IN A DIGITAL PRINTING PRESS

FIELD OF THE DISCLOSED TECHNIQUE The disclosed technique relates to printing presses in general, and to a method and system for aligning print-heads in a digital printing press, in particular,

BACKGROUND OF THE DISCLOSED TECHNIQUE Digital printing presses in general and ink jet based printing presses in particular (e.g., printing sheets or continuous webs of labels or packaging) are required to print a print job continuously and with minimum waste. Waste is defined as printed material which is not sellable, substrate which is not used to print the printed design (i.e., the product) or is printed with the design but not at acceptable quality, and thus does not generate revenue generation and the like. Digital printing presses typically employ print-heads to print the design on a substrate (e.g., a sheet or a web). These print-heads are typically arranged in rows, where each row of print-heads prints a respective color. These print-heads need to be aligned within a certain degree of error. Should the print-heads not be aligned, the quality of the printed design may degrade.

U.S. Patent Application Publication 2015/0174934 to Bogart, entitled “System and Process for Automatic Print Head Registration of a Digital Printing Machine” directs to a printing a calibration test page which includes overlapping registration targets created by the first and second print heads. Each of the overlapping registration targets includes a target region comprising X and Y co-ordinates. The overlapping registration targets are scanned to produce an image. The image is processed to produce a miss-registration score for each target region. An X and Y registration correction amounts for the second print head based upon the miss-registration scores and the second print head is adjust based upon the X and Y registration correction amounts.

SUMMARY OF THE PRESENT DISCLOSED TECHNIQUE it is an object of the disclosed technique to provide a novel method, system and patterns for aligning print-heads in a digital printing press. In accordance with the disclosed technique, there is thus provided a system for aligning print-heads in a printing press. The printing press includes a print-head array which includes a plurality of print-head rows. Each print-head row includes a plurality of print-heads. The print-heads in each print-head row print a color associated with the print-head row. Each print-head includes an array of nozzles. The array of nozzles includes a plurality of nozzle lines. The system includes an imager and a processor. The imager includes at least one imaging sensor. The imager is configured to acquire an image of a printed first alignment pattern and further configured to acquire an image of a printed second alignment pattern. The processor is configured to receive the image of the first alignment pattern and the image of the second alignment pattern, The processor is further configured to determine alignment of reference print-heads in at least a reference row of print-heads from the image of the first alignment pattern. The processor is also configured to determine alignment of color print-heads in rows other than the reference row of print-heads, relative to the reference print-heads, from the image of the second alignment pattern. in accordance with another aspect of the disclosed technique, there is thus provided a method for aligning print-heads in a printing press. The printing press includes a print-head array which includes a plurality of print-head rows. Each print-head row includes a plurality of print-heads. The print-heads in each print-head row print a color associated with the print-head row. Each print-head includes an array of nozzles. The array of nozzles includes a plurality of nozzle lines. The method includes the steps of printing a first alignment pattern on a substrate, the first alignment pattern includes a first print-head pattern respective of each the print-head in each of the print-head rows and acquiring an image of a printed the first alignment pattern. The method also includes the steps of determining from the acquired image of the printed first alignment pattern a respective print-head rotation angle for each print-head, and determining horizontal and vertical displacement respective of each the print-head in at least a reference row of print-heads, from the acquired image of the first alignment pattern. The method further includes the steps of printing a second alignment pattern on the substrate, acquiring an image of the printed second alignment pattern and determining horizontal and vertical displacement of each print-head other than the reference print-heads, relative to a reference print-head, from the acquired image of the second alignment pattern.

In accordance with a further aspect of the disclosed technique, there is thus provided a pattern for aligning print-heads in a printing press. The printing press includes a print-head array. The print head array includes a plurality of print-head rows. Each print-head row includes a plurality of print-heads. The print-heads in each print-head row print a color associated with the print-head row. The pattern includes, for each print-head in each row of print-heads, a print-head pattern. The print-head pattern includes at least two dots. The at least two dots in the print-head pattern correspond to respective at least two nozzles from two different nozzle lines in a respective print-head. A theoretical vertical distance and a theoretical horizontal distance between two of said at least two printing nozzles is known.

In accordance with a further aspect of the disclosed technique, there is thus provided a pattern for aligning print-heads in a printing press. The printing press includes a print-head array. The print-head array includes a plurality of print-head rows. Each print-head row includes a plurality of print-heads. Each print-head in a print-head row is associated with a respective row location. Each print-head Includes an array of nozzles. The array of nozzles Includes a plurality of nozzle lines. The pattern includes a plurality of color print-head patterns and a plurality of reference print-head patterns. Each reference print-head pattern printed by a respective one of the reference print-heads. Each oolor print-head pattern Is printed by a respective color print-head In a print-head row other than the reference row. Each color print-head pattern Is associated with a respective first reference print-head pattern and a respective second reference print-head pattern. The first reference print-head pattern and the second reference print-head pattern printed by a respective reference print-head above and below the respective color print-head pattern in the vertical direction. The first reference print-head patern, the second reference print-head pattern and the color print-head pattern printed by a same at least one nozzle In the array of nozzles of a respective print-head thereof. The color print-head pattern, the first print-head pattern and the second print-head pattern are printed by a respective print-head having a same row location.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: Figure 1 is a schematic illustration of a printing press system constructed and operative in accordance with an embodiment of the disclosed technique;

Figures 2A and 2B are exemplary print-head arrays which are aligned in accordance with another embodiment of the disclosed technique;

Figures 3A-3F are schematic illustrations of a method for determining the print rotation error of a print-head, in accordance with a further embodiment of the disclosed technique;

Figures 4A-4G are schematic illustrations of a method for aligning a row of print-heads, operative in accordance with another embodiment of the disclosed technique;

Figures 5A, 5B, 5C and 5D are schematic illustrations of a method for aligning an exemplary print-head array operative in accordance with a further embodiment of the disclosed technique; Figure 6 is a schematic illustration of an exemplary print-head row printing a respective color target-A, in accordance with another embodiment of the disclosed technique;

Figures 7A and 7B are schematic illustrations of an exemplary Target-A and an exemplary Target-B, in accordance with a further embodiment of the disclosed technique; and

Figure 8 is a schematic illustration of a method for aligning print-heads In a digital printing press, operative in accordance with another embodiment of the disclosed technique. DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosed technique overcomes the disadvantages of the prior art by providing a method and system and patterns for aligning print-heads in a digital printing press. The printing press includes a print-head array. The print-head array inciudes a plurality of print-head rows, where each print-head row inciudes a plurality of print-heads. The print-heads in each print-head row print a color associated with the print-head row. Each print-head in a print-head row is associated with a respective row location.

