Traditionally, infrared-system-performance evaluation techniques have relied on detected signal-to-noise analysis for a single element of the scene. This approach includes the imaging-optical system; however, the geometrical optics effects have been deemed sufficient to describe the contributions of the optical system. The diffraction-based image-forming theory generally in use in the visible wavelength region is applied to the infrared system to assess its performance. The theory of incoherent image formation is applied to self-radiating bodies that have the wavelength-dependent radiance of a Planckian radiator. Within the traditional approximations in use in the infrared system design, a closed-form solution is obtained for the detected incidence. It is shown that the detected incidence may be expressed in terms of effective radiance, the only term included in the traditional infrared system design, and a correction term which is introduced by the diffraction effects.