X-radiograph:  
The x-radiograph is the image produced from x-radiography, using x-rays to penetrate all the layers in a painting structure. The degree to which the x-rays penetrate depends on the density of the materials. For example, lead white paint, which is a metal-containing pigment, will appear white on the x-radiograph. The use of x-radiographs provides conservators and art historians with information about the sequence of paint layers and the construction of a painting. In addition, artists' changes and damages to a painting's structure may become visible that are not recognizable by the naked eye.