GB2435572A

GB2435572A - Selecting a set of inks for colour process printing

Info

Publication number: GB2435572A
Application number: GB0603691A
Authority: GB
Inventors: Simon John Whomsley
Original assignee: AVA CAD CAM GROUP Ltd
Current assignee: AVA CAD CAM GROUP Ltd
Priority date: 2006-02-24
Filing date: 2006-02-24
Publication date: 2007-08-29
Also published as: GB0603691D0

Abstract

Selecting a set of inks for colour by printing a digitally generated preview of a separated source design as it would appear if printed using a given combination of production process substrate and inks is selected. A target design is also selected and individual areas, which may be pixels or objects, of the preview are compared, area by area, with corresponding areas of the target design. The results of that comparison are evaluated according to user defined criteria including average colour difference, maximum colour difference and ink cost. The comparison process is repeated for further previews in order to determine a set of inks providing a close match to the target design.

Description

A METHOD OF SELECTING A SET OF INKS FOR COLOUR PROCESS

PRI NTING

The present invention relates to a method of selecting a set of inks for Colour Process Printing.

Definitions: Inks: This word is used here to signify the colorants which (usually in liquid form) are printed onto the substrate (or carrier material).

Conventional Printing: refers to any one of a number of printing processes (including gravure, flexo, offset litho, rotary screen and others) whereby a number of different coloured inks are mechanically applied by rollers, each of which carries a physical impression of part of the design to be printed, in sequence, onto a substrate or print carrier. The term is usually used to distinguish such processes from Digital Printing.

Digital Printing: refers to any one of a number of printing processes (including ink jet, thermal transfer etc) whereby digital design and colour information is translated into a print on paper or other substrate or print carrier through the interaction of computer software with the print heads of a digital printer. A three dimensional physical impression of the design is not usually required for digital printing.

Spot Colours! Spot Colour Printing refers to conventional printing with inks selected from a large number of available recipes (usually different combinations of Base Colours supplied by the manufacturer). These Spot Colours may form part of a standard range' of recipes or may be mixed from different combinations of (usually) three Base Colours on an ad hoc basis from time to time.

Process Colours! Process Colour Printing refers to Conventional Printing using fixed sets of inks (each of which may be a Spot Colour recipe or a Base Colour) which are intended to be used (usually in the same order on press) for a number of different jobs. Traditionally sets of four colours (Cyan, Magenta, Yellow and Black or CMYK) have been used but fixed Process sets may include any number of colours. Advantages of Process Colour Printing over Spot Colour Printing include reduced set up time on press (because the same inks remain on each printing station from one job to the next) and reduced separation time. Disadvantages of Process Colour Printing against Spot Colour Printing include reduced gamut availability and more difficult reproduction of individual (e.g. packaging brand) colours across multiple jobs and/or substrates.

Base Colours. Are inks or colorants as supplied by the manufacturer.

Existing Methods In existing methods of Process Printing a target design (TD) is separated into the individual colours which will be Conventionally printed (one after the other on a flexo, gravure, litho or rotary screen printing press) in order to produce a print which represents as accurate a match as possible to the original TD. One method of separating a design for printing is using a profile. There are many different ways of creating a profile but the International Color Consortium (ICC) have defined standard profile specifications which can be used for this task and these are widely known and used (for example in Gretag Macbeths "ProfileMaker" and Adobe's "Photoshop" software applications). Profiles of this type may be designed for standard four colour Process (CMYK) Printing or may have any other number of specified colours in which case they are generically referred to as n channel profiles'. The printed target from which the profile is generated will usually include examples of the different colours used combining as overprints. A printing method/press can be profiled by printing a standard test design, or target, using a given set of inks and then reading in the result, usually with a spectrophotometer, to produce a profile. One of a number of internationally accepted standard targets is typically used for this purpose. Ideally, a profile, as well as being specific to one set of inks, is specific to the substrate upon which the design will be printed. Since the profile contains information relating to the colour that would be produced for a given combination of the profiled inks, a design can be separated using this information. The profile can be then used to produce a digital simulation (on a calibrated computer monitor and/or digital printer) of how the design would look if conventionally printed with the print method/inks/substrate combination used to create the profile.

