Soft ultraviolet illumination (also known as black light) is a very convenient method for finding those regions of surface which have been altered. In particular, it is known that older varnish exhibit solid green glow when exposed to black light while more fresh material appears to be more dark. Thus, utilizing conventional black light units one can perform a quick analysis of the varnish condition.
Finding these retouched regions can be used for performing selective spectroscopic analysis. By comparing the materials found in both retouched regions and those which have not been altered one can differentiate between the materials used by the painter and those utilized by the restorer.