This paper presents a compact and low-power digital implementation of the Roughness Index, which provides an estimate of the fixed-pattern noise present in an infrared video stream. This noise is caused by imperfections in the fabrication of the focal plane array and its readout circuitry, in addition to other time-variant processes. The index is continuously used during normal operation of the imager to activate calibration mechanisms that compensate for these limitations. Our FPGA-based prototype computes the Roughness Index on infrared video at 30fps and a resolution of 720x480 pixels. On a low-cost Xilinx Spartan 3E XC3S500 FPGA, adding our circuit to an existing system increases logic resource utilization by only 6% and power consumption by 1.59 mW. © 2012 IEEE.