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.
