The temporal distribution of photons produced when a short duration (a few picoseconds) pulse of light is transmitted through a highly scattering medium is known as the temporal point spread function (TPSF). After travelling through several centimetres of soft tissue, the TPSF will extend over several nanoseconds. Early technical validation studies, performed using laboratory picosecond lasers and a variety of sophisticated and expensive detector systems, such as streak cameras, were discussed in the 1997 review (Hebden et al. 1997). Since then, advances in time-correlated single-photon counting (TCSPC) hardware and pulsed laser diodes have significantly reduced the cost and complexity of time-resolved measurement, and facilitated the development of multi-detector systems. TCSPC involves c