Figure 2
From: New insights into the origin of remote PPG signals in visible light and infrared
Skin represented as multiple stacked horizontal layers for (a) normal conditions and (b) for compression. The normal skin is modeled as epidermis (EPI); capillary loops (CL); upper plexus (UP); reticular dermis (RD); deep plexus (DP); and subcutis (SC). The compressed skin has reduced blood content and allows the light to penetrate deeper into the skin. On a contact-based setup, compression is achieved by loading the finger pad against the probe. The figure was built by A. Moço upon the skin layers of Madhero88, via Wikimedia Commons (en.wikipedia.org/wiki/File:Skin_layers.png, CC BY-SA 3.0).
