Chromatic Adaptation Transforms to Accommodate Printed Color Matching in Paper with Optical Brightening Agents(OBA) Quanhui Tian a, Zhen Liu b

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1 Applied Mechanics and Materials Online: ISSN: , Vol. 200, pp doi: / Trans Tech Publications, Switzerland Chromatic Adaptation Transforms to Accommodate Printed Color Matching in Paper with Optical Brightening Agents(OBA) Quanhui Tian a, Zhen Liu b University of Shanghai for Science and Technology, No. 516 Jungong Road, Shanghai, , P.R.China a Tianqh2000@yahoo.com.cn, b lunaprint@163.com Keywords: Color chromatic adaptation, Color matching, Color appearance, Printed color Abstract The color reproduction is based on visual perception. The psychophysics experiments present the light source; illuminants and view background affect the results of color matching. Paper is a typical substrate for printed color matching and effects printed color greatly. Now almost all of print papers contain optical brightening agents, which changes significantly the color appearance of printed color and makes color shifting of printed color matching. This paper used CAT02 chromatic adaptation models to transform the measurements of printed color in paper with and without OBA, compared the effect of printed color matching between before and after transforming with display, present the method using chromatic adaptation transforms to improve the accurate of printed color matching. Introduction The Chromatic Adaptation Transform (CAT) is capable of predicting corresponding colors, which are defined as pairs of colors that look alike when one is viewed under two different illuminants. The most obvious example is white paper always appears white regardless of which illuminant it is viewed under [1,2]. The printed color matching is based on visual perception and is influenced by the profiles of substrate material. Paper is a typical substrate for printed color matching, which effects color appearance of printed color greatly. Especially, there is a significant color difference of different paper which contains optical brightening agents (OBA) [3-5]. OBA is a kind of chemical additive to correct for the natural yellow color of pulp. It works by absorbing ultraviolet wavelengths of the illuminant that are not visible to the human visual system and fluoresce at blue wavelengths [6]. ISO 3664 and ISO specify standard measurement and viewing conditions to deal with substrates including OBA. However no color measurement devices available are able to accurately comply with the standards mentioned in those documents. All the print viewing booths available show variations in the spectral power distribution of the light source been used in these booths and also it deviates from the spectral power distribution recommended by the CIE [3].In that, the effect on the appearance of colors due to variation in the UV light in the lighting conditions printed on substrates with and without OBA is a problem for printed color matching. So this study focuses on printed color matching in paper with OBA, and uses chromatic color adaptation transform CAT02 [7,8] to analyze the effect of the different amount of OBA on the appearance of printed color and to get a method to accurately simulate the visual appearance of the printed colors in paper with OBA to improve printed color matching. Method & Experimental Methodology.The col i or perception of printed color in different paper produces chromatic adaptation. Namely, CAT02 was selected because it is compatible with CMCCAT97 and was optimized using all available data sets except the McCann et al. set, which includes a very chromatic adapting illuminant[8]. In that, the CAT02 functions were used to predict the tristimulus of each All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/05/16,11:49:18)

