ABSTRACT
This chapter addresses the use of natural daylight in place of artificial light for architectural applications. The scientific community recognizes the Commission International de l’Eclairage (CIE) standard spectral response of the eye for daylight called the photopic response. The luminous efficacy provides a “conversion factor” to convert more readily available (or modeled) irradiance data into illuminance data. Approximate values of luminous efficacy for the sun under various conditions shows that there is at least a 40% variation in the luminous efficacy of natural sunlight, depending on solar elevation and the solar component of interest. The National Renewable Energy Laboratory (NREL) data manual presents monthly summaries of incident illuminance for horizontal windows (skylights) and vertical windows facing north, east, south, and west. These values were computed using the Perez anisotropic diffuse model. Indicatrix functions describe the theoretical distortion of the uniform hemispherical luminance spatial distribution on the sky dome as scattering centers are introduced into the radiation field.
