Figure 5.

PPG parameters used for the calculation of the ratio of ratios (R). The AC amplitude at a generic wavelength $\lambda$ is obtained from the difference between minimum and maximum absorption (squares) during the cardiac cycle. The DC component is the average light intensity. These two parameters are extracted from red and infra-red wavelengths for calculation of the ratio of ratios (R) in equation (5) (Kyriacou and Allen 2021). Reprinted from Kyriacou et al 2022, Copyright (2022), with permission from Elsevier.