NIRs

NIRS, or near-infrared spectroscopy, is a non-invasive optical technique used to monitor tissue hemodynamics by measuring changes in the concentrations of oxygenated and deoxygenated hemoglobin. It relies on the differential absorption of near-infrared light by HbO2 and HbR, combined with light scattering in tissue, to estimate blood volume and oxygenation.

Light in the near-infrared range (roughly 650 to 1000 nm) is used because it penetrates scalp and skull with limited attenuation. Instruments emit light into tissue and detect transmitted or reflected photons. The data are analyzed with models based on the Beer-Lambert law (or its modified forms) to derive relative or absolute concentrations of HbO2 and HbR. NIRS systems vary by sensing mode: continuous-wave (CW) devices estimate relative changes, while frequency-domain (FD) and time-domain (TD) systems can provide absolute measurements and information about photon time-of-flight.

Functional near-infrared spectroscopy (fNIRS) focuses on brain activity by detecting task- or state-related changes in HbO2 and HbR in cortex. It offers good temporal resolution (seconds) and portability, with typical spatial sensitivity of a few centimeters and limited penetration depth, primarily reflecting superficial cortex.

Applications span neuroscience research, neonatal and intraoperative monitoring, rehabilitation, and increasingly consumer health, sports analytics, and industrial process sensing, including material and food analysis.

Limitations include sensitivity to scalp and skull blood flow, motion artifacts, and baseline variations; depth resolution is limited; absolute quantification requires careful calibration and modeling; standardization across devices is ongoing.

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