Sun represents the most powerful natural source of e.m. radiation for EO. Its emittance can be approximated by that of a blackbody at about 6000 K but just its reflected component (SOR) is actually available (and just during daytime) for EO. The maximum of SOR falls in the visible spectral range. Its contribution in the thermal infrared range is neglectable but in the medium infrared SOR is still significant enough and superimposed to Earth's thermal emission.