WO2013029666A1

WO2013029666A1 - Color measurement unit

Info

Publication number: WO2013029666A1
Application number: PCT/EP2011/064948
Authority: WO
Inventors: Markus BARBIERI; Stefan BARBIERI
Original assignee: Barbieri Electronic Ohg Des Barbieri Siegfried & C.
Priority date: 2011-08-31
Filing date: 2011-08-31
Publication date: 2013-03-07

Abstract

The present disclosure relates to a color measurement unit (1) for measuring a color on a printed item (9), comprising: a casing (10) having a first face (11) intended, in use, to face a printed item (9), the first face (1) being provided with a window (15); a spectral sensor (21) configured to measure color spectrum data of a light (253) received through the window (15), the received light (253) being a light reflected by the printed item (9) or a light transmitted through the printed item (9); a camera (23) configured to take an image of the printed item (9), the camera (23) being movable together with the spectral sensor (21). The color measurement unit (1) is configured to take the image together with measuring the color spectrum data. The present disclosure relates also to a measurement apparatus (6, 60, 7, 8), a printhead (81) for a printer (8), and a printer (8), all of these including a color measurement unit (1). The present disclosure relates also to a method for measuring a print quality of a printed item (9).

Description

COLOR MEASUREMENT UNIT

FIELD

The present disclosure relates to the field of color measuring on a printed item. To be more specific, the present disclosure relates to a color measurement unit for measuring a color on a printed item, for example on a test chart. For example, the measurement is a reflective or transmissive measurement.

BACKGROUND

In order to calibrate a digital printer to reproduce colors with a desired quality and accuracy, a sheet of test color patches arranged in a predetermined pattern is printed by the printer.

Such a printed sheet is a color measurement chart or test chart, which comprises one or more rows of fields or patches having different reference colors, i.e. different test colors.

The printed sheet is scanned by a measurement device like a spectrophotometer, which gauges the color patches and provides a set of data characterizing the printed colors of the patches of the test chart. These data are spectral values of the colors. The set of data, which is representative of the producible color gamut of the printer, is compared with the known expected data for the corresponding reference colors and then it is used to calibrate the printer to correctly reproduce colors.

Known spectrophotometer devices are commonly used in the color measuring field for printer calibration and for printer control.

A spectrophotometer device provides a color measurement of the light which comes from a whole measurement region of the relevant color patch; for example, the light is reflected by the color patch or is transmitted through the color patch. The measurement region has an area of some square millimeters.

Therefore, the obtained data allow to establish how much a printed patch color, as perceived by a person, differs from a desired color perception. These data are highly valuable for calibration and for obtaining a printer characterization profile. However, when an anomalous deviation between a measured printed color and a respective reference color is detected, the spectral data give no indication about the reasons of such a deviation, for example if it is due to a fault of the printer or to any other reason.

Moreover, the spectral data are representative of a mean color over a whole measurement region of the patch. Therefore, the known measurement devices are not able to detect an inhomogeneous color distribution over a same patch or to point out small flaws and other print defects within the measurement region. SUMMARY

The present disclosure is based on the technical problem of providing a color measurement device which is useful for improving the prior art devices by overcoming at least one of the disadvantages mentioned with reference to prior art and/or being able to achieve further advantages.

The solution to the technical problem can be obtained by a color measurement unit as defined in independent claim 1.

The solution to the technical problem can be obtained also by devices including such a color measurement unit, as for instance a measurement apparatus as defined in claim 19, a printhead as defined in claim 22 or a printer as defined in claim 23, and by a method as defined in claim 24.

Secondary features of the subject of the present disclosure are set forth in the corresponding dependent claims.

To be more specific, a color measurement unit according to the present disclosure comprises a spectral sensor and a camera, which are movable together. I n particular, the spectral sensor and the camera are in a fixed mutual spatial relation.

The color measurement unit is configured to take or register an image of a printed item together with measuring color spectrum data of the same printed item.

Therefore, during a measuring phase on the printed item, for example on a test chart, color spectrum data and images of the printed item can be simultaneously obtained with a single scan of the printed item.

The camera allows to obtain a number of images of several regions of a same color patch of the test chart. This is useful for detecting inhomogeneous color distribution over a same patch, by comparing the images of different regions. Moreover, print defects can be indentified in a similar way by comparing images of different regions of a same color patch.

Moreover the camera, which for example is a digital camera, allows to obtain images with a spatial resolution which is much higher than the spatial resolution of the spectral data.

In other words, the camera allows to measure an "image quality" or "print quality" of a printed item and to provide measured values of parameters that are prescribed by image quality regulations; for example, the prescribed parameters include color uniformity, drop width, drop overlap, etc.

An image of the printed item can be an electronic image, i.e. a set of digital data which is representative of an optical image or a picture of the printed item.

In one embodiment, the resolution of the digital camera is sufficient for obtaining an image wherein primary color print patterns of the printed item can be discriminated from each other. In other words, the resolution of the digital camera allows to have data about how colors are obtained, i.e. about how each primary color has been printed by a respective printing element of a printhead. For example, the printing element is a printhead nozzle.

This is useful for checking the functioning and the calibration of each printing element. In fact an actual print pattern of the printing element, which is derivable from the image taken by the camera, can be compared with an expected or theoretical print pattern for the same printing element.

