WO2006120460A2

WO2006120460A2 - A method of producing a print for preview purposes

Info

Publication number: WO2006120460A2
Application number: PCT/GB2006/001742
Authority: WO
Inventors: Paul Foxley Miller
Original assignee: AVA CAD/CAM GROUP Ltd
Current assignee: AVA CAD/CAM GROUP Ltd
Priority date: 2005-05-13
Filing date: 2006-05-12
Publication date: 2006-11-16
Anticipated expiration: 2007-11-13
Also published as: WO2006120460A3; GB0509759D0

Abstract

In a method of producing an image of a coloured design for preview purposes, a substrate is selected and a pad layer (2) is printed on the substrate (3). A series of known tonal values (1) are printed on the pad layer (2) and colour information read from the image produced by means of a spectrophotometer. The data obtained is then manipulated in a computer in order to generate a digital preview of the appearance of the image. Alternatively, where the substrate is transparent, the known tonal values may be printed on the substrate and the pad layer printed on top of the tonal values or the known tonal values printed on the substrate and a backing layer laminated with the substrate. The method enables the appearance of an image to be accurately predicted.

Description

A METHOD OF PRODUCING A PRINT FOR PREVIEW PURPOSES

The present invention relates to a method of producing a print for preview purposes. There are many variants for producing prints, but all work on the same basic idea (except for inkjet where inks mix on printer) e.g. Gravure, Flexo, Rotary and Silk Screen, Offset.

In colour printing, multiple coloured inks are printed on top of each other. Most printing processes are only able to reproduce a limited range of tones from an ink: usually a solid print and nothing. In order to generate a wide range of tones, a technique known as Halftoning is used where the image is printed as a series of dots of different sizes. From a distance, the dots appear to merge into a continuous range of tones. Colours other than those used in the inks can be obtained by printing two or more inks on top of each other (for example, Cyan and Magenta produce a Blue when combined). This is called overprinting.

For the reproduction of images, certain sets of colours work better than others. For instance Cyan, Magenta and Yellow. A particular set of these colours used together is called a process set. Most process sets use four colours - Cyan, Magenta, Yellow and Black, but there are six, seven and eight colour variants.

Spot colours are colours which are not part of a process set. They are used for several reasons: 1 ) to obtain colours which could not otherwise be achieved (i.e. that are outside the Gamut of the process colours) 2) to ensure that a particular colour is reproduced accurately (e.g. for 'Brand¹ colours in packaging applications)

3) to enable multiple colourways of a design to be produced without having to generate new process separations for each one (particularly in the Textile and Wallcovering industries)

Developing and testing separations and colourways (Proofing) is expensive because cylinders/screens have to be produced, inks mixed and press time used to produce proof prints.

When an ad-hoc approach to colour is taken (inks and settings adjusted on-press to achieve the desired result) it is difficult to reproduce a design at a later date or at a different location as the exact specifications for the print are unknown.

Simulation of final design (a virtual or digital proof) can save time and money if the simulation is an accurate representation of the final print. Existing ICC profile based solutions can work well for process colour prints, but do not handle spot colours well

Since prediction of the final appearance of a printed design without having to print it is such a useful idea, there have been many attempts to do it, with varying degrees of success. (Color Technology for Electronic Imaging Devices, Henry. R. Kang. Pub. SPIE Press. ISBN 0-8194-2108-2 provides a good overview of the field).

Dot-Based Models: e.g. Murray-Davies. Yule-Nielsen, Clapper-Yule. All based on simple model of halftone process, where print is either printed or not. These only apply to single colour prints. The Neugebauer Equations are based on the observation that, if you are printing using N inks, then the overprinted result will (given perfect halftone dots) be made up of 2^N possible colours - the colours obtained by printing all the possible permutations of solid overprints of the inks. Using the Demichel dot overlap model can determine the area of each overprint in the final print and from that, determine what the result looks like.

In practice, the dots are not perfect and the Demichel dot area calculations don't predict the actual overlaps. Also, the Neugebauer equations are not suitable for predicting the colour of spot colour overprints because of the need to measure the solid colour overprint permutations for the set of inks used.

There are a wide range of extensions to the above models that attempt to improve their agreement with reality, but all suffer from problems predicting the colour of designs with spot colours.

Layer-Based Models are based on prediction of effect of a uniform layer of ink printed on a substrate. This does not take halftoning into account, so is only useful for uniform layers of colour.

Beer-Lambert law The Beer-Lambert law applies to non-scattering colourants (e.g. Dyes in solution). The Kubelka-Munk theory predicts the effect of a layer of a certain thickness of ink with scattering coefficient S and absorbtion coefficient K. K and S are usually obtained by printing a patch of colour over a white and a black substrate and then solving the Kubelka-Munk equations. There is also a simplified version of the K-M theory which uses a single constant (K/S). This version is used for predicting the colour of thick layers of ink.

Various hybrid schemes have been proposed that use a combination of dot and layer based models to predict the final outcome (e.g. Using Kubelka- Munk equations to predict overprint colours for a Neugebauer model from known colours).

