RIS analysis of Ukiyo-e prints

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Overview

Reflected Image Spectroscopy (RIS) was carried out with a visible/near infrared hyperspectral camera, which acquires completer spectra (in the same range as the FORS equipment, about 400-1000 nm) for every pixel of the instrument's sensor. The pixel sizes vary depending on lens and distance between camera and print, but is usually well under 100 microns. The spectral Angle Mapping (SAM) algorithm can be used to map areas of a print that display identical or nearly identical spectra, thus potentially showing the distribution of specific colorants or even simple mixtures of colorant (such as a red and blue colorant used create a purple color)

provides a reflectance map across a selected region, which could include an entire print. The RIS data consists of individual spectra recorded at adjoining points that are usually well under 0.1 mm in diameter. The instrument used here is a Nerios HERA visible/near infrared hyperspectral camera (measuring in the 400-1000 nm range). RIS data are typically evaluated by highlighting areas of the object that was scanned which show identical or very close to identical reflectance spectra. Thus, the data can be a means of indicating where a specific colorant is present over an entire print. For example, the images below show where different colors are distributed in one print.

Each individual spectrum from a RSI scan can be viewed. One of these or a group of adjoining spectra from a print can be selected as a “reference” spectrum. An algorithm (Spectral Angle Mapper, or SAM) is used to examine all spectra in the scan and determine which ones most closely match the reference spectrum. With appropriate further data processing, the map of the results will accurately show areas of the print that contain the same colorant (or mixture of colorants) as the “reference” spectrum (or group of spectra). The specific colorant or mixture of colorants in the “reference” spectrum or spectra are identified by comparison with spectra from reference materials or from data obtained by other analyses (such as EEM). The value of this technique is that it can indicate where a specific colorant, which may have only been identified or hypothesized in one small area by one of the point analysis techniques, is present within the entire print. The points analyzed by XRF, FORS and EEM spectroscopy on a print are chosen to include all distinct colorants that appear by close visual examination to be present on the print, but the points may not always have included all colorants, and the points do not show the distribution of a colorant over the entire print.

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