Y. Kuznetsov, M. Ermoshina
North-West Institute of Printing of the St. Petersburg State University of Technology and Design
The relationship, degree and effect of the variety of black ink functions use in CMYK printing is discussed on the background of prepress evolution from analogue scanners of 60ies up to precise digital color control of today. There are also presented the results of comparative colorimetric analysis of CMY and CMYK printing revealing the black ink particular effect of expanding the CMY gamut by providing the chromatic colors which aren’t available for any combination of the other three process ones.
Key words: color halftone printing, color gamut, CMY, CMYK, UCR
Looking back in the latest history of graphic technology developments one can find that thirty-forty years ago there were a number of scientifically approved recommendations on direction and degree of tone and color values correction for print quality improvement. However, even at the times of color electronic prepress, there was a lack of means for proper control in providing the desired variations. In 70ies of last century it was used, for example, to indicate in advertising specification of a scanner its ability to replace the CMY achromatic component by the black ink as “up to 75%” (against the 65% of the competing model). The clear knowledge of what had to be done with an image data faced, as well, the lack of means to realize the task in some other relations.
The digital image processing of today allows for practically unlimited print parameter variation in any direction with the discretion of just 25 square microns of ink coverage. However, quite a contrary situation of adequate resources but lacking in knowledge of what should be done is often met and the need arises of additional research or training which could substantiate the recommendations and performing methods for effective use of such precise, recently appeared control facilities [Kuznetsov, 2012].
There is variety of purposes and reasons of black ink use in CMYK printing. It can be in different degree applied for reproduction of achromatic, chromatic colors as well as the achromatic component of chromatic ones. Its use can also differentiate from the vast, stationary image area to sharp edges and fine details. So, there is theoretically infinite continuum of CMY to CMYK transformations which can result in the same or better colorimetric print values.
The beginning of K-ink use within the CMY triad stems from the times of photoengraving, camera prepress. The facilities of black control according to certain rendering intent and, especially, in isolation from its other effects on resulting color were rather restricted. However, the mostly heuristically found, scanty collection of black ink settings is until now used in wide practice. One of the reasons is in some isolation of numerous participants (publishers/advertisers, prepress operators, quality managers, printers…) from each other. Lack of facilities or time for finding the optimal adjustments which would match the job/process specifics makes them to follow the narrow path of guaranteed standard parameters (Euroscale, SWOP…) or of settings stipulated by the available ICC profile.
The other reason of non-optimal black ink use is in vague interpretation of its settings and their relationship essence in “black boxes” of prepress software applications or commenting manuals. When appealing to their “help” option the user is sometimes sent to get an advice from a printer. In this relation E. Enoksson [Enoksson 2004] notes, for example, that only about a quarter of the Swedish print houses have people ever heard of the UCR and GCR functions of Photoshop.
Not so much “help” the user can get from academic sources. Problems start here from providing the proper definitions for these functions because of similar sense of their abbreviations meanings. For example both Under Color Removal and Grey Component Replacement indicate in fact the “removal of chromatic inks (CMY) achromatic component by replacing it with the black (K) one”. Meanwhile the “Complete Color Glossary” defines the UCR procedure as related just to the dark neutral colors [Southworth, Southworth 2004]. In “Handbook of Print Media” one can find the attempt to distinguish UCR, GCR, UCA functions by the examples of varying just the volume of CMY achromatic part replaced by black ink [Kipphan, 2001], though this volume can be varied within each of these procedures as well. At last, the “Digital Color Imaging Handbook” stands out GCR as a “generalization” of UCR and K addition [Bala, 2003]. There were also attempts to modify UCR under the names of such procedures as PCR (Programmed Color Removal), ICR (Integrated Color Removal) etc. and the new names are still proposed [Enoksson 2004].
The number of other explanations of fourth ink application suffers from mixing the purposes and methods of these purposes destination on the background of vast variety of CMYK combinations continuum. So, it is not out of place to separately discuss the following black ink functions:
- replacement the achromatic component of three chromatic inks combination;
- reproduction the image achromatic colors;
- expanding the print color gamut.
