In seekers that never resolve targets spatially, it may be adequate to calibrate only with sources that have known aperture irradiance. In modern missile interceptors, the target becomes spatially resolved at close ranges, and the seeker's ability to accurately measure the radiance at different positions in the scene is also important. Thus, it is necessary to calibrate the seekers with extended sources of known radiance. The aperture irradiance is given by the radiance integrated over the angular extent of the target in the scene. Thus radiance calibrations and accurately presenting the targets spatially produces accurate irradiances. The accuracy of the scene radiance is also important in generating synthetic imagery for testing seeker conceptual designs and seeker algorithms, and for hardware-in-the-loop testing with imaging projection systems. The routine procedure at the Air Force Research Laboratory Munitions Directorate's AFRL/MNGG is to model and project the detailed spatial and radiometric content of the scenes. Hence, accurate depiction of the radiance in the scene is important. AFRL/MNGG calibrates the complete projection system (synthetic image generator and scene projector) with extended sources of known radiance, not unresolved sources of known irradiance. This paper demonstrates that accurate radiance calibrations and accurate spatial rendering do provide accurate aperture irradiances in the projection systems. In recent tests conducted by AFRL/MNGG, the projection system was calibrated in terms of radiance, and the aperture irradiances were determined both as they were observed in the synthetic images that drove the projection system and in the images of the projection system measured by the unit under test. The aperture irradiances were compared with the known truth data and errors were determined. This paper presents results of analyzing the errors associated with the observed aperture irradiances.