Dual-Indicator Imaging Characterization of Luster in Chinese Freshwater Nucleated Pearls: Surface Luminance and Interference Color

Abstract

Pearl luster is commonly assessed by expert visual grading, which is subjective and may show limited interlaboratory reproducibility. We developed a dual-indicator imaging method for Chinese freshwater nucleated pearls using surface luminance and interference color. Ninety pearls visually classified as weak-, medium-, or strong-luster were measured by multi-view luminance imaging and interference-color imaging. A background-corrected brightness index (GL) was derived from peak luminance, background luminance, and full width at half maximum, while an interference color index (IGI) was calculated from standardized hue and chroma dispersion in the upper interference-color region. GL showed a strong association with expert luster grade, whereas IGI also exhibited a significant positive relationship and provided complementary information for samples that were difficult to classify from GL alone. In leave-one-out cross-validation, combining GL and IGI improved classification accuracy from 73.3% for GL alone to 78.9%, and inclusion of a GL×IGI interaction further increased accuracy to 80.0%. These results indicate that surface-reflection intensity is the primary determinant of perceived pearl luster, while interference-color variation provides secondary diagnostic information. The proposed dual-indicator framework may therefore serve as an objective complement to conventional expert grading.