  Apart from the difficulty of producing highly scattering samples, a major
challenge in the observation of Anderson localization of 3D light is
identifying an unambiguous signature of the phase transition in experimentally
feasible situations. In this letter we establish a clear correspondence between
the collapse of the conductance, the increase in intensity fluctuations at the
localization transition and the scaling analysis results based on the Thouless
number, thus connecting the macroscopic and microscopic approaches of
localization. Furthermore, the transition thus inferred is fully compatible
both with the results based on the eigenvalue analysis of the microscopic
description and with the effective-medium Ioffe-Regel criterion.
