A homogeneous medium of semi-infinite geometry with the physical source and the detector located on the medium boundary is depicted in Fig. 1(b). In the semi-infinite geometry, it becomes convenient to set the physical source at , and the detector at . The physical source at launches photon into the medium at an initial direction orthogonal to the medium-applicator interface, and the resulted photon diffusion is treated as being excited by an equivalent “real” isotropic source located at one step of reduced scattering, represented by , into the medium. The “real” isotropic source thus has the coordinates of . The effect of the medium-applicator interface on photon diffusion as modeled by the EBC21,23,24 sets zero the photon fluence rate on an imaginary boundary located at a distance of away from the physical boundary, where is a coefficient23 related to the refractive index differences across the physical boundary. This EBC is conventionally accommodated by setting a sink or a negative “image” of the “real” isotropic source, with respect to the extrapolated boundary. The “image” source for the semi-infinite geometry has the opposite strength of the “real” isotropic source and locates at , which has the coordinates of . The distances from the detector to the “real” isotropic source and the “image” source are denoted by and , respectively. The notations of and also apply to other medium geometries studied in this work when involving a boundary. In all studied geometries, the straight distance between the physical source and the detector is referred to as the “line-of-sight” source–detector distance, and hereafter specified as source–detector distance. It is with respect to this , which is to remain identical across different medium geometries considered in this work, that the DPF is to be expressed, facilitating the evaluation of the effect of shape and dimension of the nonsemi-infinite-domains on DPF in comparison to the semi-infinite geometry on DPF.