  Disordered nanostructures are commonly encountered in many nanophotonic
systems, from colloid dispersions for sensing, to heterostructured
photocatalysts. Randomness, however, imposes severe challenges for
nanophotonics modeling, often constrained by the irregular geometry of the
scatterers involved or the stochastic nature of the problem itself. In this
article, we resolve this conundrum by presenting a universal theory of averaged
light scattering of randomly oriented objects. Specifically, we derive formulas
of orientation-and-polarization-averaged absorption cross section, scattering
cross section and asymmetry parameter, for single or collection of objects of
arbitrary shape, that can be solved by any electromagnetic scattering method.
These three parameters can be directly integrated into traditional unpolarized
radiative energy transfer modelling, enabling a practical tool to predict
multiple scattering and light transport in disordered nanostructured materials.
Notably, the formulas of average light scattering can be derived under the
principles of fluctuational electrodynamics, allowing analogous mathematical
treatment to the methods used in thermal radiation, non-equilibrium
electromagnetic forces, and other associated phenomena. The proposed modelling
framework is validated against optical measurements of polymer composite films
with metal-oxide microcrystals. Our work sets a new paradigm in the theory of
light scattering, that may contribute to a better understanding of light-matter
interactions in applications such as, plasmonics for sensing and photothermal
therapy, photocatalysts for water splitting and CO2 dissociation, photonic
glasses for artificial structural colours, diffuse reflectors for radiative
cooling, to name just a few.
