In the theoretical analysis of the optical readout IR imaging, the incoherent area light source with a certain size is typically simplified into an ideal point light source, which leads to analytical errors. In this paper an area light source model is established, and using the Fraunhofer diffraction theory the influence of the optical detection sensitivity on area light source is studied. The dependence of the optical detection sensitivity on light source radius and reflector length of the focal plane array is found. And optimized design criteria for the size of light source and the length of the reflector are proposed. According to theoretical analysis, the experimental verification is carried out, and the results are consistent with theoretical analyses.