The most basic part of painting examination is the comparative evaluation of photographic results taken in different light spectra. These images are taken under normal, infrared and ultraviolet light and they help to reveal traces of previous retouch, varnish, under- or overpainting and posterior signatures.
Normal (under normal light) photograph (350-750 nm)
The photograph of details visible to the naked eye and of the entire painting is shot with flashlights in circumstances of a photographic studio.
UV (ultraviolet luminescent) photograph (below 350 nm)
It shows the layers of the painting which have been added to the surface afterwards: restored areas, retouched details, restored varnish and fake signatures become visible. (E.g. while old varnish appears in UV spectrum as green, newer varnish might luminesce in an orange colour.)
IR (infrared reflectography) photograph (750-950 nm)
IR reflectography shows what is hidden beneath the visible layer in a light spectra which is invisible to the naked eye: underdrawings, sketches, details of darkened areas or even non-visible signatures.