
400 Handbook of Biomedical Optics
vanishingly small aer a few centimeters of propagation in bio-
logical tissue. Rather than exclusively detecting early photons,
time-gated instruments now record photons arriving within
dierent delay windows, either in transmission or in reectance
geometries. e window is delayed temporally step by step rela-
tive to the laser pulse emission, so that the complete TPSF can be
recorded (Figure 20.3). Because of the low intensity contained in
individual light pulses, the light from each delay window needs
to be integrated over many cycles.
20.3.3.2 Light Sources
Gated systems are based on pulsed sources of ...