A color matching experiment using two displays: design considerations and pilot test results

Various recent studies have shown that observer variability can be a significant issue in modern display colorimetry, since narrow-band primaries are often used to achieve wider color gamuts. As far as industrial applications are concerned, past works on various aspects of observer variability and metamerism have mostly focused on cross-media color matching, an application context that is different from color matching on two displays, both in terms of human visual performance and the application requirements. In this paper, we report a set of three preliminary color matching experiments using a studio Cathode Ray Tube (CRT) display with broadband primaries, and a modern wide-color gamut Liquid Crystal Display (LCD) with narrow-band primaries, with and without surround. Two principal goals of these pilot tests are to validate the experimental protocol, and to obtain a first set of metameric data of display color matches under different viewing conditions. In this paper, various experimental design considerations leading to the current test setup are discussed, and the results from the pilot tests are presented. We confirm the validity of our test setup, and show that the average color matches predicted by the 1964 CIE 10° standard observer, although acceptable as average matches, can often be significantly and unacceptably different from individual observer color matches. The mean, maximum and the 90 percentile values of the standard observer-predicted color difference of individual observer color matches were 1.4, 3.3 and 2.6 ∆E*00 respectively. Introduction When two color stimuli produce the same visual response, a visual match is obtained. Two stimuli with very different spectral power distribution can give rise to identical cone response, leading to a metameric match. However, such a match established by one observer can, and quite often does, lead to a mismatch for a different observer, as the second observer has a different set of color matching functions than the former. This phenomenon is commonly termed as observer metamerism. The topic of observer metamerism has sparked renewed interest in the recent years with the proliferation of wide-gamut displays. Whether based on LED-backlight or employing laser primaries, all these displays compete with each other in achieving more vivid, more saturated and brighter colors. On the flipside, these displays are particularly susceptible to observer variability [1][2], since their peaky, narrow-band primaries cause noticeable shift in chromaticities of perceived colors with relatively minor change in the visual characteristics of the observer. This can be a nontrivial issue in critical color matching tasks, for example in post-production applications. Thus, it is of interest to study the effect of observer variability in color matching across traditional and modern displays, and to acquire experimental data in such a context, which can be subsequently used to better model the observer variability. There is another reason why such experiment is of high relevance for our current work. In 2006, CIE’s (Commission Internationale de l’Eclairage) technical committee TC 1-36 published a report [3] on the choice of a set of Color Matching Functions (CMF) and estimates of cone fundamentals for the color-normal observer. Based on a theoretical analysis, current authors recently showed [4] that the long-wave sensitive (x-) color matching function derived from the CIE 2006 model did not accurately predict the average Stiles-Burch observer data [5] for two subgroups in the age ranges of 22-23 and 49-50. It was also shown that the short-wave sensitive (z-) function of the 10° standard observer produced significant deviation with respect to the intra-group average observer for all three age groups studied, namely the age groups of 22-23, 27-29 and 49-50. In this case, CIE 2006 model prediction was closer to the real average observer data. A new set of color matching data will enable an indirect, nevertheless independent verification of the aforementioned observations, and will allow us to investigate their significance, if any, in the context of modern display colorimetry. The preliminary set of color matching experiments described in this paper is part of our current study that investigates the effect of observer variability in comparing colors on a modern, wide-gamut display with narrow-band primaries, and a conventional display with broadband primaries (for example, a CRT display). The pilot tests were intended to validate our experimental protocol, while the final goal of our experiments is to obtain new color matching data in the context of modern display colorimetry. Nevertheless, our approach is guided by the fundamental principles of human color vision. Thus, our experimental paradigm is a hybrid version of classical color matching experiments and the experiments from applied studies. The test setup was carefully planned, and several experimental design choices were made. These are explained in this paper, and the results from the pilot tests are analyzed. Background Fifty years ago from the time of writing this paper, Stiles and Burch conducted the most comprehensive, and arguably the most authoritative large-field color matching experiment [5] till date, which eventually led to the 1964 CIE 10° standard observer functions. Since then, and even earlier, numerous researchers have conducted color matching experiments with a variety of experimental setups and goals [6] (page 288), full review of which is beyond the scope of this paper. Here, we focus on some of the past research works that were highly relevant for advanced applied colorimetry. The reader must bear in mind however, that there also exists a vast amount of literature dealing with more classical color vision studies. In a seminal work, Thornton [7] performed several 10° color matching experiments with his visual colorimeterspectroradiometer instrument, using disparate sets of spectral primaries. Among some other highly consequential inferences, he found “...the mathematical constructs we know as the CIE Standard Observers, while they have been vastly helpful during their long tenure, when used in the computation either of perceived brightness or of matching condition of two lights, often result in an approximation that is visually unacceptable”. A motivation of the current work is to verify this observation in the context of modern display colorimetry. Soon after Thornton’s study, North and Fairchild [8] came up with an innovative experimental setup for color matching, where the observers mixed a tungsten light source filtered by interference filters and the primaries of a CRT, in order to match a diffuse tungsten-halogen source simulating daylight. This was a Maxwell-type [6] (page 293) color matching in a 2° bipartite field. Color matching data collected using this instrument was used to assess observer variability. The authors showed [9] that the CIE recommendations on observer metamerism, also known as the CIE Standard Deviate Observer [10] resulted in a prediction of the order of variability within a single observer, while the variability between different observers was significantly larger. Several years later, Alfvin and Fairchild [11] performed a color matching experiment where observers made matches between different colors presented in reflective and transmissive color reproduction media and on a broadband display. They found that inter-observer variability was twice as large as intraobserver variability, and was significantly under-predicted by the CIE Standard Deviate Observer [10]. In contrast with Thornton’s observation, they concluded that the existing CIE Standard Colorimetric Observers were a good representation of the population of normal trichromats. In another significant study on cross-media color reproduction, Oicherman et al. [12] performed a color matching experiment using spatially separated computer display and surface color stimuli, in an experimental setup that attempted to replicate real-life industrial setting for a color matching task. One of the most significant conclusions was that the variability of matches between spatially separated stimuli cannot be predicted by the variability of individual color matching functions. The authors suggested that in the context of asymmetric cross-media color matching, observer variability could be better modeled by an optimized color difference equation. The experiments described in this paper deal with matching colors presented on a display with broadband primaries and a second display with narrow-band primaries. This application context is different from cross-media color reproduction, both in terms of human visual performance and the application requirements.