A method and apparatus for creating a control strip

Abstract: A control strip (25) including an analog representation of a control color image useful in controlling the reproduction of a main color image, characterized in that the analog representation of the control color image was provided by the method of providing a digital representation of the control color image, and transforming the digital representation of the control color image into the analog representation of the control color image.

Description: FIELD OF THE INVENTION The present invention relates to electronic color prepress and press systems and techniques generally. BACKGROUND OF THE INVENTION Control strips are particularly useful in monitoring the setting up of printing presses for new printing jobs by allowing careful comparison of the parameters of a printed product to the same parameters of a proof signed by a customer. Parameters which are compared include ink hues and densities, hues of secondary colors, dot gain/loss and halftone dot deformation as a result of slur or doubling. Control strips are also useful for maintaining printing parameters over a period of time to enable the printing of faithful copies of a proof of an original. Conventional control strips for offset or gravure color printing include off-the-shelf control strips which cannot be modified by a user. A wide variety of such strips are sold, under tradenames such as Chromalin by Dupont or Matchprint by 3M, to cater to different printing needs. Often printing workshops are required to stock large numbers of control strips to cover ordinary printing jobs. State of the art control strip patterns are described in the following documents: Elyjiw, Zenon. "GATF Compact Color Test Strip", Research Project Report No. 6079, Graphics Arts Technical Foundation, Inc., Pittsburgh, PA: 1968. GATF Quality Control Device Catalog , Graphic Arts Technical Foundation, Pittsburgh, PA: 1989. The application of control strips in the conventional printing and proof process is now briefly explained. A control strip is manually mounted with a color separation film on a plate. The plate is exposed and then developed. This stage is performed for each color separation needed to print copies of the original. A color proof is produced in a similar fashion. The plates are placed in a predetermined color sequence on a printing press, and the printing run is initiated. If the printing quality of output sheets is not satisfactory, as evidenced by comparison to the proof, then operation parameters of the printing press are changed accordingly to compensate for any deviations from the desired quality. This technology presents various problems which have been discussed in the literature, such as the problem of mutual registration of color separations discussed in U.S. Patent No. 4,469,025 to Loffler et al, automatic evaluation of the control strip discussed in U.S. Patent No. 4,448,533, and in-process control to deal with the problem of variation between copies discussed in U.S. Patent No. 4,852,485. State of the art technology has eliminated the film stage by enabling direct exposure of plates from digital information stored in a color electronic pre-press system. This technology is known as computer-to-plate (CTP) technology. A commercially available device for performing CTP is the Raystar Imagesetter, commercially available from Scitex Corporation, Herzlia, Israel. Similarly, a proofing process known as direct digital colour proofing (DDCP) enabling direct exposure of proofs from digital information stored in a colour electronic pre-press system has been developed and is known in the art. The Approval proofer, commercially available from KODAK (RTM), is an example of a DDCP device. Postscript is a format defined by Adobe Systems Inc. , California, USA, for defining page parameters of images for output by a printer of film imagesetter. Initially, Postscript was used for black and white page printing. Electronically mountable control strips for black and white pages are known and are discussed in "Digital Control Wedge", UGRA Mitteilungen, vol. 1, April 1990. The output of this type of process is a printed page or film provided by an image setter or printer. In a paper entitled "Der Druck auf die Qualitat nimmt zu", published in Der Druckspiegel 3/1991, pages 304-306, the CPC 2 press control system is described which controls the operation of a press in accordance with the results of the comparison of a proof and a printout both comprising an analog representation of the control strip. The present invention seeks to provide a control strip generated from a digital precursor as well as apparatus and methods for user generation of a control strip by creating a digital precursor of a control strip. The user generated control strips are preferably capable of being printed by a CTP device and a DCCP device. The present invention provides a method of calibrating a colour printing press comprising, generating a digital representation of at least one colour separation representing an image and an associated control strip in accordance with at least one screening parameter, providing a proof and a printed output of the image and the control