Title: Colour System Calibration Sheet/ Vignettes and
Transitions FIELD OF THE INVENTION This invention relates to devices for the calibration of colour systems. BACKGROUND OF THE INVENTION
Colour systems employ colour scanners to digitize coloured originals, such as reflective or transparency prints. , The digitized data can be manipulated to produce a desired result, for _ instance a change in skin tone or a change in the amount of light in a photograph to simulate a change in the time of day. Scanners are used most often by so-called colour houses whose main clients would be advertising agencies or large corporations. The colour houses produce glossy brochures and the like for their clients using the scanried images amongst other text or graphics.
Scanners are very expensive. Any time for which a scanner is out of operation, or incorrectly operating, costs its owner, or the client which ends up absorbing the cost, a great deal of money.
Improperly calibrated scanners are a source of a good deal of lost production time. A scanner operator needs to know the proper settings for his or her machine. An original scanned in using an improperly calibrated scanner can result in a distortion of information in the digitized data. The digitized data may go through a number of graphic manipulations before it is realized that the distorted information is detrimental to the final product. The lost time is not only in performing the wasted graphic manipulations. An example of how original content may be distorted would be where the scanner settings were set to not
SUBSTITUTE SHEET -recognize red properly. In the digitized version any red hue would appear as a different hue. Should that hue also have appeared in the original image then it will be extremely expensive, time-consuming, and non-productive to reconstruct the original red hue without altering the other hue.
Presently available devices for calibrating scanners are calibration sheets having a palette of square coloured boxes. Each box having a single colour or hue. The boxes are typically arranged so that there is a transition from a hue of one colour to a hue of another through a series of the unicoloured boxes.
A scanner operator scans in the calibration sheet and outputs ,a scanned version of the sheet. The original and scanned versions are compared to determine where there are potential calibration errors.
Discrete palette calibration sheets employ a fixed number of boxes, each box having one colour or hue, for comparison purposes. From each box, or colour or hue, to the next is a visible colour difference. Inherent in a discrete palette calibration sheet is missing calibration information. The visible colour difference between adjacent boxes means other colours between those boxes are not being represented. An operator calibrating using the discrete sheet can calibrate the scanner in such a way as to accurately reproduce the colours in the calibration sheet while still having a scanner which is incorrectly calibrated for colours not on the sheet. These calibration errors would not become evident until the colour for
TITUTE SHEET which the scanner is incorrectly calibrated appears on a scanned original.
Another problem with the discrete boxes is the difficulty in comparing a number of boxes of visibly distinct colours. The eye has a problem in readily discerning whether there is a calibration error and where the calibration error lies.
The use of discrete boxes creates a further problem as the area of each box is relatively small and a calibration error is very easily overlooked.
Similar calibration problems exist for proof recorders and colour monitors. In addition recorders and monitors typically operate on four primary colours whereas scanners operate on three true primary colours, the fourth colour, black, being made up of the three primary colours. Calibration sheets to date have not taken into consideration the effect on these other devices in the use of this fourth primary colour.
It is an object of the present invention to overcome one or more of the above-mentioned problems.
SUMMARY OF THE INVENTION
In a first aspect the invention provides a colour vignette for use in calibrating a colour system or components thereof, the colour vignette comprising, a representation of a visually continuous transition from one colour to another.
In a second aspect the invention provides a calibration sheet for use in calibrating a colour system or components
T thereof,the sheet comprising, a plurality of vignettes, each vignette representing a visually continuous transition from one colour to another.
In a third aspect the invention provides a calibration sheet for use in association with the calibration of a colour system or components thereof, the sheet comprising, at least one pair of juxtaposed visual transitions from a true primary colour to the colour black, wherein one transition of the pair uses a secondary colour while the other transition uses the colour black.
