This paper resulted from activities associated with a Hong Kong RGC funded project, 'A new schema to quantify solar access and daylight availability for dense urban environments' (2002-05), on which the author was a Co-investigator. Key to that project was the proven accuracy of the simulation engine (ie Radiance) used to predict realistic measures of daylight availability. A proponent of the long-standing daylight factor measure of daylight availability for Hong Kong had earlier published findings that the Radiance system gave unreliable results in dense urban settings. This paper definitively rebuffed those claims, proving that the findings from that researcher's study were due to assumptions about the model representation and not the intrinsic accuracy of the simulation system itself. This paper was a significant factor in obtaining a follow-on Hong Kong RGC funded project with the author again as Co-investigator. This project 'Precision modelling of window apertures in cities for energy saving, comfort and amenity', will begin in late 2007.