Similar to how functional Magnetic Resonance Imaging (fMRI) measure changes in oxy and deoxy hemoglobin with powerful magnets, functional Near-Infrared Spectroscopy (fNIRS) does a similar measurement with near-infrared light. Oxygenated and de-oxygenated hemoglobin molecules absorb specific wavelengths of near-infrared light at different rates (molar extinction coefficients).

The first fNIRS systems used powerful lasers to generate light at these specific wavelengths. Some systems still do that but this requires heavy fiber optic cables to route the light from the laser to the head. Modern systems now use specially tuned LEDs allowing for light weight sources to be placed on the head.

By shining light at these specific wavelengths into the scalp of a subject, we can measure how much of that light travels via paths into the scalp and through the outer cortex (via diffusion and diffraction along the “photon banana” path). The light that makes it through is quantified and digitized by silicon photodiode detectors (very sensitive photosensors).

The spectroscopic analysis technique allows us to plug these quantities of light using the Modified Beer-Lambert Law to calculate changes in hemoglobin concentration. When neurons in one part of the brain fire more, they have a higher metabolic demand leading to more oxy-hemoglobin being sent to that part of the brain and a decrease in deoxy-hemoglobin. This change in concentrations can happen over the course of a few seconds. Like fMRI experiments, fNIRS experimental design requires setting up contrasting conditions to show relative blood flow changes and this may rely on block designs or more dispersed trials.