WO2007091278A1

WO2007091278A1 - Method and system for colour printing

Info

Publication number: WO2007091278A1
Application number: PCT/IS2007/000005
Authority: WO
Inventors: Ingi Karlsson
Original assignee: Ingi Karlsson
Priority date: 2006-02-06
Filing date: 2007-02-05
Publication date: 2007-08-16
Also published as: IS8286A; IS2275B

Abstract

A method for printing any of a large number of colours as a continuous spot colour from basic process colors such as conventional CMYK process colours, as well as a method for setting up a reference chart that functions as a guide to quickly and readily print as a solid spot colour a predetermined colour. Also provided is a printing kit comprising a chart or set of prints comprising a plurality of halftone separated single colour patches, a dataset listing the halftone composition of each patch colour and listing relative amounts of basic process colours for printing the respective colours as solid spot colours, and preferably containing solid spot colour patches that correspond to the halftone colour patches.

Description

Method and system for colour printing

FIELD OF INVENTION

The invention is within the field of colour printing, in particular for multi-colour printing on offset printing presses.

TECHNICAL BACKGROUND

Conventional colour printing suffers from many problems and limitations. Accurate reproduction of a desired colour is a continuing challenge and very often trial and error and the experience of operators of printing machines determine the outcome.

Most conventional colour printing is achieved by four-colour halftone printing, i.e. printing rasters of basic colours (series of evenly spaced dots in varying size and shape, varying in direct proportion to the intensity of tones they represent) on top of each other, typically the four basic colours of the CMYK system, cyan, magenta, yellow and black (key colour). Other colour systems for halftone printing also exist e.g. the Hexachrome® system by Pantone Inc. where basically 2 additional colours are added to CMYK (Orange and Green) to broaden the colour spectrum and consequently close matches of more colour tones such as e.g. from the Pantone colour palette, can be achieved using this method. In addition, worth mentioning is the TRUMATCH 4-colour matching system, which provides visual tools for colour matching on-press and on printers/copiers. This patented, digital system is the 4-colour palette in many software products for graphic design and illustration, e.g. from Adobe, Corel, Deneba, InScribe, Macromedia, and Quark. The system uses a logical organization of process colour by HSB (Hue/ Saturation/Brightness), showing 50 hue families in the order of the spectrum, with 40 proportioned tints and shades of each hue plus 4-colour greys. See, www.trumatch.com. TRUMATCH is strictly for use in 4-colour printing and consequently uses halftones to produce the various colour shades of the system.

For single solid colour prints, specialty mixed inks can be used to obtain deeper, more solid colour appearance. Such specialty colours are provided e.g. from Pantone as the Pantone Matching System (PMS) that consists of 14 basic colours represented in a colour formula guide which represents each colour of the system and describes how to physically mix the 14 basic colours together to obtain well over a thousand different colours/colour shades defined in the colour formula guide, each colour consisting of 1 or more of the Pantone base colours in addition to transparent white for lighter colour shades. US Patent 5,161 ,974 (1992) describes a system of a total of 20 base colours, each consisting of 2 process colours mixed by parts. In addition lighter shades of colour can be achieved in this system using halftones (i.e. so called tints).

Alternative affordable systems for reproducible quality colour printing while reducing the need for keeping a stock of specialty colours would be appreciated.

SUMMARY OF INVENTION

The present invention provides in one aspect a method for printing a desired colour as a continuous spot colour, the method comprising a simple reiterative process making possible the printing of solid spot colours that are mixed from basic process colours used in four colour printing.

