The ILUMA model was developed for the prediction of natural illumination under realistic atmospheric conditions that includes clear skies, partly cloudy and overcast conditions, precipitation, and fog. The calculation of transmission of visible light through the atmosphere is based upon a three-layer radiative transfer model developed by Shapiro. This model responds to input values of the types and amounts of high-, middle-, and low-level clouds. The fractional transmittances are used to adjust Brown's empirical data that provide illumination as a function of zenith angle for clear conditions. Approximate orbit calculations are used to determine the angles of the sun and the moon. These calculations are based upon the assumption of an elliptical orbit and employ extrapolations from known positions at a reference date, 1 January 1977. These calculations provide values of zenith angles that are probably accurate to within 0.1 degree, which are adequate of illumination calculations. The total illumination is computed as the sum of the contributions for the sun, the moon, and the background sky. A value of 0.00015 footcandle is assumed for the illumination from the background sky. The contribution for artificial light sources, for example, nearby towns, is neglected as are the effects of shadows from natural and man-made objects. Available data taken at military installations in the United States were used to evaluate the model. The results of these comparisons, which show that the model predicts reasonable values, are included in tabular form.

PcEosael Overview
Alphabetical List of EOSAEL Modules
Functional List of EOSAEL Modules

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