Where Spot Colour Printing (which may be combined with Process Colour Printing) is used, other methods of profile generation and design simulation, including the use of computer algorithms or models to predict or simulate a design's appearance based upon measurements (or calculations based upon measurements) of more limited press samples/proofs of individual Spot Colour inks (as opposed to the sets of inks used in creating a profile as described above) may be used. The advantage of this alternative (Spot Colour) method is that less advance printed information is required where a large range of Spot Colours would render the process of creating profiles of the type described above for every possible combination of two or more Spot Colours impractical. The disadvantage of the Spot Colour method of previewing designs is that the effect of combining two or more of the inks used is necessarily calculated using much less information about what actually happens when the colours are combined than is available from a typical ICC type profile.

When a design is printed using a set of inks on a substrate an image is produced. Areas of a design may contain colours which cannot be produced using (any combination of) the particular set of inks on the chosen substrate.

These areas are said to be out of the achievable colour gamut for that print process (out of gamut'). When printing, it is desirable to choose an ink set where most or all areas of the design are reproducible within the colour gamut achievable. The TD and the print (or the digital proof in advance of the conventional print if this is considered reliable) are usually checked visually to determine if a design has been changed due to some areas being out of the printable colour gamut. This process can be aided by the addition of gamut warnings which identify areas of the design which are out of gamut (for example by displaying them in an obviously distinctly wrong colour on the computer monitor and/or digital printer). In addition, a spectrophotometer can be used to read areas of the design and these can be compared against the same area of the TD and a calculation of colour difference such as delta E can be used to determine colour shift. If sufficient areas of the design are out of gamut, it may be necessary to choose a different ink set. In this case, existing methods would require reiteration of the entire process of separation and prediction until you choose one which yields the an acceptable degree of colour difference and/or an acceptable production cost. Even if the printer has already produced many different profiles based upon combinations of ink sets, substrates, and other production settings (engraving, press speed, annilox size etc) or could easily do so, the process of selecting the most appropriate combination for each job would be extremely lengthy and time consuming because the design would need to be re-separated, proofed (digitally or conventionally) and evaluated for each possible set of variables until a satisfactory result is achieved. For this reason, in practice, the printer tends to choose a profile and stick with it. This frequently results in either poor matching of the final print to the TD, or extensive and expensive on press adjustment (of inks etc) in an attempt to achieve an acceptable match (or both of these).

According to the present invention, there is provided a method of selecting a set of inks for colour printing comprising the steps of selecting a digitally generated preview of a separated source design as it would appear if printed using a given combination of production process, substrate and inks, selecting a target design, comparing individual areas area by area of the preview with corresponding areas of the target design, evaluating the results according to user defined criteria, repeating the comparison for further previews in order to determine a set of inks providing a close match to the target design.

In a preferred embodiment of the invention, the areas of the design compared may be pixels, when raster based, or objects, when vector based.

Comparison is thus carried out for each pixel or objects as the case may be.

User defined criteria may include average colour difference, maximum colour difference and/or ink cost. Average colour difference is the average value when considering all pixels or all objects. Maximum colour difference is the maximum value when considering all pixels or objects.

In a preferred embodiment of the invention, relating to a Process Colour Printing workflow, a target image may be automatically separated and rendered using each of a number of (previously created) ICC type profiles.

This may be done either at full design resolution or at a reduced resolution if, for example, greater speed is required. Digitally generated previews of the design as it would appear when printed using each of the profiles (which may contain variations in the inks used, the substrate printed on, the production process or all of these) are quickly evaluated as described above and a selection of the most appropriate profile is automatically made based upon the lowest average colour difference, the lowest maximum colour difference and the lowest cost of production based upon ink costs and ink usage for that profile/design printing combination. Alternatively, the user may be assisted to make a selection of profile based upon data extracted and presented through the method of the invention. Information gathered can be presented to the user to assist in making the selection manually. This information may be presented visually or numerically or both. In a variation of this embodiment, the invention could also be used to select the optimum/most appropriate ink set (and/or print method, substrate) for a series of designs, the advantage of doing this being that the unproductive time to clean a printing machine from its previous inks set and then set it up with the new ink set/settings can be substantially reduced In order that the invention may be more clearly understood a number embodiments thereof will now be described, by way of example with reference to the single figure of the accompanying drawing which shows the process steps in a workflow from a picture stage to a colour difference stage.

Referring to the drawing, a source image is shown at A. This may be expressed as an image made out of pixels or as one made out of objects. At B three device values (0, 0,100%) of the image shown at A are shown.

