GB2490965A

GB2490965A - Colour image reproduction using grey balance difference between source printing and current printer conditions

Info

Publication number: GB2490965A
Application number: GB1108513.1A
Authority: GB
Inventors: Lee Badham
Original assignee: Bodoni Systems Ltd
Current assignee: Bodoni Systems Ltd
Priority date: 2011-05-20
Filing date: 2011-05-20
Publication date: 2012-11-21
Also published as: WO2012160346A8; WO2012160346A1; GB201108513D0

Abstract

A method for reproducing a colour image comprises: identifying S2 a grey balance of a source printing condition (e.g. ICC profile) of an image file to be printed; identifying a printing condition of a printing unit to be used to print the image file; converting S6 the grey balance of the source printing condition to a grey balance of the printing condition of the printing unit; establishing a difference between the grey balances of the source printing condition and of the printer; and applying S7 the difference to colour values in the image file, producing a set of colour values to be printed. The respective grey balances may be first and second values of black, cyan, magenta or yellow (CMYK); grey balance differences may be added to, or subtracted from, image file color values. Combinations of non-black colourant corresponding to defined profile connection space values of the grey balance (e.g. L*a*b* values) may be determined. Printer condition can be measured from plural printed CMYK colour patches.

Description

Colour matching

Field of the invention

The invention relates to colour matching in printing operations.

Background to the invention

Printing units generally produce printed images by applying a combination of differently coloured base colourants onto a substrate. For example, it is common to apply a combination of cyan, magenta, yellow and black (CMYK) colourants to a substrate in order to create a colour image. By printing different combinations and patterns of the base colourants onto the substrate, the appearance of the printed image can be varied as required.

The combinations and patterns of the printed base colourants are generally specified in a printing instruction which is supplied to the printing unit. The printing instruction may, for example, specify the amount of each individual base colourant that should be printed onto the substrate and the locations at which it should be printed in order to produce the image. The amount, or dot ratio, of each individual base colourant (e.g. CMYK) may be represented in the printing instruction by a numerical value for each colourant.

However, the different characteristics of individual printing units, together with the differing conditions under which printing units operate, mean that supplying the same standard printing instruction to different printing units will generally result in the printing units producing images which appear to be different in terms of colour.

The different appearance of the images may be caused by the individual base colourants, such as the cyan, magenta, yellow and black inks or toner, varying slightly in colour from printing unit to printing unit. For example, manufacturing tolerances may result in different batches of yellow colourant having shghtly different colour properties to each other. The same may apply to any or all of the other individual colourants. These colour differences may be increased still further when different colourant manufacturers are used.

In addition to these manufacturing variations, the colours of the individual colourants may also be dependent on temperature and other environmental factors.

This makes it challenging to consistently reproduce a colour image even in a situation where the same printing unit and colourants are being used.

Another potential reason for images printed according to the same printing instruction appearing differently is the use of different substrates. This is because the perceived colour of a printed image is dependent on the properties of the substrate upon which it has been printed. For example, an instruction to print a particular set of numerical CMYK colour values will result in images which appear to be different in colour if the images are printed onto differently coloured papers.

A slight change in the colour of the substrate, for example from a yellow tinted white' paper such as newspaper to a blue tinted white' paper, will cause images printed according to the same set of CMYK colour values to appear to be different in colour. Other properties such as the substrate's absorbency, texture and finish type will also affect the perceived colour of an image.

Therefore, in order to accurately reproduce the same image using different printing units and/or under different printing conditions, it is necessary to tailor the printing instruction to the printing unit and the conditions being used. This is often referred to as colour management. Several colour management techniques exist.

A first of the existing colour management techniques revolves around the use of ICC profiles. These describe how a particular device inputs colour, for example in the case of a scanner or camera, or outputs colour, such as in the case of a printing press, inkjet printer or other printing unit when printing onto a particular substrate.

The ICC profiles connect individual base colourant combinations, such as a particular set of CMYK values, to colour values in a standard colour space such as CIE LAB or GTE XYZ. The standard colour space provides a profile connection space' across which a printing instruction can be tailored to take account of the characteristics of a particular printing unit and the conditions being used for printing. By providing source' and destination' ICC profiles to a colour matching module, the numerical CYMK values of a source printing instruction can be adapted so that the printed CMYK values will more accurately represent the colour of the image.

