The purpose of this paper is to investigate the signal from an IR detector in a system employing a moving space filter. The space filter may be a reticle or simply a "flying spot" aperture. When the IR detector output is caused by background radiation, the Wiener spectrum description is presumed adequate. The linear scanning case is considered in greater detail. A calculation of the detector output spectrum is made, assuming a generally accepted Wiener spectrum and a scanning rectangular aperture.