In a fourth aspect the invention provides a calibration sheet for use in association with the calibration of a colour system or components thereof, the sheet comprising at lease one pair of juxtaposed visual transitions from a secondary colour to the colour black, wherein one transition of the pair uses a primary colour while the other transition uses the colour black.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example to the accompanying drawings, which show the preferred embodiment of the present invention, and in which:
Figure l. is a reflective proof of a calibration sheet according to the preferred embodiment of the present invention;
TITUTE SHEET Figure 2. is a block diagram of a colour system; and Figure 3. is a graphical representation of a csfTour cube.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Fig. 1, a calibration sheet 1 in reflective proof form according to the preferred embodiment of the present invention has a number of separate groups; a first linear vignette group (vignettes 24-A through 24-Z) a second linear vignette group (vignettes 23, 22, 21), a circular vignette group (vignettes 20-a through 20-e and 19-f through 19-j), a third linear vignette group D (vignettes 18, 17, 16) and a discrete group (boxes 15-A through 15-Z to 1-A through 1-Z) .
It will be understood by those skilled in the art that the calibration sheet 31 can take on different forms, including reflective and transparency prints, reflective and transparency proofs, monitor displays, and digital and film separations. Although reference will be made herein to the reflective proof form of a calibration sheet 1 the invention is not limited thereto.
The term vignette as used herein is meant to describe a representation in any form whatsoever of a visually continuous transition from one colour to another. A linear vignette is a vignette where the transition occurs in a single diπfension. A circular vignette is a vignette where the transition occurs in two dimensions.
SUBSTITUTE SHEET Referring to Fig. 2, a colour scanner 32 is typically used as part of an overall colour system 33 having a number of components. The system 33 can take input from different sources; the scanner 32, a colour scope 35, external storage 37 or system storage 39. The scanner 32 usually scans either photographic reflective or transparency originals, either prints or proofs, not shown. The scanner 32 produces digital separations in four primary colours from the scanned originals. The scanner itself actually uses three colours to create these separations, the fourth is a combination of the other three and as it may be separately controlled in some components of the colour system 33 it will be referred to as a primary colour. These separations are transferred to a system processor 41. The processor 41 can either store the separations• in system storage 39 or external storage 37 for later use. Alternatively a system operator can manipulate the separations using the scope 35 through a keyboard 43 or a graphics tablet 45. The digital separations are displayed for view during manipulation on a first monitor 45 for the system operator.
The output of the system 33 can be either displayed on a second monitor 47, output as a series of four separations in positive or negative film from the scanner 32, output as photographic reflective or transparency full colour proofs from a recorder 49 or stored in system or external storage 39, 37. It is from the scanner films that the finished prints are made for brochures and the like.
TE SHEET Each of the output components, the scanner 32, recorder 49 and monitors 45, 47 are able to be separately calibrated. It is most desirable to have the output components calibrated to represent colours and the hues identically. A calibration sheet 32 is advantageous as it can be used to calibrate each component. For example, in order to calibrate the scanner 32 for a given system 33 the calibration sheet 31 would be scanned in by the scanner 32 and output as separation films. The films can be used to create a proof in an external proofing process, not shown. The sheet 31 and proof would be compared to determine calibration errors. Although the following description of a calibration is made with reference to a scanner 32, it applies also to other system 33 components. Certain differences will be pointed out where necessary.
A scanner 32 typically operates on black K, cyan C, magenta M and yellow Y as its four primary colours, black actually being a combination of each of the other primary colours. Cyan C, magenta M and yellow Y will referred to collectively as "true" primary colours where black is specifically excluded from the reference.
Using colour cube theory in order to explain colour, reference will be made to Fig. 3. A colour cube 51 represents all the possible colours the system 33 is able to scan and output. The true primary colours cyan C, yellow Y and magenta M flow from white . White is a secondary colour, however it is actually the lack of any colour. The true secondary colours
TUTE SHEET red R, blue B, green G flow from black K, each secondary colour takes a corner diagonally through the cube 51 to its opposite primary colour. The corners of the cube 51 represent the greatest degree of purity reproducible for a given primary or secondary colour in the cube 51. In most systems some primary and secondary colours have a greater degree of purity than others so the cube 51 for every system may not be absolutely regular. Additionally it is possible to use other primary colours to create other secondary colours, however these hues of primary and secondary colours were selected to emulate printing standards of North America.