Currently, establishing an ICC profile for a particular destination' printing unit requires the unit to print a very large number of known combinations of base colourants, generally represented in a printing instruction by sets of CMYK values, onto a known substrate under known printing conditions. Each combination of base colourants is referred to as a colour patch and the patches together can be referred to as a colour chart. A known technique is to print a colour chart of between approximately 800 and 1600 patches, which generally occupies substantially the whole area of an A4 or A3 sized substrate. The patches represent colours across the visible spectrum. Once printed, the colour of each patch is measured in a profile connection space, such as CIE LAB, using suitable equipment. This allows conversions between the known combinations of printed base colourants (e.g. individual sets of CMYK values) and colour values in the profile connection space (e.g. L*a*b* values) to be made for the printing unit concerned. For example, a look up-table can be compiled which links sets of printable CMYK values to L*a*b* values in the CIE LAB profile connection space. An interpolation process can be used to determine profile connection space values (e.g. L*a*b* values) for sets of CMYK values which have not been printed as a colour patch in the measured chart.

If the number of measured patches is too low, for example less than 800, the accuracy of this interpolation process can be unreliable in certain colour regions and thus the quality of the printed images is unsatisfactory. The process on the whole is both time-consuming and expensive.

Furthermore, the large number of patches required for accurate colour conversions between CMYK values and corresponding values in the profile connection space makes it necessary for the chart to be printed as a separate print job, before the main print run, so that the printing unit can be configured for the main print run on the basis of the colour values measured from the chart. Due to its large size, it is impractical for the chart to be re-printed during the course of a main print run and, as such, it is not possible to dynamically adjust the application of colourant to the substrate in order to take account of changes in the properties of the colourants during the print run.

A second existing colour management technique is to define a target printing condition using CIE LAB references for the colours of the primary colourants (e.g. CMYK) and Tonal Value Increase (TVI) for the tint percentages. The actual printing condition of the printing unit can then be adjusted and aligned to the target values. This technique is used in the cpressSIGN colour management software produced by Bodoni Systems. A problem with the technique is that it can be difficult to adjust the colours of the primary colourants, particularly in digital printing processes which use pre-manufactured toner.

A third existing colour management methodology is called Neutral Print Density.

The method involves setting target colour values for the primary colourants and having a predefined density target for Cyan, Magenta and Yellow. The method is known to be slow and requires a separate print run to setup and perform. It is only suitable for use on a litho press, where density adjustments are relatively easy to perform.

The invention provides a faster, simpler, more flexible and less expensive technique for accurately producing images having the desired colour appearance.

Summary of the invention

According to the invention, there is provided a method of reproducing a colour image, comprising identifying a grey balance of a source printing condition of an image file to be printed, identifying a printing condition of a printing unit to be used to print the image file, converting the grey balance of the source printing condition to a grey balance of the printing condition of the printing unit, establishing a difference between the grey balance of the source printing condition and the grey balance of the printing condition of the printing unit and applying the estabhshed difference to colour values in the image file to produce a set of colour values to be printed at the printing unit.

The grey balance of the source printing condition may comprise a first value of black, the grey balance of the printing condition of the printing unit may comprise a second value of black and the difference between the grey balances may comprise the difference between the first value and the second value.

The grey balance of the source printing condition may comprise a first value of cyan, the grey balance of the printing condition of the printing unit may comprise a second value of cyan and the difference between the grey balances may comprise the difference between the first value and the second value.

The grey balance of the source printing condition may comprise a first value of magenta, the grey balance of the printing condition of the printing unit may comprise a second value of magenta and the difference between the grey balances may comprise the difference between the first value and the second value.

The grey balance of the source printing condition may comprise a first value of yellow, the grey balance of the printing condition of the printing unit may comprise a second value of yellow and the difference between the grey balances may comprise the difference between the first value and the second value.

Applying the established difference to colour values in the image file may comprise adding or subtracting the difference between the colour values of the grey balance of the source printing condition and the colour values of the grey balance of the printing condition of the printing unit to or from the colour values in the image file.

Establishing the grey balance of the source printing condition may comprise reading the printing condition of the image to be printed to determine combinations of non-black colourants which correspond to defined profile connection space values of the grey balance.

The defined profile connection space values of the grey balance may be L*a*b* values.

Identifying a printing condition of the printing unit to be used to print the image file may comprise printing a plurality of CMYK colour patches and measuring the colour of the colour patches in a profile connection space.

