Sometimes you get ready for bed and turn off the light, but you can’t sleep. You open your eyes and all you see is black. You need several moments to get used to the dark and then you are able to see better. This is called ”dark adaptation” and it’s what helps our eyes see in the dark.
A person with a healthy set of eyes probably takes night vision – and the role of the biochemical, physical and neural mechanisms – for granted. Let’s have a look at how your eye actually operates in these conditions. Your eye takes in various forms of light using two kinds of cells: cones and rods, which are found on the retina at the back of the eye. Together they make up the sensory layer that helps the eye detect colors and light. The rod and cone cells exist throughout the entire retina, with the exception of the small area called the fovea. This has only cone cells, and its primary function involves focusing on detail. What’s the difference between rods and cones? Basically, cones enable us to perceive color and detail, while rod cells help us visualize black and white, and are light sensitive and detect movement.
Considering these facts, if you want to see something in the dark, you’ll be better off if you look at something off to the side of it. That way, you’re avoiding the use of the fovea, which only has cells that are less sensitive to low light.
Your pupils also dilate in response to darkness. It requires less than a minute for your pupil to fully enlarge; however, it takes approximately 30-45 minutes for you to achieve full light sensitivity. During this time, sensitivity to light increases by a factor of 10,000 or more.
Dark adaptation occurs when you walk into a darkened movie theatre from a well-lit lobby and struggle to find somewhere to sit. But soon enough, you get used to the situation and before you know it, you can see. This same thing occurs when you’re looking at stars at night. At the beginning, you won’t see many. If you keep gazing, your eyes will dark adapt and millions of stars will become visible. Despite the fact that your eyes require a few noticeable moments to get used to the darker conditions, you will immediately be able to re-adapt upon returning to bright light, and this resets any dark adaptation that had developed where it was darker.
This explains why a lot people have difficulty driving their cars at night. If you look right at the ”brights” of a car heading toward you, you may find yourself briefly blinded, until you pass them and your eyes readjust to the night light. A helpful way to prevent this sort of temporary blindness is to avoid looking directly at headlights, and learn to use your peripheral vision in those situations.
There are a number of things that could, hypothetically lead to trouble seeing at night, including: not getting enough Vitamin A in your diet, cataracts, glaucoma, or some other visual obstruction. Should you begin to notice difficulty with night vision, call to make an appointment with one of our eye care professionals who will be able to locate the source of the problem.