Categorical observers for metamerism

A method is proposed to generate categorical colour observer functions (individual colour matching functions) for any field size based on the CIE 2006 system of physiological observer functions. The method combines proposed categorical observer techniques of Sarkar et al with a physiologically-based individual observer model of Asano et al and a clustering technique to produce the optimal set of categorical observers. The number of required categorical observers varies depending on an application with as many as 50 required to predict individual observers' matches when a laser projector is viewed. However, 10 categorical observers are sufficient to represent colour-normal populations for personalized colour imaging. The proposed and recommended categorical observers represent a robust and inclusive technique to examine and quantify observer metamerism in any application of colorimetry.     

Study on metamerism degree evaluation based on wavelength sensitive cone weighting algorithm

Metamerism phenomenon can be used in illuminant detection to ensure the accuracy of light source. A method based on Long‐, Middle‐, Short‐ wavelength cones(LMS) weighting algorithm to evaluate metamerism degree is proposed. The chromatic relationship between the degree of metamerism mismatch and the light source is studied. Herein, the consistency between the metameric indices (MIs) and CIE1976 L*a*b* color difference ranking is analyzed by SRCC, KRCC, PLCC and RMSE. A statistically sampling method to obtain practical LMS cone fundamentals to evaluate metamerism degree is employed. The analysis results obtained show that the method based on LMS weighting algorithm has good evaluation ability and stability in simulation experiments and statistically sampling experiments, which are in line with visual characteristics of human. Proposed method meets the requirements of selecting metameric pairs used in light source detection. The analysis results have certain guiding significance.     

An observer‐metamerism sensitivity index for electronic displays

The effect of observer metamerism induced by electronic displays depends to a large extent on their primary spectra (red, green, and blue in the most common case). In particular, for narrow‐band primary spectra whose peak wavelength lies in the range of high variability of the observer's color‐matching function, some observers can experience very large differences between actual surface colors (e.g. in a light booth) and displayed colors if the monitor is optimized for the International Commission on Illumination (CIE) 1931 standard observer. However, because narrow‐band light‐emitting diodes lead to larger color gamuts, more and more monitors with very narrow band primaries are coming onto the market without manufacturers taking into account the associated problem of observer variations. Being able to measure these variations accurately and efficiently is therefore an important objective. In this paper, we propose a new approach to predict the extent of observer metamerism for a particular multiprimary display. Unlike existing dedicated models, ours does not depend on a reference illuminant and a set of reflectance spectra and is computationally more efficient.     

The usability of the CIE colour‐matching functions in the case of CRT monitors

During the past years, several papers have been published that question the use of the CIE colour‐matching functions in the case of metameric samples. Visually matching samples produced on CRT (Cathode Ray Tube) monitors are metameric to most colour stimuli created by illuminating reflecting materials. As CRT monitors are often used in colour design applications, it seemed important to check how well CIE colorimetry will predict such colour matches. To investigate this problem, we set up an experiment in which painted samples were matched with samples produced on a CRT monitor. The colour of incandescent lamp irradiated Munsell samples were visually matched to the mixture of the RGB primaries of a CRT monitor. Both the reflected colour stimuli of the Munsell samples and the emitted stimuli of the monitor were measured spectroradiometrically. Our results imply that there is an observer‐dependent variability among the matches, but we could not find a major difference between the tristimulus data of the hard copy and soft copy presentations that would indicate errors in the CIE colour‐matching functions. The measurement accuracy, quantization errors of the monitor, and the achieved accuracy of the colour matches are treated in this study. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 436–441, 2001     

Metamer density estimated color correction

Color correction is the transformation of response values of scanners or digital cameras into a device- independent color space. In general, the transformation is not unique due to different acquisition and viewing illuminants and non-satisfaction of the Luther–Ives condition by a majority of devices. In this paper we propose a method that approximates the optimal color correction in the sense of a minimal mean error. The method is based on a representative set of reflectance spectra that is used to calculate a special basic collection of device metameric blacks and an appropriate fundamental metamer for each sensor response. Combining the fundamental metamer and the basic collection results in a set of reflectances that follows the density distribution of metameric reflectances if calculated by Bayesian inference. Transforming only positive and bounded spectra of the set into an observer’s perceptually uniform color space results in a point cloud that follows the density distribution of device metamers within the metamer mismatch space of acqcuisition system and human observer. The mean value of this set is selected for color correction, since this is the point with a minimal mean color distance to all other points in the cloud. We present the results of various simulation experiments considering different acquisition and viewing illuminants, sensor types, noise levels, and existing methods for comparison.     