These often represent red, green and blue (RGB) but a source image may contain cyan, magenta, yellow and black (CMYK) or other different combinations involving the same or different numbers of colours. C represents the source profile of the source image shown at A. This may be an algorithm, table or matrix which translates the source device values at B into the colour space which in this case are the Lab values. Lab is a CIE (International Commission or Illumination) standard colour space and is also an ISO standard in which "L" stands for lightness and "a" and "b" are opponent colour co-ordinates, "a" representing the red green axis and "b" the blue yellow. This Lab representation in the colour space is shown at D, and is the result from the source profile.

E and F represent the separation stage. E represents the separation profile in the form of a table or matrix which converts from the colour space (in this case Lab) to a device value. F represents the device values calculated by E. In this case, this comprises four values 82, 0, 10,50% but the number of values may vary and depends upon the number of colours under consideration.

G and H represents the digital proofing stage. G represents the proofing profile which is an algorithm, table or matrix which converts device values from E of the separation stage into the colour space. This resultant colour space of the proofing profile is shown here at H as Lab as previously described but other forms of colour space can also be used.

The comparison stage I follows the digital proofing stage and determines colour difference value by comparing the Lab colour space of source image at D with that of the target image at H. Thus the workflow shows a picture at stage A and a colour difference value at stage I. This colour difference value (delta E) is the value between the calculated Lab from the source image (stages A to D) and the predicted proof (stages G and H) of the separation stage (stages E and F) of the source image (stages A to D) An alternative embodiment is concerned with the evaluation of a proposed production profile for a design which is intended to be printed using one or more Spot Colours. An n channel profile is created and used to generate a preview of the printed design. Using the above described process, this preview is automatically compared with the original target and evaluated using the selected criteria. Pre-selected numeric values may be stored in order to provide an automatic pass' or fail' assessment of the proposed production profile. In a further alternative embodiment the user in the above example may be assisted to make a selection between several alternative production profiles based upon data which has been automatically gathered using the inventive process. At the election of the printer/user this might involve, for example, displaying a number of alternatives side by side (on a computer monitor and/or digital print out) together with visible gamut warnings and/or other numerical data relating to colour differences and/or cost for each alternative.

In a still further alternative embodiment, a design which is intended to be printed using one of more Spot Colours may be previewed using one dimensional tonal information for each of the Spot Colours (either alone or in addition to an n channel or CMYK profile) for which little or no overprinting information is available. Again, the inventive process may be used to assess the acceptability of the proposed production process, substrate, inks combination (or of a number of possible alternative combinations).

Advantages of the above described embodiments over other available methods of comparison (including visual inspection and/or measurement of solid colour areas of the design using a spectrophotometer) include greater speed, lower cost and increased objectivity and accuracy.

It will be appreciated that the above embodiments have been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims

CLAIMS

1. A method of selecting a set of inks for colour printing comprising the steps of selecting a digitally generated preview of a separated source design as it would appear if printed using a given combination of production process, substrate and inks, selecting a target design, comparing individual areas area by area of the preview with corresponding areas of the target design, evaluating the results according to user defined criteria, repeating the comparison for further previews in order to determine a set of inks providing a close match to the target design.

2. A method of selecting a set of inks for colour printing as claimed in claim 1, in which the areas of the design compared are pixels when raster based.

3. A method of selecting a set of inks for colour printing as claimed in claim 1, in which the areas of the design compared are objects when vector based.

4. A method of selecting a set of inks for colour printing as claimed in claim 2 or 3, in which user defined criteria include one of the following: colour difference, maximum colour difference and ink cost.

5. A method of selecting a Set of inks for colour printing as claimed in claim 4, in which colour difference is the average value when considering all pixels or all objects.

6. A method of selecting a set of inks for colour printing as claimed in claim 4, in which maximum colour difference is the maximum value when considering all pixels or all objects.

7. A method of selecting a set of inks for colour printing as claimed in any preceding claim, in which the target design is automatically separated and rendered using each of a number of previously created International Colour Consortium (ICC) type profiles.

8. A method of selecting a set of inks for colour printing as claimed in claim 7, in which the target design is automatically separated and rendered at full design resolution.

9. A method of selecting a set of inks for colour printing as claimed in claim 7, in which the target design is automatically separated and rendered at reduced design resolution.

10. A method of selecting a set of inks for colour printing as claimed in any preceding claim, in which the set of inks is selected for a series of designs.

11. A method of selecting a set of inks for colour printing substantially as hereinbefore described with reference to the accompanying drawing.