**Definition:**Frequency-domain near infrared spectroscopy (FD-NIRS) is an fNIRS technique that sinusoidally modulates a near-infrared light source at a given frequency into tissue, and detects the phase shift as well as the attenuation in amplitude with the aim of estimating the absorption and scattering coefficients of the medium. The modulation frequency needs to be higher than the shift induced by the time of light propagation inside the medium (on the order of nanoseconds), which depends on the light speed in the medium. Usually this is on the order of 100 MHz. FD spectroscopy can vary the modulation frequency (Multi-Frequency FD) and measures the amplitude attenuation and phase shift at different frequencies. In some cases, it is possible to measure attenuation and phase at one single frequency and use modeling assumptions to estimate the absorption and scattering coefficients and in turn, quantify chromophore concentrations.Definition: Frequency-domain near infrared spectroscopy (FD-NIRS) is an fNIRS technique that sinusoidally modulates a near-infrared light source at a given frequency into tissue, and detects the phase shift as well as the attenuation in amplitude with the aim of estimating the absorption and scattering coefficients of the medium. The modulation frequency needs to be higher than the shift induced by the time of light propagation inside the medium (on the order of nanoseconds), which depends on the light speed in the medium. Usually this is on the order of 100 MHz. FD spectroscopy can vary the modulation frequency (Multi-Frequency FD) and measures the amplitude attenuation and phase shift at different frequencies. In some cases, it is possible to measure attenuation and phase at one single frequency and use modeling assumptions to estimate the absorption and scattering coefficients and in turn, quantify chromophore concentrations.

**Alternative definition:**

**Synonym:**Frequency-domain spectroscopy, Frequency-domain diffuse optical spectroscopy

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