The present invention relates to a way of determining or confirming color printing quality of samples. The invention maybe used for quality control of printing processes, for example.

It is known to manually evaluate printing quality of random samples from a printing process to ensure that a desired printing quality is reached. A person may visually compare a selected printed sample to a physical target. This method of quality control is highly subjective. Further, it may be bothersome to have to provide physical targets, in particular if samples having the same target specifications are to be printed by different printers at different locations, for example in different printing facilities or in different countries.

There is a need for an improved way of carrying out quality control of printing processes with respect to compliance with color targets.

According to an aspect, the invention provides a system for determining a color printing quality of samples. The system comprises a color measurement device, an automatic handling system, a control system controlling the automatic handling system and the color measurement device, and a detection device. The color measurement device is configured to carry out a color measurement at one or more predetermined positions on a sample while the sample is in a measurement region. The automatic handling system is configured to align the color measurement device with the one or more predetermined positions on the sample by adjusting the relative positioning between the sample in the measurement region and the color measurement device. The detection device is configured to detect sample information from the sample. The control system is configured to control the automatic handling system for aligning the color measurement device with the one or more predetermined positions on the sample based on the sample information detected by the detection device.

The control system may be configured to determine a sample type of the sample based on the sample information detected by the detection device.

The system may be provided with samples of different types without having to indicate to the system the current sample type before every single measurement. The system may be able to determine the sample type by itself based on the detected sample information and may process the sample accordingly.

Carrying out color measurements at one or more predetermined positions on a sample by a color measurement device allows to judge in an objective and reliable manner whether the colors at the predetermined positions sufficiently correspond to color targets.

Due to the automatic handling system aligning the color measurement device with the one or more predetermined positions on the sample, it is not required that an operator is continuously present during measurement of samples. The automatic handling system may ensure that alignment between the color measurement device and the one or more predetermined positions on the sample is carried out accurately and in a reproducible manner.

As the control system controls the automatic handling system for aligning the color measurement device with the one or more predetermined positions on the sample based on sample information detected from the sample by the detection device, the alignment process is guaranteed to take into account properties of the actual sample to be measured. Further, as alignment between the color measurement device and the one or more predetermined positions on the sample is carried out based on sample information detected from the sample by the detection device, the system may be used to automatically process different types of samples which may yield different sample information. In particular, different types of samples may be processed without the need to manually indicate which kind of sample is currently to be processed.

The automatic handling system may be configured to position the sample in the measurement region. It may be possible to evaluate multiple samples in an automated manner without the need for an operator to be present.

The color measurement by the color measurement device may yield a definite identification of a color at the corresponding predetermined position on the sample. A definite identification of the color may facilitate comparing different measurement results or comparing measurement results with target values. In particular, the color measurement may yield a definition of the measured color in a 3-dimensional color space. For example, color measurements may yield color values expressed in the CIE L*a*b* color space. The CIE L*a*b* color space is a color space defined by the International Commission on Illumination (CIE). It may express color as three values: L* for the lightness from black to white, a* from green to red, and b* from blue to yellow. Due to the specific design of the CIE L*a*b* color space, color values expressed in the CIE L*a*b* color space may easily be compared to each other in a way that gives a meaningful estimate of the color difference perceived by a user. Alternatively, the color measurements may yield color values expressed in any other 3-dimensional color space. However, the color measurements might also yield color values expressed in a non-3-dimensional color space. To facilitate comparison of different measurement results or comparison of measurement results with target values, the color measurement may yield a definition of the measured color in a device independent color space. However, use of a device-dependent color space would also be possible.

The system may further comprise a data memory storing information for a plurality of sample types. The information may, for example, be stored in the form of a table. The information may be changeable to account for changes in the properties of certain sample types, or to add new sample types.

