The hemispherical emittance of the selective absorbing coating on the outside of the inner glass tube of an all-glass evacuated collector tube has been determined, using calorimetry at steady state in the temperature range 50-300 degrees C and gas pressure range 1.0 x 10(-3)-1.6 Pa at the jacket between cover and inner glass tubes. Calculated gas heat flux q(c) and equivalent emittance epsilon(g) based on the theory of gas conduction at medium and low pressures have been determined. The calculations agree well with experiments. The experimental results indicate that heat losses of all-glass evacuated collector tubes due to gas convection and conduction are negligible when the gas pressure in the tube is less than 5 x 10(-2) Pa.