The implementation of acromatic principles in reproduction and black printer changes.

Agic, Darko ; Mandic, Lidija ; Strgar-Kurecic, Maja 等

1. INTRODUCTION

The black printer is developed in the same time as colour C M Y (cyan, magenta, yellow) separations. The intention of broadening the basic colours with black was to improve the reproduction meaning contrast, density range, some details etc. As the speed of printing machines grew higher, the total coverage amount was unable to dry in necessary time, what caused various technical and procedure difficulties. Implementation of UCR (under colour removal) or CCR (complementary colour removal) principles, meaning substitution of the common achromatic part in colours with black tint coverage (K), solved numbered problems including charge of chromatic dyes/tints in printing process (Kipphan, 2001). In some other situations caused decreasing in high densities (increasing of lightness L), deviations in chroma or hue for tertiary colours reproduction. Colorimetric measurements, determining Lab values and density measurements were carried out.

2. BACKGROUND

UCR was still possible to be used in photomechanical operations as a separate mask made, but also was implemented as a hardware function in older analogue scanners. It was practised for grey scale or neutral tones defined as C M Y coverage combination to be substituted with black (K) coverage in desired amount (Yule, 2000). This is quite important as reproductions are often judged on their neutral or grey regions. Digital scanners were able to distinguish reduction for grey scale (UCR) and other tertiary colours (GCR), also applied in various graphic arts computer programs (Johnson, 1984). Contemporary approach primary considers that as an achromatic substitution process for both principles.

Theoretically it is a simple process of substitution, but due to non ideal inks and substrates, various physical and optical deviations such as additivity failure, light scatter, boundary effects, have influence on the final result. One combination corresponds only to the target printing profile. In various manuals for processing in graphic arts (scanners or operating programs) the achromatic possibility is offered, but the amount of substitution is gave up to the technologist, or eventually some value for concerned process is recommended. In today's practice when ISO standardisation (ISO 1996) recommends achromatic methods, it is of significant interest to get more information about these issues.

Currently the most important color space based on the opponent-color theory is known as CIELAB (Berns, 2000). The L*, a* and b* coordinates can be calculated from the tristimulus values XYZ normalized to the white by equations (1)-(3):

L* = 116[(Y/[Y.sub.n]).sup.1/3] - 16 (1)

a* = 500[[(X/[X.sub.n]).sup.1/3] - [(Y/[Y.sub.n]).sup.1/3]] (2)

b* = 200[[(Y/[Y.sub.n]).sup.1/3] - [(Z/[Z.sub.n]).sup.1/3]] (3)

Where [X.sub.n], [Y.sub.n], [Z.sub.n], are tristimulus values of light source.

3. EXPERIMENT

Two patches, grey and tertiary, were generated on monitor for this experiment. The prints were printed on Xeikon 32D digital printer. The grey patch has following values: 90 c, 80 m, 80 y and 0 k. The corresponding RGB values are 76, 80, 70 and Lab coordinates 33, -4, and 6. Various combinations of substitutions were applied. The first combination was traditional substitution with black. The substitution was performed in several steps. In the second combination, the tone value of black was constant and the other values were calculated to adjust colorimetric values constant. In the third combination, the cyan was holding constant and the other values were calculated to keep colorimetric values constant. The CMYK values for three combination and programmed Lab values are shown in Table 1.

Programmed patches were printed and measured by spectrophotometer and densitometer. The tone values of grey patch are: 90 c, 80m, 80 y and k 0) and Lab values: 42, -14 0.3. The results are shown in table 2.

It is evident that reduction principles in recalculated profile achieve more acceptable results than standard ones. The more black substitution than in standard procedure is achieved, that result in more stable better black is performed.

For further investigation a tertiary coloured patch was generated. Its purpose was to check on that proposed principles could be adopted more common. The basic tone values recalculated values for tertiary colour patch are C 80, M 60, Y 60, and K 0. Colorimetric Lab values are 47, -5, and -1. Related R G B values are 104, 112, and 112. The patches are recalculated for K coverage 10, 30, 50 and 70.

