The Impact of Overhang on the Risk of Light Trespass in a Fully Glazed Office Building

Light pollution is an environmental problem occurring with the improper application of artificial lighting systems. Light trespass, one significant type of light pollution, is difficult to be measured and evaluated in urban areas. In Beijing, a fully glazed office building could become a possible source of obtrusive light except for the street lighting, due to a fact that the security and nightscape requirements keep the indoor lighting on overnight. This article studied the impact of external overhang on the light trespass risk using Radiance simulation in a multi-story fully glazed office building. Two typical types of external overhang have been investigated: horizontal, tilt angle - 10 degree. For each type, three overhang depths were used: 0.6m, 1.2m and 1.8m. The illuminance levels at a vertical calculation plane (with a 10m distance to façade) were calculated by Radiance, which express the magnitude of light trespass from the glazed façade. Several important findings have been achieved: averagely, the longest overhang (1.8m) reduces a relative illuminance level of 17% with the bare window as a reference, while the shortest one only has a relative reduction of 5.6%; for the protection of trespass light, the overhang works well at a height of around 5/7 façade above ground. The methods would be possibly used for the light pollution investigation in a large scale urban area.