The Rayleigh–Jeans Law is an approximation of the Planck’s law for a blackbody that states that emitted radiance is directly proportional to the blackbody temperature. Such an approximation, fits quite well with EO measurements at wavelengths higher than 1mm (microwaves). Wien’s approximation is used to describe the spectrum of the blackbody emission in the VIS-NIR spectral range lengths. The estimated errors are less than 2% at wavlengths less that 5microns. In both cases considered sources are the natural ones: the Earth at an average temperature of 300 K in the first case, the Sun at about 6000K in the second one.