Since the spectral sensor and the camera are in a fixed spatial relation to each other, the distance between a first printed region, which is measured by the spectral sensor, and a second printed region, which is simultaneously recorded by the camera, is known.

This is useful for properly organizing the obtained measures in a database wherein color spectrum data and images are associated with the respective color patches. In fact, even if the first printed region and the second printed region belong to different color patches, knowing the distance between them allows to ascribe the color spectrum data and the simultaneously recorded images to the relative color patches.

Further advantages, characteristic features and the modes of use of the subject of the present disclosure will become clear from the following detailed descriptions of preferred embodiments thereof, provided solely by way of non-limiting examples.

It is clear, however, that each embodiment described in the present disclosure may have one or more of the advantages listed above; in any case it is not required that each embodiment should have simultaneously all the advantages listed.

It is also to be understood that the scope of the present disclosure includes all the possible combinations of the embodiments mentioned above and those described with reference to the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference shall be made to the figures of the accompanying drawings, in which: - Figure 1 shows a perspective view of a test chart including a plurality of color patches;

Figure 2 shows a schematic view of how a color patch is printed by a printer; Figure 3 shows a perspective view of a first embodiment of a color measurement unit according to the present disclosure, during a measurement run on a test chart;

- Figure 4 shows a top perspective view of the color measurement unit according to Figure 3; - Figure 5 shows a bottom perspective view of the color measurement unit according to Figure 3;

- Figure 6 shows a sectional view, according to section plane VI of Figure 4, of the color measurement unit according to Figure 3;

- Figure 7 shows a schematic view of an operation pri nci ple of the color measurement unit according to Figure 3;

Figure 8 shows a sectional view, according to section plane VIII of Figure 6, of the color measurement unit according to Figure 3, from which some parts have been removed;

- Figure 9 shows a bottom perspective view of the color measurement unit according to Figure 3, in a second condition;

Figure 10 shows a sectional view, according to section plane X of Figure 9, of the color measurement unit according to Figure 3 in the second condition, from which some parts have been removed;

- Figure 1 1 shows a sectional view, as in Figure 6, of a second embodiment of a color measurement unit according to the present disclosure;

- Figure 12 shows an exploded perspective view of an enlarged detail of the color measurement unit according to Figure 1 1 ;

- Figure 1 3 shows a top perspective view of a third em bodi ment of a color measurement unit according to the present disclosure;

- Figure 14 shows a bottom perspective view of the color measurement unit according to Figure 13;

- Figure 15 shows a sectional view, according to section plane XV, of the color measurement unit according to Figure 13;

- Figure 16 shows a top perspective view of a fourth embodiment of a color measurement unit according to the present disclosure;

- Figure 17 shows a bottom perspective view of the color measurement unit according to Figure 16;

- Figure 18 shows a sectional view, according to section plane XVI II , of the color measurement unit according to Figure 16;

- Figure 1 9 shows a top perspective view of a fifth em bodiment of a color measurement unit according to the present disclosure;

- Figure 20 shows a bottom perspective view of the color measurement unit according to Figure 19;

- Figure 21 shows a sectional view, according to section plane XXI , of the color measurement unit according to Figure 19;

Figure 22 shows a first embodiment of a measurement apparatus according to the present disclosure;

- Figure 23 shows a second embodiment of a measurement apparatus according to the present disclosure;

- Figure 24 shows a third embodiment of a measurement apparatus according to the present disclosure;

- Figure 25 shows a detail of a printer includi ng a color measurement unit according to the present disclosure.

DESCRIPTION OF EXAMPLE EMBODIMENTS

A first embodiment of a color measurement unit according to the present disclosure is shown in Figures 3 to 10, where it is indicated by reference number 1.

The color measurement unit 1 is intended for measuring a color on a printed item; to be more specific, the printed item is a test chart 9.

In the example, the test chart 9 is a sheet on which patches 91 have been printed by a digital printer; each patch 91 has a respective color.

For example, the patches 91 are square-shaped and are arranged according to a plurality of parallel rows 94.

Each patch 91 is representative of a respective reference color or test color. A specific set of reference colors is chosen from standardized sets which are known in the art. Therefore, the test chart 9 is a tool for evaluating the accuracy of the printer in reproducing a specific set of reference colors.

It should be considered that a color printer produces a color region by a combination of primary color printing. For example, Figure 2 shows an enlarged view of a portion of a color patch 91 , which is obtained by superimposition of black printing 96, magenta printing 97, yellow printing 98, and cyan printing 99.

The enlarged view of Figure 2 shows a print pattern 93 for each primary color printing. In fact, each primary color is printed by a respective printing element of a printhead according to a prescribed path which, when seen in an enlarged view, appears as a print pattern. The color of the patch 91 is formed by the combination and overlapping of the primary color print patterns 93.

The color measurement unit 1 is a measuring head that, for example, can be mounted on a printer, on a printhead or on a measuring device.

The color measurement unit 1 comprises a casing 10 having a box-like shape. A first face 1 1 of the casing 10 is provided with a window 15, which for example is a hole, an aperture or a transparent portion of the casing 10. In other words, the casing 10 is opaque and external light can enter into the casing 10 only through the window 15.

For example, the casing 10 is made of opaque plastic and the window 15 is a hole provided with a lens or a protective transparent element.

For example, the window 15 is circular and it has a diameter D of 6 mm.