The terms "align”, "alignment” or “aligning” herein relates at least to finding the at least one of the vertical print position error (i.e., on a substrate) of a print-head relative to a reference, the horizontal print position error of a print-head relative to a reference, and the print rotation error of a print-head. The terms ’’align”, “alignment” or “aligning” may further herein relate to employing this difference or differences for compensating for the above mentioned print-position errors. Compensating for the above mentioned print-position errors is achieved by applying corrective actions. These corrective actions include, for example, mechanically translating the print-head, mechanically rotating the print-head, adjusting activation signals (e.g., timing signal, or deflection signal) or adjusting the image data sent to the print heads. Also the term print position error and displacement are employed herein interchangeably. The terms ‘reference color’ and ‘reference row’ are employed herein interchangeably. In general, as mentioned above, a print-head array includes a plurality of print-head rows, each printing a respective color. One or more print-head rows can print any color, for example, two print-head rows print the color black. As such, when one of these rows is selected as the reference row, the corresponding black color of that row is the reference color, white the corresponding black color of the other row is not. According to the disclosed technique, a first alignment pattern is printed on the substrate. This first alignment pattern (referred to herein as Target-A’) is employed to align the angle (i.e., the print rotation error) of each print head, as well as aligning the print-heads of a selected reference print-head row, one with respect to the other in the cross-print (X) direction and in the substrate motion direction (Y), The first alignment pattern may further be employed to align the print-heads in the other print- head row (i.e., the print-head row other than the reference print-head row), one with respect to the other in the cross-print (X) direction and in the substrate motion direction (Y). To align the print-head rows one with respect to the other, such that all the print-heads print in register (i.e., in alignment) one with respect to the other, a second alignment pattern is printed on the substrate. This second alignment pattern includes alternating print-head patterns of a print-head from the reference print-head row and a corresponding print-head from a different coior row (i.e., other than the reference row), as further elaborated below.

Reference is now made to Figure 1, which is a schematic illustration of a printing press system, generally referenced 100, constructed and operative in accordance with an embodiment of the disclosed technique. Printing press system 100 employs head alignment according to the disclosed technique. Printing press system 100 includes print-head array 102, an imager 104, a processor 106 and optionally incudes a memory 107. Imager 104 includes an imaging sensor 108 or a plurality of imaging sensors (i.e., pixel sensors). Imager 104 may be a CMOS line scan camera or a Contact Imaging Sensor (CIS). Alternatively or additionally, imager 104 may be an area camera which acquires a two dimensional (2D) image. Print-head array 102 includes an array of print-heads as further exemplified below. Printing press system 100 prints Target-A and Target-B on a substrate 112. For example, processor 106 instructs print-head array 102 to print Target-A and Target-B on a substrate 112. In Figure 1, only one example of Target-A Is depicted. Imager 104 acquires an image or images of Target-A and Target-B. Processor 106 receives the images of Target-A and Target-B (e.g., by communication with imager 104 or from a portable memory card storing the acquired images) and determines the print position error in the X direction (dX), print position errors in the Y direction (dY) and print rotation errors (dθ) of the print-heads, as further elaborated below.

Reference is now made to Figures 2A and 2B which are exemplary print-head arrays, generally referenced 150 and 160 respectively, which are aligned in accordance with another embodiment of the disclosed technique. With reference to Figure 2A, print-head array 150 includes a plurality of print-head rows, where each print-head row includes a plurality of print-heads. The print-heads in each print-head row print a color associated with the print-head row. In the example brought forth in Figure 2A, print-head array 150 includes 4 print-head row 152 _C , 152 _M , 152 _Y and 152 _K printing the colors Cyan, Magenta, Yellow and Black respectively. In general, a print-head array includes a plurality of print-head rows, each print-head row printing a respective color (i.e., there can be one or more rows printing the same color). Each one of print-head rows 152 _C , 152 _M , 152 _Y and 152 _K includes a plurality of print-heads.

Print-head row 152 _C includes a plurality of print-heads 154 _C1 , 154 _C2 .

154 _CN . Print-head row 152 _M includes a plurality of print-heads 154 _M1 ,

154 _M2 . 154 _MN . Print-head row 152 _Y includes a plurality of print heads

154 _Y1 , 154 _Y2 . 154 _YN . Print-head row 152 _K includes a plurality of print-heads 154 _K1 , 154 _K2 . 154 _KN . Each print-head includes an array of nozzles. The array of nozzles includes plurality of nozzle lines. For example, print-head 154 _C2 includes 4 nozzle lines 156 _C21 , 156 _C22 , 156 _C23 and 156 _C24 . Each nozzle line includes a plurality of nozzles such as nozzle 158. The nozzle lines are oriented in the cross-print (X) direction, In other words, the nozzle lines are oriented perpendicular to the direction of motion (Y) of the substrate on which a printed product (i.e., the printed product produced during a print job) or an alignment pattern is printed. Aiso, the nozzle lines are shifted one with respect to the other in the cross-print (X) direction.

With reference to Figure 2B, and similar to exemplary print- head array 150 (Figure 2A), print-head array 160 inciudes a plurality of print-heads rows, each print-heads row printing a respective coior. Similar to exemplary print-head array 150 (Figure 2A), print-head array 160 includes 4 print-head rows 162 _C , 162 _M , 162 _Y and 162 _K printing the colons Cyan, Magenta, Yellow and Black respectively. Each one of print-head rows 162 _C , 162 _M , 162 _Y and 162 _K includes a plurality of print-heads. Print-head row 162 _C includes a plurality of print heads 164 _C1 , 164 _C2 , .... 164 _CN . Print-head row 162 _M includes a plurality of print heads 164 _M1 , 164 _M2 , .... 164 _MN . Print-head row 162 _Y includes a plurality of print heads 164 _Y1 , 164 _Y2 , .... 164 _YN . Print-head row 162 _K includes a plurality of print heads 164 _K1 , 164 _K2 , ... , 164 _KN . Each print-head includes an array of nozzles. The array of nozzles includes plurality of nozzle lines. In print- head array 160 each print-head includes a plurality of nozzle lines. For example, print-head 164 _C2 includes 4 nozzle lines 166 _C21 , 166 _C22 , 166 _C23 and 166 _C24 . Each nozzle line includes a plurality of nozzles such as nozzle 168. The nozzle lines are oriented in the cross-print (X) direction. In other words, the nozzle lines are oriented perpendicular to the direction of motion (Y) of the substrate on which a printed product or alignment pattern is printed. Also, similar to exemplary print-head array 150 (Figure 2A), the nozzle lines are shifted one with respect to other in the cross-print

(X) direction. In general, each print-head includes at least two lines shifted one with respect to the other in the cross-print (X) direction.

The ink deposited by the print-heads in a print-head array may be subject to position errors due to mechanicai misalignment of the print-heads (i.e., in the X or Y direction or mechanical rotation), or electrical miss-calibration of the activation signals of the print-heads. Consequently, the ink deposited by the print-heads may not be correctly aligned. As such, miss-registration errors in the printed design may result. For example, if the rotation angle of the print-head is incorrect, vertical streaks may result within the width of the print-head. A print position error in the X direction may result in a vertical streak, located at one edge of the misaligned print-head, as well as color overlap with other color at the other edge of the print-head. A print position error in the Y direction may result in a horizontal streak. Also, the intended distance of the substrate from the print-heads may result in a print position error (i.e,, as exemplified above), since the heads may be aligned for one distance by not for another distance (e.g., due to a generally conical shape of the ink discharge from the print-heads).