strip, comparing the proof and the printed output, modifying at least one screening parameter according to the results of the comparison step, and repeating the generating step with the at least one modified screening parameter. The at least one screening parameters may be, screen angle, dot shape, screen ruling, dot gain or gray balance. The step of comparing may be a visual comparison, an automatic comparison which may includes a densitometric measurement. The invention also provides a colour printing press system comprising, a screen generator capable of generating a digital representation of at least one colour separation representing an image and an associated control strip in accordance with at least one screening parameter, a proofer associated with the screen generator and capable of providing a proof of the image and the control strip, output means for providing a printed output, comparing means for comparing the proof with the printed output, and means for modifying at least one screening parameter according to results produced by the comparing means, whereby the printing press can be calibrated for each new original. The at least one screening parameter may be, screen angle, dot shape, screen ruling, dot gain or gray balance. The comparing means may be a densitometer. BRIEF DESCRIPTION OF DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Fig. 1 is a generalized block diagram of an integrated pre-press and printing system useful for generating a control strip; Figs. 2A, 2B and 2C are color illustrations of control strips constructed and operative in accordance with preferred embodiments of the invention; Figs. 3A and 3B are color illustrations of portions of control strips having different screening parameters; Figs. 4A and 4B are color illustrations of control strips having custom patches; and Fig. 5 is a color illustration of a full page test form. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Fig. 1 which illustrates an integrated pre-press and printing system constructed and operative in accordance with the present invention. The system employs control strips which are compared to proof control strips in order to calibrate a press for a new original, and in order to control the printing of output sheets during a print run. The system comprises an operator interface workstation 10, such as a Prisma Workstation, commercially available from Scitex Corporation Ltd., Herzlia, Israel. The workstation l0 interfaces with a CPU 12, such as a Whisper, also commercially available from Scitex. The CPU l2 is arranged to interface with data storage apparatus such as a magnetic tape drive l4 and a disk drive l6 for receiving a digital file representing scanned or otherwise generated text and graphics information, which information will also be referred to here as "the main color image". In accordance with a preferred embodiment of the present invention a user creates a digital file on the workstation 10 which comprises a digital representation of a control strip. Alternatively, the user may retrieve a digital representation of a control strip from suitable electronic storage means such as disk 16. According to a preferred embodiment of the invention, the digital file comprises a LW (linework) representation and/or a CT (continuous tone) representation. It is appreciated that a digital representation of a control strip constructed in accordance with the method described herein may be a precursor of any of the following: a control strip 25 on a proof 27, control strips 32 on a plurality of color separation plates 30 and a control strip 40 on a printed output sheet 39. The CPU 12 outputs the digital files representing the main color image or images and the control strip to a screen generator 20, such as the screen generator described in U.S. Patent Nos. 4,350,996 and 4,456,924 to Rosenfeld, both assigned to the Applicant. The screen generator 20 preferably provides a full plate digital representation of the main image/s and of the control strips. The digital plate is characterized by screening parameters such as screen ruling, screen angle and dot shape. The screen ruling may be expressed in lines per inch and the screen angle in degrees. The dot shape may be of any suitable configuration, such as square, round, diamond or a computer optimized configuration. Alternatively, or in addition, commercially available systems exist for allowing a user to define a desired dot shape. These systems include the Scitex User Dot (U-Dot) function operative in conjunction with the Scitex Prisma workstation or the Scitex Response 300 workstation, all commercially available from Scitex Corporation, Herzlia, Israel; and the Spot function used in conjunction with Adobe's Postscript format. The screen generator 20 interfaces with a digital proofer 26 which outputs proof 27 including the control strip 25. Commercially available digital proofers include the Approval Digital Color Proofing System, by Eastman Kodak Co., Rochester, N.Y., U.S.A.; Digital Matchprint, by 3M; and the Iris, by Scitex Corporation. The screen generator 20 also interfaces with a laser beam controller 22 incorporated in a laser plotter 24, such as a Scitex Raystar plotter, commercially available from