See in the Dark

Have you ever wondered why all cats are gray in the dark?

Our eyes function in two entirely different modes, depending on how much light is available. In daylight or bright artificial light, our eyes function in day vision mode. After dark, our eyes shift to night vision mode. The physiological changes that occur in our eyes during the shift from day vision to night vision are called dark adaptation. Dark adaptation occurs slowly, typically requiring 25 minutes for 80% adaptation and 60 minutes for 100% adaptation. That’s why astronomers get upset when someone shows a bright light.


When we move from dim light to bright light, our eyes undergo physiological changes called light adaptation. But while dark adaptation occurs slowly, light adaptation occurs quickly, in two phases. During αadaptation, which requires about 1/20th of a second, the sensitivity of the retina drops by 50% or more. During βadaptation, which requires from one to several seconds, the sensitivity of the retina drops more gradually, and we recover full color vision and visual acuity.

There are many misconceptions about night vision and dark adaptation, even among astronomers. To understand the process of dark adaptation, you need to understand something about the physiology of the human eye. Our eyes have two types of light sensors, called rods and cones. Rods provide monochromatic vision, but are very sensitive to light. Cones provide full color vision, but are relatively insensitive to ...

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