DAYLIGHT | MODELLING A new frame of reference A radical new daylightmodelling schema allows the view from openings to become a material factor in the evaluation of building designs.John Mardaljevic looks at a fundamental change for lighting designers The notion that the principle behind the aperture skylight index could be used to meaningfully quantify view was, for this author, one of those rare eureka moments E uropean standard EN 17037 for daylight in buildings was approved in 2018. Its introduction asserts that: Daylight should be a significant source of illumination for all spaces with daylight opening(s).1 The word opening appears 139 times in the standard. This article gives a brief summary of a fundamental reconsideration of the way in which a building opening (or aperture) can be evaluated in terms of its potential to provide sunlight, skylight and views. The means of evaluation in each case are purely geometrical, so not subject to the vagaries of the performance gap. The approach designated aperture-based daylight modelling (ABDM) in 2019 was conceived to be a major upgrade of the methodologies traditionally used in planning guidelines for daylight/sunlight. Though radical in conception, the theoretical basis of the new approach is remarkably simple. So simple, in fact, that it may seem long overdue. One basic principle that distinguishes ABDM from traditional methods used in planning is that it is an area-based approach in other words, the connectedness across the entire opening is considered. Many, if not all, traditional methods for, say, sunlight evaluate at a point for example, BR 209 Site layout planning for daylight and sunlight.2 A second distinguishing feature (a consequence of the first) is that all ABDM evaluations take account of the size of the opening. This is worth emphasising because while self-evidently a desirable component for evaluation of, say, sunlight opening size is not a factor in the method used in BR 209. A third distinguishing feature (a consequence of the first two) is that the results for multiple openings whatever their size, aspect or orientation can be combined to give meaningful totals for, say, a particular dwelling. ABDM was conceived to allow for a seamless workflow progression from outline planning to detailed building performance evaluation for example, climate-based daylight modelling, overheating prediction, energy compliance, and so on. The approach evolved over four years: the sunlight part in 2015,3 the skylight part in 2017,4 and, finally, the view part in 2019,5 as discussed below. The sunlight beam index The sunlight beam index (SBI) is an area measure of the connectedness of a building aperture to all possible occurring sun positions for that locale and for that particular aspect of the aperture, including all possible obstructing surfaces averaged across the aperture. SBI has units of m2 hours. For any given period of time, SBI can be thought of as a measure of the crosssectional area of sunlight beam that passes through the aperture over that time. The aperture skylight index The aperture skylight index (ASI) is an area measure of the connectedness of an aperture to the sky vault in terms of the illumination received from a uniform luminance sky dome, averaged across the aperture. ASI has units of lumens. In contrast to the SBI, the ASI depends only on the size and aspect of the aperture it has no dimensions of time and it is not dependent on the azimuth (the angle between a celestial body sun, moon and the north) orientation of the aperture relative to the hemisphere of sky. 42 December 2020 www.cibsejournal.com CIBSE Dec20 pp42-44 Daylight modelling 2.indd 42 20/11/2020 17:41