Specifications for state-of-the-art projection lithography systems do not currently encompass the possible sources of variation that can and do exist in the spatial distribution of pupil fill created by the illuminators of these systems. An investigation into the types and magnitudes of these variation, for conventional and annular illumination, is presented. The variations are used to create a number of possible pupil-fill distributions. Using these pupil-fill distributions and typical amounts of low-order aberrations found in today's equipment as inputs, a simulation survey is presented of critical imaging needs in memory and logic cell fabrication. The amount of illumination-induced error predicted in placement, line-width abnormality, isolated-to- nested bias, and horizontal-to-vertical bias is used to recommend new spatial-illumination requirements for current and future needs of projection lithography.