JP2003283825A - Image data correcting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a correcting method for image data associated with a shading correction method more accurate than a conventional shading method. <P>SOLUTION: In the image data correcting method, in a step S1, a white reference object is measured. In a step S2, a black reference object and/or a gray reference object is measured. In a step S3, a correction coefficient is operated for each coordinate of each pixel by using the measured white reference data and black and/or gray reference data. In a step S4, printed matter is measured and in a step S5, resulting image data of the printed matter are corrected based upon the correction coefficient. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image data correction method relating to shading correction of a printed matter measuring apparatus for picking up an image of a printed matter placed on a table having a relatively wide range.

[0002]

2. Description of the Related Art A printed matter measuring apparatus is known which measures a printing color or color density of a printed matter and feeds back the measured data to a printing apparatus. For example, Japanese Patent Laid-Open No. 2001-35
The printed matter measuring apparatus disclosed in Japanese Patent No. 3852 discloses a printed matter placed on a table by a two-dimensional image pickup device to collectively capture image data, image-processes the image data, and measures color density of the printed matter. It is a device to perform. In the printed matter measuring apparatus, a printed matter is illuminated by a flash light source that emits light in synchronization with the image pickup means to perform image pickup.

With such a printed matter measuring apparatus, it is extremely difficult to uniformly illuminate a relatively wide range of tables. Therefore, when the two-dimensional image pickup means collectively picks up an image of the printed matter, a reading error occurs due to the difference in the illuminance distribution on the table. Such differences in illuminance distribution,
Further, it is known to perform shading correction as a method for correcting variations in the sensitivity of each image sensor. For example, as a general shading correction, a white reference is read and the output value of each image sensor is corrected so that the result matches a known value.

[0004]

As a result of an experiment conducted by the present applicant, in the shading correction method as described above,
It was found to be insufficient for accurate color measurement, and it was found that there is a problem due to the influence of the illuminance distribution, especially in the halftones such as gray levels. For this reason, there is a problem in using it for printing and plate making work that requires strict color management.

It is an object of the present invention to improve the conventional shading method and to provide an image data correction method that can obtain a good imaging result by using a black and / or gray reference object.

[0006]

According to a first aspect of the present invention, there is provided an image data correction method for shading-correcting image data picked up by CCD image pickup means, which comprises a white reference object and a black and / or gray reference object. A preparatory imaging step of imaging the image, a correction coefficient computing step of computing a correction coefficient for each coordinate of each pixel using the white reference data and the black and / or gray reference data obtained in the preparatory imaging step, and the correction A correction step of correcting each pixel value of the image data to a corrected pixel value based on the coefficient.

According to a second aspect of the present invention, there is provided a printed matter measuring apparatus for collectively picking up an image of a printed matter placed on a table by an image pickup means and measuring the color density of the printed matter based on the obtained image data. An image data correction method for shading correction of image data obtained by imaging the printed matter, comprising: a first preparation imaging step of placing a white reference object on the table and imaging the white reference object; and the table. A second preparatory imaging step in which a black reference object and / or a gray reference object is placed on the black reference object and / or a gray reference object; and white reference data obtained in the first preparatory imaging step, Black reference data obtained in the second preparatory imaging step and /
Alternatively, using the gray reference data, a correction coefficient calculation step of obtaining a correction coefficient for each coordinate position of the imaging pixel, a printed matter imaging step of placing a printed matter on a table and imaging the printed matter, and the printed matter. A correction step of correcting the pixel value at each coordinate of the image data based on an arithmetic expression in which the correction coefficient is substituted.

A third aspect of the present invention is the image data correction method according to the second aspect, wherein the arithmetic expression is a linear expression or a quadratic expression for converting a pixel value before correction into a pixel value after correction. It is characterized by being an expression.

According to a fourth aspect of the present invention, there is provided the image data correction method according to the second or third aspect, wherein the printed matter image pickup step is performed a plurality of times to perform multiple averaging.

According to a fifth aspect of the present invention, there is provided the image data correction method according to any of the second to fourth aspects, wherein the image data obtained in the printed matter imaging step is filtered by a filter of a predetermined size for each pixel. It is characterized in that it is subjected to equivalent averaging or weighted averaging.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION [Description of printed matter measuring apparatus]
An embodiment of the present invention will be described with reference to the drawings. Figure 1
FIG. 1 is a perspective view showing an example of a printed matter measuring device according to the present invention.

