The eyes have it – understanding how the human eye adapts for night vision
Canadian survival expert Mors Kochanski – the author of “Bushcraft”, really doesn’t like flashlights at all. This is even more remarkable when he discusses the merits of walking at night instead of the day on his famous 100-mile walk-abouts. Kochanski’s common-sense reasoning includes the energy saved by not having to have a fire at night to keep warm; then being able to sleep well during the heat of the day – without concern for nocturnal predators. Through a lifetime of experience without a flashlight in Northern Canada’s boreal forests, Mors knows a thing or two about night vision. Walking through the evening dusk into the darkest night, Mors simply lets nature do its magical adaptation, morphing daytime vision into the remarkable ability to see at night.
Third generation tracker Fernando Moreira, started tracking when he was eight years old in his native Portugal. Decades later, living in Reno Nevada, Fernando is a dedicated (and respected) tracker recognized throughout the United states for his tracking skills. To acquire night vision, Fernando states that you must get the human body to produce and release the chemical associated with good night vision. To make this happen, he suggests that you sit in a dark room (or with a light proof shroud over your head) and using a red filtered light - look at the surrounding red shine being careful not to look directly at the light for about twenty minutes. Fernando suggests that you may have to repeat this exercise a few times to fully activate the chemicals in your eyes.
In the book “Manhunter – a “must read” for trackers, Ian Maxwell goes into detail about “vision”. Maxwell describes the actual physiological functioning of the eye and explains how night vision works. Remarkably, the parts of the eye that allow us to clearly focus when there is a source of light, changes over to a support role in the darkness. Basically, there is a small area at the back of the eyeball where lighted vision is concentrated on specific sensors called “cones.” These sensors allow us to detect colour, details, and distant objects. In darkness, the eye primarily uses different sensors called “rods.” Dispersed over a larger area, rods are located away from the central, cone rich area at the back of the eye. The result is that if you look directly at an object in darkness, the eye has difficulty seeing it… you must train yourself to use your peripheral vision to see things at night. This is called viewing “off center”.
According to Maxwell, to gain “night vision”, there are four basic adaptations. When entering a dark area, the pupils enlarge allowing more of the available light to reach the sensors at the back of the eye; an area called the retina. Within about five to ten minutes of being in the dark, the cones become accustomed to the dim light and the eyes become a hundred times more sensitive to light. During the night vision adaptation process a chemical called rhodopsin is produced, which gives us night vision in about thirty minutes. During this adaptation process, the rods adjust to the darkness becoming 100,000 times more sensitive to light. Maxwell warns that fully adapted night vision can be lost with just a flash of white light. He recommends that to maintain night vision that strict light control is maintained. In one of our next posts we will explore what Ian Maxwell recommends for illuminating tracks at night.
The take home message from these three experts is that if we understand how our eyes adapt to low light conditions, we can continue to function at night...albeit at a diminished capacity. As night tracking is such an important topic, we will continue to explore more on night vision including what illumination to use. We may even be able to challenge some longstanding core beliefs about what lights are best.
See you on the trail,