## 3 Experimental Technique  
### BLRFS Bench-Top Instrumentation and Probing Technique  
In the provided experiments, we studied the statistical properties of spatial fluctuations of spectrally selectable fluorescence radiation propagating in the random media. In general, the methodology of the experiments was similar to that described earlier in Ref. [29], except that the detection system used was characterized by a significantly lower spectral resolution. In this case, the broadband spectral filtering of fluorescence radiation led to a significant suppression of fluctuations caused by the stochastic interference of partial components of the fluorescence radiation field. In turn, this necessitated an accurate accounting of the intrinsic noise of the detection system when analyzing the obtained data. The general scheme of the experimental setup is shown in Figure 1. The light-collecting unit (Figure 2, see also \"A\" in Figure 1) was assembled according to a collimating confocal scheme using a microscope objective (III, Nikon E plan 100X, N.A.1.25), an entrance aperture of the core of the fiber-optic patch-cord (I, Ocean Optics P100 UV-VIS) as a confocal unit diaphragm, and the lens (II) placed between the microscope objective and the entrance aperture. The input tip of the fiber-optic patch-cord was installed in the rear focal plane of the lens (II), and the surfaces of the examined samples (IV) were conjugated with the front focal plane of the microscope objective. In addition, a long-pass optical filter (V, OC590) was placed between the units II and III to block the pumping laser radiation. This configuration of optical components is a collimating unit with a linear magnification determined by the ratio of the focal lengths of the lens (\(f_{2}\approx 15\) mm) and the microscope objective (\(f_{1}\approx 1.6\) mm). In other words, within the framework of geometrical-optical approximation, this unit forms an enlarged image of a surface fragment in the rear focal plane of the lens. A more rigid consideration of the light-collecting unit in the framework of physical optics shows that with large values of the magnification and numerical aperture \(NA_{m}\) of the microscope objective the fluorescence radiation is actually collected from a single coherence area (a single speckle) of the fluorescence field within the chosen spectral interval. In fact, the characteristic size of the coherence area \(d_{c}\) can be roughly estimated as \(\lambda/\Omega\), where \(\Omega\) is the width of the angular spectrum of the collected radiation; at the same time, the size of the light-collection zone \(d_{z}\) in the front focal plane of the microscope objective corresponds to the diameter of the Airy disk and can be roughly evaluated as the doubled spatial resolution of the objective, \(d_{z}\approx 2\cdot(\lambda/2 NA_{m})\approx\lambda/\Omega\). The light collected from this single coherence area excites an ensemble of propagating modes in the multimode fiber patch-cord, which is used as the light-transferring channel. It is the mode on excitation that has the main effect on the detection conditions in the considered instrument (in particular, on \(\beta\)). Each mode corresponds to a statistically independent contribution to the overall detected signal; the statistical independence, or total decorrelation of the light fields corresponding to various modes, is established by the following condition: \(\Delta s_{m}>>l_{c}\), where \(\Delta s_{m}\) is the characteristic path-length difference between the patch-cord modes (due to existence of the intermode dispersion). Rough estimations show that in the considered system with the patch-cord length of 2 m and the patch-cord numerical aperture \(NA_{p}\) equal to 0.22, the path-length difference \(\Delta s_{m}\) is about 30 mm.