Colour scanners are capable of adjusting the primary colour content in each of the primary and secondary colours of the cube 51 at 25%, 50%, 75% and 100% of saturation.
In order to ensure the scanner 32 is properly calibrated it is necessary to align the corners of the cube 51 for a given system 33. It is evident that proper alignment of the cube 51 can be performed by ensuring each of the corner colours is at its correct purity or saturation. In order to determine whether this is the case a palette of colours containing the highest purity colours could be scanned in and printed out as is the usual method in practice today. Then the original image and output proof could be compared,, however it is difficult to see minor errors in colour and it is possible the two images would appear identical and yet the scanner could be using a misaligned colour cube 51. - 9-
This is especially so where separate adjustments control the content of colours at 25%, 50%, 75% and 100% saturation. In order to align the colour cube 51, lines can be taken along each edge of the colour cube 51 from one corner to another and placed on an original, scanned in, output and compared. Examples of such lines are shown in the following corner to corner e~dge linear vignettes; yellow Y to green G (24-A), magenta M to red R (24-B) , cyan C to blue B (24-C) , yellow Y to red R (24-D) , magenta M to blue B (24-E) , and cyan C to green G (24-F) . Similarly there are linear vignettes representing the transitions along the edges from red R to magenta M to blue B (23) , the edges from green G to yellow Y to red R (22), and the edges blue B to cyan C to green G (21) .
Each of the above corner to corner edge linear vignettes show the colours at 100% saturation.
In order to check and set the grey levels of colour within the cube 51, diagonal lines from each of the true primary colour corners through the cube 51 to its opposing secondary colour corner at 100% saturation have been included as diagonal linear vignettes, namely yellow Y to blue B (24-G), magenta M to green G (24-H) and cyan C to red R (24-1) . To check the adjustment on the 75%, 50% and 25% saturation settings, those particularly used to set grey levels, diagonal linear vignettes through the cube 51 at those settings for yellow Y to blue B (24-J, 24-M, 24-P) , magenta M to green G (24-K, 24-N, 24-Q) and cyan C to red R (24- L, 24-0, 24-R) respectively have been included. Errors in colour
SUBSTITUTE SHEET setting stand out most at levels approaching grey and the above diagonal settings are particularly helpful in showing calibration errors in the scanner. Of course, the errors may be from a combinations of incorrect settings, however the incorrect settings will be highlighted by the colour content of the grey levels.
Using the knowledge that errors in colour setting are most evident at grey levels yellow Y, magenta M and cyan C to black K (24-S, 24-T, 24-U) and partial white W to yellow Y, magenta M and cyan C (24-V, 24-W 24-X) , respectively colour transitions diagonally across a number of the faces of the cube 51 are shown as diagonal face linear vignettes. The white diagonal face vignettes (24-V, 24-W, 24-X) are partial in that they show only a part of the diagonal to highlight the grey area in those diagonals. The diagonal face vignettes will show errors in a combination of adjustments which should be evident in the corner to corner edge vignettes (24-A through 24-F) however the errors may be more prominent in the diagonal face vignettes (24-S through 24-X) and these vignettes will serve as a cross-check.
Vignettes 24-Y and 24-Z represent changes in grey scale and serve a similar function to those of vignettes 24-S through 24- X however the vignettes 24-Y, 24-Z concentrate specifically on grey tones.
Although linear vignettes can contain most of the information necessary to determine the cause of calibration errors, due to their one dimensional nature and also to space limitations of their widths these errors are sometimes still difficult to detect. The circular vignettes of 20-a through 20- c and 19-f through 19-j tend to highlight errors to a, greater degree than due linear vignettes. Calibration errors will show up as circular outlines at a given radius around the centre of a circular vignette. These are more readily detectible by visual inspection then would be a single horizontal transition error in a linear vignette. The inventor has included in his calibration sheet only* 10 circular vignettes in due to space limitations, however any number of circular vignettes such as those covering the colour transitions of linear vignettes 24-A through 24-Z could be used.
