Transparent heat mirrors: influence of the materials on the optical characteristics

Transparent heat mirrors of the antireflecting metal type with a dielectric/metal/dielectric structure are studied. It is found that dielectrics of lower refractive index give a higher cutoff wavelength, but the transition becomes more gradual. The angular behavior of these mirrors is also analyzed. The optimum mechanical efficiency obtainable with a plain collector and a carnot engine is given for different concentrations and refractive indices. Moreover it appears that using Al instead of Ag introduces appreciable absorption losses (~25-35%).