To characterize the spectral coupling between a lens based Focal Plane Array and its Quasi-Optical system, Coherent Fourier Optics methodology is employed. A lens absorber design is proposed and compared to an existing bare absorber-based imaging system. It is shown that the performance of the two imagers is comparable in terms of aperture efficiency, Point Spread Function, and normalized throughput. Moreover, the proposed lens absorber achieves 5 folds better NETD in comparison to the bare absorber due to its smaller physical area. A prototype of the proposed geometry is under fabrication as a proof of concept.