I n the em bod i ment shown , the fi rst face 1 1 is a bottom face of the color measurement unit 1. In use, the first face 1 1 faces the test chart 9.

A spectral sensor 21 is positioned inside the casing 10 to receive light entering through the window 15. To be more specific, the spectral sensor 21 is a spectrophotometer and is configured to measure color spectrum data of the received light.

In the embodiment shown, the spectral sensor 21 is positioned facing the window 15 and it has a respective optical axis 210 which crosses the window 15. In particular, the optical axis 210 is perpendicular to the first face 11.

The color measurement unit 1 comprises an illumination system for illuminating the test chart 9 with an illuminating light 251. A portion 253 of the illuminating light 251 is reflected by the test chart 9, enters into the casing 10 through the window 15, and is received by the spectral sensor 21 . When a color patch 91 is illuminated by the illuminating light 251 , the spectrum of the reflected light 253 is related to the color of the patch 91.

Therefore, the color measurement unit 1 is suitable for reflective measuring, since the light received by the spectral sensor 21 is a portion 253 of the illuminating light 251 which is reflected by the test chart 9 along the optical axis 210.

In the embodiment shown, the illumination system is positioned inside the casing 10 and is configured to point an illuminating light 251 to the window 15. In other words, the illuminating light 251 crosses the window 15 in a first direction and the reflected light 253 crosses the window 15 in a second direction.

In particular, the illumination system comprises a plurality of illuminating devices 25. Each illuminating device 25 emits a respective illuminating light beam 252 which is at an angle A of 45 degrees with the reflected light 253, i.e. with the optical axis 210. Then the illuminating light beam 252 is at an angle of 45 degrees with the first face 11 and the test chart 9 as well.

The overall illuminating light 251 is constituted by the combination of the illuminating light beams 252 of the illuminating devices 25.

A measuring technique adopting such an angle of 45 degrees for the illuminating light is known in the art.

The illuminating devices 25 are positioned inside the casing 10 and around the window 15, i.e. on a circumference 151 centered on the window 15. The illuminating devices 25 are angularly spaced from each other according to a regular distribution. In the embodiment shown, the illumination system comprises three illuminating devices 25 positioned on said circumference 151 at an angle B of 120 degrees from each other.

Each illuminating device 25 is able to produce an illuminating light beam 252 which well approximates a standard light source denoted as D50 by the International Commission on Illumination.

For example, each illuminating device 25 comprises seven LEDs 26 different from each other. The light intensity of each LED 26 can be adjusted independently from the light intensity of the other LEDs 26.

The combination of the lights emitted by the LEDs 26 is an illuminating light beam 252 which almost perfectly replicates a D50 standard light source. US patent application US 2011/062873 gives an example of a similar LEDs combination.

In an alternative embodiment (not shown), the light 253 received by the spectral sensor 21 is a portion of a light transmitted through the test chart 9, which for example is a film. In other words, the color measurement unit is suitable for transmissive measuring as well. In such an embodiment, the illumination system is not included in the color measurement unit 1 , or the illumination system is positioned outside the casing 10 and/or the window 15 is positioned between the illumination system and the spectral sensor 21 , so that in use the test chart film 9 is housed between the illumination system and the window 15.

The color measurement unit 1 further comprises a camera 23 to take an image of the test chart 9; in particular the image represents a portion of a color patch 91. The color measurement unit 1 is configured to take the image through the camera 23 together with measuring the color spectrum data through the spectral sensor 21. To be more specific, the camera 23 is a digital camera.

The digital camera 23 is positioned in a fixed spatial relation with the spectral sensor 21 , so that the digital camera 23 is movable together with the spectral sensor 21 without changing the mutual spatial relation. This means that the spectral data measured by the spectral sensor 21 can be related with the image which is simultaneously taken or recorded by the digital camera 23.

In the embodiments shown in Figures 3 to 1 1 , the digital camera 23 is mounted inside the casing 10. Moreover, the digital camera 23 is configured to receive an image of the test chart 9 through the window 15, i.e. the image is associated with the reflected light 253 which is received through the window 15.

The color measurement unit 1 comprises also an optical device which is configured to split up the reflected light 253 between the spectral sensor 21 and the digital camera 23.

To be more specific, the digital camera 23 has a respective optical axis 230. The reflected light 253 received through the window 15, which has a respective direction 150, is split up into a first light portion 253a and a second light portion 253b. The first light portion 253a has a direction along a first optical axis coinciding with the optical axis 210 of the spectral sensor 21 ; the second light portion 253b has a direction along a second optical axis coinciding with the optical axis 230 of the digital camera 23.

In the embodiments shown in Figures 3 to 1 1 , the first optical axis 210 crosses the window 15 and is aligned with the direction 150 of the reflected light 253, i.e. the spectral sensor 21 is oriented perpendicular to the window 15, whereas the digital camera 23 is positioned at a side between the window 15 and the spectral sensor 21 , i.e. at a side of the reflected light direction 150. For example, the second optical axis 230 is perpendicular to the first optical axis 210.

In the embodiment shown in Figures 6 and 7, the splitting optical device is a semi- transparent mirror 28 which reflects only a part 253b of the incident light 253, whereas the remaining part 253a of the incident light 253 is transmitted through the semi-transparent mirror 28. Semi-transparent mirrors are already known per se. The semi-transparent mirror 28 is positioned between the window 15 and the spectral sensor 21 , and faces the digital camera 23. The semi-transparent mirror 28 is at an angle M of 45 degrees with the direction 150 of the incident light 253, so that the transmitted part 253a is along the first optical axis 210 and the reflected part 253b is along the second optical axis 230.