As can be seen colorimetric lightness by recalculated values do not considerably change, and chromatic coordinates a and b slightly shift to achromatic with increasing amount of black tint coverage amount coverage, even more than implied.

[FIGURE 1 OMITTED]

The overall visual response also does not change considerably also, so for possible discrimination the behaviour of patches containing black ink combination determination in the region from 700 to 1000 nm. is performed, fig 1.

4. RESULTS AND DISCUSSION

It is obvious that achromatic model of reproducing has various interesting features. In pure reproduction process the total coverage amount is reduced, but increase in lightness is occurred, what means lower density, as shown in table 2 and 3. Higher lightness (L) in standard graphic communications means lower density and reduced density range. The reason is in optical and physical properties of dyes and substrates, such as additivity failure, unwanted light dispersion and adsorption and boundary effects. If some complex recalculation is used, meaning keeping the colorimetric values as possible constant, lightness mainly stays unchanged. Not only for reasons claimed by ISO standardisation for chromatic reduction meaning total coverage decrease and lowering technical and technological conditions, such a reduction means more for practical purposes as other colorimetric values are more stabile (mainly unchanged).

Similar situation occurred by tertiary colour, when complex recalculation was used. The very interesting situation is that by preserving colorimetric values, what includes a visual response, higher black ink amount can be implemented for chosen reproduction profile. In such combination some coloured patches optically can appear near similar, but vary with the black (K) coverage amount. For now that that can be practised in generated (tertiary) bars and stripes, for a real picture a separate model algorithm should be made. From visual/IR response it can be concluded that reproduced colours containing less black coverage have lower reflection (higher density) in visual part and higher in IR part. Colours with more black coverage have higher reflection (lower density) in visible part, and lower reflection (higher density) in IR part of spectrum, so in certain conditions they can be distinguished. This behaviour is particularly closely linked with structure of dyes.

5. CONCLUSION

A grey and a tertiary colour were generated in this experiment. Various combinations of substitutions were applied. The aim was to preserve colorimetric conditions as much as possible the same as origin. The results show that colours with more black coverage have higher reflection (lower density) in visible part, and lower reflection (higher density) in IR part of spectrum. The more black substitution than in standard procedure is achieved, that result in more stable better black is performed. It leads to conclusion that in certain conditions they can be distinguished.

6. REFERENCES

Berns, R.; (2000). Handbook of Billmeyer and Saltzman's Principles of Color Technology, Wiley, ISBN 9780471194590

ISO 12647-2: Graphic technology-Process control for the manufacture of half-tone 1996-01-10

Johnson, A. (1984), Practical Implementation of optimum Colour reproduction, The Journal of Photographic Society, Vol.32, No.4, (July, 1984) pp.146

Kipphan, H.; (2001). Handbook of Printed Media, Springer Verlag, ISBN 3540673261, Boston

Yule, J.A.C. ;(2000), Principles of Color Reproduction: Four color printing and the black printer, GATF Press, Pittsburg

Tab. 1. Achromatic substitution for a
grey patch and programmed Lab values

Comb 1. step 2.step 3.step 4.step 5.step

1 C 70 50 30 10
 M 60 40 20 0
 Y 60 40 20 0
 K 20 40 60 80
 L 45 51 50 40
 A 0 1 0 -1
 b 4 2 0 -1

2 C 86 79 67 37 23
 M 73 64 48 10 0
 Y 77 71 62 41 30
 K 20 40 60 80 84
 L 33 33 33 33 33
 A -5 -4 -4 -4 -4
 b 6 6 6 5 5

3 C 70 50 30 10
 M 66 40 20 0
 Y 60 40 20 0
 K 55 73 81 86
 L 31 31 32 33
 A 0 1 0 -1
 b 3 1 0 -1

Table 2: Recalculated coverage c m y k
values for a grey patch

Comb 1. step 2. step 3. step 4. step

2 C 77 70 58 32
 M 58 46 29 0
 Y 56 49 37 16
 K 10 30 50 70
 L 46 46 46 45
 A -4 -2 -5 -5
 b -1 -2 -2 -1