A large-dimension experimental setup, relying on the principle of the box method, for measurements of infrared emissivities of ground surfaces is described and discussed. Emissivity is obtained over an area of 1 m2 and is spectrally integrated according to the radiometer bandwidth. Analysis of the different radiance contributions to the signal and careful calibration procedure led to correction factors which have been experimentally determined. The asolute accuracy in the <10.5--12.5> μm band is 0.5%. Tests with sample material (SiO2, SiC) have shown excellent agreement with the corresponding quantities, calculated from the absolute emissivity signature measured in the laboratory. A similar observation was achieved for bare, flat, and dry surfaces studied in natural conditions as well. However, the results involving agricultural soils have given rise to roughness and moisture considerations. They have also led to the enhancement of the complexity of field measurement interpretation and the need to consider heteroeneity aspects. ¿ American Geophysical Union 1990