Significance: Spatial frequency domain imaging (SFDI) is a wide-field diffuse optical imaging technique for separately quantifying tissue reduced scattering (μs ' ) and absorption (μa) coefficients at multiple wavelengths, providing wide potential utility for clinical applications such as burn wound characterization and cancer detection. However, measured μs ' and μa can be confounded by absorption from melanin in patients with highly pigmented skin. This issue arises because epidermal melanin is highly absorbing for visible wavelengths and standard homogeneous light-tissue interaction models do not properly account for this complexity. Tristimulus colorimetry (which quantifies pigmentation using the L * "lightness" parameter) can provide a point of comparison between μa, μs ' , and skin pigmentation.
Aim: We systematically compare SFDI and colorimetry parameters to quantify confounding effects of pigmentation on measured skin μs ' and μa. We assess the correlation between SFDI and colorimetry parameters as a function of wavelength.
Approach: μs ' and μa from the palm and ventral forearm were measured for 15 healthy subjects with a wide range of skin pigmentation levels (Fitzpatrick types I to VI) using a Reflect RS® (Modulim, Inc., Irvine, California) SFDI instrument (eight wavelengths, 471 to 851 nm). L * was measured using a Chroma Meter CR-400 (Konica Minolta Sensing, Inc., Tokyo). Linear correlation coefficients were calculated between L * and μs ' and between L * and μa at all wavelengths.
Results: For the ventral forearm, strong linear correlations between measured L * and μs ' values were observed at shorter wavelengths (R > 0.92 at ≤659 nm), where absorption from melanin confounded the measured μs ' . These correlations were weaker for the palm (R < 0.59 at ≤659 nm), which has less melanin than the forearm. Similar relationships were observed between L * and μa.
Conclusions: We quantified the effects of epidermal melanin on skin μs ' and μa measured with SFDI. This information may help characterize and correct pigmentation-related inaccuracies in SFDI skin measurements.
Keywords: absorption coefficient; colorimeter; epidermis; melanin; pigmentation; scattering coefficient; skin; spatial frequency domain imaging.