Time-domain fNIRS

Time-domain fNIRS illuminates tissue with picosecond light pulses and measures the arrival-time distribution of transmitted photons (the temporal point-spread function). Because photons that travelled deeper arrive later on average, the shape of that distribution encodes depth: late-gated photons are preferentially cortical, letting TD systems separate brain from scalp signal and estimate absolute concentrations of oxy- and deoxy-haemoglobin rather than only relative changes.

This depth discrimination and absolute quantification are what continuous-wave fNIRS lacks, at the cost of expensive pulsed lasers and fast single-photon detectors. Frequency-domain fNIRS achieves related information by modulating light intensity and measuring amplitude and phase. Both remain slower and lower-resolution than intracranial optics but push non-invasive optical recording toward quantitative, depth-resolved use.