In the embodiment shown in Figure 1 1 , the splitting optical device is a beamsplitter prism 29. As shown in the exploded view of Figure 12, the beamsplitter prism 29 is divided into two parts 29a, 29b, each of them having a slanted planar surface 290 perfectly fitting to the slanted planar surface 290 of the other part. At least one of the slanted surfaces 290 is semi-reflective, i.e. it reflects a part 253b of the incident light 253 and transmits the remaining part 253a of the incident light 253. Therefore, only a part 253a of the incident light 253 is transmitted through the beamsplitter prism 29, whereas the remaining part 253b of the incident light 253 is reflected by the slanted surfaces 290.

The beamsplitter prism 29 is positioned between the window 15 and the spectral sensor 21 , and faces the digital camera 23. The slanted surfaces 290 of the beamsplitter prism 29 are at an angle P of 45 degrees with the direction 150 of the incident light 253, so that the transmitted part 253a is along the first optical axis 210 and the reflected part 253b is along the second optical axis 230.

The beamsplitter prism 29 is useful for keeping the transmitted light 253a aligned with the incident light 253. In fact, the beamsplitter prism 29 is useful for avoiding a slight misalignment between transmitted light 253a and incident light 253 which, for a semi-transparent mirror 28, is caused by refraction through the thickness of the mirror 28.

It is evident that the positions of the spectral sensor 21 and the digital camera 23 may be reverse, i.e. the spectral sensor 21 may receive the reflected light part 253b and the digital camera 23 may receive the transmitted light part 253a.

In an alternative embodiment shown in Figures 13 to 15, the digital camera 23 is outside the main casing 10 including the window 15 and the spectral sensor 21. In fact, the digital camera 23 is inside a second casing 13 mounted on the main casing 10 or connected to the latter by a connection member. The main casing 10 and the second casing 13 are in optical communication with each other, so that said second light portion 253b reaches the digital camera 23.

Since the main casing 10 and the second casing 13 are fastened to each other, the spectral sensor 21 and the digital camera 23 are movable together as a single body; therefore, the fixed spatial relation between the spectral sensor 21 and the digital camera 23 is maintained when the color measurement unit 1 is moved.

Also in this case, the positions of the spectral sensor 21 and the digital camera 23 may be reverse.

In an alternative embodiment shown in Figures 16 to 21 , a second window 17 is provided on the first face 1 1 of the casing 10. The second window 17 is spaced- apart from the first window 15, i.e. the second window 17 is at a distance W from the first window 15.

The second window 17 is similar to the first window 15 and is configured to receive light reflected by, or transmitted through, the test chart 9. In this embodiment the digital camera 23 receives an image of the test chart 9 through the second window 17; in other words, the recorded image is associated with a light received through the second window 17.

For example, the digital camera 23 is positioned facing the second window 17; its optical axis 230 crosses the second window 17 and is aligned with the direction of the respective reflected light. To be more specific, the optical axis 230 of the digital camera 23 is perpendicular to the first face 11 , i.e. the digital camera 23 is oriented perpendicular to the second window 17.

The data measured by the spectral sensor 21 and the image simultaneously recorded by the digital camera 23 are not pertinent to a same measuring region of the test chart 9 as for the previous embodiments; in fact, the data and the simultaneous image are pertinent to two different measuring regions of the test chart 9, the two measuring regions having a fixed distance W from each other. Since the distance W is preset, the relative positions of the two measuring regions are known and the position of one region (for example, the region recorded by the digital camera 23) can be calculated from the position of the other region (for example, the region measured by the spectral sensor 21).

The illumination system is configured to point an illuminating light also to the second window 17. For example, additional illuminating devices 25 are positioned around the second window 17 likewise for the first window 15.

In an alternative embodiment shown in Figures 19 to 21 , the digital camera 23 is inside a second casing 13 which is mounted on, or connected to, the main casing 10. The second window 17, through which the digital camera 23 receives an image of the test chart 9, is provided on a first face 14 of the second casing 13 that is intended to face the test chart 9 in use.

In this embodiment the spectral sensor 21 and the digital camera 23 are in different casings 10, 13 and they receive light through different windows 15, 17. However, the main casing 10 and the second casing 13 are fastened to each other, therefore the spectral sensor 21 and the digital camera 23 are movable together as a single body. The fixed spatial relation between the spectral sensor 21 and the digital camera 23 is maintained when the color measurement unit 1 is moved.

Moreover, since the distance W between the windows 15, 17 is preset, the position of one measuring region (for example, the region recorded by the digital camera 23) can be calculated from the position of the other measuring region (for example, the region measured by the spectral sensor 21).

For all the embodiments described above, the digital camera 23 can be a black-and- white camera or a color camera.

The resolution of the digital camera 23 is sufficient for recognizing the print patterns 93 on the printed item 9, i.e. the patterns followed by each printing element of the printer which printed the test chart 9.

The digital camera 23 has, for example, a resolution of 300 dots-per-inch (DPI) or greater. This resolution value is referred to the measuring region of the test chart 9 which is recorded in use; in other words, the pixel number of the digital camera 23 (for example, 2 Megapixel) required to obtain such a resolution depends also on the length of the optical path and on the features of any lens or similar optical device between the respective window 15, 17 and the digital camera 23.

