Figure 1.

(a) Reflectance spectra from a variety of human skin (data from NCSU spectral database), and from one male primate, namely Mandrillus sphinx (Sumner & Mollon 2003). Also shown here and in (b) are the maximal sensitivities for the M and L cones for routine trichromats. (b) Absorption spectrum for oxygenated and deoxygenated haemoglobin (from Scott Prahl, Oregon Medical Laser Center, http://omlc.ogi.edu). (c) Blood space for skin spectral modulation, showing the two principle variables that affect skin colour in the short term: haemoglobin oxygen saturation (x-axis), and haemoglobin skin concentration (y-axis). ‘High’, ‘baseline’ and ‘low’ values for these two variables were chosen, and the figure shows the nine skin spectra for all pairs of these parameter settings. Colours code the approximate direction of colour shift from baseline (centre). (d) Relative change from baseline for L−M and S−(L+M) for the nine model skin spectra varying over blood space from (c). Shown now are the filtered skin spectra actually reaching the retina. (e) Example skin colour modulations from modulations of blood variables. Data points show positions in this colour space for RGB values of skin under these conditions, along with the average values. See electronic supplementary material for the extended legend for this figure.