**Definition:**Time-domain near-infrared spectroscopy, in the context of fNIRS, refers to measuring the distribution of photon times of flight through tissue and fitting the resulting histogram to a theoretical model to calculate tissue optical properties, including absorption and scattering coefficients. TD-NIRS, and its Fourier domain analogue swept frequency NIRS, permits gathering a richer data set than either single frequency-frequency domain NIRS or continuous wave NIRS. However, the cost of TD-NIRS hardware per detection channel. tends to be higher. Definition: Time-domain near-infrared spectroscopy, in the context of fNIRS, refers to measuring the distribution of photon times of flight through tissue and fitting the resulting histogram to a theoretical model to calculate tissue optical properties, including absorption and scattering coefficients. TD-NIRS, and its Fourier domain analogue swept frequency NIRS, permits gathering a richer data set than either single frequency-frequency domain NIRS or continuous wave NIRS. However, the cost of TD-NIRS hardware per detection channel. tends to be higher.

**Alternative definition:**

**Synonym:**Time-resolved fNIRS

**References:**

**Related terms:**temporal point spread function, diffuse time of flight curve, absorption coefficient, scattering coefficient