Table 1 Summary on non-invasive optical methods used for quantification, imaging and structural characterization of melanin in the skin

Single-photon excited NIR fluorescence

Single-photon excited NIR fluorescence of melanin

Bulk and depths resolved measurements of melanin

^{15,16,17,18,19,20}

Diffuse reflectance spectroscopy

Broadband absorption of melanin in visible and NIR spectral range

Assessment of bulk concentration of cutaneous melanin with ≈ 1 mm resolution

¹⁸

Two-photon excited fluorescence lifetime imaging (FLIM)

Short fluorescence lifetime (< 0.2 ns) of melanin and quasi-selective excitation at ca. 800 nm

Imaging of melanin in the basal layer with submicron resolution

^25,26,27

Confocal laser-scanning microscopy

Enhanced elastic scattering on melanosomes

Imaging of melanin in the basal layer with submicron resolution

^32,33,34

Pump-probe microscopy

Time-resolved excited-state absorption and ground state bleaching of melanin

Imaging of melanin subtypes (i.e. eumelanin and pheomelanin), differentiation between aggregation modes of melanin oligomers, evaluation of malignancy

^{24,28,29,30,31}

Optical coherence tomography

Enhanced elastic scattering on melanosomes

Assessment of melanin near the basal layer with ≈ 10 µm resolution

^35,36

Optoacoustics

Broadband absorption of melanin in visible and NIR range

Bulk measurements and depth-resolved localization with ≈ 10 µm resolution

^37,38,39

Raman spectroscopy

Features of Raman scattering spectrum of melanin, proteins and lipids in 1,000–1,800 cm⁻¹ region

Bulk quantification of melanin and depth-resolved imaging (submicron resolution) of melanin in vivo. Assessment of biochemical properties of melanin in vitro. Evaluation of malignancy

^{40,41,42,43,44}