[HOF Auto Rookies]

I have seen some LOA's from JSA when the autograph for example on a baseball is extremely faint, so they took normals pictures of the signatures, then it looked like they also had what almost looked like a negative image where you then could see those "hidden" autographs. What equipment is used to provide those images? And I believe there are some where it can trace the image for verification purposes if it is faint as well.

Thanks