The control system may be configured to control the automatic handling system for aligning the color measurement device with the one or more predetermined positions on the sample based on the information from the data memory for a sample type of the sample. The stored information for a particular sample type may, for example, comprise one or more of a sample size, a sample shape, and information on the location of the one or more predetermined positions on the sample. The stored information on one or more of the sample size, the sample shape, and the location of the one or more predetermined positions on the sample may be used by the control system to determine control instructions for the automatic handling system to control the automatic handling system for aligning the color measurement device with the one or more predetermined positions on the sample. As an alternative, the stored information for a particular sample type may comprise control instructions for the automatic handling system to control the automatic handling system for aligning the color measurement device with the one or more predetermined positions on the sample of the particular sample type. The control instructions for the automatic handling system for different sample types may thus be determined and stored in advance. The stored information for a particular sample type may also comprise control instructions for the color measurement device. For example, the control instructions for the color measurement device may comprise information on a timing of starting a color measurement.

The information for the plurality of sample types may be stored in a local data memory provided near the color measurement device and the automatic handling system. In addition or as an alternative, the information for the plurality of sample types may also be stored in a remote data memory that is accessed by the control system via a communications network. The communications network may comprise one or more of a LAN network, a mobile communications network, and the internet. In particular, the information for the plurality of sample types may be stored in a central server and may be accessed from a plurality of different locations.

The detection device may comprise a camera configured to obtain image information of the sample as the sample information. The image information may be used by the control system to control the automatic handling system for aligning the color measurement device with the one or more predetermined positions on the sample.

The control system may analyze the image information to find the one or more predetermined positions on the sample. The control system may deduce control instructions for the automatic handling system from the image information to control the automatic handling system for aligning the color measurement device with the one or more predetermined positions on the sample.

In particular, information indicating the sample type may be deduced from the image information. Such information could be used, possibly in combination with stored information on sample types, to align the color measurement device with the one or more predetermined positions on the sample. For example, an identifier code or a sample shape may be deduced from the image information. It is also conceivable to, in addition or as an alternative, directly obtain the location of one or more predetermined positions on the sample within the measurement region from the image information obtained by the detection device. In this case, it would not be necessary, but still possible, to have stored information on particular sample types.

The control system may be configured to determine the location of the one or more predetermined positions on the sample based on the image information from the camera by machine vision. This may ensure that alignment of the color measurement device with the one or more predetermined positions on the sample is carried out in consideration of the actual locations of the one or more predetermined positions within the measurement region. The location of the one or more predetermined positions on the sample may be determined based on the image information in relation to one or more features of the sample. For example, the location of the one or more predetermined positions on the sample may be determined in relation to an edge or a corner of the sample. The location of the one or more predetermined positions on the sample may be determined based on the image information in relation to a reference that is fixed with respect to the measurement region. For example, the location of the one or more predetermined positions on the sample may be determined in relation to a predetermined point in the measurement region. For example, the location of the one or more predetermined positions on the sample may be determined in relation to a calibrated zero point of the alignment system or to a calibrated zero point of the camera.

In addition or as an alternative to a camera, the detection device may comprise, for example, a barcode scanner or an RFID reader to detect sample information from the sample.

The color measurement device may comprise a spectrophotometer. For example, the spectrophotometer CM-26dDC from Konica Minolta could be used.

A measurement opening of the spectrophotometer may have a diameter smaller than 10 mm, or smaller than 7 mm, or smaller than 5 mm, or smaller than 4 mm, or smaller than 3 mm. The measurement opening of the spectrophotometer may have a diameter of at least 1 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or more than 4 mm.

A printed structure can be provided at each one of the one or more predetermined positions on the sample. The printed structures may each be printed in one particular color, the printing quality of which is to be assessed. In particular, the colors of the printed structures may be colors that are used in a printing at another position of the sample.

