Time-varying characteristics of the luminance of components near the tunnel opening and luminance L 20 in the access zone

The time-varying characteristic of the access zone luminance L20 and that of the component luminance near the tunnel opening are of significant importance in the design and operation of tunnel lighting. Through measuring typical components' luminance and L20 in actual tunnels, the present study analyzed the temporal changes and primary factors of components' luminance and those of L20 under various driving directions. Theoretical analyses revealed luminance differences of the component under different driving directions is primarily influenced by the main parameter. Specifically, the main parameter of luminance of the sky is the shortest angular distance between a sky element and sun (chi), that of the road is the azimuth angle measured between the two vertical planes containing the directions of direct sunlight and observation (beta), and that of the surroundings is the absolute value of the azimuth difference between the normal to vertical plane and the sun (aZ). Experimental results demonstrate that, temporal variations of the above main parameters are significantly correlated with those of corresponding component's luminance. Experimental results show that the luminance peak of the component is the highest at the eastward or westward driving direction, followed by the southward and northward driving direction, respectively. The ratio between component's luminance peak when driving east (or west) and the average of component's luminance peak when driving south and north is as follows: 3.5:1 for the sky, 1.8:1 for the road, 2.8:1 for the end wall, and 1.4:1 for the vegetation. Additionally, the luminance peak of the components when driving east and west both occurs around 8:00 and 16:00. When the driving direction is southward, the luminance peak of sky and road occurs around the noon, and the surroundings' luminance remains high throughout the day. While for the northward driving direction, the luminance peak of surroundings occurs around the noon, and the luminance of sky and road remains high during the day. The recommendations of CIE/ CEN and IES on the luminance peak of the components in different driving directions merely reflect the sequence of the component luminance under different driving directions at around 12:00, which has a significant discrepancy with the current experimental results. Furthermore, the peak of L20 and its occurrence time are primarily influenced by components with the larger area proportion and higher luminance within the 20 degrees field. The temporal variation of primary parameters of the components can be used to effectively predict the occurrence time of the peak of L20. In addition, by combining the surface reflectance of the typical components near the tunnel opening, an accurate prediction of L20 can be further realized.