METHOD FOR SHEET INSPECTION INCLUDING BRIGHTNESS ADAPTATION

Abstract

A method for inspecting images on printed products of a printing substrate processing machine includes using at least one image sensor of an image recording system, in the course of the image inspection process, to record and digitize the printed products that have been produced. The computer compares the recorded digital printed images that have been created in this way to a digital reference image. In the case of deviations between the recorded digital printed images and the digital reference image, the printed products that have been found defective are removed. The computer divides the digital print images and the reference images into respective image parts and compensates for brightness differences between the image parts.

Claims

1. A method for inspecting images on printed products of a printing substrate processing machine, the method comprising the following steps: using at least one image sensor of an image recording system, during an image inspection process, to record and digitize printed products having been produced; using a computer to compare recorded digital printed images created during the image inspection process to a digital reference image; removing defective printed products when deviations occur between the recorded digital printed images and the digital reference image; and using the computer to divide the digital printed images and the reference images into respective image parts and to compensate for brightness differences between the image parts.

2. The method according to claim 1, which further comprises using the computer to employ a correction value subtracted from a respective brighter one of the printed image and the reference image to compensate for the brightness differences between the image parts.

3. The method according to claim 2, which further comprises using the computer to calculate the correction value by: determining an average brightness value for the image parts; comparing the average brightness values of a respective image part of the digital printed image and the reference image; and calculating the correction value by subtracting the two average brightness values from one another when a deviating occurs.

4. The method according to claim 3, which further comprises subtracting the two average brightness values from one another by: using the computer to always subtract the average brightness value of the recorded digital printed image from the average brightness value of the digital reference image; when a result of the subtraction is numerically negative, using the computer to subtract the calculated correction value from the recorded digital printed image; and when a result of the subtraction is numerically positive, using the computer to subtract the calculated correction value from the digital reference image.

5. The method according to claim 3, which further comprises using the computer to separate the respective image parts into color separations thereof and to establish the average brightness value for the respective color separations.

6. The method according to claim 4, which further comprises using the computer to separate the respective image parts into color separations thereof and to establish the average brightness value for the respective color separations.

7. The method according to claim 1, which further comprises providing a sheet-fed printing machine as the printing substrate processing machine, and providing printing sheets as the printing substrates.

8. The method according to claim 7, which further comprises using the computer to divide the printed images and reference images into image parts only in regions within a range of influence of a trailing edge of a sheet.

9. The method according to claim 1, which further comprises providing the image parts of printed images and reference images with identical sizes.

10. The method according to claim 1, which further comprises providing the image parts with a horizontal stripe shape.

11. The method according to claim 1, which further comprises providing the image parts with a polygonal shape.

12. The method according to claim 11, which further comprises selecting the polygonal shape as a rectangular, a square or a triangular.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0029] FIG. 1 is a block diagram of an example of an image recording system in a sheet-fed lithographic offset printing machine;

[0030] FIG. 2 is a diagrammatic, side-elevational view illustrating a properly functioning image recording process with illumination;

[0031] FIG. 3 is a side-elevational view illustrating an image recording process with unsatisfactory illumination at the trailing edge of the sheet;

[0032] FIG. 4 is a plan view of a sheet illustrating an example of an image section (image part) of an ideal reference image;

[0033] FIG. 5 is a plan view of a sheet illustrating an example of an image section (image part) of a recorded printed image including a darker area at the trailing edge of the sheet and an image defect;

[0034] FIG. 6 is a plan view of a sheet illustrating an example of a differential image between the recorded printed image and reference image as a result of an image inspection process for the image section (image part);

[0035] FIG. 7 is a plan view of a sheet illustrating an example of a brightness-compensated image section (image part) of the ideal reference image;

[0036] FIG. 8 is a plan view of a sheet illustrating an example of a brightness-compensated image section (image part) of the recorded printed image;

[0037] FIG. 9 is a plan view of a sheet illustrating an example of a differential image as a result of the image inspection process based on the brightness-compensated image sections (image parts); and

