- Distinction between spatial and temporal coherence characteristics of the interferogram from low-coherence microscopic systems is investigated. It is shown that the coherence region is determined by the temporal coherence only when the path length difference is scanned in a region where the reference and sample beams are collimated, that is, containing a single spatial frequency. Spatial coherence has an effect when the path length scan is performed with a non-collimated beam, such as a defocus scan of a lens where the focal depth of the lens mainly determines the coherence region. In the latter case, the fringe size for a monochromatic light is smaller than half the wavelength by a factor determined by the spatial coherence. A new analytic relation is derived for the fringe size showing that it has better validity than the Ingelstam relation in the high NA region.