DETAILED DESCRIPTION

A dual purpose colour system is provided consisting of a plurality of colours where each colour of the system (hereinafter referred to as "The Colour") is made up from one or more of the process colours Cyan, Magenta, Yellow and Black (CMYK) with, a) on one hand the correct halftone percentages for printing said continuous colour in traditional 4-colour printing along with, on the other hand, b) a formula for physically mixing by weight (grams and/or parts) and printing The Colour as a single spot colour, consisting solely of the same process colours (colourants) used for achieving the continuous colour, optionally adding transparent ink (lacquer) or white ink to ensure proper ink film thickness for visually matching the continuous halftone version of The Colour described in paragraph a) here above. In one aspect the invention provides a method for providing reference (a formula) for printing a desired colour as a continuous spot colour on a selected surface type comprises the steps of: a) providing a sample print colour patch on a selected surface type printed with halftones made up from one or more basic process colours used in traditional four- colour printing (Cyan, Magenta, Yellow, Black - CMYK); b) mixing a spot colour from said one or more basic process colours, based on the halftone composition of the sample print, i.e. the desired colour printed on the same or substantially similar surface type as the selected surface type, optionally using transparent ink (lacquer) or white ink to ensure optimal ink film thickness; c) printing a test print on said selected surface type with said mixed spot colour and comparing (preferably using a spectrophotometer to measure the L*A*B* value of the colour) to the sample print colour patch (and it's L*A*B* value), and if needed, adjusting the relative amounts of said one or more process colours and/or transparent ink (laquer) or white ink in the mixed spot colour, if the test print does not essentially visually match the sample print colour patch; d) repeating step c) until the test print essentially matches the desired colour when printed using the appropriate ink film thickness for said surface; e) measuring the density of the obtained spot colour print using a pre-defined density indicator (preferably C, M, Y or K); f) documenting the relative amounts of said process colours, and transparent ink (lacquer) or white ink, if present, in the spot colour mix, the halftone colour composition and the ink density of each of the respective process colours of the original sample print colour patch such that the same colour can be printed on demand as a continuous spot colour on the same selected surface type from said basic process colours or as a halftone composition, depending on the application.

Thereby, the desired colour has been printed as a continuous spot colour and the same colour (unique name or number) can be printed readily again on the same selected surface type by using the same ink mix composition that has been determined for said colour.

In a preferred embodiment a spectrophotometer is used for the colour comparison and matching; the sample print colour patch is measured with the spectrophotometer and a numerical value is obtained (suitably expressed in L*a*b^* coordinates), the test print printed with the mixed spot colour is subsequently measured and the measurement value compared to the reference value of the sample print patch; if the values do not match the process is repeated as described above.

When the matching spot colour mix has been obtained, the relative amount of the process colour inks as well as the amount of transparent ink (lacquer) or white ink in the spot mixture is registered together with the density value for the spot colour print. These values are registered pair-wise with the identity of the reference colour (e.g. with reference to the halftone composition, respective density of each process colour and L*a*b* coordinates of the reference colour). By systematically using this method to obtain a series of colour recipes for spot colours, a colour matching system may be produced that allows simple selection and straightforward printing of a large number of colours that can be achieved either by printing the chosen colour as a unique spot colour or as a halftone composition, - depending on the application.

The selected surface type may, e.g., be selected from glossy-surface paper, matte surface paper or regular non-surface-layered paper or any other material, paper or other material, such as plastic, metals or even clothing.

The basic process colours used in accordance with the invention are suitably one or more of the basic process colours cyan, magenta, yellow and black (CMYK), preferably as defined by the international standard ISO 2846-1 :1997 optionally in addition to transparent ink (lacquer) or white ink, to ensure correct ink film thickness for the respective surface. Other process colour sets can however as well be used for reference, such as Hexachrome colours (CMYKOG, Pantone, USA).

Preferably, in order for printers to be able to apply the method regardless of their location or scale, a reference colour chart is created, provided and printed, comprising a plurality of halftone colour patches of a suitable minimum size such that an individual patch can be measured with a conventional spectrophotometer. Preferably, the colour chart is a standardized colour chart such as e.g. the ECI 2002 chart, a superset of ISO 12642:1996 that by itself contains a very large number of colour patches where each individual patch has a known L*A*B* value. By using this, or a similar reference chart, optionally in addition to individual halftone patches selected especially with the purpose of being a part of said stand-alone colour matching system, an output profile such as an ICC (International Colour Consortium) profile may be created and used over and over again for each chosen type of paper or other material. Using the ICC profile, ensuring that all output equipment (plate setter/film setter) is correctly calibrated and adjusted and maintaining correct density - i.e. the same density used when creating the ICC profile, results in that correct halftone values can be printed with some assurance on the chosen paper or other material, resulting in the visually correct halftone colour. ICC profiling or proper adjustment of output is a premise for printing the halftone versions of any colour selected for said colour matching system. For optimal results when printing halftone spots, ICC output profiles and documentation on usage and adjustment of output and printing machine are preferably provided with said colour matching system, allowing even the smallest print houses to produce visually correct halftone versions of any colour of said colour matching system.

