In the Dark of the Night
At the beginning of my night wake about 2:00 this morning, I let the dogs out for a ramble under an overcast sky with the moon shining somewhere above it. As I opened the door I caught a glimpse of a deer running away from the house, down the driveway. The glimpse was, as we often say, out of the corner of my eye.
As it were, I had just learned of the existence of something called "night vision" just yesterday from one of the members of the Alleghany Arts and Crafts. Technically speaking, it's called scotopic vision.
We all probably know that the eye uses rods and cones to sense light. Cones, the ones we use for day vision, are concentrated in the central visual field. Rods, the ones we use for night vision, are distributed through the visual field and dominate heavily in the periphery. The density of rods is lower, however, in the center of the retina where the cones dominate. Accordingly, scotopic vision works best if the viewer does not look directly at an object but rather look slightly off to the side, or "out of the corner" of their eyes.
I gave this interesting fact a test last night with that deer running down the driveway and, sure enough, it was much more clearly visible if I didn't look directly at it.
Just how much light is required for night vision? Not much.
Day (photopic) vision works in the range of 900,000,000 (sun) to 1 (common candle) candela per square meter.
Mixed (mesopic) vision works down to 0.001 (moonless clear night) candela per square meter.
Night (scotopic) vision goes down to 0.000003 candela per square meter. And, that's somewhere around starlight on an overcast night.
Below that, baby, and you are imagining things!
The deer I saw last night was in light near the scotopic-mesopic transition.
As it were, I had just learned of the existence of something called "night vision" just yesterday from one of the members of the Alleghany Arts and Crafts. Technically speaking, it's called scotopic vision.
We all probably know that the eye uses rods and cones to sense light. Cones, the ones we use for day vision, are concentrated in the central visual field. Rods, the ones we use for night vision, are distributed through the visual field and dominate heavily in the periphery. The density of rods is lower, however, in the center of the retina where the cones dominate. Accordingly, scotopic vision works best if the viewer does not look directly at an object but rather look slightly off to the side, or "out of the corner" of their eyes.
I gave this interesting fact a test last night with that deer running down the driveway and, sure enough, it was much more clearly visible if I didn't look directly at it.
Just how much light is required for night vision? Not much.
Day (photopic) vision works in the range of 900,000,000 (sun) to 1 (common candle) candela per square meter.
Mixed (mesopic) vision works down to 0.001 (moonless clear night) candela per square meter.
Night (scotopic) vision goes down to 0.000003 candela per square meter. And, that's somewhere around starlight on an overcast night.
Below that, baby, and you are imagining things!
The deer I saw last night was in light near the scotopic-mesopic transition.
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