Abstract
Hyperspectral technology has been used to identify pigments that adhere to the surfaces of polychrome artifacts. However, the colors are often produced by the mixing of pigments, which requires that the spectral characteristics of the pigment mixtures be considered before pigment unmixing is conducted. Therefore, we proposed an experimental approach to investigate the nonlinear degree of spectral reflectance, using several mixing models, and to evaluate their performances in the study of typical mineral pigments. First, five mineral pigments of azurite, malachite, cinnabar, orpiment, and calcite were selected to form five groups of samples, according to their different mass ratios. Second, a fully constrained least squares algorithm based on the linear model and three algorithms based on the nonlinear model were employed to calculate the proportion of each pigment in the mixtures. We evaluated the abundance accuracy as well as the similarity between the measured and reconstructed spectra produced by those mixing models. Third, we conducted pigment unmixing on a Chinese painting to verify the applicability of the nonlinear model. Fourth, continuum removal was also introduced to test the nonlinearity of mineral pigment mixing. Finally, the results indicated that the spectral mixing of different mineral pigments was more in line with the nonlinear mixing model. The spectral nonlinearity of mixed pigments was higher near to the wavelength corresponding to their colors. Meanwhile, the nonlinearity increased with the wavelength increases in the shortwave infrared bands.
Highlights
The use of pigment analysis to determine the type and proportion of pigments on the surface of artifacts has become a research focus
We assume that the types of pigment endmember and their corresponding spectra are known, and we use different unmixing algorithms to perform abundance inversion
0.0467 In Table 8, the results indicated that the similarity between the reconstructed and measured spectra obtained by the linear model was poorer than that obtained by the nonlinear models in terms of the mean reconstruction error (RE) and the mean spectral angle distance (SAD)
Summary
The use of pigment analysis to determine the type and proportion of pigments on the surface of artifacts has become a research focus. Mineral pigments were commonly used in polychrome cultural relics with relatively stable properties [1]. Some mineral pigments, such as malachite and azurite, exhibit unique absorption specificities that can be used to identify them [2]. The different colors on the surface of polychrome artifacts are created by pigments being mixed together or layered on top of one another. It is hard to recognize the types of pigment using only the human eye. The identification of the pigments used on the surfaces of artifacts needs modern techniques
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