Abstract

In this study, we present a quantitative characterization of skin pigmentation through the analysis of 1005 diffuse reflectance spectra associated with Fitzpatrick skin phototypes (I–VI) of Hispanic individuals. We employed complementary metrics, including the individual typology angle (ITA), the melanosome volume fraction (fme), the structural similarity index measure (SSIM), and the percentage of spectral overlap. Our findings revealed that the average ITA values decreased progressively from (51.76 ± 6.40)◦ in phototype I to (−33.63 ± 15.91)◦ in phototype VI, confirming the inverse correlation between the typological angle and the degree of skin pigmentation. The volumetric fraction of melanosomes (fme) – derived from a quadratic relationship with the chromaticity coordinate (L∗) and exhibiting a RMSE of 0.0016 – ranged from (1.85 ± 0.46)% in phototype I to (10.83 ± 2.19)% phototype VI. These results provide a quantitative estimate of intra-phototype variability, which becomes more pronounced in darker skin tones. Furthermore, SSIM values (0.42 to 0.94) and spectral overlap (0.00% to 52.05%) confirmed high structural similarity between adjacent groups (especially between phototypes IV and V) and total divergence between extreme phototypes (I vs VI). Notably, intermediate SSIM values (0.74–0.86) were observed between second-adjacent phototypes (e.g. III vs V), suggesting a gradual spectral continuity across the scale. Finally, the total melanin content (µg mg−1) was determined from fme, achieving a correlation coefficient of ρ = 0.996, highlighting the chromatic heterogeneity and morphological stability essential for clinical and technological applications in diverse populations.