Daylight coefficients for practical computation of internal illuminances

Abstract

The daylight coefficient approach developed by Tregenza and Waters relates the luminance of an element of sky to the illuminance it produces at a point in a room. Its use can speed up daylight calculations. Once a set of daylight coefficients has been computed, it is possible to find daylight illuminances under a large number of sky luminance distributions with minimal extra effort. Because it relates to a point source of light the daylight coefficient technique can also be used for the calculation of reflected sunlight; and it is especially suitable for innovative daylighting systems with complex optical properties. This paper describes computer programming strategies to make effective use of daylight coefficients. It outlines the practical implementation of such a program to calculate year-round internal illuminances. To realise the efficiency benefits of the technique, it is important to calculate as few individual daylight coefficients as possible. This can be done by using Gaussian integration and by separating out the calculation of direct sunlight and internally reflected light. The paper describes the interface between the sky modelling technique and simulation of internal reflection, and the modelling of blinds and innovative daylighting systems. The special problems of external obstructions and ground-reflected light are also discussed.