Technical aspects and data processing of reflectance imaging spectroscopy for Operation Night Watch

Reflectance imaging spectroscopy (RIS) from visible to short-wave infrared (400 to 2500 nm), also known as hyperspectral imaging, is becoming another more widely used technique for the non-invasive study of polychrome art objects such as paintings. RIS is one of the many non- invasive imaging techniques used during “Operation Night Watch”. “Operation Night Watch” is the largest research project ever started by the Rijksmuseum in Amsterdam, and has the goal to investigate the conservation history, current condition, painting technique and artists’ materials of the Rembrandt iconic masterpiece Night Watch (1642) a canvas painting 3.78 m by 4.53 m in size. Due to the large size of the painting and its historical importance, the research is realized inside of a glass chamber that had been built around the painting in the “Gallery of Honor” of the Rijksmuseum, in front of the visiting public. RIS on the Night Watch was performed using two different hyperspectral line scanning imaging spectrometers (VNIR 400 to 1000 nm, 2.5 nm and SWIR 900 to 2500 nm, 6 nm). The complete painting was being scanned at a spatial resolution of 0.2 mm. The cameras including their lighting systems are mounted a on novel 2-D scanning platform, which includes auto focus systems that kept the line-scanning hyperspectral cameras in focus during scanning. The RIS results combined with the other non-invasive imaging techniques provide an in-depth understanding of Rembrandt’s painting technique and the actual condition of the painting and will serve as a foundation for the conservation treatment proposal for the Night Watch.