Abstract
Assessment of skin color by colorimetry can objectively measure skin-disease progression and help ensure that clinical trials recruit diverse populations. Colorimeters report color in the three-dimensional L*a*b* space (L* = lightness/darkness, a* = green/redness, b* = blue/yellowness). Empirical studies have shown that (1) human skin L*, a*, b* values cluster in a banana-shaped region of this space and (2) the angle between the point (L* = 50, b* = 0) and an individual’s (L*, b*) coordinates—the Individual Typology Angle (ITA)—can act as a melanin surrogate.
We sought to establish a causal link between cutaneous chromophore concentrations and L*a*b* values. Using a three-layer skin model and the adding-doubling method, we varied melanosome fraction (Mfrac, 0–43 %) and blood-volume fraction (Bfrac, 0.2–7 %) to generate diffuse-reflectance spectra that were subsequently converted to L*a*b*. At constant Mfrac, varying Bfrac produced distinct, non-overlapping iso-melanin curves that reproduced the empirical banana and its boundaries in the L*–b* plane. These curves furnish a theoretical basis for the ITA–melanin relationship while revealing that ITA is unreliable when Bfrac is low.
The results show how simple colorimeter readings can provide quantitative insight into underlying melanin and hemoglobin, offering a path toward improved non-invasive assessment of skin chromophores.