Quality evaluation of tourmaline red based on uniform color space

Abstract

Based on the uniform color space $$\hbox {CIE}\,1976\hbox {L}^{{*}}\hbox {a}^{{*}}\hbox {b}^{{*}},$$ the red colors of 310 tourmalines were conducted with the research of color colorimetry. In terms of the quantitative analysis of color indexes such as lightness, chroma and color hue, we put forward the eight-class five-level systems for quality evaluation of tourmaline red by K-Means cluster analysis. The systems were sorted from superior to inferior: Fancy Vivid, Fancy Intense, Fancy Deep, Fancy and Fancy Dark. Through Fisher discriminant, the accuracy was proved to be up to 98.7%. Furthermore, we analyzed the effects of different standard light sources (D $$_{65},$$ A and CWF) on tourmaline red in detail. It was pointed out that D $$_{65}$$ could better reflect the color of non-self-luminous object, which was suitable for quality evaluation of tourmaline red. Light source A significantly improved the chroma and hue of tourmaline red for red tourmaline display. Light source CWF reduced the chroma of tourmaline red, making the hue significantly deviated from the red. Therefore, CWF was not suitable for tourmaline red lighting. Finally, the work discussed the effects of non-color background lightness and color background on tourmaline red to derive the following results: (1) in the process of non-color background lightness transformation, the lightness and chroma of tourmaline red were sensitive and changed significantly, with high synchronism; whereas, the color hue has unobvious change. The higher lightness and chroma of tourmaline red led to larger effects of non-color background lightness transformation to visual lightness and saturation. It was thus proved that the non-color background was suitable for quality evaluation of tourmaline red. (2) In the process of color background transformation, color metal alloy background has significantly higher lightness (average lightness 83.81) and chroma (average chroma 39.21) than tourmaline red, with obvious orange hue (average hue angle 74.48). Therefore, color metal alloy background significantly enhanced the lightness and chroma of tourmaline red, thus greatly contributing to quality evaluation of tourmaline red. However, color metal alloy background made orange hue obvious, namely strengthening mixed hue of non-mainstream red, which was not conducive to quality evaluation of tourmaline red. Therefore, color metal alloy background was only suitable for inlaid jewelry material or gemstone display rather than quality evaluation.