The heat transmission through double glazed windows is for a large extent determined by the heat barrier created by the gap between the two sheets of glass. The gap must be small enough to avoid convection, the filling gas should have minimum thermal conductivity. When these conditions are satisfied there remains the need to minimize radiation. The amount of heat radiated by a surface is proportional to its emissivity. The emissivity of normal glass is close to one (0.9) and to reduce it one applies reflecting coatings on the glass.
New coatings are still being developed although this technique has already a very wide application.
The determination of the emissivity can be done by spectrophotometry or by means of radiometry. Measurements are difficult and of relatively high uncertainty.
The objective of the project was therefore to study a simplified method (based on the use of reference materials) and with an uncertainty not higher than a few per cent.
A new procedure has been developed and tested which allows the evaluation of the total normal and hemispherical emissivity of the surface of solids in the temperature range between 50 C and 100 C, in a considerably shorter interval than with conventional procedures. The receiver system has been calibrated in such a way that no comparison measurements must be carried out between sample and blackbody; this calibration can be checked and controlled with reference samples. The uncertainty is of 0.02 for the total normal emissivity and of 0.03 for the total hemispherical emissivity. Suggestions are given to extend the useful temperature range.