In FIG. 1, a printed matter measuring apparatus includes a table 1 on which a printed matter (printing paper) is placed, and a suction means 2 (FIG. 3) for sucking and fixing the printed matter placed on the table 1.
), A stand 3 for supporting the table 1, illumination means 4 provided on the left and right of the table 1, an image pickup means 5 provided above the table 1, and a light-shielding roof member 6
And a control calculation means 7 for controlling the printed matter measuring apparatus.
And a touch panel 8 for operating this printed matter measuring apparatus.

The table 1 is a flat-table-shaped table member which has an area on which a relatively large printed material of A3 size or more can be placed and which is inclined by 0 to 20 degrees from the horizontal. 1 is provided with a suction plate 1a having suction holes 13 and suction grooves 14 (both are shown in FIG. 2) for sucking and holding a printed matter. The table 1 and the suction plate 1a are formed in a dark color so as not to be transparent even when a double-sided printed matter is placed thereon, and a fine uneven surface treatment such as matte treatment or satin treatment so as not to generate unnecessary reflected light. Is preferably applied. Although the suction plate 1a is provided on the table 1 in this embodiment, the suction groove 13 and the like may be directly processed on the table 1. Also, the suction plate 1a
Two positioning members 11 and 12 for positioning the printed matter are fixed on the top.

The suction means 2 will be described with reference to FIGS. 2 and 3. 2 is a top view of the table 1, and FIG. 3 is a piping diagram showing a piping path of the adsorption means 2.

As shown in FIG. 2, a plurality of suction holes 13 and a plurality of suction grooves 14 for sucking printed matter are formed on the surface of the suction plate 1a constituting the suction means 2. It should be noted that some of the suction holes 13 and the suction grooves 14 are denoted by reference numerals for the sake of clarity.

The suction holes 13 are divided into three groups corresponding to a plurality of areas a to c indicated by dotted lines so as to suck the areas corresponding to the size of the printed matter.
When distinguishing the suction holes of each set, they are denoted by 13a to 13b.

Of the areas a to c, the size of the area a is set so as not to exceed the minimum size in accordance with the size of the minimum size printed matter to be measured. The printed matter is positioned by the positioning members 11 and 12 with the lower left side of the table 1 as a positioning origin O.

As shown in FIG. 3, the suction holes 13a to 13c are connected to a vacuum pump 18 via a solenoid valve 15, a solenoid valve 16 and a solenoid valve 17, respectively. Further, a vacuum switch 19 which is turned on when a predetermined vacuum degree is reached is connected in the middle of the piping between the vacuum pump 18 and the solenoid valves 15 to 17. In this embodiment, the solenoid valves 15-1
The suction holes 13 to be evacuated can be switched by appropriately combining and opening and closing 7. As a result, the suction area can be set according to the size of the printed matter.

The suction grooves 14 are formed on the suction plate 1a so as to communicate with any of the suction holes 13, respectively. Further, the suction groove 14 includes a main suction groove 14a (one of which is indicated by a thick line in the drawing) which is formed substantially along a diagonal line of the suction plate 1a, and a plurality of sub-suctions branched from the main suction groove 14a. Groove 14b
It consists of and. Note that some sub suction grooves 14b
Is directly branched from the main suction groove 14a and further branched from the sub suction groove 14b, but here, it is divided as the same sub suction groove 14b.

The main suction groove 14a extends from a corner of the suction plate 1a where the positioning origin O is located to a diagonal corner so that the main suction groove 14a is located approximately on a diagonal line. On the other hand, the sub suction groove 14b is branched from the main suction groove 14a with a spread of about 60 to 20 degrees. This is because the vacuum exhaust expands to the left and right around the diagonal line. Further, it is preferable that the distance between the sub suction grooves 14b is set to about 100 mm or less so that air is not easily accumulated. Further, both of the suction grooves 14a and 14b reach the right side or the upper side of the table 1, respectively.
It is in a state of being open to the end side of a.

Returning to FIG. 1, the pedestal 3 is a casing for supporting the table 1, and inside thereof, electrical components such as the control calculation means 7 and the vacuum pump 18 are stored. Further, the front side of the gantry 3 has a drawer (not shown) for inserting various preliminary members.

Next, the structure of the illumination means 4 will be described with reference to FIGS. 4 is a schematic view showing a cross section of the illumination means 4 as seen from the front of the device, and FIG. 5 is one illumination means 4
FIG. 3 is a perspective view of the table 1 as viewed obliquely from above, and is partially transparent and omitted for clarity.

