JPH08256231A - Stain identifying device for printed matter - Google Patents

Abstract

PURPOSE: To provide the stain identifying device of a printed matter capable of accurately identifying even stains localized on either one of the surfaces of the printed matter. CONSTITUTION: The respective picture data of the entire front and back surfaces of the printed matter P are respectively inputted by picture input parts 1 and 6, specified area extraction parts 3 and 8 respectively extract the picture data inside specified areas from the inputted respective picture data, feature amount extraction parts 4 and 9 respectively extract physical feature amounts for the extracted picture data inside the respective specified areas and stain judgement parts 5 and 10 judge the respective stains at the front and back of the printed matter P by comparing the extracted respective feature amounts with a reference value set beforehand. Then, a front and back evaluation part 11 compares the respective judged results of the stain judgement parts 5 and 10 and selectively outputs one of the judged results as a final stain identified result for the printed matter P based on the compared result.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention identifies a stain such as discoloration, creases, wrinkles, stains, print fading, holes, tears, and breaks in a printed matter inspection device for inspecting the printed state of a printed matter. The present invention relates to a stain identification device for printed matter.

[0002]

2. Description of the Related Art A conventional stain identification device for printed matter captures image data of one surface of a printed matter over the entire surface of the printed matter by an image input device using a television camera, a line sensor, or the like. The stain is identified by comparing the image data with preset reference data.

Specifically, for example, JP-A-60-17 is used.
As disclosed in Japanese Patent No. 6191, a method of binarizing a printing area and a non-printing area, accumulating extracted pixels, and comparing the difference with a reference value is considered.

Further, as disclosed in JP-A-59-57107 and JP-A-59-184807, physical quantities relating to stains are measured from image data of the front and back of a printed matter, and A method is considered in which each measurement result is added and the addition result is compared with preset reference data.

[0005]

Conventionally, as described above, as a method for identifying stains on a printed matter, a method for measuring a feature amount relating to stains on one side of the printed matter, or a stain on each side of the printed matter. A method has been performed in which each characteristic amount is measured and the sum of the measurement results on the front and back sides is used as the characteristic amount relating to stains on the printed matter, but they have the following problems.

With the method of measuring the characteristic amount relating to the stain only on one surface of the printed matter, when the other surface which is not measured is stained, the accurate stain cannot be identified.

Further, in the method of adding the physical quantities of the front and back of the printed matter and comparing with the reference value, there is a problem that the characteristic quantity cannot be reflected in the evaluation when either one is significantly soiled. Therefore, it is an object of the present invention to provide a stain identification device for printed matter, which can accurately identify stains that are biased on either surface of the printed matter.

[0008]

SUMMARY OF THE INVENTION A stain on a printed matter according to the present invention includes first image input means for inputting image data on one side of a target printed matter and image data on the other side of the printed matter. A specific area on one surface of the printed matter is extracted based on preset information from the second image inputting means for inputting and the image data input by the first image inputting means, and the extracted area is extracted. A specific area on the other surface of the printed matter based on preset information from the first specific area extracting means for extracting the image data in the specific area and the image data input by the second image input means. Second specific area extracting means for extracting image data in the extracted specific area, and image data in the specific area extracted by the first specific area extracting means. From the image data in the specific area extracted by the second specific area extracting means and the second characteristic area extracting means for extracting the characteristic value relating to the stain on one surface of the printed material from the other surface of the printed material. By comparing the feature amount extracted by the second feature amount extracting means for extracting the feature amount relating to the dirt and the first feature amount extracting means with a preset reference value,
The first stain determination means for determining stains on one surface of the printed matter and the feature amount extracted by the second feature amount extraction means are compared with a preset reference value to obtain the other of the printed matter. The second stain determining means for determining the stain on the surface of the surface is compared with the determination result of the first stain determining means and the determination result of the second stain determining means, and either one of them is determined based on the comparison result. And an evaluation unit that selects the determination result as the final stain identification result for the printed matter.

