## 2 Materials and Methods  
Figure 1A shows the layout of the experimental setup consisted of an illumination module and a data recording system. The illumination module (Figure 1B) included eight light-emitting diodes (LEDs) operating at a wavelength of 525 nm with a power of 3 W for each LED. The data recording system contained three detectors. First, a digital monochrome CMOS camera (GigE uEye UI-5220 SE of  
the Imaging Development Systems GmbH, Obersulm, Germany) for recording a sequence of subject's face images with a size of 752 x 480 pixels at a frame rate of 39 frames per second. Second, a digital electrocardiogram (EKG) (KAP-01-\"Kardiotekhnica-EKG\" of the Incart Ltd., Saint Petersburg, Russia) for recording electrocardiograms (ECG) at the sampling rate of 1 kHz in two leads. Third, an electronic USB-thermometer (RODOS-5 of the Olympic Ltd., Zelenograd, Moscow, Russia) with three semiconductor temperature sensors (DS18B20 of the Maxim Integrated, San Jose, CA, USA) for the monitoring of skin temperature in the forehead, cheek and laboratory room. The subjects' face illumination and recording of video data were done through cross-polarized polarization films to minimize motion artefacts [13]. Synchronization of the digital camera and ECG was provided with an accuracy of 1.0 ms. Temperature data were recorded every 2 s synchronously with the recording of video frames. A detailed description of the experimental setup is given in our recent paper [14]. Video frames, ECG and temperature data were downloaded to the computer's hard drive for subsequent offline processing. A distinctive feature of our imaging photoplethysmography (IPPG) system would be the video recordings of the subject's skin under green illumination. We used this illumination because the greatest light modulation resulting from the heart rate was observed at this wavelength [15; 16], despite its small penetration depth due to strong absorption by red blood cells [17]. Green light does not interact directly with pulsatile arteries and arterioles, but pulsatile arterial pressure mechanically deforms the upper layer of the capillary bed, thus modulating absorption and scattering coefficients of the light, according to the recently proposed model of PPG waveform formation [18]. Therefore, parameters of the microcirculation in the upper layer of the dermis were assessed in our experiments. The experiment was conducted according to the following protocol. The subject was in the supine position while the video camera was mounted above his or her head. After 15 min of relaxation time for the subject, their baseline data (video, ECG and skin temperature) were recorded for about 5 min. Then a warm compress was applied to their forehead. The compress was a 3 by 4 cm sealed bag made of transparent plastic, filled with water, and preheated in a thermostat to a temperature of 38-39 \({}^{\circ}\)C. The compress was applied without interrupting the video recording, ECG and temperature. The heated compress was on the subject's forehead for about 6 min, and then it was removed, again without interrupting the data recording. Thereafter, we recorded the whole set of data for more than 5 min to evaluate the relaxation processes. Air temperature in the laboratory room during the whole experiment was kept at the level of 23 \(\pm\) 1 \({}^{\circ}\)C.  
Recorded video, ECG and temperature data were processed by using custom software implemented in the MATLAB(r) platform. The algorithm of PPG signal processing was described in detail in our previous papers [14; 19]. Briefly, we calculated the spatial distribution of the blood pulsation amplitude (BPA) over the facial area. To this end, in one of recorded images, we manually selected an area for analysis containing the forehead and cheeks. This area was divided into a network of small regions of  
Figure 1: Schematic view of the experimental setup (**A**) and photograph of the illuminating module including video camera (**B**).