Abstract: Thermal images of an object are produced outdoors so as to be significantly differentiated temporally and/or geometrically and be inclusive of natural background such as would be illuminated by sunlight. A calibration of the thermal imager (e.g., radiometer) against theoretical blackbody radiation is effected using measured incident radiation, resulting in a calculated pixel intensity. A system transfer function calculation of the pixel intensity is effected, resulting in a radiance. A matrix inversion calculation of all of the radiances considered together is effected, resulting in a bi-directional reflectance distribution function (BRDF). Advantageously, the BRDF is determined from the target in a direct but mathematically-enhanced manner, thereby making economic use of natural illumination and obviating the need for modelling, simulating or reconstructing the target.