Recipe prediction systems can predict the colour of bulk ink samples, using recipe information and data from measurements of the base colourants (and sometimes measurements of previous matches). Recipe prediction systems use versions of the layer based models described above. Generally, Recipe Prediction systems don't predict the colour of the ink in the final production process but instead use a different process in the lab for generation of samples. It would, however, be possible to use an existing recipe prediction system with appropriate data to predict the colour of 100% tone (flat colour) prints onto the production substrate under production printing conditions. This is acceptable for simulating flat colour designs, but not for tonal designs because the appearance of halftones of an ink depends on the characteristics of the ink and of the printing process (substrate, haltone screen, press settings etc.). The hue of a halftone of an ink can be

(and frequently is) different to the hue of the solid chip. Different printing processes will result in different colours of tones with the same ink.

Some systems attempt to model the behavior of printed colours by measuring overprints and tones of inks made up of a small set of base colourants, which are then used as the components of recipes for all other colours printed. The recipe of an ink, together with the measurements of the base colourants is used to predict the behaviour of the ink. In practice, this doesn't work very well as the behaviour of mixes of ink (mixed in bulk off- press, that is, not overprinted on press) is not the same as a mixture of the behaviour of the recipe components.

ICC profiles are not a way to predict the colour of overprinted designs, instead ICC profiles are a standard way to describe to Colour Management Software how a particular device (e.g. Monitor, Scanner or Printer) and/or set of inks behaves.

Usually, ICC profiles are used for process colours but the ICC model is quite general and can be applied to any set of up to eight inks. These usually require a target printed with all process colours together. They are not suitable for spot colour applications because the number of permutations of colours is too high. It would be necessary to print and measure a new target for nearly every design.

According to one aspect of the present invention, there is provided a method of producing an image of a coloured design for preview purposes including the steps of selecting a substrate, printing a series of known tonal values, reading colour information from the image produced by means of a spectrophotometer and manipulating the data received in a computer in order to generate a digital preview of the appearance of the image.

The substrate is advantageously opaque and spectrophotometer measurements are taken from the tonal value side.

The substrate is advantageously transparent and spectrophotometer measurements are taken from the substrate side.

A pad layer may be printed on the substrate and the known tonal values printed on the pad layer. Alternatively, the known tonal values may be printed on the substrate and the pad layer printed on top of that.

According to another aspect of the present invention there is provided a method of producing an image of a coloured design for preview purposes, including the steps of selecting a substrate, printing a series of known tonal values on the substrate, laminating a backing layer with the printed substrate, reading colour information from the image produced by means of a spectrophotometer and manipulating the data received in a computer in order to generate a digital preview of the appearance of the image.

Spectrophotometer measurements are advantageously taken from the substrate side.

In a preferred embodiment of the invention, the known tonal values are printed as rectangular colour chips having, for example, between ten and twenty steps and as a single spot layer on the substrate. The size of each chip is sufficient to allow accurate measurement with the spectrophotometer where a pad layer is printed, the colour of the layer may be blue or black.

The collection of data from the spectrophotometer is advantageously automated and spectrophotometer measurements are taken using an X-Y table in accordance with a program semi-automatically. The manipulation of the data and the data itself may be made available to a remote user over the internet for example.

In order that the invention may be more clearly understood, embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:-

Figure 1 shows a series of tonal values represented as a rectangular colour chip;

Figure 2 diagrammatically shows, in cross section, a first sample prepared for the collection of data;

Figure 3 diagrammatically shows, in cross section, a second sample prepared for the collection of data and Figure 4 diagrammatically shows, in cross section, a third sample prepared for the collection of data. Referring to the drawings, a sample is prepared for each ink, substrate printing condition and engraving combination.

To prepare each sample, a series of known tonal values as rectangular colour chips (usually between ten and twenty steps, is printed as a single spot layer. The size of each chip should be sufficient to allow accurate measurement with a Spectrophotometer.

The series of tonal values 1 is printed over a black pad layer 2, printed on the substrate 3. The same series of tonal values is printed over a dark blue pad layer 2, printed on the substrate.

Where, as in the example shown in Figure 2, the substrate is opaque, the sample is measured from the ink side in the direction of the arrow A in

Figure 2. Measurements may be made using an X-Y table, data collection may be automatic and software may operate to add data collected to the database semi-automatically.

Where, as in the example shown in Figure 3, the substrate is transparent the tonal values 1 are printed on the substrate 3 and the pad layer 2 is printed on top of the tonal values 1 . Measurements are made in the same way as with the arrangement of Figure 2 except that measurements are taken through the transparent substrate in the direction of arrow B.

Where, as in the example shown in Figure 4, the substrate 3 is to be laminated, for example with a white or metallic backing 4 the measurement is made in the same way as with the arrangement of Figure 2, that is on the substrate side through the ^' substrate in the direction of arrow C. The laminate may be white, black, blue or other colour.