If the digital camera 23 is a color camera, the resolution is sufficient for discriminating between primary color print patterns 93, i.e. for recognizing each pattern 93 of the different color printing elements of the printer.

When carrying out a measurement, the color measurement unit 1 is positioned above a color patch 91 to be measured. To be more specific, the first face 1 1 of the casing 10 faces the color patch 91 and the window 15 is at a color patch region to be measured. The window 15 is smaller than the color patch 91 (for example, the window 15 has a diameter D of 6 mm and the color patch 91 is a square having a side length of 1 cm), therefore only a measurement region of the color patch 91 is considered during a single measuring step.

The first face 1 1 may touch the test chart 9 and rest on it, or the first face 11 may be slightly raised from the test chart 9.

The illumination system creates an illuminating light 251 that goes out through the window 15 and illuminates the measurement region of the color patch 91. The color patch 91 reflects the illuminating light 251 ; a reflected portion 253 of the illuminating light 251 enters into the casing 10 through the window 15 along direction 150.

This reflected light portion 253, whose spectrum is related to the color of the patch 91 , is split between the optical axis 210 of the spectral sensor 21 and the optical axis 230 of the digital camera 23.

The spectral sensor 21 measures the color spectrum and obtains data which are representative of the color of the measurement region of the patch 91. This type of color spectrum measurement is already known in the art.

At the same time, the digital camera 23 takes an image of the measurement region of the patch 91 . To be more specific, the image is a digital image that can be processed by an electronic processing unit.

The color spectrum data and the image, which pertain to a same measurement region, are saved into an internal memory or data storage, and/or are transmitted to a processing unit.

The color measurement unit 1 is then moved to a new measurement region and new color spectrum data and image are obtained for the new measurement region. For example, the spectral sensor 21 and the digital camera 23 have a sampling rate of 200 Hz, therefore spectral data and images of 200 measurement regions are obtained each second while the color measurement unit 1 is moved relative to the test chart 9 during a measurement run.

It is evident that several measurement regions can be measured for a same color patch 91.

A database, wherein color spectrum data and images are associated with respective measurement regions, can be obtained.

Each measurement region can be related with a respective color patch 91 , therefore the color spectrum data and the image can be compared with expected features and values for the same color patch 91. For example, a distance-measuring device gives a distance of the color measurement unit 1 from a starting point. The color patch 91 to which the measurement region is referred can be determined from the measured distance and the known side length of the patches 91.

Alternatively, a sharp variation of the color spectrum data between consecutive measurement regions is processed as an indication of a transition from a color patch 91 to an adjacent color patch 91. Therefore, knowing the color patch sequence, each measurement region can be associated with the respective color patch 91.

In the embodiments where the spectral sensor 21 and the digital camera 23 receive light from two different windows 15, 17, also the distance W between the two windows 15, 17 is taken into account to associate the color spectrum data / images with the respective color patches 91.

A print quality can be measured and determined from the obtained measurements. For example, a deviation between the color spectrum data and reference values for the same color patch 91 is calculated. The extent of such a deviation is related to the ability of the printer in reproducing a test color as desired, i.e. with a desired print quality.

The digital images are processed by the processing unit. For example, the images pertaining to a same color patch 91 are compared with each other or with a reference image in order to detect defects which are below the sensitivity of the spectral sensor 21.

In fact, due to the greater resolution of the digital camera 23, each image allows to examine small portions of the color patch 91. Each of such small portions is much smaller than the size of the window 15 and of the single measurement region; for example each small portion has a size of few hundredths of millimeter, depending on the resolution of the digital camera 23.

The images obtained by the digital camera 23 allow to measure also parameters which are prescribed by regulations in the technical field, such as color uniformity, drop width, drop overlap, etc.

Therefore, thanks to the digital camera 23 it is possible to find out print defects that cannot be discovered by the spectral sensor 21.

For example, it is possible to identify the actual print pattern 93 produced by each printing element of a printhead. The comparison between the actual print pattern 93 and the expected print pattern is useful for checking if the printing element works correctly and/or if the printing elements are properly aligned with each other. These types of check are possible even if the digital camera 23 is a black-and-white camera.

If the digital camera 23 is a color camera, the print patterns 93 of the color printing elements can be recognized, i.e. primary color print patterns 93 in a same colored region can be discriminated from each other.

The processing unit is configured to recognize the primary color print patterns 93 in the image and to check if each print pattern 93 matches the expected pattern for the same color and/or if the color print patterns 93 are properly overlapping each other. Therefore, a print quality can be established from the deviations between actual spectral data / images and the reference expected value / images. The printer settings are iteratively changed and adjusted until a desired print quality is met.

In a calibration phase or in an initial measurement phase, the spectral sensor 21 may be used also to measure an ambient light, for example by directing the first face 1 1 and the window 15 toward a ceiling or a light source in a measurement room. This is useful for having data about the ambient light under which the test chart 9 (and also the final printed product) is watched, because the ambient light can be different from a D50 standard light.

In one embodiment, the light intensity of each LED 26 can be adjusted so that the combination of the lights emitted by the LEDs 26 is an illuminating light 251 which replicates the measured ambient light. In other words, the illumination system of the color measurement unit 1 is able to simulate or reproduce the ambient light, which is used during the measurement phase instead of a D50 standard light.

