WO2004070331A1

WO2004070331A1 - Colour chart and production of the same

Info

Publication number: WO2004070331A1
Application number: PCT/SE2004/000155
Authority: WO
Inventors: Emmi Enoksson
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-02-06
Filing date: 2004-02-06
Publication date: 2004-08-19
Anticipated expiration: 2005-08-06
Also published as: SE0300317D0; SE524867C2; SE0300317L

Abstract

A colour chart for imageable information carriers such as CTP printing plates, photographic film, digital printing processes for a digital output unit or a printing substrate, in which the colour chart is intended to be brought into contact with the imageable information carrier, where the colour chart comprises test regions, with gradually changing tones, where the tones are adjusted such that a certain tonal region is given priority, and a method for the production of such a colour chart. The aim of the invention is to provide a colour chart and a method for the production of such a colour chart that creates the conditions required for improved image reproduction in the final printed work. Images with significant information in dark regions demonstrate in these regions a significantly improved gradation or definition of detail. The colour chart is adapted in this manner to a particular image or type of image (classified according to the region of brightness in which the information for which high definition is required lies).

Description

Technical Area

The present invention concerns a colour chart for a imageable information carrier such as printing plates, photographic film or a printing substrate, and a method for the production of such a colour chart. To be more precise, the present invention concerns such a colour chart and method where the colour chart is intended to be transferred onto the imageable information carrier and where the colour chart comprises test regions with progressively changing tones. Introduction

Reproduction of images remains a cause of problems in the printing industry. A major advance along the way has been the introduction of ICC profiles, which help to keep the reproduction of colour under control. The ICC format enables the colour space of colour units to be determined, and in this way it is possible with the help of colour engines to optimise, for example, output for the simulation of a print. In order to be able to print an image that is on a computer, it is necessary to convert the image from RGB-colours (R=red, G=green, B=blue - the colours stored in the computer) to CMYK-colours (C=cyan, M=magenta, Y=yellow, K=black - the colours that are printed). The conversion process is known as "colour separation". This separation is currently carried out with the aid of ICC profiles that contain an amount of data in order to make the conversion possible. The creation of ICC profiles for output units such as printing presses and printers is based on the printing/output of a colour chart and the measurement of this chart with a spectrophotometer. The values that are measured (the current values) are compared with the target values (a reference table) and the result is obtained in the form of colour tables. Many different suppliers with many different programs for colour control are active in the market, which readily causes confusion for the user since it is not easy to select the correct colour chart (IT.δ-chart) that is suitable for in-house production within different printing methods such as offset technology, digital printing, etc. These colour charts differ with respect to the number and the size of the test regions. However, one thing that is common to all colour charts on the market is that they are general and do not take into consideration that there are different types of image, i.e. to what extent important information lies in the dark or the light regions of the image. The problem with this is that the colour charts are not used optimally, with the consequence that too few values of tones are analysed with relevant values. The aim of the invention

The aim of the invention is thus to provide a colour chart and a method for the production of such a colour chart that creates the conditions required in order to be able to control the image reproduction in a manner adapted to the image, such that improved gradation and greater detail definition can be achieved in the finished work. Summary of the invention

The aim specified above is achieved through the characteristic parts of claims 1 and 4.

Brief description of the drawings

Figure 1 shows a diagram where the tones of a colour chart according to a first embodiment of the invention are compared with a traditional colour chart.

Figure 2 shows a diagram in which the tones of a colour chart according to a second embodiment of the invention are compared with a traditional colour chart. Figure 3 shows a colour chart according to the prior art with the tones evenly distributed over the complete range of tonal values (0 to 100%). Detailed description of the invention

The present invention with be here described with reference to particular embodiments, but the present invention is not limited to these. It is only limited by the claims. The aim of the invention is thus to provide a colour chart and a method for the production of such a colour chart that creates the conditions required for improved colour reproduction in the finished work. This takes place through the colour charts being adjusted or adapted to different types of image characterised by the range of tones in which the important information lies. The colour chart is intended for use with an imageable information carrier such as a printing plate, a printing cylinder or a printing press for computerised printing, known as "Computer to Plate/Press-technology", traditional photographic film or a printing substrate. The colour chart is also intended to be used for digital printing processes for digital output units. Tailored colour charts have been produced based on technical and visual analyses of different categories of image. A number of experiments have been carried out with extreme images, the information in which is located either in the dark areas or in the light areas. Standard colour charts have been used such that they can be compared with the colour charts that have been produced adapted for a particular type of image. Intermediate results show that the characterisation of the image with the colour charts produced gives a more correct analysis of the important image information with respect to image gradation and detail reproduction. The new image-adapted colour chart helps to preserve details in shadow regions, which gives a more distinct image and a greater agreement with the original. Several images with similar distributions of tone have been analysed with similar results, which proves repeatability.

