A model for the calculation of rms response nonuniformity in a direct-injection readout mercury cadmium telluride hybrid focal plane array is presented. The model includes contributions from detector material composition variations, processing-induced variations, threshold voltage variations in the input structure of the readout multiplexer, and transfer efficiency variations in the case of CCD-based readouts. The model is then used to analyze the role of the buffered direct-injection (BDI) circuit in controlling the nonuniformity. It is predicted that a minimum BDI gain of 10 is required to reduce the nonuniformity of long-wavelength infrared focal plane arrays (LWIR FPAs) to its minimum value limited by the compositional variations in detector material. This gain requirement is expected to go up with a reduction in the resistance-area product of photodiodes resulting from either imperfect processing or increased cutoff wavelength (λco > 9.43 μm).