Possibilities of energy-resolved X-ray radiography for the investigation of paintings

Abstract

X-ray radiographic images of paintings often show little or no contrast. In order to increase the contrast in radiographic images we measured the X-ray spectrum of a low power X-ray tube, after passing through the painting, with a high energy-resolution SDD detector. To obtain images, the detector is collimated with a 400 μm diameter pinhole and the painting was moved through the beam in the x and y-direction using a dwell time of a few seconds per pixel. The data obtained consists of a data cube of, typically, 200 × 200 pixels and a 512-channel X-ray spectrum for each pixel, spanning the energy range from 0 to 40 keV. Having the absorbance spectrum available for each pixel, we are able, a posteriori, to produce images by edge subtraction for any given element. In this way high contrast, element-specific, images can be obtained. Because of the high energy-resolution a much simpler edge subtraction algorithm can be applied. We also used principal-component imaging to obtain, in a more automated way, images with high contrast. Some of these images can easily be attributed to specific elements. It turns out that preprocessing of the spectral data is crucial for the success of the multivariate image processing.

Mobile depth profiling and sub-surface imaging techniques for historical paintings—A review

“…From the same research group Cabal et al [87] employed the same experimental set-up for elemental imaging via K-edge subtraction and contrast enhancement by Principal Component Analysis (PCA). Their general conclusion, which is in good agreement with the results presented by other groups, is that elements with an absorption edge significantly below 10 keV can only be imaged if only low amounts of stronger absorbing, heavier elements are present.…”

In situ macro X‐Ray fluorescence scanning on a Leonardo da Vinci portrait

“…For example, by performing X‐ray radiography (XRR), some pigments, such as copper, iron or manganese, are poorly visible when the preparatory layer is lead‐based, due to the high mass attenuation coefficient of lead that maximizes the contrast for lead distribution and obscures the low and medium Z elements in the pigment distributions. This effect occurs also when the painting support is made of a strongly absorbent material (as wood) [6–9] . Contrary to XRR analysis, neutron‐based imaging techniques as neutron activation autoradiography (NAAR) represent a very suitable tool for the visualization of elements such as manganese, copper and mercury, due to their susceptibility to activation.…”