The Flow of Light into Buildings

Abstract

A knowledge of angular relationships is essential for studying the view through a window and for calculating daylight factors. A perspective drawing contains information about the various angles subtended at a chosen view-point and thus lends itself to the study of natural lighting. However the traditional method of specifying daylight—in terms of a horizontal working plane—can give a misleading impression of the lighting conditions, which depend largely on light reaching vertical surfaces. Similarly the advent of luminance design in artificial lighting has created the need for some three-dimensional concept to replace horizontal illumination as a criterion of adequate lighting. Illumination can be analysed in scalar and vectorial terms. The calculation of scalar and vector illumination is discussed; for natural lighting each can be determined from perspective drawings. For artificial lighting the scalar illumination can be calculated from British Zonal data; this provides an index of adequacy for many lighting purposes. The vector illumination indicates the directional qualities of the lighting. The ratio of vector to scalar illumination may serve as an index of modelling.