Experimental setup for PPG measurements
We developed a setup for high-magnification and high-resolution imaging of the forearm area where ICTs were performed (Figure 2 ). To achieve a uniform glare-free illumination, we installed two 5 W LED-based light sources with green (central wavelength λ = 520 nm, FWHM 30 nm) illumination that corresponded to the hemoglobin absorption, thus enabling the high contrast of blood vessels against the blood-free imaging field. Our imaging system included a long-working-distance (70 mm) microscope tube with continuously adjustable 1.5–2.5× zoom and a monochrome CMOS camera (IDS uEye UI-3060CP-M-GL Rev.2, 1/1.2”, 1936×1216 pixels, frame rate up to 166 fps). For uniformly sharp imaging, we flattened the skin surface with a 2.5 mm thick glass plate located on a separate stand. To minimize residual distortions in the image edges, we limited the resolution to 1000×1000 pixels. Thus, our setup allowed imaging of skin areas in the range of 1.5×1.5 mm2 to 2.5×2.5 mm2with resolution high enough to detect single capillaries of 5–10 µm in diameter.