Optical Limits in Skin Reflectance Measurement: Quantifying Melanin-Dependent Constraints on Erythema Detection

ABSTRACT 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. We conclude with guidance on analytical methods, sampling strategies, and extended wavelengths to improve measurement validity across the full range of human skin pigmentation. Graphical Abstract Objectives: kin measurements are not inherently unbiased: melanin’s broadband absorption progressively masks hemoglobin’s spectral signature, reducing the sensitivity of visible-range erythema detection in darker skin. Summary Points Visible-range skin reflectance measurements capture real optical information but do not directly measure chromophore concentrations; interpreting these signals requires understanding light-tissue interactions. Melanin and hemoglobin absorb at overlapping wavelengths, creating optical entanglement that limits the separability of pigmentation and erythema signals from reflectance data. Hemoglobin spectral signatures are systematically attenuated as melanin increases, reducing the sensitivity of erythema detection in darker skin. This represents objective bias arising from physics, not subjective bias in assessment. Derivative analysis of full reflectance spectra can reveal hemoglobin-related structure that persists in darker skin even when standard indices show no signal. Melanin index distributions should be reported to assess sample representativeness, as demographic diversity does not guarantee adequate coverage of the pigmentation range relevant to optical measurement. Limitations Spectral overlap between melanin and hemoglobin is a physical property of these chromophores; no visible-range instrument or algorithm can fully overcome this constraint. These optical limits apply to any visible-range sensor, including spectrophotometers, clinical photography, and AI systems trained on photographic data. Existing datasets remain sparse at the high end of the melanin index distribution, limiting validation of measurement approaches in the darkest skin.