2.2. Light Wavelength
The interaction of light with biological tissue can be quite complex and may involve scattering, absorption and/or reflection. Anderson and Parrish examined the optical characteristics and penetration depth of light in human skin [5]; within the visible region, the dominant absorption peak corresponded to the blue region of the spectrum, followed by the green-yellow region (between 500 and 600 nm) corresponding to red blood cells. The shorter wavelengths of light are strongly absorbed by melanin. Water absorbs light in the ultraviolet and longer IR regime; however, red and near-IR light pass easily. As a result, IR wavelengths have been used as a light source in PPG sensors.