For small unresolved or partially resolved objects, the prediction of the performance of an infrared (IR) sensor system depends not only on the optics and the atmosphere, but also on the geometry of the sensing elements within the detector. The apparent scintillation caused by subpixel displacements of the detector from the optimum position is investigated. The effect is linked to the system point spread function (PSF) as well as the shape and size of the object. The effect of the shape and size of the sensing element, the optics, and the viewed object are simulated. The simulation also evaluates the effect that vibrations or pointing accuracy have on the performance of the sensor system for different types of targets. Results are presented that show that the variation in contrast of objects due to the shape and size of the active portion of a detector and the position of the target projection on the detector can be significant, and should be taken into account when estimating system performance. Although these effects are most significant in wide field of view surveillance systems, narrow field of view systems are also described and evaluated.