As shown in the figure, the illumination means 4 has a flash light source 20 provided below the table 1, a slit member 21 for cutting excess light, and an illumination light emitted from the flash light source 20 on the surface of the table 1. Parabolic mirror 22 for folding back
And a cover 23.

Two flash light sources 20 are provided on the lower surface of the table 1, and are arranged so that their light emitting directions are directed to the outside of the table 1. The flash light source 20 has a problem that the illuminance becomes non-uniform due to the shape of the light emitting portion. This is because, for example, when a U-shaped arc tube is built in as the flash light source 20, the distribution of the emitted light is biased depending on the installation direction of the U-tube. In the present invention, in order to correct the non-uniformity of the illuminance due to the shape of the light emitting portion, the light emitting portion of the flash light source 20 is covered with the semi-cylindrical diffuser member 24. The light scattering member 24 is formed of, for example, a transparent resin material having a scattering layer that scatters light.

The slit member 21 is a light-shielding plate having an opening at its substantially central portion and shields extra light emitted from the flash light source 20, especially light reflected by the light-scattering member 24 in an unnecessary direction. Is. That is, the slit member 2
1 can give a desired directivity to the light passing through the opening. In this embodiment, the opening has an elongated trapezoidal slit shape extending in the front-rear direction of the apparatus in order to make the illuminance distribution uniform.

The parabolic mirror 22 is a mirror member having a parabolic surface which is curved so as to irradiate light uniformly on the surface of the table 1, and the illumination light emitted from the flash light source 20 is reflected on the surface of the table 1. It is a means for folding back and guiding light. The height and angle of setting the parabolic mirror 22 are set so that the illumination light with which the table is illuminated has a shallow angle with respect to the table. For example, the incident angle α formed by the ray axis (the arrow in the figure) of the illumination light from the parabolic mirror 22 toward the table center and the perpendicular to the table is 2α >> θ, where θ is the angle of view of the image pickup means 5. To be large enough.

The parabolic mirror 22, as shown in FIG.
A mirror surface support 22a having a parabolic surface curved along the front-rear direction of the apparatus, a mirror surface 22b adhered to the parabolic surface of the mirror surface support 22a, and a pair for holding the mirror surface support 22a in the cover 22. Holding means 22c.

The mirror surface support 22a is a three-dimensional object made of, for example, styrofoam, which can be easily processed.
2b is formed by attaching a metal tape having a high reflectance to the parabolic surface. Of course, one flexible reflecting mirror may be curved and held, but the above configuration has an advantage that a paraboloid having an arbitrary shape can be easily formed.

The cover 23 is a light-shielding cover for preventing the illumination light emitted from the flash light source 20 from leaking out of the apparatus, and the transparent plate 25 is provided only at a position facing the table 1. An ND filter 26 is attached to the translucent plate 25 for adjusting the amount of light for making the illuminance distribution uniform.

As shown in FIG. 6 (A), the ND filter 26 decreases the light quantity in the vicinity of the table as compared with the light quantity in the central part of the table which is remote, so that the density temporarily or gradually decreases from the top to the bottom. It is a filter plate with a concentration gradient so as to increase gradually. Note that FIG. 6B is a graph showing the amount of change in the transmittance of the illumination light due to the density gradient.
As shown in the figure, the ND filter 26 has the transmission plate 25.
On the other hand, when it is attached, the transmittance is set to decrease temporarily from top to bottom due to the density gradient of the ND filter 26. This ND filter 26 is a transmission plate 2
It may be provided, for example, in the opening of the slit member or the like, without being attached to 5.

Returning to FIG. 1, the image pickup means 5 is a digital camera constructed so that incident light is divided into RGB three primary colors by a dichroic mirror and each of them is received by a separate CCD array. That is, from printed matter to RG
Image data can be obtained for each color plane of B.
The image pickup means 5 is fixed downwardly toward the table 1 by a pillar 24 supported by the gantry 3, and picks up an image of the printed matter through an opening (not shown) provided in the light-shielding roof member 6.

The light-shielding roof member 6 is a light-shielding member that is supported by the pedestal 3 and is supported by two pillars 25 extending on the table 1 with its upper end curved, and has a shape curved in the front-rear direction of the apparatus. This light-shielding roof member 6 is a table 1
It is provided so as to block the light that is specularly reflected and incident on, for example, the light installed on the ceiling of the room.

The control calculation means 7 comprises a microcomputer and is stored in the gantry 3. The control calculation means 7 controls the entire printed matter measuring apparatus and the image pickup means 5.
It is used for the calculation of the image data obtained by the above, and is connected to each part of the printed matter measuring apparatus by a cable (not shown).

The touch panel 8 is a liquid crystal monitor having a pressure-sensitive input function, and functions as an input unit for operating the printed matter measuring apparatus and a display unit for displaying the calculation result. Of course, instead of the touch panel 8, a CRT
The display means such as and the input means such as the keyboard may be used in combination.

In the printed matter measuring apparatus, first, the printed matter is placed on the suction plate 1a and suction-held. Then, the flash light source 20 is turned on, and in synchronization with this, the image pickup means 5 picks up an image of the printed matter. The captured image data is subjected to predetermined image processing to calculate the color density of the printed matter.

[Explanation of Image Data Correction Method] Next, a method for correcting image data in the printed matter measuring apparatus will be described. FIG. 7 is a flowchart showing the image data correction procedure in the present invention.

First, in step S1, a white reference object, for example, a blank printing paper is placed on the suction table,
The image is taken by the imaging means 5. Similarly, in step S2, a black reference object or a gray reference object is placed and imaging is performed in the same manner. FIG. 8 is a graph showing the distribution of CCD output values (only the R image of RGB is displayed) when the white reference object and the gray reference object (halftone dot area 60%) are imaged. In FIG. 8, the vertical axis represents the CCD output value, and the horizontal axis represents the left and right positions on the table. That is, the numerical value on the horizontal axis is the distance (mm) from the left end of the table. In the printed matter measuring apparatus of the present embodiment, although the illuminance distribution on the table is increased to about 80%, the difference in illuminance distribution and the optical shot noise, even when the reference object having the same density is imaged as shown in the figure, A measurement error occurs due to disturbance noise such as amplifier noise and variation in sensitivity of each CCD pixel.

In the case of shading correction for a general white reference object, first, the CCD output value for the white reference object in FIG. 8 is obtained for each pixel or each table coordinate, and this output value is used as the reference CCD output value ( A value obtained by dividing by the ideal CCD output value corresponding to the color of the white reference object is obtained as the correction coefficient. Then, the shading correction can be performed by multiplying the measurement result by the correction coefficient.

FIG. 9 shows an example in which such white shading correction is performed. The vertical axis of FIG. 9 represents the CCD output value corrected by the conventional white shading correction. As shown in the figure, the measurement result of the white reference material is relatively well corrected, but there is a problem that it cannot be completely corrected at the gray level of 50% which is a halftone.

As a result of an experiment conducted by the applicant, the relationship between the actually measured output value of the CCD and the corrected CCD output value is a linear approximation y.
= G (x + f) or a quadratic approximation y = ax ² + bx + c
It turns out that can be approximated by. In the present invention, in order to derive this approximate expression, the correction coefficients g to f or a to c are obtained by using a black reference object and / or a gray reference object in addition to the white reference object. Hereinafter, it is referred to as black and white shading correction in distinction from the conventional white shading correction.

The reference material used for the gray reference material is R
As long as it has a halftone in each value of GB,
Although different color reference materials may be used for each of RGB, it is preferable to use a gray color in which three colors are mixed, more simply.

Returning to FIG. 7, in step S3, a correction coefficient for performing black and white shading correction is calculated. The case where the first-order approximation formula and the second-order approximation formula are used as this method will be described respectively.

<Example Using First-Order Approximation Formula> In this example, assuming that the corrected CCD output value is E and the uncorrected CCD output value is D, the following relation is established by the first-order approximation formula. Here, using the correction factors g and f, E = g · (D + f) ··· (1).

Since the correction coefficients g and f in this case are estimated by the gain amount and the offset amount of the black reference data when the white reference data is used as a reference, they are as follows.

G = ([black reference data] + f) / ([white reference data] + f) ... (2) f = (k. [White reference data]-[black reference data]) / (1-k) .. (3) However, k is a coefficient determined from a theoretical output value range of the CCD element, and in this embodiment, for example, k = 2048 gradation / 52428 gradation = 0.03906.

In step S3, the white reference data and the black reference data are substituted into the equations (2) and (3) for each pixel position to obtain the correction coefficients g and f. As a result, the arithmetic expression (1) used for correcting the image data is obtained. A gray reference material may be used instead of the black reference material. (However, in this case, it is necessary to set the value of the coefficient k in correspondence with the gray density.)

<Example of using quadratic approximation formula> In this example, assuming that the corrected CCD output value is E and the uncorrected CCD output value is D, the following relation is established by the quadratic approximation formula. To do. Here, it is assumed that E = a · D ² + b · D + c ··· (4) using the correction coefficients a to c.

When using the above-described quadratic approximation formula, at least two types of reference objects, for example, a black reference object and a gray reference object are imaged in step S2. As a result, white reference data, black reference data, and gray reference data can be obtained. For example, the white reference data before correction is W ₀ ,
The corrected white reference data is W ₁ , the black reference data before the correction is K ₀ , the black reference data after the correction is K ₁ , the gray reference data before the correction is G ₀ , and the gray reference data after the correction is W _1. If G ₁ is given, W ₁ = a · W ₀ ² + b · W ₀ + c K ₁ = a · K ₀ ² + b · K ₀ + c G ₁ = a · G ₀ ² + b · G ₀ + c is obtained. The correction coefficients a, b, and c can be obtained by solving the above in a simultaneous manner. As a result, the arithmetic expression (4) used for correcting the image data is obtained. It should be noted that a gray reference material having another density may be used instead of the black reference material.

In step S3, if the correction coefficient according to the above equation (1) or (4) is obtained, it is stored for each coordinate and for each RGB color, and the preparation before measurement is completed. To do. Then, in step S4, the actual printed matter is imaged, and in step S5, the obtained image data is corrected for each coordinate value by using the arithmetic expression (1) or (4).

FIG. 10 is a graph showing the result of shading correction by the quadratic approximation formula. As shown in the figure, it can be seen that not only the white reference data but also the gray reference data of the halftone is corrected well.

[Other Embodiments] 1. In the above-described embodiment, the number of times the printed matter is measured is not limited, but by performing the measurement a plurality of times, it is possible to reduce variations in the measurement results due to disturbance noise and the like. For example, FIG. 11 is a graph showing variations in measurement results when the black and white shading correction is used. In this graph, a gray reference material having a halftone dot area of about 60% is measured a plurality of times, and the variation (difference between the maximum output value and the minimum output value) for each white level is plotted for each measurement.

On the other hand, the printed matter was measured three times,
When the corrected CCD output values obtained in each case were subjected to multiple averaging, the variation was reduced as shown in the figure. By thus multiplexing the number of measurements, it is possible to reduce the influence of disturbance noise and the like. It should be noted that the multiplexing of the number of measurements is effective if it is performed twice or more.

2. In the above embodiment, the image data is corrected for each pixel. Also at this time, an error occurs due to the influence of noise between each pixel. Such an error can be reduced by equivalently averaging the image data with a 3 × 3 filter in advance. 12
Represents the dispersion of the measurement results when the image data before correction is subjected to 3 × 3 equivalent average filter processing (that is, the value of a predetermined pixel is calculated as an average of 9 pixels around 3 × 3). Also in this case, it can be seen that the variation in the measurement result is reduced. The filter is not limited to 3 × 3, and weighted averaging may be performed instead of equivalent averaging.

3. In the above embodiment, the first-order approximation formula or the second-order approximation formula is used, but another approximation formula may be used. For example, the interpolation accuracy can be improved more than the quadratic approximation formula by using a monotonically increasing approximation formula such as a power function shown below. In the following equation, E is the corrected CCD output value, D is the uncorrected CCD output value, and n, m and p are the correction coefficients. E = n · D ^m + p ··· (5)

In this case, three types of reference objects of white, black and gray are imaged and, for example, the white reference data before correction is W ₀ ,
The corrected white reference data is W ₁ , the black reference data before the correction is K ₀ , the black reference data after the correction is K ₁ , the gray reference data before the correction is G ₀ , and the gray reference data after the correction is W _1. If G _{1 is set} , a simultaneous equation such as W ₁ = n · W ₀ ^m + p K ₁ = n · K ₀ ^m + p G ₁ = n · G ₀ ^m + p is obtained. Since the simultaneous equations are non-linear, the correction coefficients n, m, and p can be obtained by a convergence calculation such as Newton's method, but there is a drawback that the processing time required for the calculation increases.

4. In the above embodiment, each correction coefficient is calculated for each coordinate unit of each pixel, but may be calculated for each predetermined area unit.

5. The present invention can be implemented not only by a batch imaging type printed matter measuring apparatus using a table, but also by a scanning type printed matter measuring apparatus that scans and moves a printed matter and an imaging unit relatively.

[0058]

According to the present invention, more accurate shading correction can be performed as compared with the conventional white shading correction, and it is particularly suitable for a printing plate making operation in which color management is severe.

[Brief description of drawings]

FIG. 1 is a perspective view showing an example of a printed matter measuring apparatus including a flash lighting device according to the present invention.

FIG. 2 is a top view showing an arrangement of suction holes and suction grooves on a suction plate.

FIG. 3 is a piping diagram showing a piping path of an adsorption means.

FIG. 4 is a cross-sectional view showing a schematic configuration of illumination means.

FIG. 5 is a perspective view in which a part of the illuminating means is seen through.

FIG. 6 is a diagram showing an example of an ND filter and a graph showing its transmittance.

FIG. 7 is a flowchart of an image data correction method according to the present invention.

FIG. 8 is an example of measurement data when a white reference object and a gray reference object are read.

FIG. 9 is an example of measurement data when a white reference object and a gray reference object are read.

FIG. 10 is an example of measurement data when conventional white shading correction is performed.

FIG. 11 is an example of measurement data when black-and-white shading correction according to the present invention is performed.

FIG. 12 is a data example showing variations in measurement data when the number of measurements is increased and a multiple average is taken.

FIG. 13 is a table of data showing variations in measurement data when averaging processing is performed by filter processing.

[Explanation of symbols]

1 table 2 Adsorption means 3 mounts 4 Lighting means 5 Imaging means 6 light-shielding roof material 7 Control calculation means 20 flash light source 21 Slit member 22 Parabolic mirror 23 cover 24 Diffuser 26 ND filter

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kiyotada Amemori             4-chome Tenjin, which runs up to Teranouchi, Horikawa-dori, Kamigyo-ku, Kyoto             1 Kitamachi No. 1 Dai Nippon Screen Manufacturing Co., Ltd.             Inside the company (72) Inventor Shigeo Murakami             4-chome Tenjin, which runs up to Teranouchi, Horikawa-dori, Kamigyo-ku, Kyoto             1 Kitamachi No. 1 Dai Nippon Screen Manufacturing Co., Ltd.             Inside the company (72) Inventor Kyoko Kawada             4-chome Tenjin, which runs up to Teranouchi, Horikawa-dori, Kamigyo-ku, Kyoto             1 Kitamachi No. 1 Dai Nippon Screen Manufacturing Co., Ltd.             Inside the company F term (reference) 5B047 AA01 AB04 BA02 BB03 BC07                       BC09 BC12 BC14 BC23 CB04                       CB22 DA04 DC01 DC06                 5C072 AA01 BA08 CA02 CA09 DA06                       DA09 EA05 FA07 FA08 FB12                       FB18 LA12 RA16 UA02 UA17                 5C077 LL04 MM03 MM27 PP06 PP44                       PP45 PP46 PQ12 PQ18 SS01

Claims (5)

[Claims] 1. An image data correction method for shading-correcting image data picked up by a CCD image pickup means, comprising: a preparatory image pickup step for picking up a white reference object and a black and / or gray reference object; Obtained white reference data and black and /
Alternatively, a correction coefficient calculation step of calculating a correction coefficient for each pixel coordinate using gray reference data, and a correction step of correcting each pixel value of the image data to a corrected pixel value based on the correction coefficient, Image data correction method consisting of. 2. A printed matter measuring apparatus that collectively picks up an image of a printed matter placed on a table by an imaging means and measures the color density of the printed matter based on the obtained image data. An image data correction method for shading-correcting the image data, wherein a white reference object is placed on the table and a white reference object is imaged; Alternatively, a second preparatory imaging step for placing a gray reference object and imaging the black reference object and / or the gray reference object; white reference data obtained in the first preparatory imaging step;
A correction coefficient calculation step for obtaining a correction coefficient for each coordinate position of the image pickup pixel using the black reference data and / or gray reference data obtained in the preparatory imaging step, and placing the printed material on the table An image data correction method comprising: a printed matter image pickup step of picking up an image of the printed matter; 3. The image data correction method according to claim 2, wherein the arithmetic expression is a linear expression or a quadratic expression for converting a pixel value before correction into a pixel value after correction. 4. The image data correction method according to claim 2, wherein the printed matter imaging step is performed a plurality of times to perform multiple averaging. 5. The image data according to claim 2, wherein the image data obtained in the printed matter imaging step is subjected to equivalent averaging or weighted averaging by a filter of a predetermined size for each pixel. Correction method.