Coherence of radiation from incoherent sources: I. Sources on a sphere and far-field conditions.

Models for the coherence of quasi-monochromatic light from spherical incoherent sources are constructed by placing incoherent monopole and dipole sources on the surface of a sphere. All models allow for relatively simple numerical calculations of coherence functions for arbitrary source sizes and positions of observation points. We show analytically and confirm numerically that the far-field regime for transverse coherence is formed at distances larger than the source size, regardless of the wavelength.