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.