Glazing daylight transmittances: a field survey of windows in urban areas

The effect of urban air pollution on glazing daylight transmittance was investigated by measuring the percentage loss in glazing transmittance of a number of windows throughout a large city in the UK. A total of 430 windows in a range of building types and locations were used to create the database for this window survey. The measurements were taken under overcast sky conditions to minimise errors that could occur during periods of rapidly changing sky luminance. In addition, two adjacent photocells were used so that simultaneous comparative measurements were taken. In general it was observed that the loss in transmittance for a vertical window did not usually exceed 10%. The factors that most significantly reduced glazing daylight transmittance were (i) the function/use of the building or the rooms within, (ii) the inclination of the window and (iii) the shading of the window by overhangs. It is, therefore, suggested that future daylight design guidelines should include these factors.