昼光模拟器的研制

论述了氙灯型昼光模拟器的设计思想;理论模型;技术指标;应用效果等。     

基于旋转主成分分析的颜色组分预测研究

针对传统主成分分析法在建立光谱色彩空间时的“负指标”问题,提出了一种基于旋转主成分分析的颜色组分预测方法。该方法在最大程度保持主成分对原始光谱空间信息的累积方差贡献率同时,将初始特征向量转换为一组可作为实际基础颜色组分的全正向量,这些特征向量中的元素应按列向0或1分化。实验结果表明,这种新的预测方法不仅能很好地揭示目标图像的真实颜色成分,而且还具有较高的光谱数据重构精度。     

Toward a more accurate and extensible colorimetry. Part V. Testing visually matching pairs of lights for possible rod participation on the Aguilar-Stiles model

The spectral power distributions of strongly metameric visually matching pairs of lights, at visual fields of 10° and 1.3° and luminances from 0.2–120 cd/m₂, are examined for possible rod participation, on the Aguilar-Stiles model, in the visual matches. We develop a method for computing visual jnds of predicted rod intrusion; i.e., an analytical expression for rod mismatch. With this, we compute possible rod mismatch for 922 metameric pairs of lights, each pair pronounced a visual match by one or another of eight normal observers, by using the Trezona expression for pupil diameter or by arbitrarily assigning pupil diameters of 4, 6, 8, or 10 mm. We find some evidence of possible rod participation in the matches, under certain conditions, in small parts of the (brightness)-(fieldsize)-(spectral composition) three-space. On the Aguilar-Stiles model, rod participation is not an appreciable contributor to the troublesome tristimulus errors documented, in earlier parts of this series of articles, to be present at high and low field brightness and at large (10°) and small (1.3°) visual fields. © 1998 John Wiley & Sons, Inc. Col Res Appl, 23, 92–103, 1998     

How strong metamerism disturbs color spaces

Systems for arranging and describing color include “color spaces” and “color order systems.” In a color space, tristimulus values R, G, and B are computable for every light (every point in the space). In familiar color spaces, such computation makes use of three functions of wavelength (the color-matching functions that define one of the CIE Standard Observers), one function corresponding to each of R, G, and B. In the presence of strong metamerism (marked spectral difference between the spectral power distributions of a pair of visually matching lights), the color-matching functions may report that one light of the pair has an entirely different color from that of the other member of the visually matching pair of lights. The CIE Standard Observer embodying those color-matching functions “sees” the two visually matching lights as entirely different in color, that is, it reports entirely different sets of R, G, and B for the two visually matching lights, and, thus, an entirely different chromaticity. In an example given here, each of the CIE Standard Observers assigns a strong green color to lights that are seen by normal human observers as a visual match to a hueless reference white. On the other hand, color order systems comprising sets of real objects in a specified illuminant, and which are assembled (visually arranged) by normal observers, as are the Munsell and OSA sets, do not suffer from the type of trouble discussed here. Color spaces depending on mathematical functions of R, G, and B are at risk: both Standard Observers are shown to plot visually identical lights at widely varying points in familiar color spaces (e.g., delta E*_ab = 40–50). © 1998 John Wiley & Sons, Inc. Col Res Appl, 23: 402–407, 1998     

New metamers for assessing the visible spectra of daylight simulators and a method of evaluating them

Standard sets of metamers are used to test daylight simulators intended for visual appraisal or measurement of color. A new set is proposed and a method is given for evaluating the sensitivity of test metamers to various spectral defects in the visible spectrum. Ideal illuminant spectra were perturbed in specified ways to find the amount of defect tolerated without exceeding a given standard quality category. The amount of perturbation was compared to that tolerated by existing standard metamers, to assure that new metamers grade simulators on the same internationally accepted scale of quality. The method was used to validate metamers designed to assess simulators of CIE Illuminant D50, over the spectral range 400–700 nm, and new metamers designed to assess simulators of CIE Illuminants D50, D55, D65, and D75, over the spectral range 380–780 nm. © 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 322–330, 1999