Authors: Zhihui Li*, Honglei Ji*
*TCL Electronics Holding Limited., Shenzhen, Guangdong, China
In recent years, some groups recommend using the LCh color model to analyze color changes. For example, Rtings, a well-known evaluation website in North America, uses the hue and saturation changes of red, green, and blue as a way to evaluate the viewing angle of a TV. However, few people will explore the applicability and difficulties of this color analysis method. This p a p e r analyzes the advantages and difficulties of the LCh color model through the analysis of the theory and the results of human factors experiments.
We should explain the history of CIE LCh color model at first. In 1976 CIE created two color spaces,which named as Luv color space and Lab color space respectively. L means lightness,a&b means coordinates in CIE Lab color space[1]. The picture below is the CIE lab color space.
Along with the Lab color space, the Lch color model was also introduced. The LCh color model, which has no independent color space or chromaticity diagram,is based on the Lab color space to evaluate color changes[2]:
L: brightness coordinate, its number is the same as L in CIE Lab;
C: Saturation, the vertical distance from the brightness axis;
h: hue angle. An angle equal to 0° indicates that the chromaticity is on the +a axis, and 90° indicates that the chromaticity is on the +b axis.
The CIE LCh color space separates brightness, hue, and saturation, which is consistent with the visual perception of color.
Is it convenient to use LCh color model to evaluate color changes? Are there any difficulties in using the LCH color model? we conducted subjective experiments and data analysis to research the application difficulties of the LCH color model. We used a 65-inch LCD TV which resolution is 3840*2160 to carry out the subjective perception experiment. Before the subjective experiment, we performed a fine color calibration on this TV to make it have excellent picture quality performance.See the following test data,this TV has good picture quality and precise color control capabilities:
Since the human eye is more sensitive to changes in skin color[3], the subjective experiment is based on changes in the hue and saturation of the skin color. This skin color is selected from Macbeth chart wich is named as dark skin(R:G:B 115:82:68), we adjust the hue and saturation by changing the ratio of R/G/B in the skin tone. So we can get multiple pairs of colors with different hues or different saturations.The background under color pairs is middle gray(R:G:B 128:128:128):
We invited 5 subjects to participate in the subjective experiment, the distance between the participants and the TV’s center is 50cm to ensure that obsercer angle is 2°.The experiments was conducted in the dark room which illumination is less than 1lux. Participants were asked to answer two questions.The results were recorded in table 2(% represents the proportion of participants who agree with this option):
1.Have you observed the difference between reference color and sample color?
2.The degree of color difference is unacceptable to you.
Perception experiment result:
1. When the dark skin tone is basically unchanged and only the saturation changes, dC 0.95 starts, all participants report that they perceive the color difference and believe that dC ≤ 0.82 is the saturation change threshold for this experiment.
2. When the saturation of the dark skin is basically unchanged, only the hue changes, starting with dh=-3.2 or dh=3.7, all participants report that they perceive color differences and believe that dh ≤-9.4 and dh ≥ 14.8 are the hue changes for this experiment threshold.
3. From the perspective of theoretical combing and comparing with some standards, the threshold of our experiment is obviously larger, which is related to the participant’s understanding of color and lack of professional training and experience accumulation. From the above research results, we believe that if there is no knowledge of the LCh color model theory and the ability of judging color expertise, it is meaningless to unilaterally propose a standard based on this color model.
References
[1] ISO, Colorimetry part 4:CIE 1976 Lab Color Space,p.2 (2008).
[2] CIE, Colorimetry, vol. 1, p. 16-19 (2004).
[3] Zhihui Li,Honglei Ji and Tianying Deng, Research of visually sensitive colors as viewing angle change on TFT-LCD displays, ICDT Technical Digest, p. 3 (2020)
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Symp Digest of Tech Papers – 2021 – Li – 28 4 Research on Difficulties of LCh Color Model Application
