Minimum acceptable transmittance of glazing

This study identifies a minimum transmittance of glazing suitable for use in modern offices, based on the criterion that 85% of people consider the transmittance acceptable after briefly viewing a window fitted with the glazing. Minimum acceptable transmittances were measured for a range of conditions by having 25 subjects view a real sky and scenery through the window of a model office. The sky and scenery were viewed through clear glass, a brightness-enhancing glass and a brightness-reducing glass, under two sky conditions and at two interior illuminances. The relationship between the subjects' responses and the physical conditions was modelled using logistic regression. It is concluded that the minimum acceptable glazing transmittance lies in the range 25% to 38% The variation in minimum acceptable transmittance is associated with the sky conditions and the type of glass used, the brightness-enhancing glass being acceptable at a lower transmittance than the spectrally neutral and brightness-reducing glass types.