Fig. 2

Varnish removal imaged by UV, visible light, and average fluorescence lifetime contrast. Four different varnish removal methods were used on the same painting: A swab with no mechanical action (Swab NMA), B swab with mechanical action (Swab MA), C gel with no mechanical action (Gel NMA), and D gel with mechanical action (Gel MA). In the left column are photos under UV illumination, in the middle column under white light illumination, and in the right column are the intensity-weighted average fluorescence lifetime images. The fluorescence lifetime range is 1.4 ns to 3.0 ns (false-colour scale in A) and the scale bars 5 mm (A) apply to (A−D)