An experimental verification of the correlations existing in literature for the calculation of the coefficient of emissivity inside the atmospheric transparency window, situated in the wavelength band 8-14 mu m, has been performed. In addition, the authors propose a new correlation for the calculation of the coefficient epsilon(in), easily applicable since it presupposes the knowledge of the global emissivity of the sky, for which there are a lot of correlations in literature. (c) 2005 Elsevier Ltd. All rights reserved.