  The total emissivity of the absorbing surfaces is a critical parameter in the
calculation of the radiative thermal losses in solar thermal collectors. This
is because the radiative heat losses have a significant economic impact on the
final cost of the electricity produced in a solar thermal plant. This paper
demonstrates the need to calculate the total emissivity from spectral
emissivity measurements at the working temperature of the solar thermal
collector, instead of using extrapolated values from spectral emissivities
measured at room temperature. Usual uncertainties produced by the estimation of
the total emissivity, in which its temperature dependence is only introduced by
the Planck function, are analyzed.
