  We demonstrate that optical transmission matrices (TM) of disordered complex
media provide a powerful tool to extract the photonic interaction strength,
independent of surface effects. We measure TM of strongly scattering GaP
nanowires and plot the singular value density of the measured matrices and a
random matrix model. By varying the free parameters of the model, the transport
mean free path and effective refractive index, we retrieve the photonic
interaction strength. From numerical simulations we conclude that TM statistics
is hardly sensitive to surface effects, in contrast to enhanced backscattering
or total transmission based methods.
