The sunlight beam index: a new metric to quantify the sunlight potential of arbitrarily complex building apertures

This paper describes a fundamental rethinking of the basis for the evaluation of the sunlight potential of spaces. It aims to provide a methodology to answer the question: how much sunlight can enter a room? The measure proposed is the cross-sectional area of sunlight beam that: (a) passes through a window; and, (b) enters the main volume of the internal space. The new measure – called the sunlight beam index – is described, and examples are given for a realistic residential house. The sunlight beam index (SBI) is determined for all full year on a timestep basis (e.g. 15 minutes) but can be aggregated into monthly or yearly totals. The annual total provides a single measure for: one window; a group of windows; or, all the windows for an entire dwelling. Thus the measure is ideally suited for rating, planning and/or guideline purposes