To assess the efficacy of electrochromic enhancement, prints as-deposited and subsequent to enhancement were graded according to a five-point scale devised by Bandey17 (see the
Supporting Information). A total of 50 samples were prepared using variations in protocol (different sweat inducement times, donors, ambient conditions and deposition pressure) and aging
times (2 h to 39 days). While not from real crime scenes, this is a plausible sample set, with a range of initial print quality spanning the grading scale. Each print was regraded after optimal electrochromic enhancement. Since the darkest polymer state is accessed at 0.9 V, samples were assessed in situ commencing at this applied potential and then at 0.1 V decrements down to -0.2 V. This has the practically important attribute of exploiting maximum optical density (at 0.9 V) for underfilled trenches or lower optical density (at E < 0.9 V) for fully- or overfilled trenches. The outcome (see the Supporting Information, Figure S1) for the sample set used
was a shift of the mode from grade 2 (evidentially not useful) to grade 3 (evidentially useful) and an increase in the fraction of
useable fingerprints by 40%.