Since the brightness outside a tunnel is significantly higher than inside during the day, drivers require a certain amount of time for their vision to adjust before they can clearly see the interior-a phenomenon known as "adaptation lag." Without an appropriate transition, the "black hole effect" occurs, causing a temporary loss of normal visual function and posing a safety risk.
The black hole effect is a visual issue encountered before entering a tunnel and represents a critical aspect of tunnel lighting design. Consequently, tunnel lighting is typically categorized into entrance, interior, and exit lighting. Daytime entrance lighting levels are determined based on factors such as external brightness, vehicle speed, the field of view at the entrance, and tunnel length. The threshold zone is designed to eliminate the black hole effect, while the transition zone features gradually decreasing lighting levels to allow drivers time for visual dark adaptation.