In general, vertical position of printed dot or dots is achieved by controlling the timing of the signal applied to the nozzle depositing the ink which produces the dot. As such, adjusting the vertical position of a print head may be achieved either by mechanically adjusting the vertical position of the print-head, or by adjusting the timing of the signal applied to nozzles in the print-head. Also, horizontal position of printed dot or dots is achieved by controlling a deflection signal applied to the nozzle depositing the ink which produces the dot. As such, adjusting the horizontal position of a print head may be achieved either by mechanically adjusting the horizontal position of the print-head, or by adjusting the deflection signals applied to nozzles in the print-head. Adjusting the rotation angle of printed dot or dots may be achieved either by mechanically rotating the print-head printing the dots or by adjusting both the timing signals and the deflection signals applied to nozzles in the print-head.

Reference is now made to Figures 3A-3F which are schematic illustrations for a method of determining the print rotation error of a print-head 200, in accordance with a further embodiment of the disclosed technique. The rotation angle of a print-head is also referred to herein as ‘slope’. Printed-head 200 is similar in construction to either one of print-heads described above. With reference to Figure 3A, print-head 200 is angularly aligned. The term ‘angularly aligned’ relates herein to a print-head with nozzle lines (e.g., nozzle lines 156 _C21 , 156 _C22 , 156 _C23 and 156 _C24 in Figure 2A or nozzle lines 166 _C21 , 166 _C22 , 166 _C23 and 166 _C24 in Figure 2B) parallel and/or perpendicular to the cross-print direction (i.e., the X axis). In printed-head 200, nozzle 202 and nozzle 204 define a triangle 205. Thus, dot 206 and dot 208, printed by nozzle 202 and nozzle 204 respectively, define a triangle 210, similar to triangle 205. With reference to Figure 3B, print-head 200 is rotated in the counterclockwise direction relative to the X axis. Thus, dot 212 and dot 214, printed by nozzle 202 and nozzle 204 respectively, define a triangle 216, which is different from triangle 210.

With reference to Figure 3C, print-head 200 is rotated in the clockwise direction relative to the X axis. Thus, dot 218 and dot 220, printed by nozzle 202 and nozzle 204 respectively, define a triangle 222, which is different from triangle 210.

With reference to Figures 3D-3F, triangle 210, is a right angle triangle defined by the points ABC, where the points A and B correspond to nozzles 202 and 204. The height of triangle 210 is the Theoretical Vertical Distance (TVD) over the Y axis between nozzle 202 and nozzle 204. The width of triangle 210 is the Theoretical Horizontal Distance (THD) along the X axis between nozzle 202 and nozzle 204. The TVD and the THD are determined from known mechanical design parameters of the print-head. With reference to Figures 3E, triangle 210 and triangle 216 define a triangle 224 (triangle DAE), where the angle between the heights of triangle 210 and triangle 216 is denoted a (i.e., angle Similarly, triangle 210 and triangle 216 define a triangle 226 (triangle BAG), where the angle between the heights of triangle 210 and triangle 222 is also denoted α (i.e,, angle The angle α is the rotation ancle (l.e., the slope) of print-head 200. The angle o is estimated from the

TVD the THD and the Actual Horizontal Distance (AHD) between the two dots. For example, the angle α Is determined according to the following:

For small value of a, sin α ≈ α. Thus, the rotation angle of print-head 200 can be estimated according to the following: In view of the above, the rotation angle of a print-head which includes a plurality nozzle lines, may be determined by printing two dots corresponding to two nozzles from two different nozzle lines in the print-head, where the theoretical vertical distance and the theoretical horizontal distance between the two printing nozzles Is known. The actual horizontal distance between the two dots is measured from an image of the two dots (e.g., determined by processor 106 in Figure 1 from an Image of Target-A acquired by Imager 104), When THD-AHD Is positive, the rotation angle is referred to as a positive angle. When THD-AHD is negative, the rotation angle Is referred to as a negative angle. The method described above for determining the rotation angle of a print-head relies only on known parameters of the print-head. As such, this method Is Invariant to extrinsic Influences such as camera rotation, camera sensors miss-calibration (e.g. when more than one sensor is employed by the camera) and the like, Reference Is now made to Figures 4A-4G, which are schematic illustrations of a method for aligning a row of print-heads 250, operative in accordance with another embodiment of the disclosed technique. Print-head row 250 prints, for example, the odor black, and Includes a plurality of print-heads 254 _K1 , 254 _K2 , .... 254 _NK , Each print-head includes a plurality of nozzle lines. In print-head row 250 each one print-heads

254 _K1 , 254 _K2 . 254 _CK includes, for example, 4 nozzle lines 256 _K21 ,

256 _K22 , 256 _K23 and 256 _K24 . Each nozzle line includes a plurality of nozzles such as nozzle 258. The nozzle lines are oriented in the cross-print (X) direction. In other words, the nozzle lines are oriented perpendicular to the direction of motion (Y) of the substrate. Also the nozzle lines are shifted one with respect to other in the cross-print (X) direction. Herein, when print-heads such as print-heads 254 _K1 , 254 _K2 , .... 254 _NK are aligned one with respect to the other, these print-heads shall be referred to as being in a “nominal position”. The term “position" relates herein to the location and orientation of the print-heads.

Print-head row 250 prints a pattern which includes a print-head pattern for each one of print-heads 254 _K1 , 254 _K2 , .... 254 _NK . Print-head pattern includes at least two dots printed by each one of print-heads 254 _K1 , 254 _K2 , .... 254 _NK , where two of the at least two nozzles are located at different nozzle lines in the respective print-head. Also the theoretical vertical distance and the theoretical horizontal distance between the two printing nozzles are known. In the example brought forth in Figures 4A-4E, the upper left nozzle in the first row and the third nozzle from the left in the second row printed a respective dot. Print-head 254 _K1 printed dots 262 ₁₁ and 262 ₁₂ , print-head 254 _K2 printed dots 262 ₂₁ and 262 ₂₂ , print-head 254 _KN printed dots 262 _N1 and 262 _N2 . Printed dots 262 ₁₁ , 262 ₁₂ , 262 ₂₁ , 262 ₂₂ , 262 _N1 and 262 _N2 form at least a part of a first alignment pattern employed for print-head alignment referred to herein as “Target-A” 261. In general, Target-A 261 includes, for each print-head in each row of print-heads, a print-head pattern (i.e., a print-head target-A) which includes at least two dots, where at least two dote in the print-head target-A correspond to respective at least two nozzles from two different nozzle lines in the respective print-head, and where the theoretical vertical distance and the theoretical horizontal distance between two of the at least two printing nozzles is known. Target-A is further elaborated below. In Figure 4A, print-heads 254 _K2 and 254 _KN are depicted as rotated about their respective axis, as well as shifted in the X and Y directions one with respect to the other and with respect to print-head 254 _K1 . Consequently, the location dots 262 ₂₁ and 262 ₂₂ printed by print-head 254 _K2 and of dots 262 _N1 and 262 _N2 printed by print-head 254 _KN differ from their expected location (i.e., when all printed-heads in print-head row 250 are aligned). In Figure 4A, the actual dots printed by each one of print-heads 254 _K1 , 254 _K2 and 254 _KN Is designated by filled circles and the expected location of these dots is designated by empty circles.

