  We extensively investigate in-plane light diffusion in systems with thickness
larger than but comparable with the transport mean free path. By exploiting
amplified spontaneous emission from dye molecules placed in the same holder of
the sample, we obtain a directional probe beam precisely aligned to the sample
plane. By comparing spatial intensity distribution of laterally leaking photons
with predictions from random walk simulations, we extract accurate values of
transport mean free path, opening the way to the investigation of a previously
inaccessible kind of samples.
