Light pollution index (LPI): An integrated approach to study light pollution with street lighting and façade lighting

Rather than being complementary, street lighting and façade lighting are typically designed independently of each other. As a result, light from street lighting luminaires might spill onto building façades and influence characteristics of the façade lighting including required light levels and color. Conversely, façade lighting might reflect onto the street and contribute to the street illumination. This manuscript presents an integrated approach to analyzing the interaction between the street and façade lighting in consideration of light pollution control. A generic street model with various calculation grids was studied. It was found that a moderately lit façade contributes to a horizontal illuminance of 5 lux or more on the street and a vertical illuminance of 4 lux or more 1.5 m above the sidewalk. An over-illuminated street may result when both street and façade lighting exist without reference to each other. The size of the light-pollution calculation grid and the use of a 3-D illuminance graph were studied. It was found that fluctuations in the calculated amount of light pollution could occur unless the size of the pollution calculation grid is large enough to capture all of the upward flux. A method of achieving the optimal size of the calculation grid is demonstrated. Shortcomings of using light pollution percentage as a light pollution index are highlighted and an alternative light pollution index is proposed.