2.3. RS-Based Measurement
For the RS-based measurement, a commercial spectrophotometer (CM−700d, Konica Minolta, Inc., Tokyo, Japan) composed of an integrating sphere was used (At each visit, participants were instructed to measure 10 trials that consisted of three repetitions of strong pressure, three repetitions of weak pressure, and four repetitions of arbitrary pressure using their right thumb. For each trial, fewer than 5 s were taken from the measurement click to the data acquisition. Because the pressure between the thumb and glass on the measurement site was negatively correlated with the blood volume, high pressure enables blood to diverge from the measurement site to minimize the noise due to the hemoglobin. However, the quantitative control of pressure is not useful because the relationship between physical pressure and the blood clearance may vary among individuals. Accordingly, HbI was monitored instead of pressure to distinguish between trials of strong and weak pressure. The vicinity of minimum and maximum HbI values indicate the conditions of strong and weak pressure, respectively. The maximum and minimum HbI values measured at each participant’s finger are clear. This study’s protocol was specifically designed to obtain data across a wide range of hemoglobin levels. On behalf of the HbI provided in the software, the absorbance at 570 nm (=The wavelength range of the carotenoid spectrum is 400–510 nm, but in previous studies [The peak size of Equation (1) is negatively correlated with HbI (