Abstract: A photomultiplier tube includes a photocathode and a primary dynode having input and output apertures. A field isolating mesh is positioned at the input aperture of the primary dynode to facilitate the collection of electrons from the photocathode of the photomultiplier to the primary dynode while simultaneously electrostatically shielding secondary emission electrons from the field of the photocathode. The field isolating mesh has a central opening that is dimensioned to maximize the throughput of photoelectrons from the photocathode to the primary dynode while providing effective field isolation in the vicinity of the primary dynode. The central opening in the field isolation mesh provides the further advantage of permitting uniform deposition of photo-emissive materials on the surface of the dynode during manufacture. In an alternative embodiment, the field isolating mesh of the primary dynode is formed in two segments.