Research on Color Reproduction Characteristics of Mobile Terminals

Transcription

1 Applied Mechanics and Materials Submitted: ISSN: , Vol. 731, pp Accepted: doi: / Online: Trans Tech Publications, Switzerland Research on Color Reproduction Characteristics of Mobile Terminals LIU Rong 1,2,a, WANG Qiang 1,b 1 Hangzhou University of Electronic Science and Technology, Hangzhou , China 2 Shanghai University of Technology, Shanghai , China a lr_print@163.com(corresponding author), b profwq@263.net Keywords: Cross media; Mobile terminal; Color rendering; Color reproduction characteristic Abstract. In consideration of color reproduction requirement of cross-media reading on mobile terminals, this article takes Ipad4, Kindle, Surface and Touchpad as samples, using self-designed Digital Test Form of color characteristics for quantitative testing, and focuses on the color reproduction characteristics and application functional requirement of the mobile terminals primarily. The color rendering, color reproduction characteristics and variation of the mobile terminals based on time dimension and space dimension are obtained through experiment, which provides high reliable basics and important value for development in high-precision and high quality digital production. Introduction Cross-media reading based on mobile terminals becomes popular with the development of digital and network technology, thus color reproduction with high quality, high-precision, high-fidelity becomes the research hot-point and the focus of technological competition in cross-media reading field, especially the high-fidelity method. The same data files will produce a little discrepancy in cross-media color reproduction as a result of color reproduction principle that differs in different mobile terminals, which initiates the corresponding researches about color reproduction characteristics and its quantitative evaluation of multidimensional mobile terminals, and also becomes technical support of both mobile terminal manufacturers and consumers on device quality improvement and terminal selection [1,2]. Zhang Jianqing [3] et al. have analyzed the brightness effect on color gamut coverage of ipad and iphone; Liu Yu, Xu Yanfang [4] et al. have studied color rendering characteristics of different monitors recently, however it s apparently rare for color reproduction characteristics of multi-dimensional mobile terminals and its quantitative evaluation. This article selects four current mainstream mobile terminals Ipad4, SurfaceRT, TouchPad B10 and Kindle Fire HD as experimental devices. In consideration of the key issues on color reproduction characteristics of multidimensional mobile terminals and its quantitative evaluation, color reproduction and its variation characteristics on the mobile terminals in terms of time dimension and space dimension can be acquired by quantitative test with the application of the Digital Test Form, which provides reliable data basics for application functional demand and color reproduction characteristics evaluation of the mobile terminals, otherwise, it also has conducive and practical value for color reproduction characteristics comparison of different mobile terminals. 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-18/02/16,18:55:09)

2 Applied Mechanics and Materials Vol Color Rendering Principle and Color Reproduction Characteristics of Mobile Terminals Color Rendering Principle of Mobile Terminals. LCD mechanism is the most commonly used in mobile terminals, which realizes color rendering through additive color mixing of red, green and blue light [5], as shown in Eq. 1. C i = k 1 R i + k 2 G i + k 3 B i (1) Where, R = G = B represents neutral color, R = G = B = Max represents standard white, R = G = B = 0 represents standard black. Where, C i represents any color and k i is a scale factor. Mobile terminals used in this study are self-luminous devices. They produce different colors through changing the optical properties which are affected by input signal so as to change the manifestations of light [6,7]. Retina Display technology based on the principle of vision persistence is applied in Ipad4; Surface RT takes the Clear Type technology to make every pixel flare on the monitor; LED technology is used in Touchpad; Kindle takes LCD technology. Color Reproduction Characteristics of Mobile Terminals. Color reproduction characteristics on the mobile terminal are mainly expressed by display color gamut, tone reproduction characteristics and neutral gray reproduction quality. Display color gamut is characterized by color gamut, which is represented with term Gp, calculated by the percentage of the mobile terminals color gamut area in the entire chromaticity diagram area (0.1952) in CIE1976L*u*v* uniform color space. Tone reproduction characteristics is characterized by γ-value, which means the relationship between the input signal level and the corresponding luminance, it reflects the important properties between physical luminance and reproduction luminance. Neutral gray reproduction is characterized by the chroma value C, which refers to the amount of color components in the displayed colors when R = G = B, a smaller value represents a better neutral gray reproduction [8,9]. Evaluation of Color Reproduction Characteristics Experiment Design and Data Measurement. A chamber with Munsell N5 neutral gray walls is used as the test environment in the experiment, the four mobile terminals are Apple ipad 4, Microsoft Surface RT, Hanvon TouchPad B10 and Amazon Kindle Fire HD, the color measurement instrument is Eye-One Pro, the color processing softwares include Profile Maker Professional 5.0.7, MATLAB 7.0 and EXCEL The applied Digital Test Form is usually used in the display color characteristic measurement which contains patches for testing the tone reproduction characteristics and the neutral gray reproduction, representing the color characteristics of the mobile terminals. The experimental design process is shown in Fig.1:

