To quantitatively evaluate skin chromophores over a wide region of curved skin surface, we propose an approach that suppresses the effect of the shading-derived error in the reflectance on the estimation of chromophore concentrations, without sacrificing the accuracy of that estimation. In our method, we use multiple regression analysis, assuming the absorbance spectrum as the response variable and the extinction coefficients of melanin, oxygenated hemoglobin, and deoxygenated hemoglobin as the predictor variables. The concentrations of melanin and total hemoglobin are determined from the multiple regression coefficients using compensation formulae (CF) based on the diffuse reflectance spectra derived from a Monte Carlo simulation. To suppress the shading-derived error, we investigated three different combinations of multiple regression coefficients for the CF. In vivo measurements with the forearm skin demonstrated that the proposed approach can reduce the estimation errors that are due to shading-derived errors in the reflectance. With the best combination of multiple regression coefficients, we estimated that the ratio of the error to the chromophore concentrations is about 10%. The proposed method does not require any measurements or assumptions about the shape of the subjects; this is an advantage over other studies related to the reduction of shading-derived errors.
We adapted the critical angle method for measuring rough surfaces under partial contact to acquire an in vivo skin refractive index (RI). Assuming that the total reflection is the simple sum of reflection from areas that are in contact and reflection from those that are not in contact, the RI can be estimated even for partial contact with a rough surface. We found that cheek skin is sufficiently soft that a sufficiently large area can be in contact and that the critical angle was detectable. The RIs of the cheeks of adult females were measured. The RI range was about 1.51 to 1.53, at a wavelength of 550 nm, without considering systematic errors. The RIs of cheeks are significantly correlated with their conductance, which corresponds to their water content. We determined the relationship between the RI and conductance within the variation of skin under normal conditions; this relationship was theoretically obtained in previous studies. In the present study, a direct in vivo measurement method was developed that enabled us to measure the RI in daily life, although this method contains errors for several reasons, including disregarding
We have developed a simple and effective method for everyday measurement of translucency with a handy spectral reflectometer using edge loss. Edge loss can be used to quantify the translucency index in terms of changes in reflectance under two types of measurement conditions. Here, a measurement condition represents the pairing of an illumination area and a measurement area. As a measure of the degree of lateral spread of reflected light, the translucency index can influence the appearance of human skin because this index represents eventual translucency. First, we estimated how edge loss changes when measurement conditions are varied. We then selected the combination of two measurement conditions of large and small edge loss to minimize errors. Finally, we estimated actual skin translucency changes before and after treatments comprising acetone-ether immersion and ultraviolet irradiation. The results were qualitatively consistent with the expectations under variations in absorbance and scattering capacity, indicating the effectiveness of this method in evaluating translucency. This method allows simultaneous measurement of translucency and reflectance as a spectrum, and also appears applicable for daily use, although common optical parameters cannot be derived using this method alone.
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