The sample may be a packaging item or an item that will later be formed into a packaging item or will be part of a packaging item. For example, the sample may be a sheet to be folded into a container. In particular, the sample can be a sheet that will be folded into a packaging item used in the tobacco industry. The sample may, for example, be configured to be folded into a box for receiving tobacco articles like cigarettes or cigars. The printed structures may be printed onto the sample for the sole purpose of color verification. Such printed structures may be positioned such that they will not be visible in the final product. For example, the printed structures may be located on a side of a sheet that will form an inside of a final packaging item. The printed structures could also be positioned at locations on a sheet that will be folded so as to not be visible in the final product. In particular, the printed structures may be positioned on a surface that will be glued to another surface such that the printed structures will be covered between the two connected surfaces.

To facilitate alignment of the color measurement device with the one or more predetermined positions on the sample, a reference point for the sample may be provided in the measurement region. The reference point may ensure that the initial position of the sample after being positioned in the measurement region has at least a certain degree of accuracy. The reference point may be defined by a mechanical stop. The mechanical stop may be engaged by the sample, when the sample is positioned in the measurement region. In particular, the mechanical stop may be engaged by an edge of the sample, when the sample is positioned in the measurement region. Alternatively, the reference point may be defined by a virtual zero point of the automatic handling system, for example.

Preferably the automatic handling system comprises a stack holder for receiving a stack of samples. The stack holder may be configured such that a first sample of the stack of samples is positioned in the measurement region. The stack holder may allow the samples to be provided to the measurement region in a particularly efficient manner.

The stack holder may comprise a lifting device configured to lift the stack of samples so that individual samples can be provided to the measurement region from below. Alternatively, the samples may be provided to the measurement region from above. In this case, the next sample may enter the measurement region from above by means of gravity after a previous sample has been removed.

The automatic handling system may further comprise a removal device configured to remove the first sample of the stack of samples. The removal device may remove the first sample after measurement on the first sample has been completed, thereby, making way for the next sample to be measured. The removal device may, for example, comprise a gripper or a roller.

According to another aspect, the invention provides a method for determining a color printing quality of samples. The method can be carried out using the system for determining a color printing quality of samples as described herein. The system for determining a color printing quality of samples may be adapted and configured to carry out the method. Features described with respect to the system may also implemented in the method, and vice versa.

According to the method for determining a color printing quality of samples, a sample is positioned in a measurement region with an automatic handling system. Sample information describing at least one feature of the sample is obtained. From the obtained sample information, control information for the automatic handling system is derived and a color measurement device is automatically aligned with one or more printed structures on the sample by adjusting the relative positioning between the sample in the measurement region and the color measurement device with the automatic handling system based on the obtained sample information. A color measurement of the one or more printed structures on the sample is carried out with the color measurement device. The result of the color measurement is compared with predetermined target color data.

Obtaining the sample information may comprise detecting the printed structures on the sample via machine vision by analyzing image information of the sample obtained by a camera. It is possible to directly detect the location of the printed structures from the image information so that no other information on the sample has to be relied upon.

Obtaining the sample information may comprise determining a sample type of the sample by analyzing image information of the sample obtained by a camera. Analyzing the image information of the sample may comprise determining a sample identifier identifying the individual sample. Analyzing the image information of the sample may comprise determining a sample identifier identifying the sample type. Analyzing the image information of the sample may comprise determining a shape of the sample. Analyzing the image information of the sample may comprise determining a size of the sample. Analyzing the image information of the sample may comprise determining a number of printed structures. Analyzing the image information of the sample may comprise determining an arrangement of printed structures. Once the sample type of the sample has been determined, corresponding information may be read out from a data memory storing information for a plurality of sample types, for example.

As an alternative, the sample information could be obtained from user input. In particular, a user could enter a sample type. The control information for the automatic handling system could be derived from the sample type based on information for a plurality of sample types that is stored in a data memory. It would also be conceivable that the user directly enters data such as a configuration of the sample, a size of the sample, an arrangement of the printed structures on the sample, or a sample identifier code.

