The infrared (IR) imaging community has a need for direct IR detector evaluation due to the continued demand for small pixel pitch detectors, the emergence of strained-layer-superlattice devices, and the associated lateral carrier diffusion issues. Conventional laser speckle-based modulation transfer function estimation is dependent on Fresnel propagation and limited to paraxial geometries, preventing the utilization of this approach on small pixel pitch IR devices. This paper presents a generalized approach for determining a focal plane array (FPA) system input power spectral density, utilizing numerical evaluation of Rayleigh–Sommerfeld speckle irradiance autocorrelation functions, speckle irradiance spectral analysis using the Wigner distribution function, and experimental error quantification incurred from making wide-sense-stationary assumptions regarding the associated laser speckle random process. The effort’s final result is an experimental demonstration of an FPA resolution estimation technique valid in nonparaxial test scenarios.