Revealing the colours of ukiyo-e prints by short wave infrared range hyperspectral imaging (SWIR)

Japanese woodblock prints, also known as ukiyo-e prints, were mainly produced during the 18th and the 19th centuries. Japanese printers, colours-skilled craftsmen, used a palette composed of traditional pigments (indigo, red safflower or orpiment for instance). This palette was enriched during the 19th century with new pigments such as Prussian blue or, later, with aniline red and violet. Ukiyo-e generally display a vivid and varied hues of colours, obtained from pure colourants or mixtures of pigments. In order to reveal the colouring materials used by Japanese printers, hyperspectral imaging, in both visible and near-infrared ranges (400-1000 nm and 1000-2500 nm) was performed on a triptych designed by Kunisada/Tokugawa III, edited in 1855. This print, composed of three distinct panels, offers a dozen hues from light red to a deep violet. The aim of this work is to assess the information obtained by the hyperspectral camera in the near-infrared range and reinforce data acquired in the visible range. Indeed, the visible spectra are generally useful for the identification of pure colours but are less relevant in the case of mixtures or for some pigments, as for example Prussian blue. Usually used for the identification of binders, near-infrared spectra are also of interest for the colours and may be useful to support and complete hyperspectral data in the visible range, such as Prussian blue mapping.