Transmission of the flux as a function of the wavelength determined by the measurements (black lines) compared to the measurements obtained from the complete radiative transfer through the cell, taking into account the real temperature variation along the optical path, a) 538 K; b) 644 K; and c) 803 K (red line), and the measurements obtained when considering the mean temperature a) 510 K; b) 610 K; and c) 750 K (blue line).