The iPPG system is schematically presented in Fig. 1(a). A monochrome CMOS camera (model: EoSens, MC 1363-63, Mikrotron GmbH, Unterschleissheim, Germany) with a spectral range of 500 to 1000 nm was focused on the palm of the participant’s hand using a standard F-mount lens (model: Nikkor 20 mm, , Nikon, Japan). Two commercial infrared (, , 10 W) light sources (model: ABUS TV 6818, ABUS Security-Centre GmbH, German) were mounted on either side of the camera to provide a uniform illumination of the target area: the palm of the left hand. Images were captured for 4 min at a rate of 200 fps and an exposure time of 4 ms, producing a raw image size of . The pixels were encoded in 10-bit gray scale, which allowed the camera to detect the weak pulsations of the microvascular tissue bed. During the image recording, a reference contact PPG signal was obtained simultaneously from a reference LED-PPG circuit, as shown in Fig. 1(b). Specifically, the reference PPG signal was initially obtained by means of a standard commercial contact PPG sensor (model: SA30014C, Shanghai Berry Electronic Tech. Co. Ltd., China) placed on the middle finger of the left hand and a DISCO4 data acquisition system (Dialog Devices, UK). This plethysmographic signal was then transferred to a custom-written LabVIEW (National Instruments Co., USA) software platform and served as the power intensity signal to trigger a red LED (, , RS Components, UK) through a DAQ (model: USB-6008, National Instruments Co., USA). The red LED probe was attached to the target area with double-sided adhesive tape during image acquisition, providing a clear and reliable reference signal for easy comparison with iPPG signals during postprocessing.