Comparison in pairs was used during assessment of the resulting printed images, in which an image printed with the aid of a traditional colour chart was compared with an image printed with the aid of a colour chart according to an embodiment of the invention. The assessment group comprised 30 persons with different degrees of knowledge about images and about image reproduction, and the members of the group were instructed to examine the gradation of the images. The assessment took place under controlled conditions with the aid of a viewing cabinet at a colour temperature of 5,000 K, where the origin of the images was not revealed to the assessment group.

Images with information in the daylight regions (light regions) were also test- printed with the image-adapted colour chart, as were images with intermediate tones. The results have shown that there is no significant difference in the quality of the images. The images with intermediate tones, however, have shown greater agreement with the original when printed with an image-adapted colour chart. Thus the experiments show that the optimal image reproduction requires an adaptation to the different types of image in order to reproduce correctly the information in the image. This adaptation is already required at the creation of the different ICC profiles. The adaptation or the adjustment is carried out in that rather than allowing the test regions in the colour chart to reproduce the tones across the complete range from 0% (light) to 100% (dark) with essentially equal steps as in the colour chart according to the prior art shown in Figure 3, allowing these to be concentrated on a part of this range. It has become clear that for images with important information in the dark regions, a colour chart that uses essentially light tones gives a very good result. Allowing most of the test regions of the colour chart to take tones in the quarter tones, i.e. in the lighter range, has proved to provide a particularly good reproduction of details, particularly in the dark regions in images of this type. The tones are a mixture of cyan, magenta, yellow and black (CMYK). A diagram is shown in Figure 1 in which the difference between an existing colour chart and the curve described above is made clear. The tones of the test regions have in this case been distributed equally within the range. The input values of the various test regions are given along the x-axis, while the output values are given along the y-axis. The tones of the existing colour chart are located mainly along a line with a gradient (k) of 1 , while the curve of the prototype is also linear with a gradient of 0.4. The input values of the prototype have thus created a greater space for the output values, which means that reproduction of the dark regions has been given a larger area, and through this, a greater richness of detail. The colour chart has in this way been adapted to the type of image. The colour chart is thus adapted according to the type of region, light to dark, in which the important information in the image lies. A further embodiment of the prototype has also been tested and is shown in

Figure 2, compared with a traditional colour chart. The principle has been the same as it is in the embodiment according to Figure 1 with the quarter tones. The only difference is that parts of the three-quarter tones, i.e. values in the darker region, have been added.

A further embodiment has been tested. The tones in this prototype lie on a curved line in which the increase in the output values is less for the first test regions, and increases subsequently towards higher degrees of saturation. This embodiment of the invention has proved to be particularly efficient in the reproduction of particularly extreme night-time images, i.e. images in which the important information lies in the extremely dark parts of the image.

The method according to the present invention thus provides the greatest benefit when printing images in which the important information lies in the dark parts of the image. It has, however, become clear that the light regions are not degraded with respect to their reproduction when using the adaptation method, which is why images known as "earth images" (images with colour saturation in which important image information lies in both light regions and in dark regions) is improved. While the method has principally been tested for printing on sheets (glossy paper), it is probable that the positive effect that has been achieved will be even greater and more obvious on simpler paper such as, for example, printing on uncoated paper, newsprint.

It is reasonable to suppose that in the future, in which it will be possible to use an automatic process to classify images according to the type of image as described above, adaptation of the colour chart will constitute a step along the way in this process, and that it can be further refined.

Claims

1. A colour chart for an imageable information carrier such as a printing plate, a printing cylinder or a printing press for computerised printing ("Computer to Plate" or "Computer to Cylinder"), photographic film, a digital printing process for a digital output unit or a printing substrate, where the colour chart is intended to be transferred onto the imageable information carrier/plate, where the colour chart comprises test regions, with gradually tones, cha racterised in that the tones in the test regions are selected such that the region of lighter tones is given priority, in order to improve the contrast in dark regions of the image with significant information.

2. The colour chart according to claim 1, characterised in that the gradual change in the tone in the test regions is constant.

3. The colour chart according to claim 1, characterised in that the gradual change in the tone of the test regions is small at the beginning of the tonal region, and subsequently increases.

4. A method of the production of a colour chart for an imageable information carrier such as a printing plate, a printing cylinder or a printing press for computerised printing ("Computer to Plate" or "Computer to Cylinder"), photographic film, a digital printing process for a digital output unit or a printing substrate, where the colour chart is intended to be brought into contact with the imageable information carrier/plate, where the colour chart is provided with test regions with gradually changing tones, characterised in that the tones of the test regions are selected such that the region of light tones is given priority, in order to improve the contrast in dark regions of the image with significant information.

5. The method for the production of a colour chart according to claim 4, characte rised in that the tones are principally laid in the region of light tones, known as the quarter tones.

6. The method of the production of a colour chart according to either claim 4 or 5, characterised in that the gradual change of the tone of the test regions is made constant.

7. The method for the production of a colour chart according to either claim 4 or 5, cha racterised in that the gradual change of the tone of the test regions is made small at the beginning of the tonal region and subsequently increases.