Converting the grey balance of the source printing condition to the grey balance of the printing condition of the printing unit may comprise inputting, to a colour matching module, the source printing condition, the printing condition of the printing unit, a rendering intent and one or more profile connection space values of the grey balance of the source printing condition; receiving from the colour matching module one or more profile connection space values of the grey balance of the printing condition of the printing unit; and reading, from the source printing condition, one or more combinations of non-black colourants which correspond to the one or profile connection space values of the grey balance of the printing unit.

Establishing the difference between the grey balance of the source printing condition and the grey balance of the printing condition of the printing unit may comprise identifying the difference between a combination of non-black colourants corresponding to the grey balance of the source printing condition and a combination of non-black colourants corresponding to the grey balance of the printing condition of the printing unit.

The source printing condition may be an ICC profile.

The image to be printed may contain a CMYK ICC profile or has a known set of colour characteristics.

According to the invention, there is provided an apparatus configured to reproduce a colour image, comprising: means for identifying a grey balance of a source printing condition of an image file to be printed; means for identifying a printing condition of a printing unit to be used to print the image file; means for converting the grey balance of the source printing condition to a grey balance of the printing condition of the printing unit; means for establishing a difference between the grey balance of the source printing condition and the grey balance of the printing condition of the printing unit; and means for applying the established difference to colour values in the image file to produce a set of colour values to be printed at the printing unit.

The apparatus may comprise means for performing a method as recited in any of claims 2 to 13.

According to the invention, there is provided an apparatus comprising: at least one processor; at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to: identify a grey balance of a source printing condition of an image file to be printed; identify a printing condition of a printing unit to be used to print the image file; convert the grey balance of the source printing condition to a grey balance of the printing condition of the printing unit; establish a difference between the grey balance of the source printing condition and the grey balance of the printing condition of the printing unit; and apply the established difference to colour values in the image file to produce a set of colour values to be printed at the printing unit.

For exemplary purposes only, embodiments of the invention will now be described with reference to the accompanying figures in which:

Brief description of the figures

Figure 1 is a schematic illustration of an apparatus for printing an image specified according to a source printing condition; figure 2 is a flow diagram showing a method of printing an image file having a source printing condition using a printer having a destination printing condition which is different to the source printing condition; figure 3 is an illustration of an adjustment curve for converting black colourant values in a source printing condition of an image file to black colourant values in a destination condition of a printing unit; figure 4 is an illustration of an adjustment curve for converting cyan colourant values in a source printing condition of an image file to cyan colourant values in a destination printing condition of a printing unit; figure 5 is an illustration of an adjustment curve for converting magenta colourant values in a source printing condition of an image file to magenta colourant values in a destination printing condition of a printing unit; and figure 6 is an illustration of an adjustment curve for converting yellow colourant values in a source printing condition of an image file to yellow colourant values in a destination printing condition of a printing unit.

Detailed description of the invention

A method and apparatus I for accurately reproducing a colour image specified in an input printing instruction is described below.

Referring to figure I, an apparatus I configured to reproduce the image comprises a controller 2 which, in turn, comprises one or more processors 3. The controller 2 may be configured to operate under the control of computer readable code so that, when executed by the one or more processors 3, the computer readable code causes the processor(s) 3 to adjust an image file for printing in accordance with the method described below. The computer readable code may be stored on a memory medium 4 of the apparatus I or may be stored in an external memory. The memory medium 4 may comprise ROM, RAM or any other suitable storage means.

The apparatus I is electronic and is preferably comprised in a printing unit 5 such as a printing press, laser printer or inkjet printer. However, the apparatus 1 may alternatively be comprised externally from the printing unit 5. In this case, the printing unit 5 and external apparatus 1 ate communicatively coupled so that the apparatus I can send printing instructions to the printing unit 5 during a printing operation. Communication between the external apparatus 1 and printing unit 5 may take place wireles sly or using a suitable communication cable.

The apparatus I is configured to receive a source image file which describes an image to be printed. The source image file may, for example, be input to the apparatus I from an external source such as a computer. As is known in the printing industry, image files for printing are prepared according to a selected source' printing condition. The source printing condition is selected by the creator of the image file and is generally an ICC profile, although other types of printing condition are also possible. The source printing condition can rarely be matched at the printing unit S which is to be used to print the image and so direct printing of the source image file results in distorted or non-optimum colour in the printed image, as explained previously. The following method describes a process of adjusting the source image file in accordance with the attributes of the printing condition of the printing unit 5, also referred to as the destination printing condition', thereby allowing the image to be reproduced accurately and with high quality colour by the printing unit 5.