This is useful for carrying out the measurements under the same actual light conditions under which the test chart 9 and the final printed product are watched by an operator, a color technician or a customer.

For example, the adjustment of the light intensity of each LED 26 in order to reproduce such an ambient light is automatically performed by a control unit of the color measurement unit 1 or by a processing unit 40, the control unit or processing unit being configured to process the measured ambient light and to accordingly set the LEDs 26 for reproducing the measured light.

The color measurement unit 1 may comprise a temperature sensor and/or a humidity sensor to measure temperature and/or humidity outside the casing 10; in fact, the measured spectral data can be influenced by the local conditions at which the measurement is carried out.

The color measurement unit 1 may comprise a distance-measuring device for measuring a displacement or a movement of the color measurement unit 1 relative to the test chart 9. For example, a pivoted wheel protrudes from the first face 1 1 of the casing 10, so that the pivoted wheel rotates when the color measurement unit 1 is moved along the test chart 9. The pivoted wheel is connected to a transducer or an encoder which calculates the length of the movement on the basis of the angular displacement of the pivoted wheel. This is useful for determining the position of the color measurement unit 1 on the test chart 9 and then which color patch 91 is being measured and/or recorder.

The color measurement unit 1 may comprise one or more laser pointer(s) 18 positioned on side face(s) of the casing 10 to indicate a measurement direction 100. For example, the color measurement unit 1 comprises two laser pointers 18 positioned at opposite external side faces 12a, 12b of the casing 10. Each laser pointer 18 is configured to emit a respective laser beam 180 in a direction 100 along which the color measurement unit 1 is moved during a measurement run.

This is useful for assuring that the measurement direction 100, which is aligned with the window 15, is along a row 94 of consecutive color patches 91 to be measured. The two laser pointers 18 emit beams 180 in opposite directions, because the color measurement unit 1 can be moved in both directions. The two beams 180 are aligned with each other and with the window 15. Therefore, the laser pointers 18 indicate where measurements will be carried out.

The color measurement unit 1 may comprise one or more reference tongue(s) 19 protruding from side face(s) of the casing 10 to center the window 15 on the color patches 91. For example, the color measurement unit 1 comprises two reference tongues 19 positioned at the opposite external side faces 12a, 12b of the casing 10. The two reference tongues 19 are aligned with each other and also with the window 15, i.e. they are aligned with the measurement direction 100. Each reference tongue 19 is spike-shaped.

In use, the color measurement unit 1 is positioned so that the reference tongues 19 are aligned with the center-line of a row 94 of consecutive color patches 91 to be measured. This is useful for assuring that the measurement direction 100 is centered relative to the row 94 of patches 91 and that an erroneous measuring of patches 91 of adjacent rows is prevented.

The data and the images obtained by the color measurement unit 1 may be transferred to a processing unit via a wire, a USB port, a serial port, or a wireless (wi-fi, bluetooth,...) communication. The color measurement unit 1 may comprise an internal data storage to save color spectral data and images.

The color measurement unit 1 may comprise an internal battery or may be connected to an external power supply.

The data measured by the spectral sensor 21 may be used to calibrate the digital camera 23, in order to ensure that the digital camera 23 correctly reports colors in RGB format. To be more specific, the spectral sensor 21 is used to measure color spectrum data of color patches 91 of a reference color target and the obtained data are used to profile or calibrate the digital camera 23 after image registration of the same color patches 91.

The color measurement unit 1 may comprise a shutting member 16 for the window 15. The shutting member 16 is movable between a first position (Figure 5), in which the window 15 is open and light 253 can enter into the casing 10, and a second position (Figure 9), in which the window 15 is shut by the shutting member 16.

When the shutting member 16 is in the second position, the shutting member 16 is useful for preventing dust or dirt from going inside the casing 10 when the color measurement unit 1 is not in use.

Moreover, an internal side 16a of the shutting member 16 is provided with a calibration reference target 160, which for example is a portion with a reference white color. When the shutting member 16 is in the second position, the calibration reference target 160 is at the window 15, i.e. it faces the spectral sensor 21.

This is useful for calibrating the spectral sensor 21. For example, a first calibration measurement is carried out when the shutting member 16 is in the second position and the illumination system is off. In this case, a total darkness condition is met inside the casing 10 and the data measured by the spectral sensor 21 are referred to full black. A second calibration measurement is carried out when the shutting member 16 is in the second position and the illumination system is on. In this case, the calibration reference target 160 is measured by the spectral sensor 21 ; for example, data for reference white are obtained. Therefore, the data obtained for full black and reference white are compared with the expected reference values and the spectral sensor 21 can be calibrated accordingly.

The color measurement unit 1 can be included in different types of measurement apparatuses; some embodiments are shown in Figures 22 to 24.

A measurement apparatus for measuring a print quality of a printed product or a printed item, in particular of a test chart 9, comprises a color measurement unit 1 and a processing unit 40. The processing unit 40 is operatively connected with the color measurement unit 1 , i.e. the processing unit 40 receives the color spectrum data and the i mage data which are measu red and recorded by the color measurement unit 1.

The processing unit 40 processes and elaborates these data to determine a print quality, in particular for calibration purposes of a printer. Further details are given in the following.

