Use of infrared hyperspectral imaging (960–1680 nm) and low energy x-radiography to visualize watermarks

This paper proposes the use of near infrared (900 to 1700 nm) transmitted light imaging with a hyperspectral camera to obtain watermarks from prints. Specifically, we show that principal component analysis applied to the hyperspectral image cube collected in the near infrared was able to separate the watermark from text printed in carbon black ink on both sides of a page from the Blaue Atlas Maior of 1662. The resulting principal component image of the watermark was compared with an image obtained using a low-energy x-ray source and a phosphor plate. Low-energy x-radiography is becoming the gold standard for imaging watermarks, replacing beta radiography. The watermark obtained by transmitted near infrared hyperspectral imaging was found to possess many of the key features of the watermark revealed by the phosphor plate radiography. The method proposed here offers an additional way to extract watermarks from works of art on paper.