Ink data is stored in a central (or local) database and may be accessed over a network

In design software, each colour separation is specified as a separate layer, (the layer provides information on the tonal level used at each point in the separation, but not the ink used). The ink used on the layer is specified by name (multiple sets of inks can be used to generate colourways)

Other details of the printing process are also specified (e.g. Substrate type, press conditions, ink type and more) The design software looks up the data for the inks in the ink database using the ink name and printing conditions to identify the correct data.

Using the ink database information and the separation information in the layers, the software calculates the appearance of the final design in a device independent colour space (e.g. CIE Lab) . Where a tone is used in the design that is not present in the database, the necessary information is generated by interpolation of the known tones for that ink. Standard ICC colour management techniques are then used to display the design either on the monitor or on an ink-jet printer. Production profiling can be combined with a conventional ICC profile approach (for example when a design consist of a process colour set with some additional spot colours) by using the ICC profile to predict the appearance of the process colour part of the design (in a device independent colour space) and then using the method described above to 'print' the spot colours.

The applicants' Production Profiling measures tonal steps of known recipes under known printing conditions. The measurement data (stored in a database) is then used to predict the appearance of a printed design, which is printed using some of the known recipes and under the standardised printing conditions. The effects of overprinting are predicted via a mathematical model with parameters tuned to a particular printing process. Transparency of inks is modelled by a single parameter which is manually set for each ink. This version is adequate for applications in the Textile and

Wallcoverings industries where overprinting of colours in a design is minimised to enable the production of a range of colourways without having to engrave multiple sets of cylinders. For applications where overprinting of colours is common the simple single parameter transparency model is not always sufficient.

In a modification of the applicants' Production Profiling modelling of overprint effects and ink transparency based on measurements of actual overprints on press is added. In order to prevent the need to measure a new profile for each design (as in ICC profiling), each ink recipe is overprinted on a small number of fixed colours (including a black and usually a dark blue). Again the measurement data is stored in a database. The appearance of overprinted designs is calculated using the overprint data in the database.

By measuring a range of tones for each ink, this Production profiling process removes the need to predict the behaviour of tones on the press (which is complex).

This Production Profiling applies complex mathematical modeling to the data obtained in order to generate digital (on calibrated computer monitors and digital printing devices) previews of what a conventional print of a particular design with specified inks on a specified substrate using a specified production printing process will look like.

The Production Profiling and database may be linked and made accessible to remote users via the internet for example.

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

1 . A method of producing an image of a coloured design for preview purposes including the steps of selecting a substrate, printing a pad layer on the substrate, printing a series of known tonal values on the pad layer, reading colour information from the image produced by means of a spectrophotometer and manipulating the data received in a computer in order to generate a digital preview of the appearance of the image.

2. A method of producing an image of a coloured design for preview purposes as claimed in claim 1 , in which the substrate is opaque and spectrophotometer measurements are taken from the tonal side.

3. A method of producing an image of a coloured design for preview purposes including the steps of selecting a substrate, printing a series of known tonal values on the substrate, printing a pad layer on top of the tonal values, reading colour information from the image produced by means of a spectrophotometer and manipulating the data received in a computer in order to generate a digital preview of the appearance of the image.

4. A method of producing an image of a coloured design for preview purposes as claimed in claim 3, in which the substrate is transparent and spectrophotometer measurements are taken from the substrate side.

5. A method of producing an image of a coloured design for preview purposes, including the steps of selecting a substrate, printing a series of known tonal values on the substrate, laminating a backing layer with the printed substrate, reading colour information from the image produced by means of a spectrophotometer and manipulating the data received in a computer in order to generate a digital preview of the appearance of the image.

6. A method of producing an image of a coloured design for preview purposes as claimed in claim 5, in which the spectrophotometer measurements are taken from the substrate side.

7. A method of producing an image of a coloured design for preview purposes as claimed in any preceding claim, in which the known tonal values are printed as rectangular colour chips on the substrate.

8. A method of producing an image of a coloured design for preview purposes as claimed in claim 7, in which the rectangular colour chips have between ten and twenty steps on the substrate.

9. A method of producing an image of a coloured design for preview purposes as claimed in claim 7 or 8, in which the known tonal values are printed as a single spot layer on the substrate.

10. A method of producing an image of a coloured design for preview purposes as claimed in claim 7, 8 or 9, in which the size of each chip is sufficient to allow accurate measurement with the spectrophotometer.

1 1 . A method of producing an image of a coloured design for preview purposes as claimed in any of claims 1 to 4, in which the pad layer is either blue or black.

1 2. A method of producing an image of a coloured design for preview purposes as claimed in any preceding claim, in which the collection of data from the spectrophotometer is automated.

13. A method of producing an image of a coloured design for preview purposes as claimed in any preceding claim, in which spectrophotometer measurements are taken using an X-Y table semi-automatically in accordance with a program.

14. A method of producing an image of a coloured design for preview purposes as claimed in any preceding claim, in which data and the manipulation of data may be made available to a remote user.

1 5. A method of producing an image of a coloured design for preview purposes as claimed in claim 14, in which access to a remote user is available over the internet.