Police, firefighters, and emergency medical personnel are first responders that use thermal imaging cameras (TICs) every day. However, few performance metrics have been developed to assist first responders in evaluating thermal imaging technology. This paper describes a metric for evaluating the nonuniformity of commercial TICs. Three TICs, each employing a different thermal detection technology, were examined. Each TIC was considered a ‘black box’ while being tested. An extended area blackbody (178 mm square) was used as the thermal target, and was placed very close to the objective lens of the TIC under test. The resulting National Television Systems Committee (NTSC) video output signal of the TICs was collected, digitized, and processed. The nonuniformity was calculated using the standard deviation of the digitized image pixel intensities divided by the mean of those pixel intensities. This procedure was repeated for each TIC at several blackbody temperatures in the range from 30 °C to 260 °C. It was observed that the nonuniformity increased in a roughly linear manner with temperature. The testing procedure described herein was developed as part of a suite of tests to be incorporated into a performance standard addressing TICs used by the fire service.