**9.4 Solar control and low thermal emittance materials**

A thin homogeneous metal film is found capable of combining transmission in short wavelengths up to about 50% with high reflectance in long wavelengths (Okujagu, 1997; Wooten, 1972). The required thickness of such film, using copper, silver and gold is about 20mm. if the films are thinner, they will break up into discreet islands of strong absorptance of visible wavelengths.

Enhancement of luminous transmittance to more than 80% without significantly impairing the low thermal transmittance can be achieved by embedding the metal in anti-reflecting dielectric with high refractive index layers, such as Ti and O2. In the alternative to the metal base coatings, we may use dope Oxide semiconductor. However a wide band gap is needed in the semiconductor to permit high transmittance in the luminous and solar spectral range. To make the material metallic, electrically conducting and infrared reflecting for wavelengths exceeding a certain plasma wavelength, it requires doping to a significantly high level. Semiconductors suitable for this are: oxides based on zinc, cadmium, tin, lead and thallium and their alloys.
