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The Mechanics of Night Vision

Picture this: there's a power outage and you've got to locate a flashlight or the fuse box. At first you can't see, but gradually the things in the room begin become visible. This process is known as ''dark adaptation'' and it's what lets our eyes get used to low light settings.

Many people don't know that night vision relies on several physical, neural and biochemical mechanisms. Let's have a look at how all this operates. The retina is a layer of cells at the back of the eye. The portion of the retina directly opposite the pupil that is responsible for sharp focused vision is called the fovea. The retina is made up of cone cells and rod cells, named for their respective shapes. The rods have the capacity to function more efficiently than cone cells in low light conditions. They are absent from the fovea. What's the functional difference between rods and cones? Basically, details and colors we see are sensed by the cones, while rod cells are sensitive to light and detect movement.

This information is significant because, when you're trying to find something in the dark, like the dresser in your darkened room, instead of looking directly at it, try to look just beside it. You want to maximize the use of the rod cells in low light, and avoid relying on your cone-rich fovea, even though it seems counter-intuitive to look away from the object you want to see.

Furthermore, the pupils dilate in the dark. It takes approximately one minute for the pupil to completely dilate but dark adaptation will continue to increase for the next half hour and, as you've experienced, during this time, your ability to see will increase enormously.

You'll experience dark adaptation when you walk into a darkened movie theatre from a well-lit area and have a hard time finding a seat. After a while, your eyes get used to the situation and before you know it, you can see. You'll experience the same thing when you're looking at the stars in the sky. At the beginning, you probably won't be able to actually see that many. Keep looking; while you dark adapt, millions of stars will gradually appear. It'll always take a few moments until you begin to adjust to normal indoor light. If you go back outside, that dark adaptation will disappear in the blink of an eye.

This is actually one reason behind why a lot people prefer not to drive when it's dark. When you look at the headlights of a car heading toward you, you are briefly unable to see, until that car passes and you readjust to the night light. A helpful way to avoid this is to avoid looking right at the car's lights, and learn to try to allow your peripheral vision to guide you.

There are numerous conditions that could be the cause of inability to see at night, including: a nutritional deficiency, macular degeneration, cataracts, glaucoma, and others. Should you begin to detect that you experience problems with dark adapting, call to make an appointment with one of our eye doctors who will be able to locate the root of the problem.