X-ray machines are used to examine paintings in a similar way they are used to examine human bodies.
As with ultraviolet and infrared light, X-rays are a form of light invisible to human eyes. X-rays pass straight through some materials, but are reflected or absorbed by others. In the physician’s office, the X-ray machine shoots X-rays at the patient and has X-ray sensitive photographic film on the other the other side of the patient (Duly note that ‘film’ is an old fashioned term and technology as even X-ray machines have hit the digital age) . The X-rays pass through the patient’s skin and flesh and go to the film, but are absorbed by the bones. The result is the X-ray photograph shows the inside bones, allowing the doctor to examine the inside of the body.
Art historians get a similar insides look at paintings as X-rays go through some pants but are absorbed or reflected by others.
This helps the historian in two ways. First, the identification of certain materials can date the paint and painting. Second, it often shows what was painted underneath the first level graphics we see with our naked eyes. Famous artists are known to have had standard ways and personal styles in how they constructed their paintings, which helps the historian in judging the authenticity, and some paintings started out as dramatically different designs. X-rays have shown that Picasso’s famous ‘The Old Guitar Player’ started off as an old woman instead of an old man, and an El Greco portrait started as a still life.