Scitex Corporation Ltd., Herzlia, Israel. The laser plotter 24 provides the color separation film and/or plates 30 including the control strips 32. Subsequently, the color separation plates 30 are typically processed in a plate processor 34, such as an Econo 65/85, commercially available from Ajax International Ballerup, Denmark. The processed plates 30 are mounted in a predetermined color sequence on a color printing press 36, such as a GTO series press, commercially available from Druckmaschinien AG, Heidelberg, Germany, for printing output sheets 39 including control strips 40. The operation of the printing press 36 is preferably automatically monitored by a press control device 38, such as a CPC, commercially available from Druckmaschinien AG, Heidelberg, West Germany and described in U.S. Patent No. 4,852,485 to Brunner, the disclosure of which is incorporated herein by reference. The press control device 38 utilizes the strip 40 to control variables such as the ink density and plate pressure of the printing press 36. A detailed description of an integrated pre-press and press system is provided in Applicant's Israel Patent Application No. 97147, the disclosure of which is incorporated herein by reference. The calibration of the printing press 36 for a new original is now described. First, the proof 27, including control strip 25, and the color separation plates 30 are produced as described hereinabove. The press 36 is set up and an output sheet 39, including control strip 40, is produced. Subsequently, at least one element of control strip 40 is compared to the corresponding at least one element of control strip 25. The comparison may be performed visually, by a human operator, or, alternatively, automatically, using suitable apparatus such as a densitometer 18, which may comprise a Model 428 densitometer, commercially available from Xrite, Grandville, Michigan, U.S.A. The densitometer 18 can-detect changes in the size and color density of a printed half tone dot compared to a proofed dot. Reference is now made to Figs. 2A - 2C which illustrate color control strips constructed and operative in accordance with preferred embodiments of the invention. The control strips of Figs. 2A-2C conform to international SWOP regulations. It is appreciated that the method for calibrating control strips disclosed herein is not limited to calibration of the specific control strips illustrated in Figs. 2A-2C or even to combinations of the features thereof. Figs. 2A-2C are intended only to be examples of possible control strip configurations. Fig. 2A illustrates a control strip particularly suitable for monitoring color and registration qualities of a proof produced by a digital proofer. The control strip of Fig. 2A preferably includes the following elements: a registration target 50 comprising a plurality of relatively thin lines or curves for monitoring quality of registration; an array 52 of single color solids, two color overprints, three color overprints and four color overprints; cyan, magenta, yellow and black arrays 54, 56, 58 and 60, respectively, each comprising a plurality of patches differing in dot percentage, a gray balance array 62 and a white patch 64. Fig. 2B illustrates a control strip particularly suitable for controlling plate exposure quality. The control strip of Fig. 2B preferably includes the following elements: a registration target 70; a gray scale array 72 comprising a sequence of steps having a fixed dot percentage interval between them such as 10%, for monitoring exposure linearity; a microline array 74 comprising a plurality of positive and negative microline patches, the microlines having varying widths within a suitable range such as 6 - 50 microns; an array 76 of diagonal, horizontal and vertical lines; a highlight array 78 comprising patches having relatively high dot percentages which typically vary from approximately 1% to 5%; and a shadow array 80 comprising patches having relatively low dot percentages which typically vary from approximately 95% to 99%. Fig. 2C illustrates a printing control strip, also termed a production control strip, suitable for printing on each output sheet 39 of Fig. 1 due to its particularly narrow configuration. The printing control strip of Fig. 2C preferably includes an array 90 of single color solids, two color overprints and three color overprints; a cyan array 92 comprising cyan patches of different dot percentages, including highlights and shadows, cyan microlines and a cyan slur target, a gray balance array 94; a solid black patch 96; a magenta array 100 comprising magenta patches of different dot percentages, including highlights and shadows, magenta microlines and a magenta slur target; a solid cyan patch 102; a yellow array 106 comprising yellow patches of different dot percentages, including highlights and shadows, yellow microlines and a yellow slur target; a solid magenta patch 108; a black array 112 comprising black patches of different dot percentages, black microlines and a black slur target; a solid cyan patch 114; a gray balance array 116; and a solid white patch 118. It is a particular feature that the user can design his own control strips. A user may delete any of the