X-radiography of art objects uses the same technology as medical x-rays. Just as we can differentiate dense bone from flesh in an x-ray, art materials with different densities will appear light or dark in the x-radiograph.
For example, a charcoal underdrawing would not appear in the image, but a white paint that contains lead would block the x-rays, leaving a white spot on the final x-radiograph. This technique can explain changes made by the artist in the creation of the work, as well as methods of manufacture, the location of damages that may not be visible, and previous restoration campaigns.
Of these imaging techniques, x-radiography was the most informative for Portrait of a Man.Next: Reading the X-radiograph