The samples may be positioned in the measurement region by keeping a first end of a stack of samples at a constant level, so that the sample of the stack forming the first end of the stack is always positioned in the measurement region. This allows positioning the samples in the measurement region in a particularly reliable and fast manner. In particular, the first end of the stack of samples can be an upper end of the stack of samples. According to such arrangement, the color measurement device may be positioned above the stack of samples to facilitate alignment of the color measurement device with the printed structures on the sample. Alternatively, the first end of the stack of samples may be a lower end of the stack of samples, so that upon removal of the lowest sample, the next sample will automatically take the lowest position due to gravity.

Removing the sample from the measurement area may comprise discarding the sample of the stack forming the first end of the stack.

Preferably, the samples are printed packaging items.

The one or more printed structures may comprise one or more reference structures, each printed on the sample in a color (preferably one color per reference structure) that is also present in an image printed onto the sample. Assessing color printing quality based on analyzing printed structures of colors that are actually used in an image printed onto the sample ensures that the analysis of color printing quality is indicative of the printing quality of the actual image printed onto the respective sample.

Alternatively, color measurements could also be carried out at one or more positions on an image printed onto the sample. The one or more printed structures may comprise one or more printed structures that are part of an image printed onto the sample.

At least one of the one or more printed structures may be printed in a mixed color. The mixed color may be a color that has been generated in the associated printing process by using more than one basic color of the printing process. The basic colors of the printing process may be any color of an ink as provided to the printing process. The mixed color may be the result of mixing more than one of inks during the printing process. The mixed color may be the result of printing sufficiently small dots of different basic colors of the printing process sufficiently close to each other that the colors of the individual dots are indistinguishable to the observer. Analyzing mixed colors used in an image printed onto the sample provides a high level of quality control of color printing quality.

The sample information may comprise image information of the sample. The sample information may comprise information on the shape of the sample. The sample information may comprise information on the size of the sample. The sample information may comprise information on the location of the one or more predetermined positions on the sample. The sample information may comprise an identifier code of the sample. The sample information may comprise an identifier code of a sample type of the sample.

The target color data for comparison with results of the color measurements may be locally stored or may be accessible via a network or the internet. The target color data may be provided in the CIE L*a*b* color space. The result of the color measurement for each of the printed structures of the sample may be compared to a respective color target. Comparison may be carried out by calculating color differences between the measurement result of a printed structure and the respective color target. Preferably, the color differences are calculated in the CIE L*a*b* colorspace. In the CIE L*a*b* colorspace, the color differences dE between a measurement result of a printed structure and a color target may be calculated according to the CIEDE2000 formula specified in international standard ISO/CIE 11664-6:2014. Alternatively, color differences between the measurement result of a printed structure and the respective color target may be calculated separately for the individual dimensions of a color space in which the measurement result is defined. In particular, the color differences between the measurement result of a printed structure and the respective color target may be calculated separately for the individual dimensions (L*, a*, and b*) of the CIE L*a*b* color space.

The color target for comparison with a result of a color measurement at a specific printed structure may be determined based on the position of the printed structure on the sample. Alternatively, the result of the color measurement for the specific printed structure may be evaluated to determine a most likely color target of a plurality of possible color targets. The most likely color target may be the color target having the smallest color difference to the result of the color measurement for the specific printed structure.

