High-resolution multispectral scanning for mesoscopic investigation of discoloration of traditional Japanese pigments

Abstract

This study describes a non-invasive analytical imaging scanning technique utilizing multispectral images to study discoloration and degradation of pigments used in traditional Japanese artworks. The images have high spatial resolution which can achieve mesoscopic resolution (typically 0.1mm-10mm). Since the images are being scanned line by line instead of being recorded frame by frame, this enables accurate color and spectral recording of the material response from visible and near infrared irradiation. The multispectral images were used to reconstruct color information and spectral reflectance. The mathematical model is based on the Moore-Penrose pseuodoinverse. Using mesoscopically resolvable images, it is possible to measure the spectral reflectance of pigments ranging from μm-mm ROI unlike conventional spectrometers that requires big sampling area. The significance of mesoscopically-resolved analysis is demonstrated by investigating the discoloration and degradation of natural and artificial Japanese pigments. The pigments were heated in air at 300ºC and sampled every 10 minutes. It was observed that the pigments discolored at seemingly random clusters. The reconstruction of the spectral reflectance at different sizes of ROI reveals strong correlation with background reflection. The size of the initial discoloration sites makes it impossible to measure using conventional spectrometers. It was observed that by using sub-mm ROI, it is possible to observe reflection and absorption patterns in the pigments which does not register with mm-scale ROI. The results have shown that mesoscopically resolvable multispectral images can be used effectively to study degradation and discoloration in pigments.