Once print-heads 254 _K1 , 254 _K2 , .... 254 _CK print the respective print-heads patterns thereof, an imager 264 (similar to imager 104 - Figure 1) acquires an image 266 (Figure 4B) of Target-A 261. Image 266 includes respective representation of Target-A 261 and thus of the print-heads patterns, In Figure 4B, image 266 includes dot representations 268 ₁₁ , 268 ₁₂ , 268 ₂₁ , 268 ₂₂ , 268 _N1 and 268 _N2 of printed dots 262 ₁₁ , 262 ₁₂ , 262 ₂₁ . 262 ₂₂ , 262 _N1 and 262 _N2 . Imager 264 may also be rotated relative to the nominal orientation of the print print-heads 254 _K1 , 254 _K2 . 254 _CK . Image 266 is associated with a respective coordinate system 270. Coordinate system 270 may have sub-pixel resolution to a determined degree.

The rotation angle of print-head 254 _K1 is determined as described above in conjunction with Figure 3A-3F, employing the respective dot representations 268 ₁₁ and 268 ₁₂ and their respective locations in coordinate system 270. Similarly, the rotation angle of print-head 254 _K2 is determined employing the respective dot representations 268 ₂₁ and 268 ₂₂ and their respective locations in coordinate system 270, and the rotation angle of print-head 254 _KN is determined employing the respective dot representations 268 _N1 and 268 _N2 and their respective locations in coordinate system 270, also as described above in conjunction with Figure 3A-3F.

With reference to Figure 4C, to determine the vertical displacement (i.e,, in the Y direction) between print-heads 254 _K1 , 254 _K2 , 254 _CK , a line 272 of best fit between the dots representation corresponding to the dots printed by the same nozzle line in each of print-heads 254 _K1 , 254 _K2 . 254 _CK (e.g., line 272 is determined by processor 106 - Figure 1). In the example brought forth in Figures 4A-4E line 272 is fitted between dot representation 268 ₁₁ (printed by the first line in print-head 254 _K1 ), dot representation 268 ₂₁ (printed by the first line in print-head 254 _K2 ), and dot representation 268 _N1 (printed by the first line in print-head 254 _KN ). Due to the rotation of the imager 262, line 272 may be rotated in the coordinate system 270 associated with imager 264. The vertical displacement of print-heads 254 _K ι, 254 _K2 , .... 254 _NK is determined according to the distance between the respective dot representation of each of print-heads 254 _K1 , 254 _K2 . 254 _CK according to which the line

272 was estimated, and line 272. The vertical displacement of print-head 254 _K1 is determined according to the distance between dot representation 268 ₁₁ and line 272. The vertical displacement of print-head 254 _K2 is determined according to the distance between dot representation 268 ₂₁ and line 272, this distance is demarked 'D' in enlargement circle 274, exemplifying vertical displacement determination, The vertical displacement of print-head 254 _KN is determined according to the distance between dot representation 268 _N1 and line 272. Line 272 may be anchored to either one of dot representations 268 ₁₁ , 268 ₂₁ or 268 _N1 , for example, when one of print-heads 254 _K1 , 254 _K2 , .... 254 _NK is a reference print head. Also, the rotation of the camera can be determined from angle of line 272 in image 266. Also, once line 272 is determined, line 272 and dot representations 268 ₁₁ , 268 ₂₁ or 268 _N1 may be aligned coordinate system 270 of image 266. With reference to Figure 4D, to determine the horizontal displacement (i.e., In the X direction) between print-heads 254 _K1 , 254 _K2 , .... 254 _NK , a local grid 276 is determined between the respective dot printed by the same nozzle line in each of print-heads 254 _K1 , 254 _K2 , .... 254 _NK . Local grid 276 is determined between dot representations 268 ₁₁ ,

268 ₂₁ or 268 _N1 of each of print-heads 254 _K1 , 254 _K2 , .... 254 _NK (e.g., local grid 276 is determined by processor 106 - Figure 1). To determine local grid 276, the spacing (i.e., the relative location) between at least a selected group of dot representations 268 ₁₁ , 268 ₂₁ . 268 _N1 is determined. Thereafter, the grid spacing (i.e., the distance between the grid points) that best fits the spacing between the dot representations 268 ₁₁ , 268 ₂₁ , .... 268 _N1 is determined, for example, according to the least square criterion. The start of the grid may be anchored, for example, at a determined distance before the first dot representation in the selected group of dot representations (e.g., at half the expected distance between the first nozzle mark in the group and the preceding nozzle mark), the coefficients of the equations: y = ax + b (1) are determined where a is the grid spacing and b is the anchor point before the dot representation in the group. Thereafter, each one of print-heads 254 _K1 , 254 _K2 . 254 _NK is horizontally aligned with reference grid 276 according to the expected distance between dot representations 268 ₁₁ , 268 ₂₁ , .... 268 _N1 (e.g., the grid point closest to this expected distance).

The term ‘distance* herein refers to the selected metric employed to determine the spacing, which can be measured, for example, meters, millimeters or pixel units (i.e., not necessarily in whole pixels). Employing a local reference grid between adjacent print head targets eliminate the accumulation of errors when aligning the print head target relative to a single reference target. Also, each print-head target-A may be printed by different by different nozzles in the respective print-heads, However, to align the print-heads in a row of print-heads, one with respect to the other, the print-head target-A’s should be printed by the same nozzles in each print-head. Alternatively, when print-head target-A’s are printed by different nozzles the rotation angle of the camera, relative to the X and Y directions, should be known. When the rotation angle of the camera relative to the X and Y directions is known, the Y and X direction in the acquired image are also known and the print-head target-As may be shifted in the correct direction in the image coordinate system according to the known distances between the nozzle lines,

The method described above in conjunction with Figures 4A-4E exemplified the alignment of print-heads in a single print-head row, one with respect to the other and the rotation of each print-head employing Target-A. In general, according to the disclosed technique, Target-A is employed to align the angle of each print-head in the array of print heads, as well as to align the print-heads in a reference row of print-heads in the vertical and horizontal directions relative to each other. In some cases, when the location of the rotation axis of each of the print-heads is not known, Target-A is printed twice, once to align the rotation angle and once to align the vertical and horizontal displacement of at least the reference print-head row. Once the rotation of the print-heads have been aligned, and at least the print-heads in a reference row of print-heads have been aligned one with respect to the other in the X and Y direction, the print- head rows need to be aligned one with respect to the other. To that end, the print-head rows, other than the reference row of print-heads, are aligned with respect to the reference row of print-heads. To align the print-head rows one with respect to the other, the printing press prints a pattern referred to herein as Target-B. Reference is now made to Figures 5A, 5B, 5C and 5D, which are schematic illustrations of a method for aligning an exemplary print- head array, generally referenced 300, operative in accordance with a further embodiment of the disclosed technique. Print-head array 300 include a plurality of print-head rows 302 _C , 302 _M and 302 _Y printing the colors Cyan, Magenta and Yellow respectively. Each one of print-head rows 302 _C , 302 _M and 302 _Y includes an array of nozzles. The array of nozzles includes a plurality of print-heads, each print-head printing the respective color of the respective row. Print-head row 302 _C includes print-heads 303 _C1 , 303 _C2 , 303 _C3 . 303 _CN-1 , 303 _CN . Print-head row 302 _M includes print-heads 303 _M1 , 303 _M2 , 303 _M3 , .... 303 _MN-1 , 303 _MN . Print-head row 302 _Y incudes print-heads 303 _Y1 , 303 _Y2 , 303 _Y3 , .... 303 _YN-1 , 303 _YN . Each print-head includes a plurality of nozzles lines. Each nozzles line includes a plurality of nozzle, The nozzle lines are shifted one with respect to other in the cross-print (X) direction. Print-head array 300 prints a Target- A 305 on a substrate 304. As mentioned above Target-A includes, for each print-head in each row of print-heads, at least two dots, where at least two dots correspond to respective at least two nozzles from two different nozzle lines in the respective print-head. The theoretical vertical distance and the theoretical horizontal distance between the at least two nozzles from the two different lines is known.

