Optimize Maximal Brightness and Correlated Color Temperature for Multi-primary color displays Mang Ou-Yang a , Ting-Wei Huang *a , Yao-Fang Hsieh a , and Yi-Ting Kuo b, c a Dep. of Optics and Photonics, National Central Univ., Jhongli City, Taoyuan County , TAIWAN TEL:+886-34227151 Ext:65280 , FAX:+886-32807026, Email:oym@dop.ncu.edu.tw b Institute of Applied-Arts , National Chiao-Tung Univ., Hsihchu city, Taiwan c Jen-Teh Junior College of Medicine, Nursing and Management, Miaoli County, Taiwan ABSTRACT Color temperature(CT) conversion of triprimary color display from one white point to another on the Planckian’s locus with the maximal brightness has been proposed. However, whether converting an original white point to another white point on the isotemperature line will enlarge maximal brightness more than converting an original white point to another white point on the Planckian’s locus needs to be determined. This paper proposes a new algorithm to enlarge maximal brightness by calling the center of gravity method of color mixing in the acceptable color difference range while the CT is converted. From the prior study, we find that the apexes of color gamut boundary move along the line of center gravity of primaries while the total brightness varies, where the line of center gravity of primaries is linked by the color points mixed by two or more full primary colors and one partial primary color. And in CIE 1931 color space, the color gamut boundary expanding from white point as total brightness decreasing will touch the isotemperature line with its apexes. Therefore, the best point of CT conversion of tri-primary color display with more maximal brightness is determined by the isotemperature line and the line of center gravity of primaries. Further, the theory extends to multi-primary color displays. Lastly, the simulations prove that converting a white point to another on the isotemperature line enlarges maximal brightness more than converting a white point to another on Planck’s locus. Keywords: color temperature conversion, correlated temperature, isotemperature line, multi-primary color 1. INTRODUCTION In lighting, color displays, and other fields, color temperature (CT) is used for characteristics of visible light. The color temperature of a light source is the temperature of the Planckian’s blackbody radiator, whose radiation is the same chromaticity as the light source. In other words, color temperature is used to describe the chromaticity of the light source. The chromaticities of many nature and artificial light sources, including daylight, aren’t on this locus, hence “correlated color temperature” (CCT) is used to indicate the temperature of the blackbody, whose chromaticity is nearest that of the light source. In other words, CCT is used to instead of color temperature to describe its characteristic when the chromaticity of a light source is off the Planckian locus. In 1963, a graphical method of finding CCT on a diagram is proposed by Kelly [1]. In 1968, the methods for numerical evaluation of CCT from the spectral power distribution of the source are proposed by Robertson [2]. The CCT was defined as “the color temperature corresponding to the point on the Planckian locus which is nearest to the point representing the chromaticity of the illuminant considered on an agreed uniform-chromaticity-scale diagram” by CIE. And many researches about computing CCT and determining isotemperature line [3-9] are proposed. Images’ characteristic and human’s perception of temperature feeling against the images can be bridged by CT. However, when images are displaying with many different media devices, how can one be sure that images will appear as intended without knowing the media devices? The International Color Consortium (ICC) recommends a solution that device profile files containing media device characteristics are produced as the bridges between different devices and provide enough characterization data to accurate color rendering. Therefore, many researches about CT and CCT applications at displays and lighting have proposed recently [10-16]. And color temperature conversion is one of the key points of color management system for improving image browsing. Using gain of R, G, and B channel to adjust the CT or the white point in displays [17-19] is a direct and cost effective way. However, the maximal brightness will decrease when converting one white point to another one. Hence, the theories about how to convert color temperature with maximal Applications of Digital Image Processing XXXII, edited by Andrew G. Tescher, Proc. of SPIE Vol. 7443, 74431H · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.825822 Proc. of SPIE Vol. 7443 74431H-1