A Monte Carlo simulation code developed to model time-domain transilluminat
ion measurements with small-area detectors through an optically thick scatt
ering slab is presented. A hybrid approach has been implemented to reduce c
alculation times. Most of the scattering slab is treated stochastically, al
beit with variance reduction techniques and the isotropic diffusion similar
ity rule. The contribution to the output signal per unit area and time of p
hoton packets propagating in a thin slice near the output face of the slab
is calculated analytically after each propagation step. This approach drast
ically reduces the calculation time but produces spikes in the temporal sig
nals. (C) 1999 Optical Society of America.