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Cross sections

Optical coherence tomography (OCT)

Rembrandt created depth by working with height differences in the paint surface and by applying semi-transparent glaze layers in the shadows. By imaging these layers of paint very precisely, we gain insight into Rembrandt’s use of these techniques. However, these paint layers are covered with a protective transparent varnish layer. In order to visualize the layers underneath the varnish we use OCT, which is short for optical coherence tomography.

From medical sicence

OCT is a technique that is used in the hospital for eye examination. With OCT, 3D images of the retina are non-invasively collected to aid diagnosis of eye diseases. In addition, a great deal of research is being done to further the OCT technique within other specialties in the hospital, such as cardiology and urology. Within Operation Night Watch, the OCT technique is performed in collaboration with the department of Biomedical Engineering & Physics of the Amsterdam UMC.

Imaging with near-infrared light

OCT uses near-infrared light, which penetrates through thin and semi-transparent layers. The light is then reflected and scattered by the different layers and structures in the material, which is caused by the difference of refractive index of these layers and structures. The refractive index is a measure of the speed of light in a material and indicates the extent to which light is reflected and deflected at the interface between two materials. The back-scattered light is captured and measured using a so-called interferometer, after which 3D images are constructed by a computer. The largest advantage of OCT is the possibility to create cross-sections of the semi-transparent layers without damaging the painting. In the past, cross-sections could only be obtained by cutting out a very small piece from the painting. Therefore, OCT is a valuable addition to the other research techniques used within Operation Night Watch.

OCT images of the Night Watch

Using the OCT technique, we have collected 3D images of the impasto areas and glaze layers of the Night Watch. From these images, we have determined the paint topology and layer thicknesses with high precision. In addition, OCT was used to monitor the paint surface structure during the varnish removal tests. These results contribute to the study of Rembrandt's painting technique and determination of the restoration strategy.

3D OCT-image of the Night Watch

A 3D OCT-image of the Night Watch is shown below. This image was collected from a detail of the embroidery on the costume of Willem van Ruytenburch, the lieutenant to the right of the captain in the foreground.

  • Left: high-resolution photography of the detail on the costume of Willem van Ruytenburch. The red box indicates the location of the OCT-scan.
  • Right: 3D OCT-scan showing the topology of varnish and paint layer.
  • Below: OCT-cross section, in which we are looking into the painting. In this image, the varnish layer and the paint surface are clearly visible. The height differences, of about 0.15 mm, in the paint surface and the varnish, are clearly visible.