In direct detection ladar systems, the received irradiance statistics and, therefore, the detected photon counting statistics are determined by two parameters: the average collected irradiance value and the M parameter. The M parameter is the number of independent speckle cells, per polarization and per independent laser mode, subtended by the receiving aperture and focused onto a detector. In the 1960's Goodman analytically determined the M parameter for simple ladar geometries such as circular, square, and Gaussian shaped targets and circular and square shaped receiving apertures. This paper examines the numerical evaluation of this M parameter for arbitrarily shaped target source regions, per pixel or per pixel-range-bin, and arbitrarily shaped receiving apertures using the 2D discrete Fourier transform. This evaluation method is capable of treating the cases of high reflectivity target source regions, such as a cylinder's or cone's glint-line, and Gaussian spatial mode illumination which effectively reduce the source area. The analyses apply when round-trip atmospheric scintillation effects are negligible.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.