The circular vignettes the inventor has chosen are corner to corner edge transitions namely (with the first named colour being at the centre of the each vignette) ; magenta M to blue B
- *. (20-a) , cyan C to blue B (20-e) , yellow Y to red R (19-f) ; yellow
Y to green G (19-g) , cyan C to green G (19-i) and magenta M to red R (19-j) .
The other four circular vignettes included are the secondary colours to black namely red R, green G, blue B and white W to black K (20-b, 20-c 20-d and 20-h) . The secondary colours to black were chosen as circular vignettes as some scanners have a separate adjustment on transition specifically from secondary colours to black K. Also they show colours changing to black which tend to highlight calibration errors and they include
SUBSTITUTE SHEET combinations of primary colours which will serve as a cross-check on errors which may have been overlooked in other vignettes.
Vignettes in 18, 17, 16 include corner to corner diagonally through the cube 51 vignettes in a stretched form at the same saturation level as those of vignettes 24-G, 24-1 and 24-H. Specifically the linear vignettes of group D are blue B to yellow Y (18) , red R to cyan C (17) and green G to magenta M (16) . Being stretched, these vignettes provide more detail on the information contained in these transitions, especially the grey tones which tend to highlight calibration errors as mentioned previously.
The discrete group (15-A through 15-Z to 1-A through 1-Z) contains discrete boxes of colours. Although the discrete boxes have problems associated with" them in missing some continuity of colour information, they are included here only for specific purposes. Certain columns of the boxes form discrete linear visual transitions from one colour to another for example boxes 13-A through 1-A. Certain of these transitions namely, (13-A through 1-A and 13-B through 1-B, 13-C through 1-C and 13-D through 1-D, 13-H through 1-H and 13-1 through 1-1, 13-K through 1-K and 13-L through 1-L, 13-T through 1-T and 13-U through 1- ϋ, 13-V through 1-V and 13-W through 1-W, and 13-X through 1-X and 13-Y through 1-Y) are paired and juxtaposed. One transition of each pair is from a true primary colour or a secondary colour to the colour black using the addition of the colour black from one box to the next in the transition, for example transition 1-A through 13-A. The other transition of the pair u-ses the addition of a single one of the true primary colours where the transition is from a secondary colour or a secondary colour where the transition is from a true primary colour, for example transition 13-B through 1-B.
These paired transitions are very useful in devices having separately adjustable black content levels. These are typically recorders 49 and monitors 45,47.
The actual colour content in the transitions from a colour to black using black should not change to the eye, however, the density of the colour will become greater going to black. The colour content of the other transition should definitely change and the density of the black at the end of the transition should not be as heavy as other colours are not fully present to increase the density.
Discrete boxes have been used in the transitions of the discrete group, as comparisons are only necessary at intervals and a box rather than a continuous transition allows a greater display area of a given interval. Continuous transitions could, however, be used for this purpose as well.
Vignettes to be ultimately used in a calibration sheet 31 in reflective proof form or any other form can be produced first in digital form using a scope 35 and its manual inputs 43, 45. The vignettes may be digitally stored in external storage 37 or system storage 39. A calibration sheet 31 in reflective proof or transparency form can be produced by the recorder 49. A
SUBSTITUTE SHEET calibration sheet 31 can be output as a set of four separations to be used by the external proofing process, not shown, to create a calibration sheet 31 in proof form or to an external printing process, not shown, to create a calibration sheet 31 in printed form. Alternatively vignettes can be produced digitally using one system 33 and output as a calibration sheet 31 in reflective proof or transparency form or in digital form to be input to another system 33 through that other system's scanner 32 or through its external storage 37.
It will be evident to those skilled in the art that there will be other forms of the present invention which fall within its spirit and scope as defined by the following claims.