Sun represents the most powerful natural source of e.m. radiation for EO. The main source of its radiation is the nuclear fusion of Hydrogen into Helium which occurs in central part (“Core”) of the Sun. Outside, the energy transfer is dominated by radiative process (“Radiative zone”) then by convection (”Convective zone”). Solar radiation at the Top of the Earth Atmosphere comes from the outer layer of the sun, the photosphere, whose estimated (conventional) temperature is 6000-6300 K. 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, in daytime, superimposed to Earth's thermal emission. The high intensity of solar refelcted radiation (SOR) coupled with the high atmospheric transmittance in the VIS/NIR range, guarantee the highest signal-to-noise ratio for sensors operating in that spectral range. This huge amount of available signal, together with the development of advanced micro-sensor technology (started with the Charged Coupled Devices - CCD etc.), explains why the EO passive sensors with the highest spatial and/or spectral resolution presently achievable, are operating in the VIS/NIR range.
achievable by operating in this spectral region.