In the example brought forth in Figure 5A, each one of print-heads 303 _C1 , 303 _C2 , 303 _C3 . 303 _CN-1 , 303 _CN , print-heads 303 _M1 ,

303 _M2 , 303 _M3 . 303 _MN-1 , 303 _MN and print heads 303 _Y1 , 303 _Y2 , 303 _Y3 . 303 _YN-1 , 303 _YN prints a respective print-head pattern. Print-head 303 _C1 prints dots 306 _C11 and 306 _C12 , print-head 303 _C2 prints dots 306 _C21 and 306 _C22 , print-head 303 _C3 prints dots 306 _C31 and 306 _C32 , print-head 303 _CN-1 prints dots 306 _CN-11 and 306 _CN-12 and print-head 303 _CN prints dots 306 _CN1 and 306 _CN2 . Each one of print-head 303 _C1 , print-head 303 _C2 , print-head 303 _C3 , print-head 303 _CN-1 and print-head 303 _CN print the dots associated therewith employing the same nozzles in the respective array of nozzles (e.g., the first nozzle in the first line and the fifth nozzle in the third line the second nozzle in the second line and the last nozzle in the third line). Also, dots 306 _C11 and 306 _C12 are printed by two different lines in the array of nozzles of printed-head 303 _C1 . Similarly, dots 306 _C21 and 306 _C12 are printed by two different lines in the array of nozzles of printed-head 303 _C2 , dots 306 _C31 and 306 _C32 are printed by two different lines in the array of nozzles of printed-head 303 _C3 , dots 306 _CN-11 and 306 _CN-12 are printed by two different lines in the array of nozzles of printed-head 303 _CN-1 and dots 306 _CN1 and 306 _CN2 are printed by two different lines in the array of nozzles of printed-head 303 _CN . The theoretical vertical distance and the theoretical horizontal distance between the nozzie printing dots 306 _C11 and 306 _C12 is known. Simiiarly the theoretical vertical distance and the theoretical horizontal distance between the nozzie printing dots 306 _C21 and 306 _C22 , between the nozzie printing dots 306 _C31 and 306 _C32 , between the nozzle printing dots 306 _CN-11 and 306 _CN-12 and the theoretical vertical distance and the theoretical horizontal distance between the nozzle printing dots 306 _CN1 and 306 _CN2 are also known (e.g., from the design specification of the printing press). Similarly to print-heads 303 _C1 , 303 _C2 , 303 _C3 , .... 303 _CN-1 , 303 _CN , print-head 303 _M1 prints dots 306 _M11 and 306 _M12 , print-head 303 _M2 prints dots 306 _M21 and 306 _M22 , print-head 303 _M3 prints dots 306 _M31 and 306 _M32 , print-head 303 _MN-1 prints dots 306 _MN-11 and 306 _{MN- 12} and print-head 303 _MN prints dots 306 _MN1 and 306 _MN2 . Also print-head 303 _Y1 prints dots 306 _Y11 and 306 _Y12 , print-head 303 _Y2 prints dots 306 _Y21 and 306 _Y22 , print-head 303 _Y3 prints dots 306 _Y31 and 306 _Y32 , print-head 303 _YN-1 prints dots 306 _YN-11 and 306 _YN-12 and print-head 303 _YN prints dots 306 _YN1 and 306 _YN2 .

Dots 306 _C11 , 306 _C12 , 306 _C21 306 _C22 , 306 _C31 306 _C32 , 306 _CN-21

306 _CN-12 , 306 _CN1 and 306 _CN2 , dots 306 _M11 , 306 _M12 , 306 _M21 306 _M22 , 306 _M31 306 _M32 . 306 _MN-21 306 _MN-12 , 306 _MN1 and 306 _MN2 and dots 306 _Y11 , 306 _Y12 , 306 _Y21 306 _Y22 , 306 _Y31 306 _Y32 . 306 _YN-21 306 _YN-12 , 306 _YN1 and 306 _YN2 define Target-A. Dots 306 _C11 , 306 _C12 , 306 _C21 306 _C22 , 306cai 306 _C32 , .... 306 _CN-21 306 _CN-12 , 306 _CN1 and 306 _CN2 define cyan target-A, dots 306 _M11 , 306 _M12 , 306 _M21 306 _M22 , 306 _M31 306 _M32 , .... 306 _MN-21 306 _MN-12 , 306 _MN1 and 306 _MN2 define magenta target-A and dots 306 _Y11 , 306 _Y12 , 306 _Y21 306 _Y22 ,

306 _Y31 306 _Y32 , .... 306 _YN-21 306 _YN-12 , 306 _YN1 and 306 _YN2 define yellow target-A. in the example brought forth In Figure 5A, print-head row 302 _M , printing the magenta color is selected as the reference print-head row. An imager 314 acquires an image of Target-A. The representation of magenta target-A in the acquired image is employed determine the rotation of print-head 303 _M1 , 303 _M2 .303 _MN (i.e., as described above in conjunction with Figures 3A-3F and 4A-4E) and to align print-heads 303 _M1 , 303 _M2 , .... 303 _MN in the X and Y directions one with respect to the other (i.e., as described above in conjunction with Figures 4A-4E). The representation of cyan target-A in the acquired image is employed determine the rotation of print-head 303 _C1 , 303 _C2 . 303 _CN (i.e., as described above in conjunction with Figures 3A-3F and 4A-4E). The representation of cyan target-A is optionally employed to align print-heads 303 _C1 , 303 _C2 . 303 _CN in the X and Y directions one with respect to the other (i.e., as described above in conjunction with Figures 4A-4E). The representation of yellow target-A in the acquired image is employed determine the rotation of print-head 303 _Y1 , 303 _Y2 , ...,303 _YN (i.e., as described above in conjunction with Figures 3A-3F and 4A-4E). The representation of yellow target-A is optionaily employed to align print-heads 303 _Y1 , 303 _Y2 . 303 _YN in the X and Y directions one with respect to the other (i.e., as described above in conjunction with Figures 4A-4E).

It is noted that the nozzles printing the dots in one row of print-heads need not be the same nozzles printing the dots in another row of print-heads, For example, nozzles printing the dots in cyan target-A need not be the same as the nozzles printing the dots in yellow target-A. Also, Target-A may be employed to align the rows one with respect to the other, employing the known spatial relationship between the nozzles printing the dots. However, employing Target-A to align the rows of print-heads one with respect to the other may be subject to cumulative errors.

