Optical limits in skin reflectance measurement: Quantifying melanin-dependent constraints on erythema detection.

As skin color measurement shifts from subjective classification to quantitative assessment, the assumption that objective measures are unbiased requires scrutiny. We provide a physics-informed framework for interpreting visible-range skin measurements, clarifying terminology, describing light-chromophore interactions, and surveying tools and output metrics. The physics of light-tissue interaction constrains what any visible-range observation can reveal: melanin and hemoglobin absorb across overlapping wavelengths, and as melanin increases, hemoglobin's signature is masked. By analyzing over 15,000 spectra from the International Skin Spectra Archive, we demonstrate that this attenuation is systematic and most severe in the darkest skin, where data are scarcest. These limits arise from the physical properties of skin's biological constituents and consequently apply to any sensor operating in the visible range. This progressive masking reduces the sensitivity of visible-range erythema detection in more highly pigmented skin, representing an objective, physics-driven constraint rather than a subjective bias. We conclude with guidance on analytical methods, sampling strategies, and extended wavelengths to improve measurement validity across the full range of human skin pigmentation.