Fig. 2

a A schematic illustrating the concept of path. Here, \({\mathbb {P}}_0\) denotes a path where emitted photon is not scattered or absorbed within the field of view. \({\mathbb {P}}_1\) denotes a path where emitted photon is scattered once and \({\mathbb {P}}_2\) denotes a path where emitted photon is scattered twice. b A schematic describing the various notations used to describe a path. At each emission or scattering location, \((r_m,E_m^w,\theta _{k_m})\) denotes the location of emission or scatter, energy at the emission or after scatter and the outgoing direction of the photon, respectively. The notation of each subpath is also demonstrated in the schematic