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Night Vision: How Does it Work?


You wake from a dream in the middle of the night, or perhaps you're trying to find the light switch or door in the dark. It's happened to all of us. You need several moments to get used to the dark and then objects in the room begin to re-appear. This process, ''dark adaptation,'' allows people to adjust to the dark.

Many people don't know that night vision is dependent upon several physical, neural and biochemical mechanisms. So how do our eyes adapt to low light? Let's begin by having a closer look at the eye and its complex anatomy. Your eye contains photoreceptors that can be classified as rod cells and cone cells, at the back of the eye; or, to be precise, on the retina. Together they form the sensory layer. This is the part that helps the eye pick up light and color. The rod and cone cells are found throughout the entire retina, save for the small area opposite the pupil known as the fovea. This is made up of only cone cells, and its main function involves focusing. What's the functional difference between rods and cones? In short, cones help us see color and detail, and rod cells are sensitive to light.

So, if you want to see something in the dark, like the dresser in a darkened room, it's much better to look at the area off to the side of 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.

Another process your eye undergoes is pupil dilation. The pupil dilates to its maximum size in less than a minute; however, your eyes will continue to adapt over a 30 minute time frame and, as everyone has experienced, during this time, your ability to see in the dark will increase greatly.

Dark adaptation occurs when you first enter a dark movie theatre from a well-lit lobby and have a hard time finding somewhere to sit. After a while, your eyes adapt to the situation and before you know it, you can see. You'll experience a very similar feeling when you're looking at the stars in the sky. Initially, you probably won't be able to actually see that many. Keep looking; as your eyes continue to dark adapt, millions of stars will become visible. It takes a few noticeable moments until you begin to adapt to normal indoor light, but if you walk back out outside, that dark adaptation will be lost in a moment.

This is actually why so many people have difficulty driving their cars at night. When you look right at the ''brights'' of a car heading toward you, you may find yourself briefly blinded, until that car passes and your eyes readjust to the night light. A good way to avoid this is to avoid looking right at the car's lights, and instead, use peripheral vision to observe oncoming traffic at night.

There are numerous conditions that could contribute to inability to see at night. These include diet-related vitamin deficiencies, macular degeneration, cataracts, glaucoma, and others. Should you begin to notice issues with night vision, call to make an appointment with one of our eye doctors who will be able to get to the root of the problem.


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