# Reliability Enhancement Driven by ANN for Lighting Control System in Highway Tunnels
## 2 Research Status of Tunnel Lighting System
### Progress of Tunnel Lighting Strategies  
#### 2.3.1 Characteristics of Visual Adaption in Highway Tunnels  
Light and Dark Adaption  
The tunnel entrance and exit are transition sections in the vehicle operating environment. Driving from the outside light environment to the inside artificial light environment through the tunnel entrance, or in the opposite direction driving out of the tunnel exit, the light environment differences between inside and outside exist objectively. If this difference is stark, it is likely to result in visual lag for drivers and cause short-term visual impairment. This is often called the black hole or white hole effect [14]. The reason for this visual impairment is a result of the inherent characteristics of human eyes, which cannot instantly adapt to a sharp increase or decrease in luminance level. This visual adaptation is divided into light adaptation and dark adaptation. Light adaptation is from the dark to the light, eyes adapt to the increase in luminance level and are able to recover vision. Dark adaption refers to the changes in luminance level in the opposite direction and the visual system adapts to the low luminance level. Research has shown that dark adaptation takes place much more slowly than light adaptation [15; 16], the whole process of light adaptation is short and it usually takes only tens of seconds in an artificial light environment, this indicates that the visual discomfort for drivers caused in tunnel entrance is more severe than that in tunnel exit, i.e., the black hole effect is more dangerous than the white hole effect. If drivers fail to react appropriately to light adaption or dark adaption, the adverse effect will disturb drivers' recognition of vehicles or obstacles ahead, as well as driving stability, driving safety and comfort, meaning that increased stress is imposed on drivers, eventually resulting in operational errors and traffic accidents. Driver Adaption in Daytime and Nighttime  
As defined by CIE in 1983, photopic vision refers to the vision when the luminance level is more than a few cd/m\({}^{2}\) (normally 3 cd/m\({}^{2}\)). At this time, vision is mainly operated by cones and maximum visual response is at 550 nm in the blue-green range of the spectrum. Scotopic vision refers to the vision when the ambient luminance level is lower than 0.01 cd/m\({}^{2}\). Rod cells are the main photoreceptors for scotopic vision and the peak of spectral optical efficiency is at about 490 nm. The luminance level of mesopic vision falls in between scotopic vision and photopic vision, cones and rods respond simultaneously to changes in luminance level. In daytime or when the luminance level is greater than 3 cd/m\({}^{2}\), this is considered photopic vision with better light conditions, and obstacles ahead are easy to identify and visual recognition time is shorter than that at nighttime. At nighttime or when the luminance level is lower than 0.01 cd/m\({}^{2}\), little perceptible difference between obstacles and the background causes an increase in visual recognition time. Due to different luminance levels and the regulatory role of eyes, visual information captured by drivers differs greatly between daytime and nighttime. In low-illumination tunnel environments, frequent changes in light and dark phenomena often exist. Even if tunnel lighting is used, since lighting illumination is basically similar to environment illumination, visual recognition conditions of obstacles in the field of view are not significantly improved, which can easily lead to ambiguous visual recognition and erroneous distance judgment. In the process of visual adaptation, drivers are often unable to perceive obstacles ahead clearly and instantly, and insufficient visual information regarding obstacles will degrade the quality of visual recognition and endanger driving safety.