Assessing Daylighting Performance of Buildings Using Orthographically Projected Area of Obstructions

Recently the Hong Kong Government has issued a performance-based code governing daylighting in buildings to supplement its prescriptive code. The performance-based method uses the vertical daylight factor (VDF) at the centre of the window pane in the specification of the performance standards. However, the calculation of VDF is not an easy task and is especially not suitable in the early design stage when designers may need to evaluate the daylighting performance quickly using simple sketches and limited data. In view of this, the current performance-based code allows the use of a simplified alternative method called the unobstructed vision area (UVA) method. In this paper, a review of the UVA method is first given. Then a method based on the orthographically projected area (OPA) of all obstructions above the reference point is presented. The OPA can be determined with similar efforts as determining the UVA. Case studies comparing the performance between the OPA and UVA methods are presented. Results show that the OPA method predicts the VDF reliably and therefore is truly a simplified performance-based method.