According to earlier skin reflectance modeling, skin reflectance spectra were calculated for skin with the most common properties: Melanin (volume fraction 2% in Epidermis); hemoglobin HbO2 and Hb (150 g Hb/liter ) and Thickness of collagen tissue in papillary dermis about 0.1mm.
What are the parts of this spectrum? This spectrum can be exploded into several distinct spectra: Epidermis, Hemoglobin; there are also spectra of water and collagen substances, but they don’t play the leading role in our wavelength interval.
As you can see, melanin in the epidermis absorbs most of blue light ~470nm; hemoglobin absorbs green light ~252nm, and red light ~640nm, and infrared 850 are used to define papillary dermis thickness because these wavelengths are less absorbed by melanin and hemoglobin.