Frosty air stung my cheeks and my breath froze to my eyelashes as I zoomed down the trail. Fading sunlight and frigid temperatures haven’t kept me from skiing! An intricate pattern of blacks and whites decorated the thick winter woods. Each twig carried a shadow of white snow. Wind-driven snow accentuated the furrowed bark of trees. Subtle shadows graced the drifted terrain. I was skiing through an Ansel Adams photograph.
The pale sunlight gradually faded behind the hills, and a gibbous moon rose to take its place. I barely noticed the change at first, since the moon (reflecting light from the sun back toward us) was so bright. During the long days of summer—when bedtime comes before twilight, and mosquitoes flock at dusk—I seldom get outside after dark. Skiing under the stars, with trees casting moon shadows across my path, was a lovely treat. The contrast of trees and snow made it easy to navigate. I left my headlamp in my pocket.
As my purple-mitten clad hands swung in an easy rhythm, I noticed their color fading toward charcoal gray, finally matching the color scheme of the forest. Despite being mostly limited to seeing in grayscale now, it was still impressive that I could see at all. According to the American Optometric Association (AOA), the sensing capabilities of the human eye reach across nine orders of magnitude. This means that the darkest light we can see is one-billionth as bright as the brightest light we can see.
Two different types of light receptors in our eyes make this possible. Our cone cells function in daylight, and give us color vision with a high resolution of detail (20/20 vision if we’re lucky). Rod cells work even in very dim light, but provide limited resolution (20/200 vision), and a black and white palate. Cones are clustered in the center of our retinas, while rods form a donut around the periphery of the retina.
(A freaky side effect of our rod and cone arrangement is that if you stare directly at a small object in the dark, it will disappear! A tree trunk might be severed with its top half floating, your friend’s face might go dark, or a star you just saw might blink out. These optical illusions are caused by a small blind spot in the middle of our night vision, where the cones would normally focus if they had sufficient light. If you just look slightly to the side, the objects will reappear.)
Humans have several additional adaptions that help us to see across such a wide spectrum of light levels. Our pupils can dilate to let in more light at night, or contract to physically protect our eyes from damaging amounts of light during the day. The diameter of the pupil can contract to 1.5mm and expand to 8mm, which equates to a 30-fold range in the quantity of light entering the eye. After skiing for more than half an hour in low light conditions, my pupils were probably widely dilated.
A chemical called rhodopsin is another key to our night vision. Rods use rhodopsin to absorb photons and perceive light by converting it to electrical activity, which initiates visual impulses in the brain. (Cones use their own specific photopigment in the same way.)
When you expose your eyes to bright light, photons actually split the rhodopsin into two other chemicals. This reduces the sensitivity of our retina and protects our eyes from damage to intense light. It also diminishes our ability to see in low light. (Have you ever noticed how turning on a flashlight temporarily ruins your night vision?) Although it takes several minutes, the chemicals eventually recombine into rhodopsin and our night vision returns.
The moonlight provided plenty of light for skiing. Rods can function even on an overcast night with no moon, and the high contrast between snow and trees gave me confidence in avoiding collisions. As moon shadows from the trees slipped by under my skis, I looked down to admire the patterns. I was surprised to find that my skis still looked red!
Back inside, I did a little research and discovered that with any color except red, as you turn down the lights, you can watch as first the color turns gray (cones stop working), and then you lose the sensation of light (rods stop working). This is called the photochromatic interval. With red, however, the color and sensation of light disappear at the same time. And, as it turns out, cones can function a little even with only 50% moonlight!
As wonderful as human vision is to me, I know that many animals see in ways that we can only imagine. Many critters have better night vision, can see more colors than we do, and can even see faster than humans. I will explore those super senses in a future article.
With Winter Solstice and the shortest days of the year behind us, we’ll soon have increasing amounts of light to see by. For now, our grayscale night vision melds perfectly with the black and white winter world. By summer, when our cones have 15 hours and 43 minutes of daylight to see by, the Northwoods will once again be full of color.