A method for evaluatina the contribution of background flux reflected from a sample is shown. An example is described showing that the reflected background contribution in a thermographic examination is about 2% and the effect of opening a window is about 0.5%.
William L. Wolfe, William L. Wolfe,
"Effects Of Reflected Background Radiation On Radiometric Temperature Measurement", Proc. SPIE 0226, Infrared Imaging Systems Technology, (24 June 1980); doi: 10.1117/12.958732; https://doi.org/10.1117/12.958732