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6. Conclusion and Discussion
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6.1 Conclusion
The human visual system differs from person to person, and the CIE 1931 Standard Colorimetric Observer has been used so far for standardized color matching function. The main objective of this study was to show observer metamerism in displays with different spectral characteristics and to suggest the need for individual colorimetric observers. A color matching experiment and a color difference experiment were conducted using a total of six types of displays in a dark room. Participants with normal color vision were recruited in the experiment, and the total number of participants was 10. The experiments were analyzed to find out observer variability, the difference between each observer, by indicating on CIE 𝑢′10𝑣′10 plane and calculating color difference using △𝑢′10𝑣′10 and △𝐸00. In the color matching experiment, the color matching dataset was obtained, and observer metamerism was analyzed from various viewpoints.
The observer performance tests were analyzed in three aspects: matching accuracy performance, individual repeatability, and long-term repeatability. First, the validity of the experimental method, how well people match colors, has been proven by experimenting on a display with the same spectral characteristics. The result showed that people matched color around the reference color, so people have good matching accuracy, 0.0011 △𝑢′10𝑣′10 . Second, individual repeatability was analyzed using experiments with two trials. The result was shown although repeatability was different for each person, people had consistent repeatability overall, 0.0021 △ 𝑢′10𝑣′10 . Third, long-term repeatability was analyzed using experiments with the same display combination, conducted on different days. The color difference for long-term repeatability was lower than individual repeatability with 0.0018 △𝑢′10𝑣′10, so people have consistent long-term repeatability.
The color matching variation between observers was analyzed by the color difference between matching average and individual matching data. The experimental results show that observer variability was different according to the display type and that the variability was greater in the color matching experiment in multi-color than in white color. In addition, by showing that the color difference of observer variability is 0.0032 △ 𝑢′10𝑣′10 , which was three times larger compared to individual repeatability, it showed that the color matching results are different for each individual.
The field size effect on color matching was analyzed with the same display combination on different field sizes. The color difference for FOV impact was 0.0035 △𝑢′10𝑣′10, about twice as larger than that of individual repeatability, so the result showed that color matching was different depending on the field size.
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Lastly, the effect of display characteristics on color matching was analyzed by the color difference between reference and test displays. It was confirmed the color difference was varied depending on the display characteristics, and the average color difference was 0.0072 △𝑢′10𝑣′10, more than three times larger than that of individual repeatability. So people perceived as same in different CIE 1964 XYZ tristimulus values, and this phenomenon further showed the failure of current color matching function.
The color difference experiment confirmed the way in which people actually perceived the color matched by other people. The amount of color difference between reference and test displays was responded in 5-point Likert scale. Participants responded that the color difference was low for the color located closer to their matching result. In the case of other people's matching color, it was said that it was also in the acceptable range, if that color was adjacent to their matching result. However, they responded that the color difference was high for the distant color, and the color having the same chromaticity coordinates as the reference was evaluated as having the highest color difference. The result was shown that there is individual color matching deviations, but the 1931 Colorimetry error is larger than that. Therefore, a new color matching function, which makes reference color, matching colors, and color evaluated as a low color difference united, is needed.
In the color matching function performance test, the color prediction using four test CMFs was conducted. The test color matching functions were CIE 1931 2°, CIE 1964 10°, CIE 2015 10°, and Asano 151 lms-CMF. The performance of each color matching function was 5.84, 5.51, 4.93, and 4.54
△𝐸00 in order. Among them, the performance of Asano 151 lms-CMF was the best. However, it was revealed that the Asano 151 lms-CMF alone cannot predict the color matching data of people. Therefore, the necessity of a new color matching function was found to predict the color matching data perfectly without color matching for all colors.
As a conclusion, the colors with the same CIE 1931 tristimulus values are perceived as different due to a failure of the current color matching function. Besides, because each person's visual characteristics are different, color matching differs from person to person. The color matching of other people is also in an acceptable range to some extent, for colors adjacent to their color matching result, but in the end, the error of CIE 1931 Colorimetry is quite serious. Therefore, a new function considering the individual's physiological characteristics is needed.
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6.2 Application & Future work
In this study, color matching experiment and color difference experiment were conducted with 2 degrees and 4 degrees on simple patches. Based on this, further research is required for three points.
First, experiments were conducted only with colors of 2 degrees, and mainly 4 degrees. However, since the display that people watch in real is a full screen (Hu & Houser, 2006). Therefore, an experiment using a full patch will be required.
Second, this study was conducted only on simple patches with no pattern. It would be good to conduct with various images to see how people evaluate the real images. This experiment is necessary because what people really see is an image, not a simple patch.
Third, the necessity of a new color matching function was suggested. Therefore, using the color matching experiment results of this study, the study for the color matching function that is more suitable for human physiological characteristics is required for future work.
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