Fig. 2

PPG and its derivatives of actual human subjects. Each section was divided from the polarities of derivatives (e.g., PN mean the combination of positive 1st derivative and negative 2nd derivative). First derivative implies the velocity index of blood volume change and second derivative means acceleration of blood volume changes. PP_i, PN_i, NN_i and NP_i are based on incident wave, and PP_r, PN_r, NN_r and NP_r are occurred from reflected wave