Image Frequency Analysis for Testing of Fire Service Thermal Imaging Cameras

A growing number of first responders are purchasing infrared cameras (thermal imagers). Thermal imagers provide critical information for many firefighting operations: size up, tracking of fire growth, location of victims and egress routes, overhaul, etc…. Although thermal imagers have proven to be useful tools for the fire service over the past 10–15 years, there is no existing comprehensive performance standard for thermal imagers used in fire service applications. A suite of thermal imager performance metrics and test methods is being developed for possible inclusion in a proposed standard on thermal imagers for the fire service. The following goals apply to two of the imaging performance metrics in this suite: to explore the feasibility of a new spatial resolution metric and test method, the Random Modulation Transfer Function (MTF); and to transfer the essential characteristics of complex fire scene images to a thermal target for the effective temperature range test using image frequency content analysis. Images of humans standing in an upright position at varying distances from the thermal imager under test and wearing either street clothes or turnout gear were analyzed for frequency content. These data were then used to determine the frequency of a bar target for the bench-scale effective temperature range test.