Optical and quantum interference at the micro and nano-scales are of growing interest. Its accurate description bases on the non-paraxial propagation of the spatial correlation, in which the physical observable, determined by the square modulus of the optical and quantum wave functions, is expressed as a modal expansion on a 3D non-paraxial geometric kernel, with the spatial correlation as coefficient. The kernel plays the main role of the model and is deduced from the optical wave equation in free-space as well as from the Schrödinger equation for particle propagation in field-free regions. Two features are analyzed in detail, i.e. the physical implications on the wave and particle interference due to the 3D spatial modulations provided by the local and non-local components of the kernel at the micro and nano-scales, and the decay of the kernel terms with the propagation distance which leads to a novel criterion for the kernel accuracy. The interference modeling is implemented on a matrix algorithm and is illustrated by some examples with nano-structured masks.