Earth's itself represents the second (after Sun) most powerfull natural source of e.m. radiation for EO. Its average emittance can be approximated by that of a blackbody at about 290 K. Even if very less powerfull than Sun such a source is available for EO day and nigth. The maximum of its emission falls in the thermal infrared (around 10 micron) being Earth's emission trascurable in the VIS-SWIR range.