According to the International Committee on Illumination CIE recommendations, luminance should be monitored as the luminance contained within in a conical field of view, subtending an angle of 20º. It is recommended that a luminance photometer is mounted in the tunnel approach road approximately 120 - 200 metres from the portal, depending on the approach speed of the vehicles and their stopping distance. The purpose of monitoring the luminance at the portal is to adjust the level of light intensity inside the road tunnel to the light intensity outside so that drivers do not have to adjust their eyes quickly or become affected by the “black hole” effect where they decelerate rapidly and become a hazard to other road users.
By Codel International Ltd based in Bakewell, UNITED KINGDOM.
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