As mentioned above, once the rotation of the print-heads have been aligned, and at least the print-heads in a reference row of print-heads have been aligned one with respect to the other in the X and Y direction, the print-head rows need to be aligned one with respect to the other. To that end, the print-head rows, other than the reference row of print-heads, are aligned with respect to the reference row of print-heads. To align the print-head rows one with respect to the other, the printing press prints a pattern referred to herein as Target-B.

Target-B includes a plurality of color print-head target-B’s and a plurality of reference print-head target-B’s. Each reference print-head target-B is printed by a respective reference print-head of a reference row. Each color print-head target-B is printed by a respective color print-head in a row other than the reference row. Each color print-head target-B is associated with a first reference print-head target-B, a second reference print-head target-B and optionally a third reference print-head target-B. The first reference print-head target-B and the second reference print-head target-B are printed above and below (i.e., in the vertical direction) the respective color print-head target-B. The first reference print-head target-B, the second reference print-head target-B and the color print-head target-B are printed by the same at least one nozzle In the array of nozzles of the respective print-head, The third reference print-head target-B is printed on the left or right side (i.e., adjacent in the horizontal direction) of said color print-head target-B. The third reference print-head target-B, is printed by an at least one nozzle different from the at least one nozzle printing the color print-head target-B, but located on the same line of nozzles in the array of nozzles of the respective print-heads. The color print-head target-B, the first reference print-head target-B, the second reference print-head target-B and the third reference print-head target-B are printed by a respective print-head having the same row location. The side of the third reference print-head target-B alternates between rows other than the reference row. It is noted that a first or a second reference print-head target-B may be a third reference print-head target-B of another color and vice versa. Consequent to the above, each color print-head target-B is surrounded by reference print-head target-B's. Target-B may further include additional reference print-head target-B's.

According to another example, Target-B includes alternating columns of color print-head target-B’s and reference print-head target-B's. Thus, each color print-head target-B is associated with a first reference print-head target-B and a second reference print-head target-B printed to the left and the right (i.e., in the horizontal direction) of the respective color print-head target-B. One of the first reference print-head target-B and the second reference print-head target-B, and the color print-head target-B are printed by the same at least one nozzle in the array of nozzles of the respective print-head and by a print-head located at the same row location. The other one of the first reference print-head target-B and the second reference print-head target-B is printed by a print-head at an adjacent row location. The color print-head target-B's in each row of targets relates to a different color.

With reference to Figure 5B, 5C and 5D, print-head array 300 prints a Target-B 311 on substrate 304. Print-head row 302 _M prints at least one reference target-B line, which includes a plurality of print-heads targets-B. In Figures 5B and 5C, a print-head target-B is exemplified as a dot. Print-head row 302 _M prints dots 306 _M11 , 306 _M12 306 _M21 , 306 _M22 , 306 _M31 , 306 _M32 . 306 _MN-11 , 306 _MN-12 , 306 _MN1 and 306 _MN2 . In the example brought forth in Figures 5B and 5C, print-head row 302 _M another reference target-B line, which includes dots 312 _M11 , 312 _M11 312 _M21 , 312 _M22 , 312 _M31 , 312 _M32 , .... 312 _MN-11 , 312 _MN-12 , 312 _MN1 and 312 _MN2 .

Print-head 303 _M1 prints dots 306 _M11 , 306 _M21 , 312 _M11 , 312 _M21 . Print-head 303 _M1 prints dots 306 _M11 and 312 _M11 by a nozzle located in a first nozzle location (e.g., the first nozzle in the first line) in print-head 303 _M1 , and prints dots 306 _M12 and 312 _M12 by a nozzle located in a second nozzle location (e.g., the fifth nozzle in the first line) in print-head 303 _M1 , where the first nozzle location and the second nozzle location a located on the same line of nozzles in the nozzles array of print-head 303 _M1 . Similarly print-head 303 _M2 prints dot 306 _M21 , dot 306 _M22 , dot 312 _M21 and dots 312 _M22 , print-head 303 _M3 prints dot 306 _M31 dot 306 _M32 , dot 312 _M31 and dots 312 _M32 , print-head 303 _MN-1 prints dot 306 _{MN- 11} , dot 306 _MN-12 , dot 312 _MN-12 and dot 312 _MN-12 and print-head 303 _N prints dot 306 _MN1 , dot 306 _MN2 , dot 312 _MN1 and dot 312 _MN2 .

In the example brought forth in Figures 5B-5C, print-head row 302 _C and print-head row 302 _M prints the cyan target-B line. Print-head 303 _C1 prints dot 308ci by a nozzle located in the first nozzle location (the first nozzle in the first line in the example above) in print-head 303 _C1 , and print-head 303 _M1 prints dot 308 _MC1 by a nozzle located in a second nozzle location (the fifth nozzle in the first line in the example above) in print-head 303 _M1 . Similarly print-head 303 _C2 prints dot 308 _C2 and print-head 303 _M2 prints dot 308 _MC2 , print-head 303 _C3 prints dot 308 _C3 and print-head 303 _M3 prints dot 308 _MC3 , print-head 303 _CN-1 prints dot 308 _CN-1 and print-head 303 _MN-1 prints dot 308 _MCN-1 , and print-head 303 _CN prints dot 308 _CN and print-head 303 _MN prints dot 308 _MCN .

Also in the example brought forth in Figures 5B-5C, print-head row 302 _Y and print-head row 302 _M prints the yellow target-B line, Print-head 303 _M1 prints dot 310 _MY1 by a nozzle located in the first nozzle location (the first nozzle in the first line in the example above) in print-head 303 _M1 , and print-head 303 _Y1 prints dot 310 _Y1 by a nozzle located in a second nozzle location (the fifth nozzle in the first line in the example above) in print-head 303 _Y1 . Similarly print-head 303 _M2 prints dot 310 _MY2 and print-head 303 _Y2 prints dot 310 _Y2 , print-head 303 _M3 prints dot 310 _MY3 and print-head 303 _Y3 prints dot 310 _Y3 , print-head 303 _MN-1 prints dot 310 _MYN-1 and print-head 303 _YN-1 prints dot 310 _YN-1 , and print-head 303 _MN prints dot 310 _MYN and print-head 303 _YN prints dot 310 _YN .

In view of the above, for example, dot 306 _M11 is a first reference print-head target-B of dot 308ci (i.e., the respective color print-head target-B). Dot 306 _MY1 is a second reference print-head target-B of dot 308ci and dot 306 _MC1 is a third reference print-head target-B of dot 308ci. As a further example, dot 308 _MC1 is a first reference print-head target-B of dot 310 _Y1 (i.e., the respective color print-head target-B). Dot 312 _M12 is a second reference print-head target-B of dot 310 _Y1 and dot 31 O _MY1 is a third reference print-head target-B of dot 310 _Y1 .

Imager 314 acquires an image 316 of Target-B. image 316 includes a representation of Target-B and thus of the print-heads targets-B. The representation of Target-B includes dots representations dots representations 318 _M11 , 318 _M21 , 318 _M21 , 318 _M22 , 318 _M31 , 318 _M32 .

