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It is often complex and difficult to predict the daylighting of an atrium building. The atrium is usually expected not only to be an attractively daylit space in its own right, but also to deliver light to adjoining spaces. Most of the light reaching these spaces will have been reflected within the atrium. This paper outlines current guidance on daylighting design for atria and reviews published techniques to calculate daylight within them, including a methodology to predict daylight factors within atria and the surrounding spaces.

Sky luminance distribution is one of the most important quantities for calculating daylight in buildings and the associated energy savings. At the Building Research Establishment, Garston, sky luminance distributions have been measured on a long term basis for the first time in the UK. These data have been used to assess a number of models which aim to predict the hourly variation of sky luminance distribution. It is concluded that the three best models analysed would all perform well in a computer program to simulate internal daylight levels and lighting energy use.

Within the UK, regional variations in climate and latitude (1,2) can cause differences in the relative amounts of daylight available; yet existing methods for predicting daylight in interiors are mainly based on daylight data from south-east England. There is increasing interest in using daylight to save lighting energy in buildings; however, it is possible that the benefits of daylighting may vary significantly from region to region throughout the UK. This paper reports on a study of the magnitude of this variation in lighting energy use, based on the model of manual switching in daylit spaces derived by Hunt(3,4). Analysis of meteorological daylight data(5) for five locations has produced, for the first time, curves which can be used to predict hours of use of manually controlled lighting in daylit buildings in various parts of the UK. These curves are presented, together with an analysis of the regional differences in lighting use for various example situations. The significance and probable causes of these variations are discussed, and the need for more daylight measurements in different parts of the country is explored. [ABSTRACT FROM PUBLISHER]

Luminous efficacy relates two important quantities-daylight and solar radiation. Its use enables calculations of daylight availability and passive solar gain in buildings to be integrated to a large extent. Luminous efficacy also enables daylight data to be generated where directly measured illuminances are not available. This paper reviews measured and calculated luminous efficacies obtained by authors in various parts of the world. For direct sunlight luminous efficacy increases noticeably with solar altitude (70-105 lm/W). For diffuse skylight average luminous efficacies range from around 130 lm/W (clear) down to around 110 lm/W for overcast skies. Global efficacies are around 105 lm/W for clear skies and for average conditions. In each case'measured luminous efficacies are not constant but can vary over a substantial range depending, for example, on solar altitude, atmospheric aerosol and water vapour content, and type and amount of cloud. These effects are examined in detail and their influence on luminous efficacy discussed. Finally an Appendix describes the experimental measurement of luminous efficacy and outlines the types of experimental error that can occur. [ABSTRACT FROM PUBLISHER]

Daylight in buildings can save energy by reducing artificial lighting energy consumption. An accurate estimate of internal daylight availability is required for energy use predictions and to include the effect of window orientation this must be based on an adequate model of sky luminance distribution. For such energy calculations the 'average sky' (based on measurements for a wide range of real skies) is recommended as a replacement for the CIE overcast sky. This paper analyses luminance data obtained by Wegner to show that the luminance distribution of an average sky is of the form L = ae-by + c where y is the angle in degrees between the sun and the element of sky under consideration, b is approximately 0.025 and a and c are functions of solar altitude alone, and, significantly, do not depend on the altitude of the element of sky. Using this formula together with a direct solar component the paper derives expressions for horizontal and vertical illuminances, and the average illuminance inside a side-lit room; and briefly outlines further work, under way at BRS, to evaluate the model for weather conditions in the UK and to apply it to problems of daylighting design. [ABSTRACT FROM PUBLISHER]

This article reports that a project sponsored by the BRE Trust and CIBSE SLL has analyzed the utility of new metrics for daylight design, based around climate-based daylight modeling (CBDM). CBDM employs concepts like useful daylight illuminance (UDI) to categorize daylight provision. The author feels the target illuminance above which daylight is considered sufficient, and the maximum illuminance, above which it is considered excessive, vary between authors and standards.