Thanks to the discovery of infrared radiation, it became possible to photograph what seemed invisible, but the results of this analysis can only be perceived by the human eye with the help of a photographic plate. Infrared rays allow you to detect the previously invisible state of the work of art, absorbing or reflecting the color matter that makes up the picture. The photograph reveals to us an invisible inscription, a drawing, an unfinished stage of work. However, the results are unpredictable, and decoding the image obtained in the photo is often very complex and difficult. Nevertheless, it becomes possible to read the inscriptions, sometimes located on the reverse side of the picture. In addition, infrared radiation makes it easier to determine the nature of the pigment, complementing the results of observations made under a microscope or physico-chemical method. Based on the Encyclopedia of  art ArtWiki