Ultrashort Laser Pulse Phenomena, 2nd Edition

Imaging 585

that can only withstand average powers of irradiation of a few mW, these noise

considerations limit the applicability of diffraction limited imaging to scattering

densities µ

s

L ≤ 35.

The multiply scattered photon path can be described by a diffusion model [20].

For L  l

t

, it can be shown that a collimated input beam of diameter d < l

t

at the

sample input broadens to a diameter d



, which is approximately given by [21]:

d



= 0. 2L, (13.3)

if we refer to the early-arriving scattered light that exceeds the detection noise

and to illumination intensities below the critical values for biomedical samples.

The quantity d



gives a reasonable measure of the resolution that can be achieved

in imaging an object buried in a dense scatterer. With a sample ...