[0038] FIG. 10 is a flow chart of the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0039] Referring now in detail to the figures of the drawings, in which mutually corresponding elements have the same reference symbols, and first, particularly, to FIG. 1 thereof, there is seen an example of an image recording system 2 implementing the method of the invention. The system is formed of at least one image sensor 5, usually a camera 5, which is integrated into the sheet-fed printing machine 4. The at least one camera 5 records the printed images generated by the printing machine 4 and transmits the data to a computer 3, 6 for analysis. This computer 3, 6 may be a separate computer 6, e.g. one or more dedicated image processors 6, or it may be identical with the control unit 3 of the printing machine 4. At least the control unit 3 of the printing machine 4 has a display 7 for displaying the results of the image inspection process to an operator 1.

[0040] FIG. 2 illustrates a mechanical configuration of a sheet inspection system and the physics for a case in which a printing sheet 8 has not left a printing nip between a blanket cylinder 9 and an impression cylinder 11. In this case, an inspection point of the camera 5 and an illumination point of an illumination unit 10 coincide and the image inspection process may proceed at optimum illumination. FIG. 3 illustrates a case in which image inspection takes place at the trailing edge of the sheet as mentioned above. This drawing likewise shows the mechanical configuration of a sheet inspection system and the physics of a case in which the printing sheet 8 has left the printing nip and flutters. It is clearly shown that in this case, the point of inspection of the camera 5 and the illumination point do not coincide and therefore the illumination is not ideal for sheet inspection purposes. This causes a recorded printing image 13 to become darker and has a corresponding effect on the image inspection process.

[0041] The invention remedies this situation, providing high-quality inspection even at the trailing edge of the sheet without any higher costs and without creating pseudo defects 15, 15a. The fundamental principle is a local adaptation of the brightness values in the recorded camera images 13.

[0042] FIG. 10 is a schematic flow chart of a preferred embodiment of the method of the invention. The first step is to create a reference image 12. Whether this is done on the basis of preprint data or by teaching in is irrelevant to the method of the invention. Then printed products 8 in the form of printed sheets 8 that have been produced are recorded and digitized by using the image recording system 2, i.e. the camera system 5 thereof, in the course of the image inspection process. This is where the method of the invention starts. The image processor 6 divides recorded prints 13 and the reference image 12 for comparison into image parts 20.

[0043] FIG. 4 illustrates an image section at the trailing edge of the reference image 12. In this case, the reference image has been taught-in as a good image 12. The figure clearly shows dark image areas 15 in the upper image area caused by the fluttering of the trailing edge of the sheet in the reference image 12 as well.

[0044] FIG. 5 illustrates the same image section at the trailing edge of the sheet on the recorded digital print 13. In this case, the print sheet 8 does not flutter as much as in the reference image 12 and consequently the trailing edge image areas are brighter. The blackened o in the lower right-hand fifth of the image is a genuine print defect 16 that needs to be detected in the image inspection process.

[0045] FIG. 6 illustrates the same image section of a differential image 14 between the reference image 12 and the printed image 13 as it is created in the course of the image inspection process. Large differences between the two images 12, 13 are shown. The marked image areas 15a in the upper image half are undesired pseudo defects 15a. The marked black o in the lower right-hand fifth of the image represents a genuine print defect 16a that has been detected. The marked deviations 15a shown in FIG. 6 in the upper image half are exclusively caused by sub-optimal illumination due to a fluttering of the trailing edge of the sheet and therefore constitute undesired pseudo defects 15a. Since the marked deviations 15a in the form of the darker areas 15 (see FIG. 4) are more pronounced on a bright image background 17 than on a darker image background 18, they are only marked as pseudo defects 15a on the bright image background 17 in the differential image 14.