Referring to figure 2, in a first step SI of the file adjustment method, the apparatus 1 reads the source printing condition from the source image file and, optionally, stores the source printing condition in the memory 4. In the specific example discussed herein, the source printing condition is the ICC profile: ISO 12647-2, coated'. However, any suitable printing condition, such as a different ICC profile, could alternatively be used.

In a second step S2 of the method, the apparatus 1 determines the grey balance of the source printing condition. As is known in the printing industry, the grey balance of the source printing condition corresponds to one or more combination(s) of non-black colourants which will create a visually neutral appearance when printed on the substrate assumed by the source printing condition. -10-

Any known method of determining these grey balance combinations can be used.

For example, where the source printing condition is an ICC profile, the ICC profile can be read by the apparatus 1 to determine one or more combinations of non-black colourants which correspond to a defined grey balance value in a profile connection space such as CIE LAB. This is discussed in detail below. Any suitable profile connection space can be used as an alternative to CIE LAB, for example CIE LCH, CIE LUST or CIE XYZ.

Hereinafter, combinations of non-black colourant will be referred to in the context of combinations of cyan, magenta and yellow (CMY). However, it will be appreciated that other combinations of non-black colourant could alternatively be used to print a neutral grey.

In this example, the grey balance of the source printing condition is defined, using the CIE LAB profile connection space, as combinations of non-black (CMV) colourants which have a* and b* values that are equal to a* and b* values of the black colourant (K). This method of defining grey balance using the a* and b* values of the black colourant (K) will be used in the subsequent discussion.

However, as is known in the printing industry, other definitions of grey balance could alternatively be used. Examples include defining grey balance as one or more mixes of non-black colourants (e.g. CMY) which have a* and b* values of zero, or defining grey balance based on the a* and b* values of the substrate onto which the image is to be printed. Paper is likely to be the most commonly used substrate, although any suitable substrate is possible.

An example of a plurality of combinations of non-black colourants which correspond to the grey balance of the source printing condition, ICC profile: ISO 12647-2 coated', is shown in Table A below. Specifically, Table A shows the combinations of non-black colourants (CMY) which have a* and b* values that are most closely matched to the a* and b* values of a plurality of tint percentages of black colourant (K). In this example, three tint percentages of black colourant (K) are used: 25%, 50% and 75%. The CMY combinations are identified by the apparatus I from the ICC profile: ISO 12647-2, coated'.

Table A

79.16 0 -1.61 22 16 l7 61.58 0 -1.16 45 35 36 40.95 0 -0.64 72 62 62 The CMY combinations given in Table A represent the correct grey balance of the source printing condition, ISO 12647-2, coated'.

As will be understood, the first column of Table A represents tint percentages of black colourant (K) whilst the second, third and fourth columns represent their corresponding L*a*b* values in the GIL LAB profile connection space. The L*a*b* values of the black colourant for any particular tint percentage can be obtained from look-up tables in the ICC profile: ISO 12647-2, coated' using known techniques.

The fifth, sixth and seventh columns of Table A specify the combinations of non-black colourants, in this case values of CMY, which most closely match the grey balance of the source printing condition. As already discussed, in this example three CMY grey balance combinations are found which respectively correspond to the a* and b* values of three different tint percentages of black (K). Optionally, a greater number of grey balance combinations could be found by looking up CMY combinations which correspond to the a* and b* values of a correspondingly greater number of tint percentages of black colourant. If the grey balance is defined in a different way, for example as the combinations of non-black colourants which have a* and b* values of zero, then a corresponding step is performed to look up the relevant CMY colourant combinations which mostly closely match the defined grey balance. -12-

The apparatus I may identify the combinations of non-black colourant (CMY) which correspond to the L*a*b* grey balance values using an iteration process. For example, the apparatus I may iterate through all possible CMY combinations in the source printing condition, in this case the ICC profile: ISO 12647-2 coated', to find the CMY combinations that are most closely matched, in terms of their values in the CIE LAB profile connection space, to the selected black tint (K) percentages. A known technique can be used. For example, the apparatus 1 may be configured to evaluate colour difference in terms of AE (deltaE) and select the CMY combination for which the iSE between the relevant tint percentage of black (K) and the CMY combination is least. Look-up tables in the source printing condition can be used to determine the L*a*b* values of any CMY combination using known techniques.

