Daylight simulation : validation, sky models and daylight coefficients.

Abstract

The application of lighting simulation techniques for daylight illuminance modelling\nin architectural spaces is described in this thesis. The prediction tool used for all the\nwork described here is the Radiance lighting simulation system.\nAn overview of the features and capabilities of the Radiance system is presented.\nDaylight simulation using the Radiance system is described in some detail. The\nrelation between physical quantities and the lighting simulation parameters is made\nclear in a series of progressively more complex examples. Effective use of the interreflection\ncalculation is described.\nThe illuminance calculation is validated under real sky conditions for a full-size office\nspace. The simulation model used sky luminance patterns that were based directly\non measurements. Internal illuminance predictions are compared with\nmeasurements for 754 skies that cover a wide range of naturally occurring\nconditions. The processing of the sky luminance measurements for the lighting\nsimulation is described. The accuracy of the illuminance predictions is shown to be,\nin the main, comparable with the accuracy of the model input data. There were a\nnumber of predictions with low accuracy. Evidence is presented to show that these\nresult from imprecision in the model specification - such as, uncertainty of the\ncircumsolar luminance - rather than the prediction algorithms themselves.\nProcedures to visualise and reduce illuminance and lighting-related data are\npresented.\nThe ability of sky models to reproduce measured sky luminance patterns for the\npurpose of predicting internal illuminance is investigated. Four sky models and two\nsky models blends are assessed. Predictions of internal illuminance using sky\nmodels/blends are compared against those using measured sky luminance\npatterns. The sky model blends and the Perez All-weather model are shown to\nperform comparably well. Illuminance predictions using measured skies however\nwere invariably better than those using sky models/blends.\nSeveral formulations of the daylight coefficient approach for predicting time varying\nilluminances are presented. Radiance is used to predict the daylight coefficients\nfrom which internal illuminances are derived. The form and magnitude of the daylight\ncoefficients are related to the scene geometry and the discretisation scheme.\nInternal illuminances are derived for four daylight coefficient formulations based on\nthe measured luminance patterns for the 754 skies. For the best of the formulations,\nthe accuracy of the daylight coefficient derived illuminances is shown to be\ncomparable to that using the standard Radiance calculation method.\nThe use of the daylight coefficient approach to both accurately and efficiently predict\nhourly internal daylight illuminance levels for an entire year is described. Daylight\ncoefficients are invariant to building orientation for a fixed building configuration. This\nproperty of daylight coefficients is exploited to yield hourly internal illuminances for\na full year as a function of building orientation. Visual data analysis techniques are\nused to display and process the massive number of derived illuminances.