2. Relationships of black and balanced CMY in achromatic component of chromatic inks combination
1.1 Volume of replacement
This relationship is often used to be illustrated by the diagram of the kind presented on figure 1, where figures 1b and 1c show the examples of partial (50)% and complete (100%) volumes of achromatic share replacement.
Figure 1. Providing formally the same chromatic color: without black ink (a); at 50% and 100% CMY achromatic share (CMYmin ) replacement by black (b, c); with further removal of equal M and C amounts on behalf of an red or orange ink in HiFi printing
This parameter variation is historically related to UCR function first of all used for the darker image areas because of total ink limitation. In fact, it can be varied from 0% up to 100% independently of the given pixel brightness, i.e. in the highlights, middle tones or shadows.
Volume and range of (CMY)min replacement is, except of ink limit, stipulated by the number of other technological, economic, operating and image quality considerations including:
- ink consumption costs;
- fidelity and stability of the grey balance within a run;
- color disbalance due to rosettes geometry variation, as well as moiré and rosettes visibility [Daels, Delabastita, 1994];
- gamut mapping intents;
- use of complementary to CMY inks in Hi-Fi printing, etc.
Nature of these reasons is well known or described in hereinabove referred literature while the last one is cleared up by the figure 1d. It shows that the 100% removal of one of CMY inks is compulsory over all tonal range to make worthwhile the use of an additional ink of opposite, complementary color (red or orange one in this example) to expand the print color gamut. This makes it clear too that the screen of complementary color can safely use the angle of its corresponding process ink for the latter has been completely removed in particular image area.
1.2 Varying the replacement volume within the tone range
The volume variation of this replacement within the tonal range can be illustrated by diagram of figure 2, where the straight line 1corresponds to printing of the whole grey scale of color image exclusively by CMY inks. For simplicity the balanced CMY inks amount changes along lines of this graph in equal proportion (C=M=Y) thereby related to the use of, so called, “ideal inks”.
With taking the line 1 for a reference the other curves of figure 2 demonstrate the possible variants of achromatic CMY and K-ink volumes relationship along the tone range. Thus, curve 2 indicate at its upper point the (CMY)min withdrawal of 40% where initial C=M=Y=100% is replaced by combination of C=M=Y=60% and K=40% at 220% of ink total. Following this curve to lighter areas the use of black ink is gradually reduced to 0% at middle tone. Starting from C=M=Y=50% and until white point the achromatic component is again reproduced only by CMY.
Curve 3 illustrates the constant replacement value of 40% along the whole grey scale. Here, for example, middle tone is presented by C=M=Y=30% and K=20% with the latter comprising 40% of achromatic value defined at this point by C=M=Y=50% of the reference curve 1.
Figure 2. Variation of balanced CMY and K values along the image grey scale:
1 – without K;
2 – gradual reduction of CMY replacement by K from 40% in shadows to 0% in middle tones;
3 - with 40% constant volume of replacement along the whole scale;
4 – with gradual growth of CMY replacement by K from 60% in shadows 100% in middle tones;
5 – without CMY along the whole scale
Gradual replacement increase from 60% in the darkest areas up to 100% in middle tones is shown by curve 4. It also indicates the reproduction of lighter part of the scale exclusively by black ink.
At last, the curve 5 coincides with horizontal axis of the diagram and relates to complete replacement volume of 100% along the whole scale. It corresponds to the so called “binary chromatic + black” strategy where CMY inks can’t be altogether found in any chromatic area of a print.
As result, this diagram describes two different dimensions of (CMY)min presentation the first of them relating to volume of K-ink use while the other one to location of certain volume application within the image tone range.
Resulting color doesn’t change at standard viewing conditions for any combination of CMY achromatic and K. It makes possible to modulate the K volume within these components relationship by the signal of additional image. Such print securing pattern will be concealed for a viewer at any “white” light but becomes apparent at illuminant which power is concentrated in close vicinity to infra-red region, where the black ink reflectance greatly exceeds that of the other process ones [Ziljak, Vujic, Pap, 2008].
3. Achromatic colors reproduction
Providing the maximum of reproducible grey levels was outlined by R. Hunt [Hunt, 1997] and stipulates the priority of black ink use for grey scale generation in color print. The whole achromatic component can be considered as the basic one for image formation while the CMY inks as auxiliary ones, i.e. responsible just for the image chromaticity. Moreover, due to well known degradation of color vision sensitivity with decreasing of detail angular, spatial dimension the small details and high contrast sharp transitions can also be reproduced with greater share of black or even, as it’s done in color TV, completely achromatic.