318 _MN-11 , 318 _MN-12 , 318 _MN1 and 318 _MN2 correspond to the first reference target-B line. Dots representations 320 _C11 , 320 _C21 , 320 _C21 , 320c ₂₂ , 320 _C31 , 320 _C32 , ..., 320 _CN-11 , 320 _CN-12 , 320 _CN1 and 320 _CN2 correspond to the cyan target-B line. Dote representations 322 _Y11 , 322 _Y21 , 322 _Y21 , 322 _Y22 , 322 _Υ31 , 322 _Y32 . 322 _YN-11 , 322 _YN-12 , 322 _YN1 and 322 _YN2 correspond to the yellow target-B line. Dots representations 324 _M11 , 324 _M21 , 324 _M21 , 324 _M22 , 324 _M31 , 324 _M32 , .... 324 _MN-11 , 324 _MN-12 , 324 _MN1 and 324 _MN2 correspond to the second reference target-B line.

As mentioned above, the print-heads in the reference print-head row were aligned one with respect to the other employing Target-A. To illustrate the use of Target-B, and with reference to Figure 5C, consider, for example, dot representation 320 _C2 is associated with the print head 302 _C2 printing the cyan color. Dot representation 318 _M21 , associated with the print-head 303 _M2 printing the reference (magenta) color, is located vertically above dot representation 320 _C2 . Dot representation 322 _MY2 , also associated with the print-head 303 _M2 , printing the reference (magenta) color, is located vertically below dot representation 320 _C2 . Thus, dot representations 318 _M21 (i.e., the first reference print-head target-B of print-head 303 _C2 ) and dot representation 322 _MY2 (i.e., the second reference print-head target-B of print-head 303 _C2 ) define a vertical line 326. The horizontal distance between dot representation 320 _C2 and vertical line 326 relates to the horizontal displacement of print-head 303 _C2 . The horizontal displacement of print-head 303 _C2 is determined from the horizontal distance between dot representation 320 _C2 and vertical line 326. The vertical displacement of print-head 303 _C2 can be determined from the vertical distance of dot representation 320 _C2 from dot representations 318 _M21 and dot representation 322 _MY2 .

Dot representations 320 _MC1 (i.e., the third reference print-head target-B of print-head 303 _C2 ), associated with reference print-head 303 _M1 , is located horizontally to the left of dot representation 320 _C2 . Dot representations 320 _MC2 , associated with reference print-head 303 _M2 , is located horizontally to the right of dot representation 320 _C2 . Thus, dot representations 320 _MC1 and 320 _MC2 define a horizontal line 328. The vertical distance between dot representation 320 _C2 and horizontal line 328 relates to the horizontal displacement of print-head 303 _C2 . The horizontal displacement of print-head 303 _C2 is determined from the horizontal distance between dot representation 320 _C2 and horizontal line 328. As mentioned above, the third reference print-head target-B Is optional and provides additional information regarding the horizontal displacement of the print-heads. When a print-head target-B is located at the edge of Target-B, for example dot representation 322 _YN associated with print-head 303 _YN , the vertical displacement of print-head 303 _YN may be determined from a horizontal line 330 defined by the horizontally nearest dot representations associated with a reference print-head, for example, dot representations 322 _MYN-1 and 322 _MYN . In general, with reference to Figure 5D, the dot representations associated with the reference row may be employed to define a reference grid 332, according to which the horizontal and vertical displacement of the print-heads in rows, other than the reference row, may be determined. The reference grid is defined by at least one horizontal line (e.g., line 328) and at least one vertical line (e.g., line 326). The horizontal and vertical displacement of said each color print-head in color print-head rows 302 _C and 302 _Y is determined (e.g., by processor 106 - Figure 1) from the location of the respective color print-head patterns (e.g., dot representation 320 _C2 corresponding to 308 _C2 ) relative to a reference grid defined by said reference print-head patterns (e.g., 318 _M21 , 322 _MY2 , 320 _MC1 and 320 _MC2 corresponding to dots 306 _M21 , 310 _MY2 , 308 _MC1 and 308 _MC2 ).

In the examples brought forth above in conjunction with Figures 3A-3F, 4A-4E and 5A-5D, Target-A and Target-B were exemplified as including dots relating to the different colors being printed. However, Target-A and Target-B are not limited to dots. Rather, each print-head may print a respective print-head pattern. For exampie, the pattern may be, for example, one or more vertical lines, or one or more horizontal lines. As a further example, the pattern may be a lattice including at least one vertical line and at least one horizontal line where the lattice is printed by a respective line of nozzles in the respective print-head.

Reference is now made to Figure 6, which is a schematic illustration of an exemplary print-head row, generally referenced 400, printing a respective color target-A 408, in accordance with another embodiment of the disclosed technique. Print-head row 400 prints, for example, the color black (K) includes a plurality of print-heads. In Figure 6, print-head row 400 includes three print-heads, print head 404 _K1 , print-head 404 _K2 , and print-head 404 _NK . Each print-head includes a plurality of nozzle lines. In print-head row 404 each one print-heads 404 _K1 , 404 _K2 , and 404 _K3 includes, for example, 4 nozzle lines such as nozzle lines 406 _K11 , 406 _K12 , 406 _K13 and 406 _KM of print head 402 _K1 . Each nozzle line includes a plurality of nozzles such as nozzle 407. The nozzle lines are oriented in the cross-print (X) direction. In other words, the nozzle lines are oriented perpendicular to the direction of motion (Y) of the substrate. Also the nozzle lines are shifted one with respect to other in the cross-print (X) direction. In Figure 6, print-heads 404 _K1 , 404 _K2 , and 404 _3K are in the nominal position.

In the example brought forth in Figure 6, color target-A 408 includes a row of print-heads targets-A. In the example brought forth in Figure 6, target-A includes a lattice for each print-head, each lattice printed by the nozzles in the print-head. Color target-A 408 includes lattices 412 _K11 , 412 _K12 and 412 _K13 and lattices 412 _K21 Lattice 412 _K22 Lattice 412 _K23 . each including horizontal and vertical lines. The vertical lines in lattice 412 _K11 are printed by at least some of the nozzles in the first nozzle line (i.e., nozzle line 406 _K11 ) in print-head 404 _K1 . Similarly, the vertical lines in lattice 412 _K12 and 412 _K13 are printed by at least some of the nozzles in the first nozzle line in print-head 404 _K1 and print-head 404 _K2 respectively.

The vertical lines in lattice 412 _K21 are printed by at least some of the nozzles in the fourth nozzle line (i.e., nozzle line 406 _K14 ) in print-head 404 _K1 . Similarly, the vertical lines in lattice 412 _K22 and 412 _K23 are printed by at least some of the nozzles in the fourth nozzle line in print-head 404 _K1 and print-head 404 _K2 respectively. The horizontal line in lattices 412 _K11 , 412 _K12 and 412 _K13 and lattices 412 _K21 Lattice 412 _K22 Lattice 412 _K23 are printed by nozzles in all the nozzles lines (i.e., not necessarily all the nozzles in the print-head) in the respective print-head (i.e., to produce a continuous line). Any two vertical lines may be employed to determine the AHD, as described above in conjunction with Figures 3A-3F.

