NIGHT SKY AND DARK ENVIRONMENTS: BEST MANAGEMENT PRACTICES FOR ARTIFICIAL LIGHT AT NIGHT ON BLM-MANAGED LANDS TECHNICAL NOTE 457 19 3.2.2 Effects on Visibility of Objects in Dark or Near-Dark Conditions The human eye functions at an enormous range of light levels, from starlight to bright sunlight (Figure 2). The eye uses two different photoreceptor cells— cones and rods—to see this range of light levels. Cones and rods function at different levels of light. Compared to rods, cones are capable of functioning at higher levels of light (referred to as photopic light levels), are used to see and distinguish between colors, have the greatest visual acuity (clarity and sharpness), and adapt to changes in light levels relatively quickly. Vision that uses the cones exclusively is referred to as photopic vision. Rods function at low levels of light (i.e., dark environments). Rods distinguish shades of gray and grayscale contrast and are more sensitive than cones to small variations in light levels, but rods cannot distinguish colors and have relatively poor visual acuity. Rods respond much more slowly than cones to changes in light levels. Vision that uses the rods exclusively is referred to as scotopic vision. In near-dark conditions (referred to as mesopic light levels), both the rods and cones function. Most nighttime outdoor and traffic lighting scenarios are in the mesopic range. Ambient light levels influence which vision system is used. In naturally dark environments, scotopic vision is used. Artificial light causes the eyes to shift from using scotopic vision for seeing in the dark to using photopic vision for seeing in lit environments. As a result, this literally changes what is seen. Importantly, because rods are very sensitive but respond very slowly to changes in brightness, the sight of a bright light in otherwise dark conditions can substantially degrade scotopic vision, thus degrading or even eliminating the ability to detect faintly lit objects for a period of time, requiring as long as 50 minutes for a full recovery (Ruseckaite et al. 2011). Lights in otherwise naturally dark environments can pose a safety concern for people moving out of the lit area into the dark. Vision can be greatly diminished as the eyes shift from photopic to scotopic vision. Figure 2. Human vision at different light levels.
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