3 82 Advanced Printing and Packaging Materials and Technologies Design standard edition file Generate test patches in Matlab Sample device back to factory setting status Import test patches to sample device Analysis of color reproduction characteristics in time dimension Analysis of color reproduction characteristics in spatial dimension Tone reproduction characteristics Neutral gray reproduction Display color gamut Quantitative analysis to obtain color rendering variation of sample device Driving the device color characteristic improvement Screening device according to the user Color "WYSIWYG" solution Fig.1 Experimental design flow chart Tristimulus values XYZ and L * a * b * colorimetric values can be acquired through measuring the displayed Digital Test Form on the mobile terminals by the use of X-Rite Eye-One Pro spectrophotometer. Data Analysis and Quality Assessment. Given color reproduction requirement on the mobile terminals of cross-media reading, this article focuses on the analysis of the color rendering, color reproduction characteristics and its variation based on time and space dimension by experimental design flow and data measurement method. Quality evaluation and different applications of the four test devices are also compared. Analysis of Color Reproduction Characteristics in Time Dimension. Color reproduction characteristics based on time dimension refer to variation of tone reproduction and neutral gray reproduction versus time. γ-value can be used to characterize the mobile terminals tone reproduction characteristic, which is calculated as the following Eq. 2. Where, Y represents the displayed luminance value; V represents the input signal level; a and L b are coefficients. Luminance values Y and signal level V are consistent with Y of tristimulus values and digital driven values of single channel respectively. After analysis and fitting of the optimized brightness values and digital control values with least squares according to Eq.3, three channel γ-values of four sample devices in different time frames can be obtained by using patches for testing the tone reproduction characteristics, as shown in Tab.1 are the R channel values: Tab.1 γ-values of R channel in different time periods Time(min) Ipad Kindle Surface Touchpad The results of G and B channels are similar to R channel by using the same method. In Tab.1, γ-value of four samples vary regularly with time, fluctuations in different degrees can be observed in the beginning of 20min, then it tends to be stable slowly, which is concerned with the erratic brightness resulted from instantaneous currents when connecting the power switch; γ=2.3 of Surface RT, TouchPad B10 and Kindle Fire HD in stable state illustrate that the displayed brightness

4 Applied Mechanics and Materials Vol values increase with the driven values changing gradient in the midtone and the highlight, which makes the bright parts exhibit good level and is favorable for bright color reproduction; while γ = 1.6 of Ipad4 indicates that the displayed brightness values vary little with the driven values changing gradient, so the shadow shows better level and is favorable for darker colors reproduction. γ curves in stable status of R channel on the four sample devices are shown in Fig.2. Fig.2 γ curves in R channel It can be taken from Fig.2 that there is a certain degree of tone compression of the four sample devices, while the tone curve compression of Ipad4 is relatively smaller due to its high luminous intensity and high saturation. That of other three devices can be improved by enhancing channel level signal or improving light sensitivity of display. Neutral gray reproduction can be characterized by chroma value C. Measuring 13 points on the device screen are shown in Fig.3. Patches tested for neutral gray reproduction can be used to obtain the maximum and the average values of C. The results are shown in Tab.2. Tab.2 C values of different time periods Time(min) MAX AVG MAX AVG MAX AVG MAX AVG MAX AVG Ipad Suface touchpad Kindle From the table above, it can be observed that C values vary regularly with time and they tend to be stable slowly after 20min. The C values of ipad 4, Kindle Fire HD and Surface RT are small which shows relatively good neutral gray reproduction; while those of TouchPad B10 are the largest, which indicates poor neutral gray reproduction. The chroma values distribution of four samples in stable state is shown in Fig.3: Fig.3 Chroma values distribution on a, b color plane The neutral gray of TouchPad B10 is far from blue, which is resulted from the level signal enhancement of B channel; while that of other mobile terminals is close to green, which is mainly

