Infrared (IR) reflectometry is used to show the possible underlying drawing or signature of an artwork. It is based on the property that has IR light to go through the different paint layers. If we illuminate the artwork with white light (broad range of wavelengths) then only the infrared components of this light will reach the underlying drawing. This one that is made with charcoal will absorb this type of radiation. Thus, using a camera sensitive to IR, as the one we have in our laboratory (1.2 mm), we can show the underlying drawing.
We can see the underlying drawing (and eventual "pentimenti" of the author) by IR reflectometry.