2 Applied Mechanics and Materials Vol printed colors based different paper white colors and two profiles were made with the transformed data of printed colors in paper with and without OBA after chromatic color adaptation. The effects of printed color were compared to verify that the more accurate color matching would be got with chromatic color adaptation transform. The procedure of this study is shown by Figure1. Print out IT8.7/4 target Printed Color samples (OBA) Whitepoint based on paper (OBA) Compare the differences Printed Color samples (NoOBA) Whitepoint based on paper (NoOBA) CAT02 Transformed data Compare the differences Transformed data Make a Profile using transformed data printed color samples (OBA.icc) transformed data (OBA_CAT02.icc) transformed data (NoOBA_CAT02.icc) printed color samples (NoOBA.icc) CMYK color image (OBA.tif) CMYK color image (OBA_CAT02.tif) CMYK color image (NoOBA_CAT02.tif) CMYK color image (NoOBA.tif) Experimental Fig. 1 The Procedure This research used two papers with and without OBA (Grammage as 220 g/m2). These papers are the similar thickness, optical and other print abilities. The same color targets (IT8.7/4 includes 1,617 color patches) were printed out with the same print conditions (Heidelberg SM74 Sheet-fed Offset press,the same plate). Spectral values of color samples were measured with MeasureTool. The instrument was X-Rite isis spectrophotometer and measurement condition was M1, which defined as D50, 2 angles, white backing, no UV-cut filter[3]. The measurement values consisted of two groups, one is the printed colors in paper with OBA, and the other is the printed colors in paper without OBA. The color differences of these two samples were computed. The CIE XYZs of printed colors were calculated from the spectral data as input data, deltas X Y Z were calculated to analyze the differences between printed color measurements in paper with and without OBA. The printed colors tristimulus values were transformed by Matlab with CAT02 transforms function. The white point to comparison the quality of color matching, the profiles made by measurements and transformed data were proofed for the same standard RGB color image (ISO300) with Adobe Photoshop CS5. Results and Analysis Visual observation & color difference computation The tristimulus differences between before and after transforming. Comparison of tristimulus differences between printed colors in paper with and without OBA, there is a significant difference. The relationship of delta X Y Z and CIE X, Y, and Z is displayed in Figure 2A. From the Figure 2A, the results indicate a magnitude of the effect of tristimulus values of Z, while tristimulus values of X and Y are impacted slightly.

3 634 Packaging Science and Technology X,Y,Z Initial -4 X,Y,Z Chromatic Color Adaptation (A) Before transform (B) After transform Fig. 2 The relationships of X, Y, Z and X, Y, Z with OBA After chromatic adaptation transforms, the tristimulus differences between printed colors in paper with and without OBA are smaller than before, especially the tristimulus values of Z are impacted greatly, which is shown in Figure 2B. The color differences between before and after transforming. The color differences between before and after transforms were decreased, shown in Figure3. Initial is the color difference of the IT8.7/4 target in paper with and without OBA before chromatic adaptation transforming. The Chromatic Color Adaptation is the color difference of the IT8.7/4 target in paper with and without OBA after chromatic adaptation transforms. The chromatic color adaptation transforms functions help to reduce the color differences. The 90% of color differences are less than 2.5 E 76 after transforms with the white colors of paper as reference white point. The 85% of color differences are less than 2 E 76, which are named as perfect color reproduction. That means the chromatic color adaptation is efficient for the printed color matching between paper with and without OBA from the distributions of E CRF Initial Chromatic Color Adaptation E Fig. 3 CRF_ E curves of printed The influence for color matching. The results of soft proofing shown in Figure 4, testify the color shifting due to OBA effect. This experiment measured the IT8.7/4 target printed in paper containing OBA with spectrophotometer under M1 measurement condition, profiled with the spectral measurement values, and observed the standard color images (ISO300) displaying on screen with soft proofing system supported by Adobe Photoshop CS4, and got the results from comparison to the visual images with different profiles.

