How is light trespass measured or calculated?
The Institution of Lighting Engineers (ILE) specifies light trespass limits for light entering windows in terms of environmental zones (ILE 2000). (See “What are lighting environmental zones?”) It is difficult to measure light trespass because the occurrences are so different. Illuminance on a vertical plane (for example vertical illuminance at the window, as depicted in Figure 4) may be appropriate in some cases. Horizontal illuminance might be appropriate in other cases (for example, horizontal illuminance on a bed). An illuminance level of 1 lux (0.1 footcandle) might be acceptable to some and completely objectionable to another. For reference, 0.3 lux (0.03 footcandle) is typical of the illuminance that results from moonlight.
In a recent study, Boyce et al. (2001) suggest that light trespass be measured in a relative manner, at property boundaries, to take into account both light coming into the property as well as light inside the property. When designing the lighting for a site, imagine a vertical plane at the boundary between the site property and an adjacent property. Measure the illuminance on the vertical plane at the property boundary, directed toward the lighting design site, 180° away from the adjacent property. Then rotate 180° away from the lighting design site, and measure the illuminance on the vertical plane, directed toward the adjacent property. Calculate the ratio of the illuminance on the plane pointing toward the lighting design site to the illuminance on the plane pointing toward the adjacent property. If the ratio of the measured illuminances is less than unity (one), this implies that the design site receives more light from the adjacent property than the design site delivers to the adjacent property. If the ratio is greater than unity, then the design site delivers more light to the adjacent property than the adjacent property delivers to the site. The larger the ratio, the more likely light trespass complaints are to occur.