In a third step S3 of the method, the apparatus I causes a colour bar to be printed at the printing unit 5. The colour bar comprises a plurality of colour patches, each of which is made up of known combinations of non-black and black colourants (CMY and CMYK). More specifically, in this example, each printed patch corresponds to a known set of printed CMYK or CMY values, which may be stored in the memory 4 associated with the apparatus 1. The colour bar may also include a blank patch in which no colourant is printed. The apparatus I may, for example, supply a printing instruction to the printing unit 5 instructing the printing unit S to print the patches. The printing instruction comprises a particular CMYK or CMY dot percentage ratio for each patch. The number of colour patches in the colour bar is significantly less than the prior art method of using a colour chart described previously. An example is a colour bar containing approximately forty colour patches. In particular, the number of patches in the colour bar can be limited to the number of patches which is sufficient to evaluate the combinations of CMY which will produce a neutral grey, i.e. the grey balance, when printed under the destination printing conditions. This is explained in more detail below. An example set of printable CMYK values for a forty patch colour bar is provided below.

-13 -

C M Y K

I Al 80.00 0.00 0.00 0.00 2 A2 0.00 20.00 20.00 0.00 3 A3 50.00 50.00 0.00 0.00 4 A4 100.00 0.00 100.00 0.00 AS 0.00 0.00 0.00 80.00 6 A6 0.00 0.00 20.00 0.00 7 B1 30.00 30.00 30.00 0.00 8 B2 100.00 0.00 0.00 0.00 9 B3 80.00 80.00 0.00 0.00 B4 20.00 0.00 20.00 0.00 11 B5 0.00 50.00 0.00 0.00 12 B6 0.00 0.00 0.00 100.00 13 Cl 80.00 80.00 80.00 0.00 14 C2 20.00 0.00 0.00 0.00 C3 0.00 50.00 50.00 0.00 16 C4 100.00100.000.00 0.00 17 CS 0.00 80.00 0.00 0.00 18 C6 0.00 0.00 0.00 20.00 19 Dl 0.00 0.00 50.00 0.00 D2 100.00 100.00 100.00 0.00 21 D3 0.00 0.00 0.00 0.00 22 D4 0.00 80.00 80.00 0.00 23 D5 20.00 20.00 0.00 0.00 24 D6 50.00 0.00 50.00 0.00 23 El 0.00 100.00 0.00 0.00 26 E2 0.00 0.00 80.00 0.00 27 E3 20.00 20.00 20.00 0.00 28 E4 30.00 0.00 0.00 0.00 29 ES 0.00 100.00 100.00 0.00 E6 80.00 0.00 80.00 0.00 31 Fl 0.00 20.00 0.00 0.00 -14- 32 F2 0.00 0.00 0.00 50.00 33 F3 0.00 0.00 100.00 0.00 34 F4 80.00 0.00 0.00 0.00 F5 0.00 20.00 20.00 0.00 36 F6 50.00 50.00 0.00 0.00 37 Gl 100.00 0.00 100.00 0.00 38 G2 0.00 0.00 0.00 80.00 39 G3 0.00 0.00 20.00 0.00 G4 50.00 50.00 50.00 0.00 The colour bar is printed onto a substrate such as paper, card, fabric or any other suitable medium under the destination' printing conditions present at the printing unit 5.

In a fourth stage S4 of the process, the apparatus I causes the printed colour bar to be measured in the profile connection space to determine the colour of each patch.

The colour of the substrate is also measured, for example via the blank patch in the colour bar. In this example, since the CIE LAB connection space is being used, this stage comprises measuring the L*a*b* values of each patch. The L*a*b* measurements can be made according to known techniques, for example by using a spectrophotometer 6 to read the L*a*b* values for each patch. The spectrophotometer 6 may be comprised in the apparatus 1 and/or the printing unit and is communicatively coupled to the controller 2 so that data can be exchanged between the two devices. For example, the spectrophotometer 6 may be installed in a paper outlet path of the printing unit 5 so that colour bars on sheets of paper moving along the outlet path can be routinely measured. Optionally, the outlet path may be separate to a main outlet path which is being used to output the results of the rest of the print job.