The color differences between the measurement result of a printed structure and the respective color target may be compared to threshold values to determine whether a deviation from the color target is still acceptable. The same threshold value may be used for all printed structures on the sample. Alternatively, different threshold values may be assigned to different printed structures of the sample. If a color difference between the measurement result of a printed structure and the respective color target is below a corresponding threshold value, it may be determined that the color printing quality of the printed structure is acceptable. If the color difference between the measurement result of a printed structure and the respective color target is calculated according to the CIEDE2000 formula, the threshold value may be between 1 and 3.5, preferably between 1.2 and 3.0, preferably between 1.5 and 2.5, preferably between 1.8 and 2.2. Those ranges may include or alternatively exclude the indicated boundary values. If the color difference between the measurement result of a printed structure and the respective color target is calculated according to the CIEDE2000 formula, the threshold value may be no more than 3, or no more than 2.5, or no more than 2, or no more than 1.5, or no more than 1. If the color difference between the measurement result of a printed structure and the respective color target is calculated according to the CIEDE2000 formula, the threshold value may be more than 0.5, or more than 1, or more than 1.5, for example. If the color difference between the measurement result of a printed structure and the respective color target is calculated separately for the individual dimensions (L*, a*, and b*) of the CIE L*a*b* color space, the threshold value for one, for two or for all of the individual dimensions may be between 1 and 3.5, preferably between 1.2 and 3.0, preferably between 1.5 and 2.5, preferably between 1.8 and 2.2. Those ranges may include or alternatively exclude the indicated boundary values. If the color difference between the measurement result of a printed structure and the respective color target is calculated separately for the individual dimensions (L*, a*, and b*) of the CIE L*a*b* color space, the threshold value for one, for two or for all of the individual dimensions may be no more than 3, or no more than 2.5, or no more than 2, or no more than 1.5, or no more than 1. If the color difference between the measurement result of a printed structure and the respective color target is calculated separately for the individual dimensions (L*, a*, and b*) of the CIE L*a*b* color space, the threshold value for one, for two or for all three of the individual dimensions may be more than 0.5, or more than 1 , or more than 1.5, for example.

A report on the quality of printing may be generated based on the results of the measurements on one or more samples. The report may, for example, comprise a number of printed structures that have a color difference from the corresponding color target of more than the respective threshold. The report may further comprise information such as an average color difference per type of printed structure.

Printed structures of different colors may be printed on different individual samples constituting a set of samples. According to such approach, printing quality of a large number of colors may be evaluated, even if there is insufficient space to provide a sufficiently large printed structure of each color on a single one of the samples. The positions of the printed structures on the individual samples may be identical for all samples of the set. Alternatively, the positions of the printed structures on the individual samples of the set by be different depending on the colors of the printed structures on the respective sample.

According to another aspect, the invention provides a use of a detection device. Features described with respect to the system or method for determining a color printing quality of samples also may be implemented in the use, and vice versa.

The invention provides a use of a detection device for detecting sample information from a sample to derive control information for controlling an automatic handling system for automatically aligning a color measurement device with one or more printed structures on the sample by adjusting the relative positioning between the sample and the color measurement device.

A sample type of the sample may be determined based on the sample information detected by the detection device. Determining the sample type of the sample may be carried out by a control system.

In the following, the invention will be further described with the help of exemplary embodiments with reference to the figures.

Fig. 1 shows a schematic top view of a sample according to an embodiment;

Fig. 2 shows a schematic perspective view of a system for determining printing quality of samples according to an embodiment;

Fig. 3 shows a flowchart of a method for determining a printing quality of samples according to an embodiment using a detection device;

Fig. 4 shows a flowchart of a method for determining a printing quality of samples according to an embodiment obtaining sample information from user input; and Fig. 5 shows a series of user interfaces for inputting sample information according to an embodiment.

Fig. 1 shows a schematic representation of an exemplary sample 1. According to the illustrated embodiment, the sample 1 is a sheet from which a packaging box for receiving tobacco articles is to be folded. However, the skilled person will recognize that the invention is also applicable to printed packaging items of other configurations, or to printed samples 1 that are not packaging items. The sample 1 has been subjected to a printing process printing an image 3 and a plurality of reference structures 5 onto the sample 1. The image 3 may, for example, contain a desired design or marketing content. The image 3 may be printed in one or more colors, which may in particular comprise one or more mixed colors. The reference structures 5 have been printed in different colors (one color per reference structure 5) that are each also represented in the image 3. Preferably, at least one reference structure 5 has been printed using a mixed color. There may also be one or more reference structures 5 printed in a basic color of the printing process.

According to the illustrated embodiment, the reference structures 5 have been printed for the purpose of being subjected to color measurements to be able to assess printing quality of the printing process. It would, however, also be conceivable to not print separate reference structures 5, but to rather carry out color measurements at specific positions directly on the image 3.

