Tunnel lighting : Method of calculating luminance of access zone L20

Abstract

To determine the luminance levels required in tunnel lighting installations a safe maximum value of the luminance of the access zone L ₂₀ has to be evaluated. In the CIE guide for the lighting of tunnels and underpasses two methods are described for estimating this value from practical experience of the tunnel orientation, the type of landscape and the proportions of sky, road and landscape in the 20° field. In this paper a new method based on the CIE philosophy is described which allows a more precise determination of this safe maximum value. It is based on the known empirical relations for the luminances and illuminances generated by daylight and the mean relative sunshine time distribution during the year for the region in question. The safe maximum value is obtained from the cumulative frequency distribution of the L ₂₀ values computed over the whole year.