Further, the stain identification device for printed matter of the present invention is
The first image input means for inputting image data of one side of the target printed matter, the second image input means for inputting image data of the other side of the printed matter, and the first image input means. First specific area extracting means for extracting a specific area on one surface of the printed matter from the input image data based on preset information, and extracting image data in the extracted specific area; From the image data input by the second image input means, a specific area on the other surface of the printed matter is extracted based on preset information, and image data in the extracted specific area is extracted. Specific area extracting means, and the first
First feature amount extraction means for extracting a feature amount relating to stains on one surface of the printed matter from the image data in the particular region extracted by the particular region extraction means, and second feature area extraction means. Second feature amount extracting means for extracting a feature amount relating to stains on the other surface of the printed matter from the image data in the specific area, the feature amount extracted by the first feature amount extracting means, and the second feature amount extracting means. A feature quantity unifying unit that compares the feature quantity extracted by the feature quantity extracting unit and selects one of the feature quantities as the feature quantity for the printed matter based on the comparison result, and the feature quantity unifying unit selects the feature quantity unifying unit. A stain determination unit for determining stains on the printed matter by comparing the characteristic amount with a preset reference value is provided.

[0010]

The stain amount is similarly determined from the image data of the one side of the printed matter by extracting the feature amount related to the stain in the region corresponding to the feature and determining the stain amount and the image data of the other side. By evaluating the results together with the results, it is possible to accurately identify stains that are uneven on one surface of the printed matter.

Further, by evaluating the feature amount extracted on one side of the printed matter and the feature amount extracted on the other side together, similarly, stains unevenly distributed on one side of the printed matter can be obtained. Even if there is, it can be accurately identified.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. First, a first embodiment will be described. FIG. 1 is a schematic diagram of a printed matter stain (for example,
1 schematically shows a configuration of a stain detection device for a printed material for identifying (color change). That is, the printed matter P to be identified
One surface (front surface) of the entire surface is optically scanned and photoelectrically converted by the image input unit 1 including a photoelectric conversion unit such as a television camera or a line sensor, and image data of the entire surface is obtained. Is read, digitized and input. The image data of the surface of the printed matter P input by the image input unit 1 is stored in the image memory 2.

In the present embodiment, the extraction of the feature amount related to the dirt is performed by specifying the region according to the feature amount, but the specific region is limited by the specific region extraction unit 3. That is, the specific area extraction unit 3 extracts a specific area corresponding to the characteristic amount extracted from the image data of the entire surface of the printed matter P. The characteristic amount extraction unit 4 extracts a physical phenomenon of stains on the surface of the printed matter P as numerical data called a characteristic amount by performing predetermined image processing on the image data in the limited specific area.

Numerical data output from the feature quantity extraction unit 4 is input to the dirt determination unit 5, and the dirt condition on the surface of the input image, for example, 1 (most beautiful) to M.
One of the numerical values (most dirty) is output, and the dirt on the surface is determined.

Similarly, for the other surface (back surface) of the printed matter P, an image input unit 6, an image memory 7, a specific area extraction unit 8, a feature amount extraction unit 9, and an image memory unit 7 for determining stains. A dirt determination unit 10 is provided, and these are configured similarly to the image input unit 1, the image memory 2, the specific area extraction unit 3, the feature amount extraction unit 4, and the dirt determination unit 5.

The determination results of the front and back of the printed matter P by the stain determination units 5 and 10 are input to the front and back evaluation unit 11, respectively. The front / back surface evaluation unit 11 outputs a stain condition of the printed matter P, for example, a numerical value of any one of 1 to M based on a preset reference value, and a final stain determination on the printed matter P is performed.

The processing of each unit will be described in detail below. The image input unit 1 reads the image data of the entire surface of the printed matter P, and the image input unit 6 reads the image data of the entire surface of the printed matter P. It should be noted that here, the case where the photoelectric conversion unit in the image input unit is fixed and the image data is read by transporting the printed matter P will be described as an example. On the contrary, the printed matter P is fixed and the image input unit is fixed. The same applies when reading is performed by moving the photoelectric conversion unit.

2 and 3 show examples of the configurations of the image input units 1 and 2. That is, the printed matter P is conveyed in the direction of the arrow in the figure while being sandwiched between the two conveyor belts 21 and 22. A reference plate 23 is fixed to the lower portions of the conveyor belts 21 and 22 with respect to the drawing, and the printed matter P is conveyed at a substantially central portion on the reference plate 23.

