Quantitative measurement of illumination invariance for face recognition using thermal infrared imagery

A key issue for face recognition has been accurate identification under variable illumination conditions. Conventional video cameras sense reflected light so that image gray values are a product of both intrinsic skin reflectivity and external incident illumination, obfuscating intrinsic reflectivity of skin. It has been qualitatively observed that thermal imagery of human faces is invariant to changes in indoor and outdoor illumination, although there never has been any rigorous quantitative analysis to confirm this assertion published in the open literature. Given the significant potential improvement to the performance of face recognition algorithms using thermal IR imagery, it is important ot quantify observed illumination invariance and to establish a solid physical basis for this phenomenon. Image measurements are presented from two of the primarily used spectral regions for thermal IR; 3-5 micron MidWave IR and the 8-14 micron LWIR. All image measurements are made with respect to precise blackbody ground-truth. Radiometric calibration procedures for two different kinds of thermal IR sensors are presented and are emphasized as being an integral part to data collection protocols and face recognition algorithms.