For the reasons previously discussed, it is likely that the substrate and/or the colourants being used at the printing unit 5 will not match those assumed in the source printing condition, in this case the ICC profile: ISO 12647-2, coated'. As -15 -such, combinations of printed colourants, for example a particular CMY combination, may appear differently than would be the case if they were printed under the source printing condition.

From the measured L*a*b* values of the known CMYK and CMY patches in the printed colour bar, the apparatus 1 is configured to create a CMYK to L*a*b* conversion profile for the printing unit 5. This constitutes a fifth stage of the process 55. The conversion profile establishes a destination printing condition' for the printing unit 5 and can be created as an ICC profile. The conversion profile comprises the relationships between the known CMYK colourant combinations in the printed patches and their measured L*a*b* values. For CMYK combinations which have not been printed in the colour bar and therefore for which there is no measured L*a*b* value, the conversion profile comprises a predicted CMYK to L*a*b* relationship. These predicted relationships are made according to standard techniques based on the known CMYK to L*a*b* relationships measured from the printed colour bar. Due to the colour bar comprising a relatively small number of patches, the prediction may not be highly accurate across the entire colour space.

However, since the colours in the colour bar are optimised for establishing a neutral grey, very accurate predictions can be made for the grey balance. The conversion profile is also sufficient to make conversions between non-black CMY colourant combinations and L*a*b* values.

Preferably, if the source printing condition, for example ISO 12647-2, coated', specifies L*a*b* values for the colours of the base colourants (e.g. L*a*b* values for C, M and Y inks or toner), the colourants in the printing unit 5 should be matched as closely as possible to these specified L*a*b* values.

In a sixth stage S6 of the process, the apparatus 1 is configured to convert the identified grey balance of the source printing condition (see Table A) to the destination printing condition of the printing unit 5. -16-

Specifically, the source printing condition, the conversion profile created for the printing unit 5 from measurement of the colour bar and the three L*a*b* grey balance values of the source printing condition (see Table A) are input by the controller 2 to a colour matching module 7 comprised in the apparatus 1. A rendering intent, for example relative colorimetric or perceptual, for the source image is also input to the colour matching module 7 at this stage. The rendering intent is selected based on the type of image which is to be printed, according to known techniques. The selection of rendering intent will decide whether the substrate colour or print contrast is used to determine the target L*a*b* grey balance value for the printing unit S (see below. Black point compensation can optionally be used.

The colour matching module 7 uses the inputted source printing condition ISO 12647-2, coated', the conversion profile for the printing unit S and the rendering intent to adapt the L*a*b* grey balances of the source printing condition to the destination printing condition of the printing unit S using known a technique.

Specifically, the colour matching module outputs sets of CMYK colour values which, if printed using the printing unit 5, would produce a visually neutral grey.

These destination' CMYK values are calculated directly from the grey balance values of the source printing condition.

An example is shown in Table B below, in which the L*a*b* values of the source grey balance given in Table A are adjusted to account for the differing aspects of the destination print condition. As shown in the third column of Table B, three CMYK destination' grey balance combinations are output by the colour matching module 7 -one for each of the source printing condition grey balances previously identified from the source profile: ISO 12647-2, coated'. From the conversion profile established from reading the printed colour bar in the fourth step S4 described above, the apparatus I reads the L*a*b* values of the destination grey balance CMYK values. These destination' L*a*b* values are shown in the fourth column of Table B. Optionally the destination L*a*b values are stored in the memory 4. -17-

Table B

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F

L41aO,b-O.64 C17M9Y9K58 L47a1b4 Due to limitations of the destination printing process, it may not be possible to achieve the source grey balance L*a*b* values under the destination printing condition, A reason for this may be that the L* values of the solid black colourant in the source and destination printing conditions are different. In the example discussed here, the L* value for solid black colourant in the source printing condition (ISO 12647-2, coated) is 16 whilst the L* value for solid black colourant in the destination printing condition is 33. Therefore, scaling is performed when making the conversion to ensure that detail in the image is not lost using known techniques.

Additionally or alternatively, the a* and b* values may be different between the source and destination printing conditions because of differences between the colours of the source and destination substrates which affect how the colour of the printed colourant is perceived.

Therefore, as shown in Table B, the target L*a*b* values for the destination condition may be different to the L*a*b* values in the source condition.