Fig. 2 shows a system 7 for determining the printing quality of the samples 1. The system 7 comprises a stack holder 9 for receiving a stack of the samples 1. The stack holder 9 is configured to be open towards an upper side so that the printed surface of the top sample 1 of the stack of samples 1 is visible. The top sample 1 is positioned in a measurement region 11 , in which the color measurements will be carried out.

The system 7 comprises a color measurement device 13 to carry out the color measurements at predetermined positions on the sample 1 corresponding to the reference structures 5. The color measurement device 13 may be a spectrophotometer. The color measurement device 13 may output color values for the reference structures 5 in a 3-dimensional color space. Preferably, the color values for the reference structures 5 are obtained in the CIE L*a*b* color space based on the color measurements by the color measurement device 13.

To be able to align the color measurement device 13 with the reference structures 5 on the top sample 1 of the stack, the color measurement device 13 may be mounted to an alignment robot 15 for moving the color measurement device 13. In the illustrated embodiment, the alignment robot 15 comprises a X-Y-stage for moving the color measurement device 13 in an X-Y-plane. The color measurement device 13 might also be moveable with respect to a Z-direction. As an alternative, the color measurement device 13 might be stationary and the sample 1 might be movable for aligning the color measurement device 13 with the reference structures 5. It would also be conceivable to move both the color measurement device 13 and the sample 1. The system 7 further comprises a removal device 17 for removing the top sample 1 of the stack of samples 1 after the reference structures 5 of the top sample 1 have been measured by the color measurement device 13. In particular, the removal device 17 may discard a sample 1 after measurement on the sample 1. In the illustrated embodiment, the removal device 17 comprises a rotatable gripper 19 for gripping the top sample 1. After gripping the top sample 1, the gripper 19 is rotated while holding the sample 1 and discards the sample 1 in a discarding area 21. Subsequently, a raising device 23 of the stack holder 9 will raise the remaining stack of samples 1 so that the new top sample 1 is positioned in the measurement region 11. In the illustrated embodiment, the stack holder 9 further comprises a mechanical stop 25 to limit the upward motion of the first sample 1 of the stack of samples 1 by engaging the first sample 1.

The system 7 further comprises a control system 27 controlling the automatic handling system (alignment robot 15, removal device 17 and raising device 23) and the color measurement device 13. In the illustrated embodiment, the control system 27 is in wireless communication with the removal device 17, the alignment robot 15, the raising device 23 and the color measurement device 13 to control the removal device 17, the alignment robot 15, the raising device 23 and the color measurement device 13. Further the control system 27 wirelessly receives measurement results from the color measurement device 13. Of course, it would also be conceivable to use wired connections.

The system 7 is suitable for determining color printing quality of samples 1 of different types. A lateral guide structure 30 of the stack holder 9 may be exchangeable, if samples 1 of different sizes are to be measured.

According to the embodiment shown in Fig. 2, the system 7 comprises a detection device 29 configured to detect sample information from the sample 1. The control system 27 is configured to control the alignment robot 15 for aligning the color measurement device 13 with the reference structures 5 on the top sample 1 based on sample information detected by the detection device 29. In the illustrated embodiment, the detection device 29 comprises a camera.

Fig. 3 illustrates a flowchart of a method for determining a printing quality of samples 1 to be used with the system 7 shown in Fig. 2.

In step 100, the method starts.

In step 101, a user manually places a stack of samples 1 to be assessed in the stack holder 9. In step 102, the raising device 23 adjusts a height of the stack of samples 1 such that the top sample 1 is in the measurement region 11. For that, the control system 27 may control the raising device 23 based on output from the detection device 29. Alternatively, the raising device 23 may be initially set by the user when inserting the samples 1.

In step 103, the detection device 29 detects sample information from the top sample 1. The sample information is transmitted to the control system 27. In the illustrated embodiment, the sample information comprises image information of the top sample 1. In step 104, the control system 27 derives control information for the alignment robot 15 from the sample information. The control information comprises control instructions for aligning the color measurement device 13 with the reference structures 5 on the top sample 1. There are multiple ways in which the control information may be derived from the detected sample information.

