Figure 2.

Schematic illustration of light propagation through the neuronal tissue. On the left side of the illustration, possible photon paths for different wavelengths are depicted (red colors represent wavelengths of λ > 800 nm (mainly absorbed by oxyHb—see Photon 1), whereas yellow colors represent wavelengths of λ < 800 nm (mainly absorbed by deoxyHb—see Photon 2). Path 3 represents a photon that undergoes some scattering events before being recorded by a detector. Path 4 represents a ballistic photon. Path 5 represents a photon that, after some scattering events, is not recorded by a detector (lost due to forward scattering). Path 6 represents a photon that is lost due to backward scattering. In the right part of the illustration, the formulas to calculate concentration changes in chromophores are shown (based on continuous-wave NIRS). The symbols have the following meanings: A: light attenuation, or ΔΑ(λ): changes in light attenuation at a certain wavelength (λ); Ι_Ιn: intensity of emitted light; Ι_Out: intensity of recorded light; ε(λ): the extinction coefficient of the chromophore at a certain wavelength (λ); Δc: changes in chromophore concentration; d: separation (distance) between source and detector; DPF(λ): differential path length factor (DPF) for a certain wavelength (λ); g(λ): scattering at a certain wavelength (λ), where g is cancelled out since it is assumed to be negligible when only light attenuation (as in continuous-wave NIRS) is considered [45,54,58].