Non-destructive inspection of The Night Watch with shearography: how good is the adhesion between the original canvas and the lining canvas?
The Night Watch is now being investigated with a technique called shearography (speckle pattern shearing interferometry). This is an optical inspection technique that is used to identify hidden defects and damage in various materials. Shearography is mostly used in aerospace to inspect composite materials and structures, for instance in parts of aeroplanes. For the anticipated treatment of The Night Watch, the main question is whether the adhesion between the original canvas and the lining canvas that was applied in 1975, is sufficient.
Research techniques
This research is part of the Operation Night Watch, which is the largest research and conservation project ever for Rembrandt’s masterpiece. Diverse research techniques have been used already to reveal information about the making process and condition of the painting. Shearography will reveal information about the internal integrity of the canvas support.
TU Delft
This shearography instrument was developed at the Aerospace Engineering Faculty of the Delft University of Technology. In the aerospace industry, this unique instrument is used for example to locate hidden damages in composite structures. To assess whether the adhesion of The Night Watch canvasses is still good, the painting is slightly heated with white halogen lamps by 1 or 2°C. As the painting cools down, the shearography instrument continuously captures images with the scattered laser light. These images, called interferograms, give micro-displacements between materials during the cooling phase and will thus show if air is present between the two canvasses. This would mean that the adhesion is not good.
Safe
The heating of the painting by 1 or 2°C is within safe limits and is comparable with the daily variations of the temperature in the gallery. It is safe for the painting and therefore this inspection is called ‘non-destructive’. During the procedure we constantly measure the surface temperature of the painting with an infrared camera to control the process.
The scattered green laser light from this instrument that you may observe is safe for visitors. Laser safety goggles are not needed due to the low intensity of the laser light (comparable with scattered light from a laser pointer).
