The Useful Daylight Illuminance Paradigm : A Replacement for Daylight Factors

The daylight factor persists as the dominant evaluation metric because of its simplicity rather than its capacity to describe reality. Defined as an illuminance ratio under the CIE overcast sky, the daylight factor is insensitive to both the prevailing local climate and the azimuth component of glazing orientation. The drive towards sustainable, low-energy buildings places increasing emphasis on detailed performance evaluation at the early design stage. Recent advances in lighting simulation techniques have demonstrated that reliable predictions of time-varying daylight illuminances founded on hourly climatic data is now attainable. These simulations predict hourly internal daylight illuminances for a full year using realistic representations for sun and sky conditions. Data from these simulations can be used to predict the performance of daylight responsive lighting controls. Of equal significance is the nature of the true daylighting performance of the space this is “contained” in the time-series of absolute illuminance values. The processing and analysis of voluminous time-series data is currently the preserve of engineers/scientists. But might not architects and designers benefit from some characterisation of daylighting performance that is founded on a time-series of realistic daylight illuminance data? The recently formulated useful daylight illuminance (UDI) paradigm is a scheme to process annual time-series of illuminance data to give a concise characterisation of the true daylighting performance of a building. Achieved UDI is defined as the annual occurrence of daylight illuminances across the work plane where the illuminances are within the range 100lux to 2000lux. These limits are based on a survey of reports of occupant preferences and behaviour in daylit offices with user-operated shading devices. The degree to which UDI is not achieved because illuminances exceed the upper limit is indicative of the potential for occupant discomfort. The UDI paradigm also gives significance to those daylight illuminances below the typical design threshold (e.g. 500lux), but which are nevertheless known to be valued by occupants and which also have the potential to displace all or part of the electric lighting. The UDI paradigm preserves much of the interpretive simplicity of the familiar daylight factor approach. In contrast to daylight factors however, UDI is founded on absolute values of illuminance predicted under realistic skies generated from standard meteorological datasets. This paper describes the application of the UDI paradigm to assess both side-lit office spaces and a large open plan area with light-well. Introduction The exploitation of daylight is recognised as an effective means to reduce the artificial lighting requirements of non-domestic buildings. In practice however, daylight is a greatly under-exploited natural resource. Significant amongst the various reasons for this may be the lack of realism of the standard predictive method: the daylight factor approach. A new schema to assess daylighting potential was introduced by the authors in a recent paper [1]. Called Useful Daylight Illuminance (or UDI), the schema preserves much of the interpretive simplicity of the familiar daylight factor approach. Useful Daylight Illuminance however is determined from absolute values of time-varying daylight illumination for a period of a full year. This method of daylight illuminance modelling we call ’climate-based’ since the (hourly) sun and sky conditions are founded on values from annual climate datasets (e.g. direct normal irradiance and global horizontal irradiance from TMY2 files). The idea of daylight autonomy has been used by others to evaluate the illuminance predictions from climate based analyses [2]. Daylight autonomy is a measure of how often (e.g. percentage of the working year) a minimum work plane illuminance threshold of 500 lux can be maintained by daylight