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The Mechanics of Night Vision

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Picture this: your house loses power and you need to find a flashlight or the fuse box. After a few moments you are able to distinguish between the objects in the room and the darkness around you. This process, ''dark adaptation,'' causes our eyes to adjust to the dark.

In order for night vision and dark adaptation to happen, several physiological, neurological and biochemical mechanisms have to take place behind the scenes. Let's talk about how your eye actually operates in these conditions. The retina is a layer of cells at the back of the eye. The portion of the retina opposite the pupil that is responsible for the point of focus is called the fovea. The retina is made up of rod-shaped and cone-shaped cells. The rods have the capacity to function even in low light conditions but those cells are not found in the fovea. You may have heard that the details and colors we see are sensed by cone cells, while rod cells help us visualize black and white, and are light sensitive.

Considering these facts, if you want to see something in the dark, like the dresser in a darkened room, you'll be better off if you look at something off to the side of it. It works by using the light-sensitive rod cells.

Another mechanism your eye implements in low light is pupil dilation. It requires less than a minute for the pupil to completely dilate but it takes about 30 minutes for you to achieve full light sensitivity. During this time, it is estimated that light sensitivity can increase by a factor of 10,000 or more.

Dark adaptation occurs when you leave a bright area and enter a dim one, for example, walking inside after spending time in the sun. Despite the fact that your eyes require a few noticeable moments to get used to the dark, you'll quickly be able to re-adapt to exposure to bright light, but if you go back into the darker setting, your eyes will need time to adjust again.

This is actually why many people prefer not to drive when it's dark. If you look directly at the ''brights'' of a car heading toward you, you are momentarily unable to see, until that car passes and you once again adjust to the night light. To prevent this, don't look right at the car's lights, and instead, try to allow peripheral vision to guide you.

There are numerous things that could, hypothetically lead to trouble with night vision, including: a nutritional deficiency, cataracts, glaucoma, or some other visual interference. If you detect that you experience trouble in the dark, call to make an appointment with one of our eye care professionals who will be able to locate the root of the problem.

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