If the desired colour is originally obtained on a specimen, the specimen may be suitably compared to a series of sample prints by manual or automatic (halftone density based and/or spectrophotometric) comparison to a chart such as the ECI 2002 chart; or the desired colour can be simply selected from such series of sample patches.

The next step of the method comprises mixing a spot colour from the basic process colours of the halftone colours used in the sample print patch, the ratio of the individual colour components may be selected based on the halftone composition of the sample print patch. The thus obtained mixed spot colour is printed on said selected surface type (e.g. glossy-surface paper, matte surface paper or regular non-surface- layered paper) and the resulting print is compared to the sample print patch, preferably by numerical comparison, e.g., as described above. If the match between the printed spot colour and the sample print is imperfect, a slight amount of one or more of the process colours in the spot colour mix is added to the colour mix and a new print is made. A transparent ink (lacquer) or white ink is optionally used to achieve the desired ink film thickness and lightness. This step may be repeated multiple times until a visual match is obtained.

When the desired spot colour is obtained on the selected surface type the density of the colour is measured, suitably by using a pre-defined density indicator for the basic process colour(s) being used (cyan, magenta, yellow or black). The measured density and the process colour composition are registered and paired up and thus the same spot colour can be produced on demand on the same selected surface type.

It should be appreciated that when the above process has been performed for a number of colours, e.g. a series of colours that can be arranged in a colour map or colour fan/book, a system is obtained whereby a multitude of colours can be readily printed as spot colours with the use of conventional process colours, e.g. the CMYK colours, which decreases the need for a stock of specialty colours for a large number of printing applications. Preferably the process is completed for a number of colours, each colour on a series of different surface types, such as, e.g., glossy-surface paper, matte surface paper and regular non-surface-layered paper; this will ensure that a desired colour (colour tone) can be achieved on various types of surfaces, resulting in virtually the same visual colour tone regardless of chosen paper type/surface.

In another aspect the invention provides a colour printing kit comprising: a chart or set of prints comprising a plurality of halftone separated single colour patches; a dataset listing the halftone composition of each patch colour and listing relative amounts of basic process colours for printing the respective colours as solid spot colours as well as the needed amount of transparent ink (lacquer) or white ink for optimal ink film thickness.

Preferably the colour printing kit of the invention further comprises a chart or set of prints comprising a plurality of solid spot colour patches that correspond to one or more and preferably all of said halftone separated halftone colour patches.

The colour printing kit is in another useful embodiment provided as an electronic dataset stored on an electronic data medium, e.g. in a computer memory, on a CD or disc or other computer memory type. Such a dataset may suitably be used in connection with a computer running a computer program that will provide parameters for printing a desired spot colour based on a colour description that is given as input to the program in the form of a halftone printing definition, L*a*b* coordinates or the like colour definition. The invention additionally pertains in yet a further aspect to a method of printing a desired colour as a solid spot colour using a colour kit of the present invention, the method comprising the steps of: providing a sample print of said desired colour or a numerical definition of the desired colour in terms of halftone composition or colour parameters such as L*a^*b^* coordinates, matching said desired colour to the closest halftone single colour patch and/or colour definition from said dataset, mixing a colour ink mix based on the relative amounts of process colours as in the data set element associated with said closest matching colour, printing said desired colour using the obtained colour ink mix, optionally mixed with transparent ink (lacquer) or white ink to ensure optimal ink film thickness and lightness.

FIGURES

Figure 1 illustrates a colour printing kit according to the invention providing values for two colours as printed by halftone printing (1a) and spot colour printing (1b).

