1. A nuclear radiation attenuator for providing a test image from a flood source of radiation for testing a scintillation camera comprising: a plurality of stackable plates formed of radiation attenuating material and selected so that a plate provides a background region of substantially uniform background radiation passed therethrough from said flood sources, each of said plates having a generally uniform thickness; said plates each including a plurality of apertures which form discrete target regions located in the background region of the plate, each target region enabling the transmittal of radiation from said flood source with an intensity that differs from the radiation passed through said background region to provide for each target region a desired target to background intensity contrast ratio between it and said background region, said apertures being regularly distributed over a plate to enable examination of the linearity of said camera, and with the size of said apertures being selected to provide an indication of the spatial resolution of said camera and wherein wherein the apertures in the plates are so located that when said plates are stacked, first target regions each have a material thickness that is different from the combined plates background region thickness and wherein second target regions have a material thickness that is different from the thickness of said first target regions and said combined plates background region.