Conveyor belts 21 and 2 corresponding to the reference plate 23
2, the reference plate 23 and a light source 24 for illuminating the conveyed printed matter P, and the reflected light from the reference plate 23 and the conveyed printed matter P are received and converted into electric signals. The photoelectric conversion unit 25 is provided, and the output of the photoelectric conversion unit 25 is input to the A / D converter 26.

Reflected light (or transmitted light) may be vertically input from the printed matter P to the light receiving surface of the photoelectric conversion unit 25. In this case, as shown in FIG. 3, the light source 24 is provided so as to prevent the angle θ from the straight line L connecting the printed matter P and the photoelectric conversion unit 25 from approaching 0 ° or 90 °. Install so that θ falls between 30 ° and 60 °. Although FIG. 3 illustrates a case where the transportation direction is illuminated from the traveling direction, the illumination may be performed from the rear side.

Here, regarding the role of the reference plate 23, FIG.
The printed matter P shown in FIG. The printed matter P shown in FIG. 4 is an image in which the character “A” is printed in the center of a blank sheet with ink other than white, and the specific area for extracting the characteristic amount is the shaded area shown in FIG. The central area of the image data.

In the background corresponding to the area for extracting the feature quantity,
A substance having a high reflectance is used for the central portion 23a of the reference plate 23. Further, the substance having a high reflectance must be a substance that is not easily soiled so that the ink or the like does not adhere to the reference plate 23 to reduce the reflectance when the printed matter P having the prints on the front and back is conveyed. That is, a substance having a high reflectance and less likely to be stained is installed on the back side of the printed material P to be conveyed.
Examples of this substance include opal and ceramic.

Further, the conveyor belts 21, 2 on the reference plate 23
The areas 27 and 28 located outside the print sheet P and corresponding to both ends in the direction orthogonal to the conveyance direction of the printed matter P are the printed matter P.
Use a color that has a high contrast with the paper. For example, in the case of white paper as shown in FIG. 4, it is black. This is because the position of the printed matter P can be easily determined when analyzing the image data.

The specific area extracting units 3 and 8 extract (cut out) a specific area using position information, density information and the like. FIG.
As an example of the printed matter P to be identified, the extraction method of the specific area will be specifically described.

First, from the read image data, the printed matter P
Extract the entire area. Here, since it is assumed that the printed matter P is being conveyed, when it is extracted at a fixed position, it may be impossible to correctly extract it due to uneven transportation. Therefore, in this embodiment, the area of the printed matter P is extracted based on the density information. Here, since the reference plate 23 is set so that the contrast between the paper color of the printed matter P and the background color is sufficiently high, it is obvious that the boundary between the printed matter P and the background can be easily extracted by setting the threshold value based on the density value. is there.

Next, a specific area for extracting the characteristic amount is extracted. As an example of the extraction method, a method of extracting a specific area using position information will be described. When extracting the shaded area in FIG. 5, that is, the central area of the image data, of the image data, this extraction area is an area limited by the distance from the center point s of the printed matter P. Center point s of printed matter P
Is an intersection of a straight line SL1 parallel to two parallel sides and having an equal distance from the two sides, and another straight line SL2 parallel to two parallel sides and having an equal distance from the two sides. A perpendicular line is drawn from the center point s of the printed matter P to each of the four sides of the printed matter P, and the inside of a rectangle surrounded by a line whose distance from the center point s is t is defined as an extraction area.

Next, a method of extracting the specific area using the density information will be described. Of the image data, the shaded area shown in FIG. 6A, that is, the print area of the image data is extracted. For this extraction area, the frequency distribution of the density values of the entire surface of the image data is obtained (see FIG. 6B), and the portion of the frequency distribution having a low density value, that is, the area corresponding to the density values H1 to H2 is extracted. It is a thing.

The density value H1 is, for example, the minimum value in the range of inputtable density values. As another method of determining the density value H1, there is also a method of setting the lowest density value among the density values of the entire surface of the captured image data as H1.

The density value H2 is obtained as follows. For a clean printed matter P without stains, for example, the density value of the boundary separating the printed area and the non-printed area is obtained by the "Otsu's discriminant analysis method" which is well known as a binarization method of a grayscale image. The density value at the boundary is H2.

