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A J DowellThe Technical Characteristics and Uses of Ano-Fol Pre-Anodized Aluminium for Durable Energy Efficient Reflectors Ano-Coil Limited U.K., The Buchholz Group, Germany Summary This paper describes the technology and properties of pre-anodized aluminium, a material ideally suited to aiding energy conservation in lighting and other applications. Reflectors are a natural application for aluminium as it has the highest reflectivity of all metals commercially available at economic cost. Electro-chemical conversion of aluminium to its oxide during anodizing grows a protective layer which is transparent and intimately linked on an atomic scale to the metal itself. This gives the advantage that there are no problems with de-lamination, undercutting or peeling of the coating. The optical properties of pre-anodized Aluminium depend partly on the characteristics of the metal from which it is made and partly on the optical behaviour of the coating. The oxide film grown in anodizing is transparent and has a refractive index of approximately 1.6. Hence light falling on the surface will penetrate the surface of the oxide and be refracted in towards the aluminium surface. It therefore passes twice through the thickness of the clear anodic film during reflection and film purity and properties are very important. Control of these is critical for top quality material and, working with their suppliers, The Buchholz Group have developed ways of maximising specular reflectance to allow optimum light control. Since it is produced in flat sheet, the optical properties of Anofol material can be measured accurately by a number of instruments to characterise light output from the surface. This theoretically provides a link between the material used for manufacturing a reflector and the photometric characteristics of that reflector, whereby a material selected on the basis of its photometric properties in flat sheet could combine with a particular reflector design to achieve a known light distribution. The most fundamental measure of light output from the surface is total reflectance and this is most accurately assessed in an integrating sphere. However, reflectance varies with the angle of the incident light, with the lowest reflectance being obtained normal to the surface and the highest at shallow, grazing angles. This is particularly relevant to anodized aluminium and data is given to demonstrate this in the paper. At a grazing angle of 85 degrees pre-anodized aluminium can show higher reflectance than a front surface silvered mirror. This combination of high total reflectance and very high specular reflectance provides maximum light output efficiency and this will be demonstrated by comparison of different materials in the same reflector form. Another energy efficient use of Anofol pre-anodized sheet arises from its very high infra-red reflectivity. This, combined with its durability, has led to the development of solar collectors using large, curved sheets. Work on the development of new factory and office radiant heating is also in progress. A further aspect, very relevant to today's issues, is the environmental impact a material has in its production and use. Recent large investment in the world's most modern coil anodizing line at Ano-Coil in the UK, incorporates the anodising process. Examples pertaining to this and the energy savings possible with the material will be described.
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