Optionally, before the fifth step S5 is carried out, the apparatus 1 may be configured to convert the image CMYK values and grey balance of the source condition to an intermediate printing condition which is more closely matched to the destination printing condition. For example, if the image to be printed has been -18 -set-up using the source condition: ICC profile ISO 12647-2, coated' but is to be printed on a newspaper printing press 5 which is not closely matched to the source condition, the CMYK values of the image file may be converted to a standard newspaper printing condition which is more closely matched to the destination printing condition. These values then become the new source' values and can be used in the adjustment process described below. An example for a standard newspaper printing condition is the ICC profile: ISO Newspaper26v4. A known technique can be used to convert between the ICC profiles.

Once the L*a*b* values of the source grey balance have been converted to the destination printing condition, the apparatus I is configured to determine adjustments which are necessary to achieve a colour match between the source and destination printing conditions. This allows the CMY and K values of the source image file to be adjusted so that, when printed at the printing unit 5, the colour of the image is optimal. A detailed explanation is given below.

Using the CMY to L*a*b* conversion profile estabhshed in step four S4 of the process and the target destination grey balance estabhshed in the fifth stage of the process S5 (see Table B, column 4), the apparatus I is configured to read which tints of black (K) and combinations of non-black colourants (CMY) need to be printed under the destination printing condition to match the target L*a*b* values for the destination grey balance. This constitutes the sixth stage S6 of the process.

The results are shown in Table C below.

Table C

42 38 29 31 70 70 61 67 -19-In a similar process to the second step 52, in the sixth step 56 the apparatus I is configured to iterate through the tints of black colourant (K) in the source printing condition, ISO 12647-2, coated' or ISO Newspaper2óv4', to find the K tints which have an L* value most closely matching the L* of the identified target grey balances of the destination printing condition (see Table B). Similarly, the apparatus I is configured to iterate through the combinations of non-black colourants (CMY) in the source printing condition, ISO 12647-2 coated' or ISO Newspaper26v4', to find the CMY combinations which have L*a*b* values most closely matching the L*a*b* of the target grey balance of the destination print condition.

An example of the differences between the CMYK values of the source and destination printing conditions is shown in Table D below. It will be appreciated that the majority of values in Table D are derived directly from Tables A and C discussed above. An extra hne, corresponding to a source condition black colourant (K) tint percentage of 10 is also included for illustrative purposes.

Table D

*Plt'uriM raxuirnmNil I rtr-s w* ratst a'u rptr-t -a' Th 6 7 8 6 4 4 -2% -3% -2% -3% 22 16 17 19 17 12 13 -6% -5% -4% -4% 45 35 35 42 38 28 31 -8% -7% -7% -4% 72 62 62 70 70 61 67 -5% -2% -1% +5% In the seventh stage S7 of the process, using the differences between the black colourant K grey balances identified for the source and destination printing conditions, the apparatus 1 is configured to estabhsh a black colourant (K) adjustment curve which can be used to adjust all source' values of K for the image file to corresponding destination' K values that should be printed at the printing unit 5. An example is shown in Figure 3. As can be seen, the values of K are -20 -plotted against each other for the source and destination printing conditions. By reading across the curve, the apparatus I is able to convert any value of K in the source image file to a value of K that should be printed at the printer 5, thereby maintaining a colour balance in the printed image.

In an eighth stage S8 of the process, a similar process is carried out for the tints of CMY. Specifically, using the differences between the non-black colourant (CMY) grey balances identified for the source and destination printing conditions, the apparatus 1 is configured to establish an adjustment curve for each non-black colourant (CMY) which can be used to adjust all source' values of C. M and Y in the source printing condition to corresponding destination' C, M and Y values that should be printed at the printing unit S. Examples are shown in Figures 4, 5 and 6.

By reading across the curves, the apparatus I is able to convert any value of C, M or Y in the source image file to a value of C, M or Y that should be printed at the printer 5, thereby maintaining a colour balance in the printed image.

The colour bar can be printed at regular intervals, for example across the bottom of an A4 or A3 sized substrate, throughout a main print run so that the grey balance of the destination printing unit S is continually matched to the grey balance of the source printing condition. In this way, variations in the colour of the colourants at the destination printing unit S can be measured during the print run and appropriate adjustments can be made to the printed CMYK values to maintain grey balance.

Furthermore, since the adjustments are based around grey balance, the method is not limited to the colours of the non-black colourants at the printing unit 5 being CMY. The method can be used whatever the non-black colourants of the printing unit 5, as long as the colourants can be combined to print a neutral grey.

