X-rays are a form of electromagnetic radiation and are shorter in wavelength than ultraviolet radiation. This non-destructive technique is commonly used for technical examination of paintings in order to assess their condition and learn about the original materials and techniques used by artists.  

X-rays can pass through paint layers and supports quite easily but the level of penetration depends on the physical properties of the material being x-rayed like its density and thickness. For instance the x-rays penetrate the low-density materials (canvas support) relatively easily and are absorbed by higher-density materials (pigments containing heavy metals like lead white or minium). Thus when reading an X-ray the denser areas of the artwork show up lighter or even white. 

X-radiography is very useful for detecting the composition changes (pentimenti), hidden paintings and over-painted areas. This examination technique can reveal the information about the composition and condition of the painting canvases, panels, wooden sculptures, the location, extent and nature of damages like tears, holes, internal cracks, pest infestation and also old repairs like inlays, fabric patches, linings and fills. 

Information collected by X-ray examination is extremely valuable to conservators as it helps to determine the conservation issues of the object and subsequent correct conservation approach.  The information revealed by this type of examination can also assist art historians in the interpretation of the art work and more specific dating.

The examination of artworks using X-rays must be done at the laboratory level and can be recorded with radiography techniques, which rely on the passing of X-rays through the object and recording the resulting image on radiographic film.

Baptism of Jesus. Panel tempera painting. 1685. Poland. X-radiograph shows changes of design that an artist has made. The lead white areas of the painting are clearly visible.Baptism of Jesus. Panel tempera painting. 1685. Poland. The painting was photographed in diffused light.