4 Applied Mechanics and Materials Vol Figure 4 shows the view of the standard color image on the screen with soft proofing. Up of two color images are two CMYK color images converted by profiles, which was made with measurements in paper with and without OBA. Down of two color images are two CMYK color images converted by profiles, which was made with chromatic adaptation transforms color data of paper with and without OBA. In this experiment, the characterized dataset is measured under M1 measurement condition, which means that dataset didn t correct and compensate OBA for color matching and the view of color images with this profile displayed the mismatching for OBA. Brighten paper looks whiter but the colors measured shift to blue, then yellowness compensation would be given as soon as color match. After chromatic adaptation transforms, the color differences between printed colors in paper with and without OBA decreased. Two color images with color chromatic adaptation transformed profiles looks more similar than before. On the comparison of two images and hard copies, these two displaying images look much similar as hard copy image in paper without OBA under D50 light source. Picked up the color samples from the converted images, and computed the color differences to draw CRF_ E curves, shown in Figure 5. CRF EBefore EAfter E Fig. 5 CRF_ E curves for color converted by before and after Fig. 4 Color image in soft proofing system Figure 5 shows the distribution of color differences of the color samples picked up from CMYK color images in different condition. The black line is the distribution of color differences between CMYK color image which named as OBA.tif and converted with measurements in paper with OBA, and the color samples picked up from another CMYK color images which named NoOBA.tif and converted with measurements in paper without OBA. The dash line is the distribution of color differences between CMYK color image which named as OBA_CAT02.tif and converted with transformed data of measurements in paper with OBA, and the color samples picked up from CMYK color images which named NoOBA_CAT02.tif and converted with transformed data of measurements in paper without OBA. From CRF curves of color differences shown in Figure 5, the color differences between two CMYK color images converted with transformed data are smaller than the others color images. The 90% tile of CRF curves are less than 1.4ΔE 76, and the largest color difference is 1.5ΔE 76. Otherwise, before transforming, the color differences between color images in paper with and without OBA are larger than those of after transforming. It means the chromatic adaptation transform function could be used for printed color matching in different paper.

5 636 Packaging Science and Technology Conclusions This paper analyzed the printed color matching in paper with and without OBA and proved the chromatic adaptation transforms model would improve the accurate of printed color matching. The CAT02 transform function was used to transform measurements of printed colors in paper with and without OBA. The color differences between colors in different paper decreased after chromatic adaptation transforming. Furthermore, the color images converted in different conditions were compared with display. The color images converted with chromatic adaptation transformed data were much close for visual perception. Finally, there are something need to further research as this paper used display to study color matching in paper with OBA and the chromatic color adaptation of printed color in different paper, which made OBA and different paper conditions simulate on screen and is different from the actual print out on paper. References [1] R.S. Hunter: The measurement of appearance, ( 2nd ), edited by John Wiley & Sons, New York(1987). [2] Information on [3] ISO/DIS Graphic technology Spectral measurement and colorimetric computation for graphic arts images (2009). [4] J. Calabria and D.C.Rich: IS&T/SID Color Imaging Conference (2003), pp [5] International Color Consortium, Summary of CIE Publication 163: The effects of fluorescence in the characterization of imaging media, White Paper 14, Level: Intermediate (2005). [6] M.F. Esteves, A.C. Noronha and R.M. Marinho: World Textile Conference - 4th AUTEX Conference (2004). [7] N. Moroney, M.D. Fairchild, R.W.G. Hunt, C. Li, M.R. Luo and T. Newmann: The CIECAM02 Color Appearance Model. Proc. IS&T/SID 10 th Color Imaging Conference (2002), pp [8] C.J Li and M.R. Luo: Testing the robustness of CIECAM02. Color Res. App., Vol.30 (2005), pp [9] V.Chovancova-Lovell and P.D. Fleming: 22 nd International Conference on Digital Printing Technologies Final Program and Proceeding (2006), pp [10] ISO 3664, Viewing conditions Prints, transparencies and substrates for Graphic Arts Technology and Photography(2000). [11] R.McDonald: Colour Physics for Industry, (2nd ed.), Bradford: Society Of Dyers and Colourists(1997). [12] C Puebla: Whiteness Assessment: Axiphos GmbH (2006).

6 Packaging Science and Technology / Chromatic Adaptation Transforms to Accommodate Printed Color Matching in Paper with Optical Brightening Agents (OBA) / DOI References [8] C. J Li and M.R. Luo: Testing the robustness of CIECAM02. Color Res. App., Vol. 30 (2005), pp /col [9] V. Chovancova-Lovell and P.D. Fleming: 22nd International Conference on Digital Printing Technologies Final Program and Proceeding (2006), pp /J.ImagingSci.Technol.(2006)50:6(550)