Analysis Methods

When measuring daylight performance in buildings one needs to have an understanding of various ways to describe overall success and failure of a particular design. This is especially crucial when evaluating design decisions using digital or physical models. This chapter is intended to provide the foundational knowledge regarding illuminance, luminance, and direct sunlight based daylight metrics. Part 1 will focus on illuminance-based metrics, that is, light measured as it falls on a surface, usually the horizontal workplane. In the United States, this measure is given the unit of footcandles, and in the metric system it is given the unit lux. 100 lux = 9.29 footcandles due to the difference between a square foot and a square meter, but we will use the simple 10:1 relationship throughout. Part 2 will provide some guidance on luminance based daylight metrics, or light measured as it is reflected from all surfaces toward a given viewpoint. Luminance is given the unit of foot-lamberts (US) or candela/m2 (metric).

It is also important to note that some metrics are considered at a given point in time (static metrics) while some metrics are considered on an annual basis (dynamic metrics). Historically, static metrics have been more common but there is momentum building for dynamic metrics, especially with regard to the measure of illuminance. Dynamic or annual daylight metrics can be calculated in at least two dramatically different ways. They can be considered with the raw architectural geometry only, assuming no interior furnishings or blinds, or they can be considered with all interior furnishings and with assumptions about users’ blind operation habits. The data represented in “Analysis Methods” considers only the raw architectural geometry with no furniture or blind operations. That is, the blinds are assumed to be open all the time regardless of sun position or sky condition.

The two buildings shown on this page will be used throughout the Analysis Methods document as test cases to show the ability of various analysis metrics to describe performance. The thin plan of AllianzKai and the deep floor plate of the AON Center provide a dramatic contrast with regard to daylight performance. However, other aspects of the buildings have been simplified, such as all blinds were removed, and the window to wall area ratios were normalized at 40% despite their differences in reality.