Those sources where the radiation has a constant phase relationship between waves spatially as well as temporally, e.g. lasers. Non-coherent optical sources emit radiation which is randomly distributed in phase, spatially as well as temporally. Most sources which are used in spectroscopy and for spectrochemical analysis conform to this latter group.
Source:
PAC, 1985, 57, 1453. 'Nomenclature, symbols, units and their usage in spectrochemical analysis - V: Radiation sources (Recommendations 1985)' on page 1456 (https://doi.org/10.1351/pac198557101453)