Oxygenated and deoxygenated hemoglobin fNIRS estimates the concentration of hemoglobin from changes in absorption of<unk><unk>  light. As light moves or propagates through the head, it is alternately scattered or absorbed by the tissue through which it travels. Because hemoglobin is a significant absorber of<unk><unk>  light, changes in absorbed light can be used to reliably measure changes in hemoglobin concentration. Different fNIRS techniques can also use the way in which light propagates to estimate blood volume and oxygenation. The technique is safe, non-invasive, and can be used with other imaging modalities. fNIRS is a non-invasive imaging method involving the quantification of chromophore concentration resolved from the measurement of<unk><unk>  (NIR) light attenuation or temporal or phasic changes. The technique takes advantage of the optical window in which (a) skin, tissue, and bone are mostly transparent to NIR light (700–900 nm spectral interval) and (b) hemoglobin (Hb) and deoxygenated-hemoglobin (deoxy-Hb) are strong absorbers of light. Absorption spectra for oxy-Hb and deoxy-Hb for<unk><unk>  wavelengths There are six different ways for<unk>  light to interact with the brain tissue: direct transmission, diffuse transmission, specular reflection, diffuse reflection, scattering, and absorption. fNIRS focuses primarily on absorption: differences in the absorption spectra of deoxy-Hb and oxy-Hb allow the measurement of relative changes in hemoglobin concentration through the use of light attenuation at multiple wavelengths. Two or more wavelengths are selected, with one wavelength above and one below the isosbestic point of 810 nm—at which deoxy-Hb and oxy-Hb have identical absorption coefficients. Using the modified Beer-Lambert law (mBLL), relative changes in concentration can be calculated as a function of total photon path length. Typically, the light emitter and detector are placed ipsilaterally (each emitter/detector pair on the same side) on the subject's skull so recorded measurements are due to back-scattered (reflected) light following elliptical pathways. fNIRS is most sensitive to hemodynamic changes which occur<unk> est to the scalp and these superficial artifacts are often addressed using additional light detectors located closer to the light source (short-separation detectors). Modified Beer–Lambert law Changes in light intensity can be related to changes in relative concentrations of hemoglobin through the modified Beer–Lambert law (mBLL). The Beer lambert-law has to deal with concentration of hemoglobin. This technique also measures relative changes in light attenuation as well as using mBLL to quantify hemoglobin concentration changes. Basic<unk>  (fNIRS) abbreviations BFi = blood flow index CBF = cerebral blood flow CBV = cerebral blood volume CMRO2= metabolic rate of oxygen CW= continuous wave DCS = diffuse correlation<unk>  FD = frequency-domain Hb, HbR= deoxygenated hemoglobin HbO, HbO2= oxygenated hemoglobin HbT= total hemoglobin concentration HGB = blood hemoglobin SaO2= arterial saturation SO2= hemoglobin saturation SvO2= venous saturation TD=time-domain
Functional near-infrared spectroscopy
Iatrochemistry
Hemoximetry
Intravascular fluorescence
Spectral imaging 
Perfusion scanning
Photoplethysmogram
Diffuse optical imaging