Thermal analyses for energy conservation or solar energy applications require knowledge of the solar radiation incident on buildings or collector surfaces which are typically inclined with respect to the horizontal. In the absence of solar radiation measurements on an appropriately tilted surface, this radiation may be estimated by one of several methods which usually require values of diffuse and direct components on a horizontal surface. Other applications, such as focusing or concentrating solar collectors, require values for the direct beam radiation, which, in the absence of direct beam measurements, may be estimated directly from the global component or indirectly from the difference between global and estimated diffuse components.

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