored in the memory with the result after the shading correction stored in the memory, the defect Identify the location. The details will be described below. First, it moves to one of the positions in the sub-scanning direction stored in the memory. The shading correction plate is read at that position, and the shading correction is performed using the shading correction data stored in the memory. The result and the shading correction execution result stored in the memory are compared for each CCD pixel, and if they match, there is no defect at the position in the sub-scanning direction. If there is a disagreement, the defective position on the shading correction plate is specified by the CCD pixel position and the position in the sub-scanning direction. Actually, it is rare that they completely match each other. Therefore, it may be determined whether the difference between the obtained result of the shading correction and the result of the shading correction stored in the memory is equal to or more than a threshold value. If the threshold value is exceeded, it is a defect. Further, depending on the size of the defect, it may hardly affect the read image. Therefore, even if the pixels determined to be defective are continuous and are larger than a predetermined number of pixels, it may be determined to be defective. Good. Next, move to another position in the sub-scanning direction stored in the memory, repeat the same process, and perform it for all positions in the sub-scanning direction stored in the memory. It is possible to specify the position of the defect existing in the. The defect position specifying operation will be described with reference to FIG. 9 as an example. When the shading correction plate is read by moving to the first position in the sub-scanning direction, for example, A, and the shading correction is performed using the shading correction data stored in the memory, the result of FIG. 9A is obtained. When this is compared with the result of the shading correction execution stored in the memory, there is a match, so there is no defect at the position in the sub-scanning direction A. Next, when the shading correction plate is moved to the position of B and the shading correction is performed using the shading correction data stored in the memory, the output of the part α is affected by the defect and the It will be the same as the data of the part without defects. This is the state of FIG. 9-B. Comparing this with Figure 9-D,
Since the mismatch occurs at the position in the main scanning direction α, the defect position on the shading correction plate can be specified as the position α in the main scanning direction and the position B in the sub scanning direction. Finally, when the shading correction plate is read by moving to the position in the sub-scanning direction C and the shading correction is performed using the shading correction data stored in the memory, the result is as shown in FIG. 9C. By comparing this with FIG. 9-D, the position of the defect on the shading correction plate can be specified as the position β in the main scanning direction and the position C in the sub scanning direction. Here, the data after the shading correction and the data stored in the memory have been described by performing a simple comparison, but as described above, it is rare that they completely match, so there is a difference between the two. It may be judged as a defect when it is equal to or more than a predetermined value. Further, since the read image may not be affected depending on the size of the defect, it may be determined that the pixel is determined to be defective only when the pixels determined to be defective are consecutive and the number of pixels exceeds a predetermined number.

The above is the method for identifying the defect position on the shading correction plate.

Next, a method for avoiding the influence of defects will be described.

The position for shading correction may be selected in the sub-scanning direction where there is no defect in the shading correction plate, but here, with a certain margin, the center of the region where there is no defect in the shading correction plate is selected. It is the position where shading correction is performed. Below, FIG.
To explain in detail. A, B, C, and D in FIG. 10 are positions in the sub-scanning direction that are recognized as defects as a result of specifying the defect positions on the shading correction plate. First, these positions are rearranged in ascending or descending order. Assuming that the data items are sorted in ascending order in FIG. 10, they are C, A, D, and B. Next, the two positions in the sub-scanning direction that maximize the difference between the positions in the adjacent sub-scanning direction are obtained. In FIG. 10, it becomes D and B. Then, finally, the center of the two positions in the sub-scanning direction (FIG. 10-E) is determined as the position where shading correction is performed.

The above description is based on the assumption that there is a position in the sub-scanning direction where there is no defect, but it is possible that the position in the sub-scanning direction where there is no defect cannot be found due to changes over time. At this time, the influence of this defect appears in the read image, but it is desired that it be as inconspicuous as possible. Therefore, the influence of the defect existing on the shading correction plate can be minimized by selecting the position having the smallest number on the reading position. Also, in advance, set an area that can soften the impression when looking at the image even if the influence of the defect appears.
It is possible to select a position having a defect in the area.
Furthermore, when there are many positions with a small number, the defect is moved to the edge of the image by selecting the position where the defect is located at the edge of the shading correction plate most so that the impression when viewing the image is softened. You can also

[0040]

As described above, according to the present invention, since no special device is added, a defect existing on the shading correction plate can be detected at a low cost, and a portion having no defect can be set as a shading correction position. By selecting, it is possible to remove image unevenness due to defects. This method can be configured with a simpler algorithm than the conventional method,
Since the defect detection of the shading correction plate is not performed immediately before reading the image, it is possible to maintain the same image reading time as the conventional device. If there is a defect, the effect of the defect can be mitigated by selecting a position where the effect is not noticeable as the shading correction position.

[Brief description of drawings]

FIG. 1 Configuration of image reading device

FIG. 2 Configuration of image reading device

[Figure 3] Purpose of shading correction

FIG. 4 Configuration of shading correction circuit

FIG. 5 is an explanatory diagram of shading correction when a defect is included.

FIG. 6 is an example of correction with shading correction data including a defect.