components, arrays and patches described above; may add suitable components, arrays or patches; may rearrange components within the control strip; may change the dimensions or orientation on the page of the control strip; or may change the parameters of any component, array or patch, such as but not limited to the dot shape thereof. Reference is now made to Figs. 3A and 3B which illustrate screening parameters of a 50% black patch 119 such as the 50% black patch of black array 112 in Fig. 2C. In Fig. 3A, the patch 119 is formed of square dots oriented at a 30° screen angle and having a screen ruling of 175 lines per inch. In Fig. 3B, the patch 119 is formed of round dots oriented at a -7.5° screen angle and having a screen ruling of 133 lines per inch. It is appreciated that the described method, which provides a digital precursor of a control strip which may be printed, allows a user to select any suitable value for any of the above screening parameters by inputting suitable options to screen generator 20 of Fig. 1. The result is a control strip with the user-selected screen parameters. The user may customize individual patches according to the particular printing application. Such customized patches may include a particularly configured registration target. Another type of customized patch is a custom color patch representing a particular color other than cyan, magenta, yellow and black, such as gold, silver, or Panton colors. The customized color patch is intended to monitor printing of a color employed in a particular printing application, generally in addition to the process colors. The custom colors are generally generated as additional plates and are overprinted in designated areas of the color image on the plate. The customized color may or may not be a combination of C, M, Y and K. Figs. 4A and 4B illustrate control strips including customized patches. The control strip of Fig. 4A is generally identical to the control strip of Fig. 2A except that, in Fig. 4A, white patch 64 of Fig. 2A is replaced by a customized color patch 120 representing a beige color important for a particular application. The control strip Fig. 4B, is generally identical to the control strip of Fig. 2C except that customized beige color patch 120 is added. It is appreciated that the term "control strip" as employed in the present specification and claims is not intended to be limited to conventional strip-like configurations but rather is intended to refer to a representation of a control color image employed mainly for controlling the reproduction of a main color image, the control color image having any suitable configuration. The term "control strip" is also not intended to exclude a representation of a control color image employed mainly for setting up a color printing device such as a press. For example, Fig. 5 illustrates a full page color control strip or test form useful for monitoring the operation of a color writing device such as a press during press set up. A particular advantage of a digitally represented control strip is that the control strip may be printed as either a positive or a negative.

Claims: A method of calibrating a colour printing press comprising, generating a digital representation of at least one colour separation representing an image and an associated control strip in accordance with at least one screening parameter, providing a proof (27) and a printed output (39) of the image and the control strip, comparing the proof and the printed output, modifying at least one screening parameter according to the results of the comparison step, and repeating the generating step with the at least one modified screening parameter. A method as claimed in claim 1, wherein the at least one screening parameter includes, screen angle, dot shape, screen ruling, dot gain or gray balance. A method as claimed in claims 2 or 3, wherein the step of comparing includes a visual comparison. A method as claimed in claims 1 or 2, wherein the step of comparing includes an automatic comparison. A method as claimed in claim 4, wherein the step of automatic comparison includes a densitometric measurement. A colour printing press system comprising, a screen generator (20) capable of generating a digital representation of at least one colour separation representing an image and an associated control strip in accordance with at least one screening parameter, a proofer (26) associated with the screen generator and capable of providing a proof of the image and the control strip (27), output means (24,30,34,36,38) for providing a printed output, comparing means (18) for comparing the proof with the printed output, and means (12) for modifying at least one screening parameter according to results produced by the comparing means, whereby the printing press can be calibrated for each new original. A system as claimed in claim 6, wherein the at least one screening parameter is, screen angle, dot shape, screen ruling, dot gain or gray balance. A system as claimed in claim 6 or 7, wherein the comparing means includes a densitometer.

Inventors: BARAK ILAN, BARAK, ILAN
Applicants: SCITEX CORP LTD, SCITEX CORPORATION LTD.
Application year: 1992
Main classification: B41F33
IPC codes: B41F33