The method for extracting the specific area is not limited to these, and it is of course possible to extract an area having a medium density value. The feature amount extraction units 4 and 9 extract feature amounts related to stains on the printed matter P from the image data in the specific region extracted by the specific region extraction units 3 and 8. Here, description will be made on the premise that the feature amount extraction units 4 and 9 basically perform the same process, but in the present embodiment, even if the feature amount extraction units 4 and 9 perform different processes, The effect is no different.

As the characteristic amount relating to the stain of the printed matter P,
For example, discoloration of printed matter, creases, wrinkles, stains, print fading, holes, tears, and breaks. Of these, in the present embodiment, the extraction method for the regions of FIG. 5 and FIG. 6A extracted by the specific region extraction units 3 and 8 will be taken as an example.

First, as one of the characteristic quantities relating to the stain of the printed matter P extracted from the area shown in FIG. 5, discoloration of the sheet of the printed matter P can be mentioned. This color change feature extraction is processed according to the flowchart shown in FIG. 7, for example. That is, a density histogram is created from the density value of each pixel of the image data of FIG. 5 (S1), the cumulative values of the adjacent density values are sequentially compared from the minimum density value, and the cumulative value of the density value d is the density value. The density value when it first falls below the cumulative value of the density value of d−1, that is, H3 shown in FIG. 6B is extracted as the first maximum point (S2).

Similarly, the cumulative values of the adjacent density values are sequentially compared from the maximum density value, and the cumulative value of the density value d is the density value d +
The density value when it is lower than the cumulative value of the density value of 1, that is, H4 shown in FIG. 6B is extracted as the second maximum point (S3). Next, the difference between the first maximum point H1 and the second maximum point H2 is calculated (S4), and the difference value is used as the color change feature amount.

The method of calculating the maximum point is not limited to the above method, and it is of course possible to obtain two peaks by differentiating the density histogram. Next, as one of the characteristic amounts relating to stains on another printed matter P extracted from the area shown in FIG. 6A, there is a blur of printing. The feature amount of the print blur is extracted according to the following equation, where v is the variance of the density values.

V = (ΣXi ² − ((ΣXi) ² / n)) / n where Xi: density value i: 1 to n n: total number of pixels Note that the feature quantity extraction method is not limited to these. For example, the holes are extracted by comparing the density value of the central portion 23a of the reference plate 23 existing on the back surface of the printed material P with the density value of the printed material P in the area of FIG. Of course, it is possible.

The dirt determination units 5 and 10 are feature amount extraction units 4 and 4, respectively.
Each feature amount extracted in 9 is input, and the numerical value corresponding to the stain corresponding to it is output. For example, as shown in FIG. 8, a numerical value corresponding to the degree of stain is set as a reference value in advance according to the extracted value (v) of the print faint, and this reference value is compared with the actually extracted extracted value. Therefore, the degree of dirt is determined.

The front and back evaluation section 11 inputs numerical values representing the degree of dirt on each of the front and back of the printed matter P judged by the dirt judging sections 5 and 10, and combines the judgment results of the front and back to obtain a numerical value indicating the overall degree of dirt. Output. For example, the rule of “selecting the dirty condition of the dirty one” is set in advance, and if the output value of the dirt determination unit 5 is more dirty than the output value of the dirt determination unit 10, the output value of the dirt determination unit 5 is , Which is output as the output value of the front / back evaluation unit 11 (that is, the final stain identification result).

The rule for evaluation in the front / back evaluation unit 11 may be, for example, "select the cleanest stain condition" or "select the average stain condition of two determination results".

In the above description, the case of identifying stains by extracting one feature amount has been described, but when identifying stains by extracting two or more feature amounts, for example, With the configuration shown in FIG. 9, it is possible to perform the same identification as in FIG. That is, a plurality (N) of feature quantity extraction units 41 to 4N and 9
1 to 9N are provided, and the feature amounts extracted by these are input to the dirt determination units 5 and 10, respectively, and dirt determination is performed.

As a method of judging this stain, for example,
As in the case of FIG. 8 described above, there are a method of presetting the degree of dirt for each of a plurality of feature amounts, a method of using a preset linear discriminant function, a method of using a neural network, and the like.