For example, the control system 27 may determine a sample type of the sample 1 based on the sample information. The sample type may be derived from the image information by evaluating characteristics of the image by image processing. For example, the sample type may be derived by detecting an identifier code for the individual sample or an identifier code for the sample type printed on the sample 1 or by detecting a sample shape or a distribution of the printed structures 5 on the sample 1.

The system 7 may comprise a data memory 35 storing a table with information for a plurality of sample types. The information for a particular sample type may comprise one or more of a sample size, a sample shape, information on the location of the one or more printed structures 5 on the sample 1 , and control instructions for the alignment robot 15. The control system 27 may derive the control information for aligning the color measurement device 13 with the printed structures 5 on the sample 1 based on the information from the data memory 35 for a sample type of the sample 1.

As illustrated in Fig. 2, the data memory 35 with the information for the plurality of sample types may be a local memory of the system 7. However, the information for the plurality of sample types may also be stored in a remote data memory 35 that may be accessed by the control system 27 via a communications network, such as a LAN network, a mobile communications network, or the internet.

As an alternative, the control system 27 may directly obtain the locations of the printed structures 5 in the measurement region 11 from the image information obtained by the detection device 29. In this case, it is not necessary to determine a sample type. The control system 27 may derive the control information for aligning the color measurement device 13 with the printed structures 5 on the sample 1 based on the obtained locations of the printed structures 5.

In step 105, the alignment robot 15 operates according to the control information to align the color measurement device 13 with a first reference structure 5 on the top sample 1.

In step 106, the color measurement device 13 carries out a color measurement at the respective reference structure 5 and sends the results to the control system 27.

In step 107, it is determined whether all printed structures 5 of the top sample 1 have been measured. If there are still printed structures 5 to be measured, the method returns to step 105 and the alignment robot 15 is controlled to move the color measurement device 13 to align with the next reference structure 5, where another color measurement is carried out by the color measurement device 13. If it is determined in step 107 that all printed structures 5 of the top sample 1 have been measured, the method proceeds to step 108.

In step 108, the results of the color measurements of the respective sample 1 are compared with target color data. The target color data may be locally stored or may be accessible via a network or the internet. The target color data may be provided in the CIE L*a*b* color space. The result of the color measurement for each of the printed structures 5 of the sample 1 may be compared to a respective color target. Comparison may be carried out by calculating color differences between the measurement result of a printed structure 5 and the respective color target. Preferably, the color differences are calculated in the CIE L*a*b* color space. In the CIE L*a*b* color space, the color differences dE between a first color and a second color, which may be a color target, may be calculated according to the CIEDE2000 formula specified in international standard ISO/CIE 11664-6:2014. Alternatively, color differences between the measurement result of a printed structure 5 and the respective color target may be calculated separately for the individual dimensions of a color space in which the measurement result is defined.

The color differences between the measurement result of a printed structure 5 and the respective color target may be compared to threshold values to determine whether a deviation from the color target is still acceptable. The same threshold value may be used for all printed structures. However, it would also be possible to assign different threshold values to different printed structures 5. If a color difference between the measurement result of a printed structure 5 and the respective color target is below a corresponding threshold value, it may be determined that the color printing quality of the printed structure 5 is acceptable. If the color difference between the measurement result of a printed structure 5 and the respective color target is calculated according to the CIEDE2000 formula, the threshold value may, for example, be no more than 3, or no more than 2.5, or no more than 2, or no more than 1.5.

In step 109, the removal device 17 is controlled to remove the top sample 1 of the stack.

In step 110, it is determined whether all samples 1 of the stack have been scanned. If it is determined in step 110 that there is still at least one sample 1 in the stack, the method returns to step 102 and the raising device 23 raises the stack to position the new top sample 1 in the measurement region 11. If it is determined in step 110 that there are no more samples 1 in the stack, the method proceeds to step 111.

