The effect of a (spatially) partially coherent quasi-monochromatic lowest order Gaussian-beam on the direct detection system is studied in weak and strong atmospheric turbulence. The analytic expression for the scintillation index has been developed and analysed as a function of the spatial correlation distance (lc) and the correlation time (ts), associated with the source, as well as the strength of turbulence and the integration time (td) of the detector. The probability of fade is also studied as a function of relative detector speed td/ts. The limiting cases of slow (td>>ts) and fast (td<<tc) detector are discussed.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.