A first embodiment of a measurement apparatus is shown in Figure 22, where it is denoted by reference number 6. The measurement apparatus 6 comprises a main body 61 and a color measurement unit 1 which is movable relative to the main body 61. The main body 61 includes the processing unit 40 and a display 63 for showing data and information about the measurements and the status of the apparatus 6. The measurement apparatus 6 comprises a guiding rail 65 which is mounted on the main body 61. The color measurement unit 1 is mounted on the guiding rail 65 and can slide along the guiding rail 65 according to a measurement direction 100. Therefore, the guiding rail 65 guides the movement of the color measurement unit 1 relative to the main body 61 during a measurement run. The data obtained by the color measurement unit 1 are transferred to the processing unit 40 via a wire or a wireless communication.

The measurement apparatus 6 is useful for carrying out measurements on a test chart 9 which is kept stationary relative to the main body 61 during a measurement run. For example, the test chart 9 and the measurement apparatus 6 are positioned on a table and the measurements are carried out by moving the color measurement unit 1 along the guiding rail 65 in a first measurement run, so as to measure a first row 94 of color patches 91 of the test chart 9. When the first row 94 has been measured, the test chart 9 or the main body 61 are moved to align the color measurement unit 1 with a second row 94 of color patches 91. The color measurement unit 1 is moved along the guiding rail 65 to measure the second row 94 of color patches 91 in a second measurement run. Further measurement runs are performed until all rows 94 of color patches 91 have been measured.

A second embodiment of a measurement apparatus is shown in Figure 23, where it is denoted by reference number 60. The measurement apparatus 60 comprises a board 62 and a couple of rulers 64a, 64b. The rulers 64a, 64b are perpendicular to each other and are movable relative to the board 62. To be more specific, the rulers 64a, 64b are slidingly mounted on rails 66a, 66b configured to allow a two- dimensional movement of the couple of rulers 64a, 64b on the board 62. Also the tilt angle of the rulers 64a, 64b relative to a lower edge 62a of the board 62 can be adjusted thanks to a pivot connection 66c between the couple of rulers 64a, 64b and the rails 66a, 66b.

The measurement apparatus 60 comprises a color measurement unit 1 which is movable relative to the rulers 64a, 64b. For example, the color measurement unit 1 can slide along each one of the rulers 64a, 64b, which then are guiding tracks or rails for the color measurement unit 1.

In the embodiment shown, a processing unit 40 and a display 63 are included in the same casing 10 of the color measurement unit 1. The measurement apparatus 60 is useful for carrying out measurements on a test chart 9 comprising color patches 91 arranged according to many rows and columns. The test chart 9 is positioned on the board 62 and fastened to the latter.

The rulers 64a, 64b are moved until the color measurement unit 1 is aligned with a first row (or a first column) of color patches 91 . A first measurement run is carried out by moving the color measurement unit 1 along the ruler 64a (or along the ruler 64b), so that the first row (or first column) is measured. Then the rulers 64a, 64b are moved until the color measurement unit 1 is aligned with a second row (or a second column) of color patches 91 and a second measurement run is carried out by moving the color measurement unit 1 along the ruler 64a (or along the ruler 64b), so that the second row (or second column) is measured. Further measurement runs are performed until all rows (or all columns) of color patches 91 have been measured.

A third embodiment of a measurement apparatus is shown in Figure 24, where it is denoted by reference number 7. The measurement apparatus 7 comprises a support table 71 , an arm 73 and a color measurement unit 1 fastened to the arm 73 and facing the support table 71. For example, a processing unit 40 is housed within a side casing 75 which the arm 73 protrudes from.

The support table 71 is movable relative to the arm 73. To be more specific, the support table 71 is equipped with a motor 77 and guiding means 78 for moving the support table 71 relative to a base 79. The arm 73 and the side casing 75 are fastened to the base 79. A test chart 9 is positioned on the support table 71 and fastened to the latter. The support table 71 moves the test chart 9 under the color measurement unit 1 , which carries out the measurement of the color patches 91. A measurement apparatus according to the present disclosure can include a color measurement unit 1 mounted in a printer 8. Such a measurement apparatus is useful for providing a single apparatus able to print and calibrate the printer 8 itself. For example, a printer 8 comprises a printhead 81 including at least one printing element 83 (for example, an ink ejector for printing a respective color or a plurality of ink ejectors each one specific for a respective color) and a color measurement unit 1.

In other words, the printing element(s) 83 and the color measurement unit 1 are mounted on a same movable carriage 85. Therefore they are moved together by a single motor device 87.

Other embodiments of the subject of the present disclosure may exist, all of these falling within the scope of protection of the claims which are provided hereinbelow.

Claims

1. A color measurement unit (1) for measuring a color on a printed item (9), comprising:

- a casing (10) having a first face (1 1) intended, in use, to face a printed item (9), the first face (1) being provided with a window (15);

- a spectral sensor (21) configured to measure color spectrum data of a light (253) received through the window (15), the received light (253) being a light reflected by the printed item (9) or a light transmitted through the printed item (9);

- a camera (23) configured to take an image of the printed item (9), the camera

(23) being movable together with the spectral sensor (21),

the color measurement unit (1) being configured to take the image together with measuring the color spectrum data.

2. The color measurement unit (1) according to claim 1 , wherein the camera (23) is configured to receive an image of the printed item (9) through said window

(15), the image being associated with the light (253) received through the window (15).