Additionally, each lattice target (i.e., print-head pattern) may include a target locator mark, such as target locator mark 414, printed by designated nozzles in each print-head, and employed for identifying the lattice target in an acquired image of Target-A and for segmenting the targets, Target locators can be included in Target-B as well as further exemplified below in conjunction with Figure 7B. In general, the spatial relationship between the target mark locator and lines in the lattice is known. As such, a target locator mark, such as target locator mark 414 can also be employed for virtual target completion in case some of the nozzles are clogged, or the electric signal controlling the nozzle is miss-calibrated (resulting in a weak or deviated nozzle), or in case the misalignment is such that lattices overlap or due to noise in the acquired image of the target.

Also, in some cases, the Field Of View (FOV) of a single imaging sensor is not sufficient to cover the entire width of the substrate. In such cases, an imager with two or more imaging sensors is employed to acquire an image of the full width of the substrate, the FOV of each adjacent pair of sensors exhibit an overlap therebetween. In order to be able to combine the images produced by each imaging sensor at least one “stitching” locator mark, such “stitching” locator marks 414i and 414 ₂ , is added for each pair of adjacent imaging sensors, to at least one of the lattices at a location in which the FOV of the imaging sensors overlap. Employing targets such as lattices with target locator marks and stitching locator marks provides redundancy in the information employed for alignment of the print-heads and thus rendering the target robust for various printing conditions (i.e., either intrinsic or extrinsic to the printing press). It Is noted that when a single color Is employed for printing, the color target-A Is also Target-A.

Reference Is now made to Figures 7A and 7B which are schematic Illustrations of an exemplary Target-A generally referenced 450, and an exemplary Target-B generally referenced 460, In accordance with a further embodiment of the disclosed technique. Target-A 450 and Target-B 460 relate to a four color process where the printing press prints the colors black (K), cyan (C), magenta (M) and yellow (Y). When more than one print-head row Is employed for the same color, then each print-head prints a respective color target-A and color targei-B at a corresponding location on the substrate. Thus, for example, two cyan target-A’s are differentiated according to their location In the acquired Image.

The description above exemplifies head alignment according to the disclosed technique employing either a three color process (CMY) or a four color process (CMYK). However, head alignment according to the disclosed technique may be employed with any number of colors, for example, with a seven color process (Cyan, Magenta, Yellow, Black, Orange, Violet, Green - CMYKOVG), with an eight color process (Cyan, Magenta, Yellow, Black, Orange, Violet, Green, white - CMYKOVGW), or a twelve color process,

Reference Is now made to Figure 8, which Is a schematic Illustration of a method for aligning print-heads in a digital printing press, operative In accordance with another embodiment of the disclosed technique. In procedure 500, a first alignment pattern (i,e., Target-A) is printed on a substrate, Target-A Includes, for each print-head in each row of print-heads, a first print-head pattern (i.e., a print-head target-A). The first print-head pattern Includes at least two dote, printed by the same nozzles In the array of nozzles In each one of the print-heads. At least two dots In the first print-head pattern correspond to respective at least two nozzles from two different nozzle lines in the respective print-head, and the theoretical vertical distance and the theoretical horizontal distance between two of the at least two printing nozzles is known. With reference to Figures 1, 4A, 5A, 8A, and 7A, printing press 100 prints a first alignment pattern (i.e., Target-A), such as Target-A 261, Target-A 305, Target-A 408, or Target-A 450 on substrate 112. For example, processor 106 instructs print-head array 102 to print the first alignment pattern on a substrate 112.

In procedure 502, an image of the printed first alignment pattern is acquired. The acquired image of the printed first alignment pattern includes representations of the first print-head patterns. With reference to Figure 1, imager 104 acquires an image of the printed first alignment pattern and provides the acquired image to processor 106.

In procedure 504, for each print-head, a print-head rotation is determined from a representation of a respective first print-head pattern, in the acquired image of the first alignment pattern. With reference to Figure 1, processor 166 determines for each print-head a print-head rotation, from the image of the first print-head pattern.

In procedure 506, the first alignment pattern (i.e., Target-A) is printed again on a substrate. With reference to Figures 1 , 4A, 5A, 6A, and 7A, printing press 100 prints a first alignment pattern (i.e., Target-A), such as Target-A 261, Target-A 305, Target-A 468, or Target-A 450 on substrate 112. For example, processor 106 instructs print-head array 102 to print the first alignment pattern again on a substrate 112.

In procedure 568, an image of the printed first alignment pattern is acquired. The acquired image of the printed first alignment pattern includes representations of the first print-head patterns. With reference to Figure 1, imager 164 acquires an image of the printed first alignment pattern and provides the acquired image to processor 108. In procedure 510, for the print-heads in at least a reference row of print-heads, the horizontal and vertical displacements one with respect to the other are determined, from the acquired image of the first alignment pattern. The horizontal and vertical displacements of print-heads in rows other than the reference row, one with respect to the other, may also be determined from the acquired image of the first alignment pattern, Furthermore, horizontal and vertical displacements of print-heads in rows other than the reference row, relative to a reference print-head (i.e., a print-head in a reference row) may also be determined from the acquired image of the first alignment pattern. With reference to Figure 1 , processor 106 determines for each print-head in at least a reference row of print-heads the horizontal and vertical displacement, from the acquired image of the first alignment pattern. From procedure 510 the method proceeds to procedure 516. in procedure 512, a second alignment pattern (i.e., Target-B) is printed on a substrate. The second' alignment pattern includes an array of second print-head patterns (print-head targets-B) where each print-head is associated with a respective second print-head pattern. Target-B was described herein above in conjunction with Figures 5A-5D., With Reference to Figures 1, 5B and 7B, printing press 100 prints a Target-B, such as Target-B 311 or Target-B 460 on substrate 112. For example, processor 106 instructs print-head array 102 to print the second alignment pattern on a substrate 112.

In procedure 514, an image of the printed second alignment pattern is acquired. The acquired Image of the printed first alignment pattern includes representations of the second print-head patterns. With reference to Figure 1, imager 104 acquires an image of the printed Target-B. in procedure 516, for each print-head in other than the reference print-head row, horizontal and vertical displacement is determined relative to a reference print-head, from the acquired image of the second alignment pattern. With reference to Figure 1, for each print-head in other than the reference print-head row, processor 106 determines horizontal and vertical displacement relative to a reference print-head , from image of the second print-head pattern.

With regards to Figure 8, it is noted that procedures 506 and 508 are optional as indicate by the dashed line from procedure 504 to procedure 510. When the location of the rotation axis of the print-head, reiative to the location of the nozzles printing the first print-head pattern is known, then a single Image is sufficient to align the rotation angle of the print-heads as well as align the horizontal and vertical displacement of the reference row of print-heads. The rotation angle alignment, as well as the horizontal and vertical displacement may be implemented logically (i.e., from the image data). Also, as indicated in Figure 8, procedures 512 and 514 may be performed at any time independent of procedures 500-510, for example, when the alignment according the first alignment pattern is performed logically (i.e,, from image, without actually performing corrective action). It is also noted that when the acquired image of the first alignment pattern is employed to determine the horizontal and vertical displacements of print-heads in rows other than the reference row, relative to a reference print-head (i.e., a print-head in a reference row), procedures 508-516 are optional. Implementing these procedures may depend, for example, on the errors involved in determining the horizontal and vertical displacements of print-heads in rows other than the reference row, relative to a reference print-head, employing the acquired image of the first alignment pattern.

It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.