It will be appreciated that modifications and adjustments can be made to the above-described process and apparatus without departing from the scope of the invention defined in the claims. -21 -

Claims

Claims 1. A method of reproducing a colour image, comprising: identifying a grey balance of a source printing condition of an image file to be printed; identifying a printing condition of a printing unit to be used to print the image file; converting the grey balance of the source printing condition to a grey balance of the printing condition of the printing unit; establishing a difference between the grey balance of the source printing condition and the grey balance of the printing condition of the printing unit; and applying the estabhshed difference to colour values in the image file to produce a set of colour values to be printed at the printing unit.
2. A method according to claim 1, wherein the grey balance of the source printing condition comprises a first value of black, the grey balance of the printing condition of the printing unit comprises a second value of black and the difference between the grey balances comprises the difference between the first value and the second value.
3. A method according to claim I or 2, wherein the grey balance of the source printing condition comprises a first value of cyan, the grey balance of the printing condition of the printing unit comprises a second value of cyan and the difference between the grey balances comprises the difference between the first value and the second value.
4. A method according to any one of claims I to 3, wherein the grey balance of the source printing condition comprises a first value of magenta, the grey balance of the printing condition of the printing unit comprises a second value of magenta and the difference between the grey balances comprises the difference between the first value and the second value.
5. A method according to any preceding claim, wherein the grey balance of the source printing condition comprises a first value of yellow, the grey balance of the printing condition of the printing unit comprises a second value of yellow and the difference between the grey balances comprises the difference between the first value and the second value.
6. A method according to any preceding claim, wherein applying the established difference to colour values in the image file comprises adding or subtracting the difference between the colour values of the grey balance of the source printing condition and colour values of the grey balance of the printing condition of the printing unit to or from the colour values in the image file.
7. A method according to any preceding claim, wherein establishing the grey balance of the source printing condition comprises reading the printing condition of the image to be printed to determine combinations of non-black colourant which correspond to defined profile connection space values of the grey balance.
8. A method according to claim 7, wherein the defined profile connection space values of the grey balance are L*a*b* values.
9. A method according to any preceding claim, wherein identifying a printing condition of the printing unit to be used to print the image file comprises printing a plurality of CMYK colour patches and measuring the colour of the colour patches in a profile connection space.
10. A method according to any preceding claim, wherein converting the grey balance of the source printing condition to the grey balance of the printing condition of the printing unit comprises: inputting, to a colour matching module, the source printing condition, the printing condition of the printing unit, a rendering intent and one or more profile connection space values of the grey balance of the source printing condition; receiving from the colour matching module one or more profile connection space values of the grey balance of the printing condition of the printing unit; -23 -reading, from the source printing condition, one or more combinations of non-black colourant which correspond to the one or profile connection space values of the grey balance of the printing unit.
11. A method according to claim 10, wherein establishing the difference between the grey balance of the source printing condition and the grey balance of the printing condition of the printing unit comprises: identifying the difference between a combination of non-black colourants corresponding to the grey balance of the source printing condition and a combination of non-black colourants corresponding to the grey balance of the printing condition of the printing unit.
12. A method according to any preceding claim, wherein the source printing condition is an ICC profile.
13. A method according to any of claims I to 11, wherein the image to be printed contains a CMYK ICC profile or has a known set of colour characteristics.
14. An apparatus configured to reproduce a colour image, comprising: means for identifying a grey balance of a source printing condition of an image file to be printed; means for identifying a printing condition of a printing unit to be used to print the image file; means for converting the grey balance of the source printing condition to a grey balance of the printing condition of the printing unit; means for establishing a difference between the grey balance of the source printing condition and the grey balance of the printing condition of the printing unit; and means for applying the established difference to colour values in the image file to produce a set of colour values to be printed at the printing unit.
15. An apparatus comprising means for performing a method as recited in any of claims 2 to 13.-24 -
16. An apparatus comprising: at least one processor; at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to: identify a grey balance of a source printing condition of an image file to be printed; identify a printing condition of a printing unit to be used to print the image file; convert the grey balance of the source printing condition to a grey balance of the printing condition of the printing unit; establish a difference between the grey balance of the source printing condition and the grey balance of the printing condition of the printing unit; and apply the established difference to colour values in the image file to produce a set of colour values to be printed at the printing unit.