[0046] The solution to this problem is the core of the method of the invention. Since the fluttering behavior at the trailing edge of the printing sheet 8 is complex and variable, it is impossible to correct brightness values 22 using fixed values. Therefore, correction values 23 are derived from the taught-in reference image 12 as follows:

[0047] As mentioned above, the image processor 6 divides the reference image (good image) 12 and the recorded digital image 13 on the current print sheet 8 into image parts 20. Then the image processor 6 separates every digital image part 20 into its three color separations RGB 21. Since the reference image 12 has been taught in on the basis of recorded digital print images 13, it is likewise available in an RGB format, allowing it to be divided into the color separations 21. If, in an alternative embodiment, the reference image 12 is created on the basis of digital preprint data, it either needs to be available in an RGB format from the start or a color transformation into this format needs to be made. Then the image processor determines an average gray value 22 for every one of the color separations 21 of an image part 20 of the reference image 12 and of the printed image 13. If the values are not the same, the image processor corrects the gray values 22 in the color separation 21 in question of the image part 20. A correction value D 23 is calculated as follows:

N=(average gray value in the reference image)
O=(average gray value in the printed image) and D=NO.

[0048] If D>0, D is subtracted from every gray value 22 of every pixel within the reference image part to darken the reference image 12 in a corresponding way. If D<0, D is subtracted from every gray value 22 of every pixel within the printed image part to darken the printed image 13 in a corresponding way.

[0049] If the method of the invention is applied to the original images 12, 13 in accordance with FIGS. 4 and 5, images 12a, 13a that have been corrected in terms of brightness are obtained as shown by way of example in FIGS. 7 and 8. In this example, brightness correction has only been made in the recorded printing 13a in FIG. 8. FIG. 7 again shows the corresponding image section at the trailing edge of the sheet on the reference image 12. FIG. 8 illustrates the same image section of the corrected recorded digital print image 13a at the trailing edge of the sheet. The drawing clearly shows regions 19, 19a formed by individual corrected image parts 20 in which brightness correction in accordance with the invention has been made. The left-hand area 19 was darkened to a lesser extent than the right-hand area 19a. A comparison between these corrected images 12a, 13a in the form of a corrected differential image 14a in FIG. 9 shows that the pseudo defects 15a disappear. FIG. 9 thus shows the same image section of the differential image 14a between the corrected reference image 12a and the print image 13a. One can see that there are no longer any marked pseudo defects 15a in the upper image halfin contrast to the uncorrected differential image 14 shown in FIG. 6. The marked black o in the lower right-hand image fifth is the actual print defect 16a that needs to be detected and remains visible as desired.

[0050] Once all color separations 21 have been corrected, they are recombined to form a complete RGB image part 20, which is then inserted into reference image 12a/printed image 13a at the original position. In this way, a localized adaptation of the brightness values is achieved for every image part 20. These processed images 12a, 13a are then used in the actual image inspection process.

[0051] Moreover, in lithographic offset printing, it has been found advantageous to select thin horizontal stripes as a geometrical shape for the image parts 20. A reason for this is that in this printing technique, the brightness fluctuation rate is high in the printing direction yet low in a direction transverse to the printing direction. Narrow horizontal image parts at the trailing edge yield better results than square image parts. However, it is possible to vary the shape of the image parts 20 as a function of the actual application. In general, square or triangular image parts are most suitable. Curved surfaces in a 3-dimensional space may more easily be approximated by triangular image parts than by square image parts, although the former are more difficult to process in programming terms.

LIST OF REFERENCE SYMBOLS

[0052] 1 operator [0053] 2 image recording system [0054] 3 control unit [0055] 4 printing machine [0056] 5 image sensor [0057] 6 image processor [0058] 7 display [0059] 8 print sheet [0060] 9 blanket cylinder [0061] 10 illumination unit [0062] 11 impression cylinder [0063] 12 good image/reference image [0064] 12a good image/reference image after brightness correction [0065] 13 recorded print image [0066] 13a recorded print image after brightness correction [0067] 14 differential image with brightness deviation [0068] 14a differential image after brightness correction [0069] 15 dark image areas [0070] 15a recorded dark image areas (pseudo defects) in the differential image [0071] 16 genuine print defect [0072] 16a detected print defect in the differential image [0073] 17 bright image background [0074] 18 dark image background [0075] 19, 19a image areas after brightness correction [0076] 20 image parts [0077] 21 color separations of image parts [0078] 22 average gray value/brightness value [0079] 23 correction value