DESCRIPTION OF A PREFERRED EMBODIMENT

Figure 1 shows an example of two colour entries, "Spot Red" and "Spot Sky Blue" from a colour printing kit in accordance with the invention. Figure 1a shows the parameters for conventional halftone printing of the two colours, showing halftone values for cyan, magenta, yellow and black (C, M, Y, K) in that order. Figure 1b shows parameters for printing the visually best possible match of said halftones as spot colours by mixing the basic colour inks C, M, Y, K, (relative values listed in that order) adding transparent ink (fifth value) to obtain correct film thickness. The values listed in the five boxes for each paper type are the relative amounts of the basic colours and transparent ink to be mixed in the spot colour ink mixtures. Note that the values in the Figure are not necessarily correct; these would need to be first obtained with the method of the invention as described herein.

The entries are depicted as pages in a handy flip book providing recipes for a number of spot colours.

Claims

1. A method of printing as a continuous spot colour a desired colour tone printed in conventional four-colour halftone printing (CMYK), said method comprising mixing by weight the same process inks used in said halftone printing of said colour and adding to the mix transparent ink (lacquer) or white ink to ensure proper ink film thickness and lightness and printing said colour as a continuous spot colour with said mix.

2. The method of claim 1 wherein the CMYK colours are defined by the international standard ISO 2846-1 : 1997.

3. The method of claim 1 wherein said continuous spot colour is printed on the same type of surface as said colour tone printed in conventional four-colour halftone printing is printed on.

4. The method of claim 3 wherein said surface type is selected from the group consisting of glossy-surface paper, matte surface paper and regular non-surface-layered paper.

5. The method of claim 1 , comprising the steps of a) providing said colour tone printed in conventional four-colour halftone printing printed on a selected surface type; b) mixing a spot colour from said one or more basic process colours, based on the halftone composition of said sample print colour patch, adding a transparent or white ink to ensure optimal ink film thickness; c) printing a test print on said selected surface type with said mixed spot colour and comparing to the sample print colour patch, and if needed, adjusting the relative amounts of said one or more process colours in the mixed spot colour, if the test print does not essentially visually match the sample print colour patch; d) repeating step c) until the test print essentially visually matches the desired colour when printed using the appropriate ink film thickness for said surface; e) measuring the density of the obtained spot colour print using a pre-defined density indicator; f) registering the density and process colour composition of the obtained spot colour print such that the same colour can be produced on demand as a continuous spot colour on the same selected surface type from said basic process colours, to thereby print the desired colour as a continuous spot colour.

6. A colour printing kit comprising:

5 a chart or set of prints comprising a plurality of halftone separated single colour patches; a dataset listing the halftone composition of a plurality of colour tones identified with colour patches or index values and listing relative amounts of basic process colours including a value for transparent ink (lacquer) or white ink, for printing the respective 10 colours as solid spot colours.

7. The colour printing kit of claim 6 comprising a chart or set of prints comprising halftone separated single colour patches of said plurality of colour tones.

15 8. The colour printing kit of claim 7 further comprising a chart or set of prints comprising a plurality of solid spot colour patches that correspond to one or more of said halftone separated single colour patches.

9. The colour printing kit of claim 8 wherein the dataset further comprises L*a*b* 20 coordinates for each colour.

10. The colour printing kit of claim 9 wherein the dataset further comprises density values for each colour.

25 11. The colour printing kit of any of claims 6-10, wherein said relative amounts of basic colours and transparent and/or white ink are obtained with the method of any of claims 1- 5

12. The colour system of any of claims 6-11, set forth in digital format as stand-alone 30 software or for use in 3^rd party software.

13. A method of printing a desired colour as a solid spot colour using the colour kit of any of claims 6 to 12, comprising the steps of: providing a sample print of said desired colour or a numerical definition of the desired colour in terms Of halftone composition or colour parameters such as L*a*b* coordinates, matching said desired colour to the closest halftone single colour patch and/or colour definition from the dataset of said colour kit, mixing a colour ink mix based on the relative amounts of process colours and amount of transparent or white ink as in the dataset element associated with the closest matching colour, printing said desired colour as a solid spot colour using the obtained colour ink mix.