fNIRS Physics
fNIRS detects oxyhemoglobin (HbO) and deoxyhemoglobin (HbR) by taking advantage of the fact that the two molecules absorb near-infrared light differently. Combining optical spectroscopy with a modified version of the Beer-Lambert law lets the system estimate changes in blood oxygenation within the cerebral cortex.
Emitting near-infrared light
Light sources — LEDs or laser diodes — and detectors — photodiodes or avalanche photodiodes — are placed on the scalp. The source emits near-infrared light, typically at two or more wavelengths between roughly 650 and 950 nm. This range is called the "optical window" because skin and skull absorb relatively little light at these wavelengths, near-infrared light can penetrate about 1.5 to 3 cm into the head, and hemoglobin is the primary absorber in this range.
| Wavelength | Sensitivity |
|---|---|
| ~760 nm (roughly 690–730 nm) | More sensitive to deoxyhemoglobin (HbR) |
| ~850 nm (roughly 830–850 nm) | More sensitive to oxyhemoglobin (HbO₂) |
These wavelengths are typically chosen to straddle the "isosbestic point" around 800–810 nm, where the absorption curves for oxy- and deoxyhemoglobin cross over. LEDs are commonly used as the light source because their incoherent output keeps retinal exposure low enough to meet Class 1 eye-safe limits without extra safety interlocks, and because they run cool and cheap enough to fit into lightweight, wearable, field-ready equipment.
How light travels through tissue
Rather than traveling in a straight line, light entering the scalp scatters repeatedly as it passes through skin, skull, cerebrospinal fluid, and brain tissue. On average, near-infrared light travels only about 1/10 mm through brain tissue before scattering again. Some photons are absorbed along the way, mainly by oxyhemoglobin and deoxyhemoglobin, while the rest eventually re-emerge at the surface, where they're picked up by a detector some distance from the source. This creates a curved, "banana-shaped" path between source and detector that samples roughly the outer 5–8 mm of the cortex — the superficial layers of the brain.
Because the amount of light reaching a detector depends on how much was absorbed along that path, measuring detected light intensity at different wavelengths — for example, more HbR absorption around 760 nm and more HbO absorption around 850 nm — lets the system distinguish changes in each hemoglobin type. Systems typically record these changes continuously, at around 5 to 20 measurements per second.
The modified Beer-Lambert law
Because tissue scatters light so extensively, the standard Beer-Lambert law has to be modified to account for a longer effective path length. It relates the change in optical density to hemoglobin concentration as:
ΔA = ε × ΔC × L × DPF
- ΔA — change in light absorption (optical density)
- ε — extinction coefficient, or how strongly HbO or HbR absorbs a given wavelength
- ΔC — change in hemoglobin concentration
- L — distance between source and detector
- DPF — differential pathlength factor, which accounts for scattering
Since the device measures light at two or more wavelengths, it has enough equations to solve for the two unknowns — the change in HbO concentration and the change in HbR concentration.
The hemodynamic response
When a region of the brain becomes more active, neurons there consume more oxygen, triggering nearby blood vessels to dilate and deliver more blood — a process called neurovascular coupling. Because the resulting increase in oxygen-rich blood outpaces the increase in oxygen actually consumed, this produces a characteristic pattern: HbO rises, HbR falls, and total hemoglobin often increases slightly. This pattern is known as the hemodynamic response, and by tracking it over time, fNIRS can infer which cortical regions are more active during a given task.
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