On input diffraction effects in infrared optical systems and the use of spatial apodization.

The problem of diffraction at the input aperture of an optical system allowing stray radiation from outside the geometric field of view to enter the system is examined. This problem can be particularly serious in remote sensing applications where it may be necessary to detect a small cold target surrounded by an extended hot background. The basic equations describing the diffraction pattern are discussed and then modified to include spatial apodization of the diffracting aperture. A considerable reduction in the magnitude of the diffraction pattern sidelobes, due to the apodization, is demonstrated. The method is shown to be capable of highly efficient rejection of stray radiation arriving at angles outside the geometric field of view of the optical system.