2.2. RRS-Based Measurement
To evaluate SCL, a peak reflecting various types of carotenoids was used. As a reference, RRS has been widely used in previous studies because the size of the peak measured at the thumb was highly correlated with the SCL. To minimize the effect on other components of the skin, the right thumb, which has a low melanin concentration and a high carotenoid concentration due to its thick stratum corneum, was selected as the measurement site. The details of the peak size calculation using Raman spectra are described in a previous study [At each visit, the participants were instructed to conduct three trials by placing their right thumb in contact with the device’s glass window for measurement. For each trial, measurement with an exposure time of 400 ms was accumulated for 50 times. The average of the measured values was used for the analyses.