5 84 Advanced Printing and Packaging Materials and Technologies due to the level signal enhancement of G channel. This situation can be improved by white-reference correction and then the control of the three-channel colors under the maximum driven values. Analysis of Color Reproduction Characteristics in Spatial Dimension. Color reproduction characteristics based on spatial dimension refer to variation within the entire display area. Gp-value is used to characterize the mobile terminals color gamut, the measuring points spatial distribution are shown in Fig.4: Fig.4 Spatial distribution of measurement points The Digital Test Form was used to obtain different measurements of devices after booting for half an hour. Gp values can not be accurately calculated since CIE1931XYZ is a non-uniform color space, thus the XYZ tristimulus values have to be converted to a uniform color space CIE1976L*u*v*. Trace points on CIE1976 u*v* chromaticity diagram with triplot of Matlab, the yellow line in Fig.5 represents the theoretical color gamut, Srgb. Fig.5 Color gamut of mobile terminals The figure indicates that the displayed color gamut is a triangular area formed by chromaticity points of three primaris. The color gamut of Ipad4 is the largest one among 4 test device, which mainly reflects the red and blue area, while other three devices represent smaller color gamut due to the weak performance in red and blue channels, we can enhance the color saturation by improving the single-channel level signal. The,, of RGB channels are known, so the color gamut area can be calculated according to Eq.3 and 4. Gp values in different measuring points of four sample devices are shown in Tab.3. Then Gp is:

6 Applied Mechanics and Materials Vol Measurement points Tab.3 Gp values in different areas Ipad Suface Touchpad Kindle From Tab.3 we can draw that the Gp values of four samples in area 4, 5, 7, 9 and 10 are consistent on the whole, which indicates better homogeneity in the middle area than the edges. It would be best to display colorful images in the middle area as color gamut in the middle area is greater than the edges. Conclusions In terms of color reproduction requirement on the mobile terminals of cross-media reading, this article takes Ipad4, Kindle, Surface and Touchpad as sample device, using self-designed the Digital Test Form of color characteristics for quantitative testing, focuses on the analysis of the mobile terminals color rendering, color reproduction characteristics and its variation based on the time dimension and spatial dimension. It also proposes quantitative analysis method of color reproduction characteristics evaluation and application functional requirement of mobile terminals. It provides improved direction and reliable data base for color reproduction characteristics and consumers to filter mobile terminals, finally it provides a better solution for achieving "WYSIWYG". Acknowledgments The authors acknowledge the support of National Science and Technology Support Program (grant No.2012BAH91F03), National Natural Science Foundation of China (grant ) and the Technology Innovation Team of Cross-media Digital Publishing Platform (grant No.ZX ). References [1] J.Thomas, J. Y. Hardeberg, I. Foucherot, etal.the PLVC display color characterization model revisited.[j]color Research and Application,2008,33(6), [2] Feng Chang, Rui-guang Wang, Xi-feng Zheng. Algorithm of reducing the non-uniformity of images in LED display panel[c].ieee Transactions on Power Electronics,2010(1): [3] Zhang Jian-qing, LIU Zhen. Color gamut coverage of the ipad and iphone is affected by the brightness[j]. Journal of Optics, 2013,33(F06): [4] LIU Yu, ZHOU Min, XU Yan-fang. The research of display color characteristic[a]. Color science and technology the second China academic conference paper printing and packaging the set[c] [5] CHANG Chih-hao, CHENG Hsing-chieh, LU Yin-jui, etal. Enhancing Color Gamut of White OLED Displays by Using Micro-cavity Green Pixels[J].Organic Eletronics,2010,11:

7 86 Advanced Printing and Packaging Materials and Technologies [6] XU Jing-ya, LIU Hao-xue, Huangmin. The research of LCD color calculation method[j]. China printing and packaging,2010,11(2): [7] LIU Ming-liang, WANG Yong-ming, ZHU Hai-hong. Determination of the color CRT display gamut[j].journal of Wuhan Technical University of Surveying and Mapping,1997,22(2): [8] Liu Quanen. Color domain and color domain coverage ration[j].video Engineering, 2004, 269(11):49~51. [9] WANG Yong, XU Hai-song, XU Dong-hui, the comparative study of Display color characteristic model.[j] Journal of Zhe Jiang university (engineering science), 2006,40(6):

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