Further, when it is desired to change the limited specific area in accordance with the feature amount, an area extraction control signal is input to the area extraction control units 12 and 13 from a keyboard (not shown) or the like,
The area extraction control units 12 and 13 enable the specific area extraction units 3 and 8 to
It is possible to specify or change the specific area to be extracted by controlling the. As the area extraction control signal, for example, a method of setting a plurality of areas to be extracted in advance and inputting numerical values and characters corresponding to the areas, and directly inputting numerical information used for extraction such as position information and density information There are ways to do it.

When the process after the image data is input is exactly the same on the front and back of the printed matter P, the process shown in FIG.
0 or the configuration shown in FIG. 11 can also be identified as in FIG. 1 is a method for simultaneously performing front / back identification processing, while FIGS. 10 and 11 are methods for sequentially performing front / back identification processing.

That is, in FIG. 10, an image input section 1 on one side of the printed matter P and an image input section 6 on the other side are provided, and respective outputs of these are input to the front / back determination section 14, where the front / back side is determined. After determining which one of the image data, the identification process similar to that in FIG. 1 is sequentially performed. In this case, for example, only one system of the image memory 2, the specific area extraction unit 3, the feature amount extraction unit 4, and the stain determination unit 5 in FIG. 1 is used.

Here, the front / back determination unit 14 performs front / back determination by a control signal indicating either the front or back from the outside (not shown), performs image processing on the input image data, and performs the similarity method, for example. Use to determine the front and back.

The result determined by the dirt determination unit 5 is temporarily stored in the determination result storage unit 15 in correspondence with the determination result of the front / back determination unit 14. Then, when the front and back determination results are gathered in the determination result accumulating unit 15, the front and back determination results are input to the front and back evaluation unit 11 together with a number indicating the printed matter P,
The stains on the printed matter P are identified as in FIG.

FIG. 11 shows an example of the configuration when only one image input unit 1 is used, and the flow of processing is the same as in FIG. Therefore, in this case, the printed matter P is conveyed twice. That is, first, one side of the printed matter P is conveyed upward, and then it is inverted and the other side of the printed matter P is conveyed upward.

Next, the second embodiment will be described. FIG. 12 schematically shows the structure of a stain identification device for printed matter according to the second embodiment, and the features up to the feature quantity extraction units 4 and 9 are the same as those in the first embodiment. That is, the feature amount extraction unit 4,
When the feature values of the front and back of the printed matter P are extracted in 9,
The respective feature amounts are input to the feature amount integration unit 16. The feature amount integration unit 16 obtains a feature amount that reflects the features of each surface, inputs the feature amount to the stain determination unit 17, and determines the stain.

The case where the density value is used as the characteristic amount will be described below as an example. The printed matter P to be identified is shown in FIG. 4, and the area for extracting the characteristic amount is the shaded area shown in FIG. 13, that is, the entire surface of the printed matter P. FIG. 14A shows a density histogram extracted as a feature amount by the feature amount extraction unit 4 on one surface for this region, and FIG. 14A shows a density histogram extracted as a feature amount by the feature amount extraction unit 9 on the other surface. (B). In this example, it can be seen from the density histogram that the surface of FIG. 14 (b) has stains that are not on the surface of FIG. 14 (a).

The feature quantity integrating unit 16 divides each density histogram into a printing area and a non-printing area by, for example, the "Otsu's discriminant analysis method" which is known as a binarization method of a grayscale image. Get the boundary density value. The density value at this boundary is H5 in FIG. 14A and H in FIG. 14B.
6 Then, based on the extracted density values H5 and H6, for example, a characteristic amount reflecting the degree of dirt on the front and back is obtained based on a preset rule such as “selecting a high density value”. It should be noted that this rule is not limited to this, and of course, for example, “selecting a low density value” or “selecting an average density value” is possible.

In the dirt determination unit 17, the dirt determination unit 5 in FIG.
As in the case of 10, the stain of the printed matter P is determined. Note that the present invention is not limited to the above embodiment,
Of course, various modifications can be made without departing from the spirit of the present invention.

[0051]

As described in detail above, according to the stain identification device for a printed matter of the present invention, the characteristic amount relating to the stain is extracted from the image data of one surface of the printed matter in the area corresponding to the characteristic, and the stain is detected. It is possible to accurately identify stains biased on one side of the printed matter by evaluating the result of the determination and the result of similarly determining stains from the image data on the other side. .

Further, by evaluating the feature amount extracted on one side of the printed matter and the feature amount extracted on the other side together, similarly, stains unevenly distributed on one side of the printed matter can be obtained. Even if there is, it can be accurately identified.