In step 110, a report on the quality of printing is generated based on the results of the measurements on all samples 1 of the stack. Also, individual reports for each sample 1 could be generated. The report may, for example, comprise a number of printed structures 5 that have a color difference dE from the corresponding color target of more than the respective threshold. The report may further comprise information such as an average color difference per type of printed structure. The report may be generated by the control system 27. Alternatively, the results of the measurements may be transmitted to a central server and may be evaluated there. In step 150, the method ends.

Fig. 4 illustrates a method for determining the printing quality of a sample 1 according to an alternative embodiment. The method according to the alternative embodiment may be carried out with a system that is similar to the system 7 of Fig. 2, but lacks the detection device 29.

Most steps of the method of Fig. 4 correspond to the respective steps of the method of Fig. 3. Only the steps that are different will be described. In particular, steps 100, 101 , 102 and 105 to 150 are the same for the methods of Figs. 3 and 4. However, step 103 is replaced with step 131 in Fig. 4 and step 104 is replaced with step 141 in Fig. 4. Further, according to Fig. 4, step 102 is carried out after steps 131 and 141.

In essence, according to the method of Fig. 4, the control information for the automatic handling system 5 is not derived based on detected sample information, but based on user input.

In step 131, after the user has placed the stack of samples 1 in the stack holder 9, sample information describing at least one feature of the samples 1 is be obtained from user input. Of course, step 131 could also be carried out before the user places the stack into the stack holder 9 in step S2.

According to an embodiment, the user could enter a sample type or data representing the sample type as sample information in step 131. In this case, step 141 could be identical to step 104 of Fig. 3 and the control information for the alignment of the color measurement device 13 with the printed structure 5 could be derived based on the sample type as described above.

According to another embodiment, the user directly enters data on the samples 1 as sample information in step 131. Fig. 5 shows an example of a sequence of user interfaces for entering the sample information according to such an embodiment. In interface A, the user selects the size and shape of the sample 1 from a number of predetermined options (two options in the illustrated case). In interface B, the user then selects positions of the printed structures 5 on the samples 1. In the illustrated embodiment, the positions of the printed structures 5 are selected by selecting one or more of preset possible positions. It would also be conceivable to let the user select from different templates having different positions of the printed structures 5, for example. In interface C, the user is presented with a representation of the selected sample configuration and may confirm the selected sample configuration.

In step 141, the control information for the alignment of the color measurement device 13 with the printed structure 5 is derived based on the sample configuration entered by the user, preferably by the control system 27.

The skilled person will recognize that the order in which some of the steps indicated in Figs. 3 and 4 are carried out may be changed without deviating from the basic functional principle of the method.

The method according to Fig. 4 is best suited to process a stack of samples 1 having only samples 1 of the same type. The method according to Fig. 3 may also process a stack of samples 1 having samples 1 of only the same type. However, the method according to Fig. 3 is also efficient with a stack of samples 1 having samples 1 of two or more different sample types, as the sample information is individually detected by the detection device 29 for each sample 1.

If a large number of different colors, in particular many different mixed colors, are used in the image 3 printed on the sample 1, there may be insufficient space on the sample 1 to add printed structures 5 for each color on every sample 1. To nevertheless be able to confirm printing quality of all involved colors, printed structures 5 of different colors may be printed on different individual samples 1 constituting a set of samples 1. The positions of the printed structures 5 on the individual samples may be identical for all samples of the set. Alternatively, the positions of the printed structures 5 on the individual samples 1 of the set by be different depending on the colors of the printed structures 5 on the respective sample 1.

The set of samples 1 or a subset of samples 1 thereof may be put into the stack holder 9 and may automatically be processed by the above-described system 7. For each specific printed structure 5 on a sample 1 of the set, a most likely color target may be determined as that color target of a plurality of color targets that has the smallest color difference to the result of the color measurement for the specific printed structure 5. The color difference between the result of the color measurement for the specific printed structure 5 and the corresponding most likely color target may be used to generate a report, as described above.