The effect of accurately balanced K and CMY combined use for b/w image is especially apparent when the latter is simultaneous printed on the same sheet with multicolor ones. Our research based on UCR offset atlas [Avatkova, 1987] has shown, for example, that at ink limit 290% the use of C=M=Y=62% in combination with 100% of K extends the grey scale optical density range on about 0.3. Various ways of such combining are formally possible. Both of these inks amounts can be uniformly distributed along the grey scale or each of them alternatively concentrated in its highlights or shadows [Kuznetsov, 2002].
It has to be assumed that such consideration to certain extent artificially separates the formations of print achromatic color and achromatic component of chromatic one. In prepress software these functions are merged in each other and altogether governed by the same curves. Moreover, achromatic color is also influenced by the so called Under Color Addition (UCA) settings. However, such discerning of these functions may look purposeful in creating or operating the updated prepress software of foreseen future.
4. Expanding the print color gamut
Discussed above combined use of K and CMY inks expand the gamut in close vicinity to L axis of LCh space where colors can still be considered achromatic with taking into account their permissible delta E, for example of 5 units, from neutral as illustrated by figure 3.
Figure 3. Black ink adds new neutrals to CMY color space in close vicinity of the lower part of its achromatic axis
At the same time, it’s practically used to add some K amount to C, M or Y solids to get new, darker chromatic colors. However the gamut expanding due to chromatic colors which exclusively appear with applying the fourth, black ink wasn’t separately considered in literature. Moreover, one can find the contrary statement of color gamut reduction with black ink introduce [Bala, 2003]. Along with all this, the issues of print gamut expanding by the use of intensive inks, addition to CMY process colors of their complimentary RGB ones within the so called Hi-Fi Color concept, etc. were widely discussed in last decades.
Our research was provided by comparative analysis of CIE Lab colorimetric data from offset atlas [Avatkova, 1987], as well as from test step wedges especially produced by variety of three and four inks combinations on digital printer. Examples of these data are visualized at LCh color space vertical slices for magenta and blue hues on figure 4. The upper line of the left diagram shows the L reduction with continuous adding the magenta tone value to substrate from 0% to 100%, while the upper line of the right one — with the similar adding of M + C.
It may look that the further reducing these colors lightness can be provided by adding to their solids the complementary process color. In the first case such color is green provided by the balanced sum of cyan and yellow, while for the blue tint it’s yellow. However, as it’s demonstrated by the lower lines of both diagrams, the much purer colors are generated by simple adding of the black ink. The increase of saturation achieves 30 chroma units for magenta at L = 30 level and about 40 units for blue at L = 23.
Figure 4. Coordinates of CMY and CMYK dark chromatic colors on meridian sections of CIE Lch space for magenta (left) and blue (right) hues.
Such difference can be physically explained by that the black ink more or less uniformly reduces reflectance without distortion of source (magenta or blue) spectrum profile, while the inks of opposite colors darken the source one by suppressing reflectance at their inherent in bands of spectrum.
The hatched areas of the both diagrams demonstrate the expanding of lower part of color gamut with the use of the fourth, K color. According our three dimensional calculations it comprises about 10%.
It’s purposeful to the black ink use in CMYK process in relation of its three basic functions:
- replacement of achromatic share of CMY originated chromatic colors;
- reproduction of achromatic colors in combination with balanced CMY;
- creating the new chromatic colors unavailable for CMY.
Within the whole its variants continuum the first of these functions is in full characterized by two dimensions:
- volume of K as the share of a reference (CMY)min;
- location of certain K volume application within the tone value range of a print.
Contrary to the first one, the other two of these functions are completely additional and have no alternatives.
New chromatic colors provided by adding black ink expands the CMY gamut on about 10%. So, the move from CMY to CMYK which took place in the 30ies of last century can be concerned as the first step to Hi-Fi color technologies aimed nowadays on a similar effect by once again increasing the number of process inks.
The results of research can help to explore the effect of described black ink functions in multicolor halftones.
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