[Brief description of drawings]

FIG. 1 is a block diagram schematically showing the configuration of a stain identification device for printed matter according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram schematically showing the configuration of an image input unit.

FIG. 3 is a side view schematically showing a main part of an image input unit.

FIG. 4 is a plan view showing an example of a printed matter.

FIG. 5 is a diagram illustrating a first example of a specific area extracted from a printed matter.

FIG. 6 is a diagram illustrating a second example of a specific area extracted from a printed matter.

FIG. 7 is a flowchart showing an example of processing performed by a feature quantity extraction unit.

FIG. 8 is a diagram for explaining an example of processing performed by a stain determination unit.

FIG. 9 is a block diagram showing a first modification of the first embodiment.

FIG. 10 is a block diagram showing a second modification of the first embodiment.

FIG. 11 is a block diagram showing a third modification of the first embodiment.

FIG. 12 is a block diagram schematically showing the configuration of a stain identification device for printed matter according to a second embodiment of the present invention.

FIG. 13 is a diagram illustrating an example of a specific area extracted from a printed matter.

FIG. 14 is a diagram illustrating an example of processing performed by a feature amount extraction unit.

[Explanation of symbols]

P ... Identification target (printed matter), 1, 6 ... Image input section,
2, 7 ... Image memory, 3, 8 ... Specific area extraction unit,
4, 9, 41 to 4N, 91 to 9N ... Feature amount extraction unit,
5, 10, 17 ... stain determination section, 11 ... front and back evaluation section,
12, 13 ... Region extraction control unit, 14 ... Front / back determination unit,
15 ... Judgment result storage unit, 16 ... Feature amount integration unit.

Claims (2)

[Claims] 1. A first image input unit for inputting image data of one surface of a target printed matter, a second image input unit for inputting image data of the other surface of the printed matter, and the first image input unit. A first specific area for extracting a specific area on one surface of the printed matter from the image data input by the image input means based on preset information, and extracting image data in the extracted specific area. From the extraction means and the image data input by the second image input means, a specific area on the other surface of the printed matter is extracted based on preset information, and the image data in the extracted specific area is extracted. Second specific area extracting means for extracting, and dirt on one surface of the printed matter from the image data in the specific area extracted by the first specific area extracting means A first characteristic amount extracting means for extracting a characteristic amount, and a second characteristic amount for extracting a characteristic amount relating to stains on the other surface of the printed matter from the image data in the specific area extracted by the second specific area extracting means. A feature amount extraction unit, and a first stain determination unit for determining stain on one surface of the printed matter by comparing the feature amount extracted by the first feature amount extraction unit with a preset reference value. And a second stain determination unit that determines stains on the other surface of the printed matter by comparing the feature amount extracted by the second feature amount extraction unit with a preset reference value. The determination result of the first stain determination unit is compared with the determination result of the second stain determination unit, and based on the comparison result, one of the determination results is the final stain identification for the printed matter. Dirt identification device of the printed matter, characterized by comprising an evaluation means for selecting as a result, the. 2. A first image input means for inputting image data of one side of a target printed matter, a second image input means for inputting image data of the other side of the printed matter, and the first A first specific area for extracting a specific area on one surface of the printed matter from the image data input by the image input means based on preset information, and extracting image data in the extracted specific area. From the extraction means and the image data input by the second image input means, a specific area on the other surface of the printed matter is extracted based on preset information, and the image data in the extracted specific area is extracted. Second specific area extracting means for extracting, and dirt on one surface of the printed matter from the image data in the specific area extracted by the first specific area extracting means A first characteristic amount extracting means for extracting a characteristic amount, and a second characteristic amount for extracting a characteristic amount relating to stains on the other surface of the printed matter from the image data in the specific area extracted by the second specific area extracting means. The feature amount extracting means compares the feature amount extracted by the first feature amount extracting means with the feature amount extracted by the second feature amount extracting means, and one of the features is based on the comparison result. A feature amount integrating means for selecting an amount as a feature amount for the printed matter, and a stain determining means for determining the stain on the printed matter by comparing the feature amount selected by the feature amount integrating means with a preset reference value. An apparatus for identifying stains on printed matter, comprising: