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Turn Out the Lights: How Does Night Vision Work?

Imagine your house loses power and you need to find a flashlight or the fuse box. It takes a couple of minutes for your eyes to adjust to the dark. This process, called ''dark adaptation,'' allows people to adjust to the dark.

In order for night vision and dark adaptation to happen, several physiological, neurological and biochemical mechanisms must take place behind the scenes. How do your eyes actually function in the dark? Firstly, let's examine the eye and its complex anatomy. 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 rod-shaped and cone-shaped cells. The rods are able to function more efficiently than cone cells in low light conditions. Those cells are not found in the fovea. You may have learned that the details and colors we see are detected by the cones, and rod cells are sensitive to light.

This information is significant because, when you're struggling to focus on something in the dark, like the edge of the last stair in a dark basement, it's much better to look at something right next to it. When you do that, you use the part of the eye that has rods, which, as mentioned above, are more responsive to light, even if there isn't much of it.

The pupils also dilate in low light. The pupil grows to it its largest diameter within 60 seconds; however, it takes approximately 30 minutes for the eyes to achieve full light sensitivity.

Here's an example of dark adaptation: when you walk into a dark cinema from a bright lobby and have trouble finding a seat. But after a couple of minutes, your eyes adapt to the situation and see better. This same thing occurs when you're looking at stars at night. At first you probably won't be able to actually see that many. Keep looking; while you dark adapt, millions of stars will become visible. It takes a few noticeable moments for your eyes to get used to normal indoor light. If you walk back out into the brightness, that dark adaptation will disappear in a flash.

This is why a lot people don't like to drive at night. When you look at the lights of an oncoming vehicle, you may find yourself briefly blinded, until that car is gone and you once again adjust to the night light. A good way to prevent this sort of temporary blindness is to avoid looking directly at headlights, and instead, try to allow peripheral vision to guide you.

If you're finding it challenging to see when it's dark, call us to schedule an appointment with our doctors who will check that your prescription is up to date, and eliminate other reasons for decreased vision, such as cataracts and macular degeneration.