In collaboration with Loughborough University, this project 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:

• Passes through a window, and
• 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 a 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.