Dipole model for far-field thermal emission of a nanoparticle above a planar substrate

•generalized dipole model within framework of fluctuational electrodynamics.•thermal emission of dielectric and metallic nanoparticles in near-field of a substrate.•impact of different temperatures of nanoparticle, substrate, and environment.•theoretical basis for modelling of all scattering type near-field thermal microscopes.•divergency free derivation of electric and magnetic dressed polarizabilities.