3. The color measurement unit (1) according to claim 2, wherein the spectral sensor (21) has a first optical axis (210) and the camera (23) has a second optical axis (230), the color measurement unit (1) comprising a splitting optical device (28, 29) for splitting up the light (253) received through the window (15) into a first light portion (253a) along the first optical axis (210) and a second light portion (253b) along the second optical axis (230).

4. The color measurement unit (1 ) according to claim 3, wherein the splitting optical device is a semi-transparent mirror (28) or a beamsplitter prism (29).

5. The color measurement unit (1) according to claim 3 or 4, wherein the first optical axis (210) crosses the window (15), the spectral sensor (21) facing the window (15) and the camera (23) being at a side between the window (15) and the spectral sensor (21).

6. The color measurement unit (1 ) according to claim 3, 4 or 5, wherein the second optical axis (230) is perpendicular to the first optical axis (210).

7. The color measurement unit (1) according to claim 1 , wherein said window is a first window (15), the camera (23) being configured to receive an image of the printed item (9) through a second window (17), the image being associated with a light received through said second window (17), wherein the second window

(17) is provided on a respective face (1 1 , 14) intended, in use, to face the printed item (9), the second window (17) being spaced-apart from the first window (15).

8. The color measurement unit (1 ) according to any one of claims 1 to 7, wherein the spectral sensor (21) and the camera (23) are inside the casing (10).

9. The color measurement unit (1 ) according to any one of claims 1 to 8, wherein the camera (23) is a black-and-white digital camera.

10. The color measurement unit (1) according to any one of claims 1 to 8, wherein the camera (23) is a color digital camera.

11. The color measurement unit (1) according to claim 10, wherein the color digital camera (23) has a resolution which is sufficient for discriminating between primary color print patterns (93) of the printed item (9).

12. The color measurement unit (1) according to any one of claims 1 to 1 1 , wherein the camera (23) is a digital camera having a resolution of 300 DPI or greater.

13. The color measurement unit (1) according to any one of claims 1 to 12, comprising an illumination system for illuminating the printed item (9), wherein the illumination system comprises at least three illuminating devices (25) positioned around said window (15) , the illuminating devices (25) being angularly spaced from each other according to a regular distribution.

14. The color measurement unit (1) according to any one of claims 1 to 13, comprising an illumination system for illuminating the printed item (9), wherein the illumination system comprises at least one illuminating device (25) including a plurality of LEDs (26) different from each other.

15. The color measurement unit (1) according to claim 13 or 14, wherein said illuminating device (25) is configured to produce an illuminating light (251 , 252) which almost perfectly replicates a D50 standard light source.

16. The color measurement unit (1 ) according to claim 14, wherein the light intensity of each LED (26) is adjustable independently from the light intensity of the other LEDs (26), whereby the illuminating device (25) is adjustable to produce an illuminating light (251 , 252) which reproduces an ambient light measured by the spectral sensor (21).

17. The color measurement unit (1 ) according to any one of claims 13 to 16, wherein the illumination system is inside the casing (10) and is configured to point an illuminating light (251 , 252) to said window (15) or to said first window (15) and second window (17).

18. The color measurement unit (1) according to any one of claims 1 to 17, comprising a shutting member (16) for shutting said window (15), the shutting member (16) being movable between a first position and a second position, the window (15) being open when the shutting member (16) is in the first position and the window (15) being shut by the shutting member (16) when the shutting member (16) is in the second position, wherein an internal side (16a) of the shutting member (16) is provided with a calibration reference target (160), the calibration reference target (160) being at the window (15) when the shutting member (16) is in the second position.

19. A measurement apparatus (6, 60, 7, 8) for measuring a print quality of a printed item (9), comprising a color measurement unit (1) according to any one of claims 1 to 18 and a processing unit (40), the processing unit (40) being operatively connected to the color measurement unit (1) and being configured to process color spectrum data and i mage data received by the color measurement unit (1) to determine a print quality.

20. The measurement apparatus (6, 60) according to claim 19, comprising a main body (61 , 62) and a guiding member (65, 64a, 64b, 66a, 66b) for guiding a movement of the color measurement unit (1) relative to the main body (61).

21. The measurement apparatus (7) according to claim 19, comprising a support table (71) for the printed item (9), the color measurement unit (1) being mounted on a base (79) and being positioned facing the support table (71), wherein the support table (71 ) is movable relative to the base (79) and to the color measurement unit (1).

22. A printhead (81) for a printer (8), comprising at least one printing element (83) and a color measurement unit (1) according to any one of claims 1 to 18, said at least one printing element (83) and said color measurement unit (1 ) being movable together.

23. A printer (8) comprising a printhead (81) according to claim 22.

24. A method for measuring a print quality of a printed item (9), comprising the following step:

- providing a printed item (9);

- measuring color spectrum data of the printed item (9);

- taking an image of the printed item (9);

- calculating a deviation of the color spectrum data from a reference value and calculating a deviation of the taken image from a reference image;

- determining a print quality on the basis of said deviations,

wherein the step of taking the image is carried out together with the step of measuring the color spectrum data.

25. The method according to claim 24, wherein the steps of measuring a color spectrum data and of taki ng an i mage are carried out at a pl urality of measurement regions of the printed item (9).

26. The method according to claim 24 or 25, wherein the step of calculating a deviation of the taken image from a reference image comprises the step of discriminating between primary color print patterns (93) of a same measurement region of the printed item (9).