FIG. 7 Shading correction plate

FIG. 8: Reading data of the shading correction plate and correction data

FIG. 9: Data after shading correction

FIG. 10 Shading correction plate

[Explanation of symbols]

10 Platen glass 11 Shading correction plate 12 manuscripts 13 Reading unit 14 lenses 15 Lighting 20 line sensor 21 Defects included in the shading correction plate 22 Home Position 62 motor

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 5B047 AA01 BA02 BB02 CA02 CA12                       DA04 DC06                 5C072 AA01 BA08 DA12 EA05 FB12                       LA15 MB04 RA16 UA02                 5C077 LL04 MM03 PP06 PP43 PP58                       PQ20 PQ22

Claims (10)

[Claims] 1. A correction plate having a length capable of covering the entire main scanning direction with a uniform reference density by a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally along the main scanning direction. A shading function that reads a (shading correction plate), stores correction data based on the output of each photoelectric conversion element, and corrects the output of each photoelectric conversion element based on the correction data when reading an image of a document. In the image reading apparatus provided with, means for reading the shading correction plate at the plurality of positions in the sub-scanning direction by the line sensor, and creating and storing the shading correction data from the minimum value of the output of each photoelectric conversion element,
Means for performing shading correction by reading the shading correction plate at a plurality of positions in the sub-scanning direction using the data for shading correction, reading the shading correction plate at a plurality of positions in the sub-scanning direction, Means for comparing with the shading correction execution result,
A method for locating defects existing in a shading correction plate. 2. A correction plate having a length capable of covering the entire main scanning direction with a uniform reference density by a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally along the main scanning direction. A shading function that reads a (shading correction plate), stores correction data based on the output of each photoelectric conversion element, and corrects the output of each photoelectric conversion element based on the correction data when reading an image of a document. In the image reading apparatus equipped with, the shading correction plate is read by the line sensor at a plurality of positions in the sub-scanning direction, shading correction data is created, and the maximum value of the correction data corresponding to each photoelectric conversion element is used. A means for creating and storing data for shading correction, a plurality of data in the sub-scanning direction using the data for shading correction. Means for reading the shading correction plate at the position of
A method for locating a defect existing in a shading correction plate, comprising means for reading the shading correction plate at a plurality of positions in the sub-scanning direction and comparing the shading correction execution result. 3. The shading correction plate according to claim 1, wherein the comparing means is for examining a mismatch between the two, and if there is a mismatch, the shading correction plate is considered to be defective. Existing defect location method. 4. The comparing means is to check by the magnitude of the difference between the two values, and if it is larger than a certain value, it is considered that the shading correction plate is defective. A method for locating a defect existing in the shading correction plate according to the item 1. 5. When there are a predetermined number or more of consecutive defective spots, these spots are again judged as defective,
5. The defect position identifying method existing in the shading correction plate according to any one of claims 1 to 4, wherein if the number of defects is less than a predetermined number, it is not judged as a defect. 6. A correction plate having a length capable of covering the entire main scanning direction with a uniform reference density by a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally along the main scanning direction. A shading function that reads a (shading correction plate), stores correction data based on the output of each photoelectric conversion element, and corrects the output of each photoelectric conversion element based on the correction data when reading an image of a document. In the image reading apparatus equipped with the shading correction plate, based on the defect position information existing in the shading correction plate, the center of the maximum area of the areas where main scanning lines without defects are continuously present in the sub-scanning direction is used as shading correction data. A method for removing the influence of defects existing in the shading correction plate, which is characterized in that 7. A correction plate having a length capable of covering the entire main scanning direction with a uniform reference density by a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally along the main scanning direction. A shading function that reads a (shading correction plate), stores correction data based on the output of each photoelectric conversion element, and corrects the output of each photoelectric conversion element based on the correction data when reading an image of a document. In the image reading apparatus equipped with, based on the defect position information existing in the shading correction plate, when there are defects on all the main scanning lines, the place where the number of defects is the smallest is the shading correction data creation position. A method of removing the influence of a defect existing in a shading correction plate. 8. A method for removing the influence of a defect existing on a shading correction plate, wherein when there are many positions where the number of defects is the smallest, the position where the defect is located at the edge of the shading correction plate is the shading correction data. The method for removing the influence of a defect existing in the shading correction plate according to claim 7, wherein the method is the creation position. 9. A method for removing the influence of a defect existing in a shading correction plate, wherein when there are many positions where the number of defects is the smallest, the position where the defect exists only within a predetermined area is used for shading correction. The method for removing the influence of a defect existing in the shading correction plate according to claim 7, wherein the data creation position is set. 10. A correction plate having a length capable of covering the entire main scanning direction with a uniform reference density by a line sensor having a plurality of photoelectric conversion elements arranged one-dimensionally along the main scanning direction. A shading function that reads a (shading correction plate), stores correction data based on the output of each photoelectric conversion element, and corrects the output of each photoelectric conversion element based on the correction data when reading an image of a document. In the image reading apparatus equipped with the, if there is a defect on all the main scanning lines based on the defect position information existing on the shading correction plate, the position where the defect exists only within a predetermined area is used for the shading correction. A method for removing the influence of a defect existing in a shading correction plate, which is characterized in that it is used as a data creation position.