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The Effects of Lighting on Vision

We can see in the bright noonday sun and in the darkness of a moonless night. Our eyes work when the brightness changes by a factor of one billion. At least three mechanisms are responsible for this. First, the physical size of the opening of the eye changes -- the iris adjusts to the light level. Second, the biochemical reactions within the eye adjust their rate with brightness. Third, the neural receptors that create the electrical signals change their "gain," the amount of signal they generate when triggered by light. All those factors affect our vision.

Acuity
- Cones in our eyes see finer detail than rods, so the sharpness of our vision goes down in darkness.
  
  There are two types of receptor cells within the eye, named "rods" and "cones" after their shapes. Cones function best in bright light, while rods work best in low light. Cones have better spatial resolution then rods, which means that cones can see finer details than rods. Because cones stop working at low light levels, in darkness our eyes cannot make out fine details, so our visual acuity goes down.
Color Vision
- Rods in our eyes cannot distinguish color, so don't do a job like this in poor lighting!
  
  The human eye contains several different types of cones. They are distinguished by the specific structure of the photosensitive chemical that triggers them. The different chemical structures mean they are sensitive to different colors. Most people have three different "flavors" of photoreceptor, which means they see at least three different colors. But it's only cones that have different photoreceptors, so when darkness makes them stop working, our eyes can't see color in low light.
Spatial Sensitivity
- The cones are concentrated in the center of the retina -- there are no rods right behind the pupil of the eye, in the center of the field of view. On the other hand, away from the center of the field of view, there are only rods and no cones. In darkness, when cones stop working, our eyes don't work directly in the center of vision. Instead, our eyes are sensitive off to the side from where we look.
Mesopic Vision
- When our cones are working in bright light, that's called photopic vision, while vision using only the rods is called scotopic vision. There's a wide range over which both rods and cones are providing signals. That range is called mesopic vision. Vision can actually improve in the mesopic range. When the iris closes down, the lens in the eye performs better. So when the cones are still responding and capable of providing detailed information, and the rods are working as well, our vision can be at its best.

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