1. In an apparatus for determining a location of at least one substantially point-like radiator of energy in a three dimensional volume comprising: said at least one substantially point-like radiator of energy; at least one detector adapted to be disposed in a known location within said three dimensional volume, and adapted to be positioned to intercept energy radiating from said substantially point-like radiator, wherein said detector comprises at least one array of elements adapted to detect said radiation and to convert said detected radiation into output signals; and at least one pattern member, adapted to be disposed between said array and said radiator, comprising plural gradations of transparency, wherein said pattern is adapted to spatially modulate radiation that is incident thereon as a function of said gradations in transparency, and to project said modulated energy onto said array in the form of at least one real image; and a signal processor adapted to convert output signals representative of said at least one modulated real image into a direction of said radiator relative to an assembly of said detector and said pattern member; the improvement that comprises: at least one substantially ideal reference image of said pattern member on said array; said signal processor being adapted to compare said modulated image with said reference image; said signal processor comprising a discrete mathematical correlation function that is adapted to characterize the degree of match between said modulated image and said reference image for a plurality of discrete displacements of said reference image with respect to said array, said signal processor being adapted to identify at least one of said displacements that produces a correlation that is characterized by a well defined maximum value; and said signal processor being adapted to convert said displacement(s) producing a maximum value into a direction of said radiator in said three dimensional volume with respect to the known location of said detector in said three dimensional volume.