RIS analysis of Ukiyo-e prints

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). RIS would not usually identify colorants not already detected by the other analytical techniques but may reveal areas where particular colorants are present that had not been determined by the point techniques.

One example is shown here. An averaged spectrum from a group of adjoining pixels is selected to serve as a reference, then SAM was used to map where this reference spectrum is located over the entire print ( or the mapped portion of the print). A white reference card is used to 'normalize' spectra or an area of the slightly yellowed paper can be used. Reflectance spectra from a point FORS analysis or an average RIS spectrum of the same small area from which the FORS spectrum came should be nearly identical to one another if the same 'normalization' was used.

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  Photographic image of print by Toyokuni
  MFA# 11.30331

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  RGB projection of hyperspectral data
  MFA# 11.30331

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  RIS points similar to yellow selection
  MFA# 11.30331

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  RIS points spectrally similar to blue 1 selection
  MFA# 11.30331

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  RIS points spectrally similar to blue 2 selection
  MFA# 11.30331

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  RIS points spectrally similar to pink selection (safflower)
  MFA# 11.30331

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  RIS points spectrally similar to red selection (madder)
  MFA# 11.30331

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.