Average daylight factor for clear sky conditions

Abstract

An average daylight factor is proposed for sunny climates on the basis of two assumptions. The first is that the illuminance on the external plane of a vertical window (as a result of direct skylight, reflected skylight and reflected sunlight) is approximately equal to the horizontal diffuse illuminance as a worst condition, where direct sunlight is not included in either quantity. Although the illuminance on the vertical window plane may vary considerably from the diffuse illuminance, such an assumption reduces the complexity of estimating the illuminance on the window under clear sky conditions. The second assumption is a new equation to predict the average internal illuminance on the working plane as a proportion of the illuminance on the external surface of the vertical window plane.