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The Mechanics of Night Vision

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Something wakes you up in the middle of the night, or you're searching for a light switch or door handle or phone in a room with the lights off. These sorts of things happen to us all the time. You notice that it is almost impossible to see anything for several moments before your eyes adjust. This process, ''dark adaptation,'' causes people to see even when it's really dark.

A person with a healthy set of eyes probably takes night vision - and the biochemical, physical and neural mechanisms - for granted. Let's have a look at how this works. Your eye has, in addition to other cells, rod cells and cone cells, at the back of the eye; or, to be precise, on the retina. Together they make up the sensory layer that gives your eye the ability to detect light and color. The rod and cone cells are found throughout the retina, with the exception of the small area called the fovea, where there are only cone cells. This section provides detailed vision, for example when reading. What's the difference between these two cell types? 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 attempting to make out an object in the dark, like the dresser in your darkened room, it's better to view it through the side of your eye. 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 response to darkness. The pupil dilates to its biggest capacity within 60 seconds but your eyes will keep adapting over a 30 minute time frame and, as everyone has experienced, during this time, your ability to see will increase greatly.

Dark adaptation occurs when you first enter a dark cinema from a bright area and have a hard time locating a seat. After a while, you adapt to the dark and see better. You'll experience the same thing when you're looking at the stars in the sky. At the beginning, you won't see many. Keep looking; as your eyes continue to dark adapt, the stars will gradually appear. It takes a few noticeable moments until you begin to adjust to regular indoor light. If you go back into the brightness, those changes will be lost in a flash.

This explains why many people prefer not to drive at night. When you look right at the headlights of an approaching car, you are momentarily unable to see, until that car is gone and your eyes once again adjust to the night light. To prevent this, don't look right at headlights, and learn to use peripheral vision to observe oncoming traffic at night.

There are numerous conditions that could, hypothetically lead to trouble seeing at night. Here are some possibilities: a nutritional deficiency, macular degeneration, cataracts, glaucoma, and others. If you detect difficulty with night vision, call to make an appointment with one of our eye doctors who will be able to locate the source of the problem.