Raman spectroscopy: identification of azo pigments. (a, b) Optical micrographs of patient 6's biopsy at two different magnifications [(a) 10× and (b) 100×] revealing the presence of micrometre-size clusters, mixture of PR2 and TiO2 (inset: magnification of a white anatase pigment cluster). (c) Corresponding Raman spectra (without correction of the luminescence background). Reference spectra from the Soprano spectral library are also given as comparison (PR2, C.I. 12310, Monoazopigment, Naphthol AS) and anatase (TiO2) downloaded from the RRUFF spectral library (ID R110109, 780 nm). (d, e) Optical micrographs of patients 2 (d) and 1 (e) revealing the presence of micrometre-size red pigments PR170. (f) Corresponding Raman spectra (excitation wavelength λexc = 785 nm, objective 100×, numerical aperture = 0.9). Both spectra are baseline corrected. Reference spectra from the Soprano spectral library are also given as comparison (PR170 C.I. 12475, Monoazopigment, Naphthol AS).