Cubic illumination

Abstract

Cubic illumination specifies the spatial distribution of illumination about a point in terms of the illuminances on six faces of a small cube centred at the point. This form of specification is proposed as the basis of a system of applied photometry. Its advantage is that once the cubic illumination has been determined, whether by measurement or calculation, a range of indices of lighting may be derived quite readily. Analysis of the cubic illumination specification comprises defining two components of the illumination solid; the vector component defined by the illumination vector, and the symmetric component, which remains after subtraction of the vector component. The planar illuminance for any orientation, and the average illuminances of regular solids, such as a cylinder or sphere, are calculated by summing the contributions of these two components. By specifying a direction of view towards the point, indices which relate to how lighting might affect the appearance of a three-dimensional object located at the point may be calculated, and by specifying a direction of view from the point, the eye illuminance, or the illuminance of a plane normal to the direction of view, may be derived. The paper gives procedures for calculating, measuring and analysing cubic illumination, and reviews applications for the derived illumination metrics.