An article on BBC radio a few days ago stated Davinci used a lot of experimental techniques and types of paint when he created the picture and many of them failed to last very long. The image is very fragile and has been placed in a enviromentaly contolled container. The problem is super fine dust is getting in and it threatens to form a film on the picture.  Because of the efforts being made to conserve the picture the ability to study it has been greatly curtailed, so this image was created.