Developing performance metrics for thermal imaging cameras

Thermal imaging cameras detect infrared radiation to semi-quantitatively determine the thermal profile of the surroundings. Previously, no standard performance measurements have been established for thermal imaging cameras in applications such as search and rescue and firefighting. The spatial contrast resolution in realistic atmospheric conditions was assessed for several TIC through full scale fires of methanol, toluene, and common household materials. Absorbing gas-phase combustion products greatly affected the ability of the TIC to measure local contrast in temperatures as a function of spatial frequency. The cost, labor, and repeatability of full scale fire testing make it difficult to include such tests as a standard metric and relatable bench scale options are explored. In this regard, performance metrics have been based on a voltage output signal to determine image quality. A test was developed using a thermal gradient target to relate the output signal to the luminance projected by the LCD screen seen by the user. The target produces a voltage ramp in the output signal which is recorded in 8 bit digital tape format and is compared to the luminance profile measured by a 16 bit CCD camera array with an IR filter. The relationships between the signals were nonlinear and unique to each imager tested. Measurements of the output signal only do not describe the final performance and a complete standard performance evaluation must measure the display luminance. A standardized test for determining the relationship between output and display luminance including gradient target design, luminance measurement system, and optical components has been proposed for the National Fire Protection Association.