Picture this: there’s a blackout and suddenly, 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 is called ”dark adaptation”.
Many people don’t know that night vision relies on the cooperation of physical, neural and biochemical mechanisms. So how do your eyes adapt to low light? Let’s start by exploring the eye and its anatomy. The retina is a layer of cells at the back of the eye. The section of the retina behind the pupil that is responsible for sharp focused vision is called the fovea. The retina comprises cone cells and rod cells, named for their respective shapes. The rod cells have the capacity to function better than cone cells in low light conditions. They are absent from the fovea. What’s the functional difference between these two cell types? Basically, details and colors we see are sensed by cone cells, while the rods are sensitive to light and detect movement.
Now that you know some background, let’s relate it to dark adaptation. If you’re attempting to focus on an object in the dark, you’ll be better off if you view it with 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.
Another way your eye responds to the dark is by your pupils dilating. It requires approximately one minute for the pupil to completely dilate but your eyes will get better at seeing in the dark over a half hour time frame. During this time, it is estimated that sensitivity to light may grow by a factor of 10,000 or more.
Here’s an example of dark adaptation: if you leave a bright area and enter a dim one, for example, walking inside after sitting in the sun. It’ll always take a few moments for your eyes to adapt to regular indoor light. If you walk back out outside, that dark adaptation will disappear in a moment.
This explains why many people have difficulty driving at night. If you look at the headlights of opposing traffic, you may find yourself briefly blinded, until that car is gone and your eyes once again adjust to the night light. A good way to prevent this sort of temporary blindness is to avoid looking directly at the car’s lights, and learn to use peripheral vision to observe oncoming traffic at night.
There are a number of things that could contribute to difficulty seeing in the dark. These include a nutritional deficiency, cataracts, glaucoma, or some other visual interference. If you notice that you experience problems with seeing at night, schedule an appointment with one of our eye doctors who will be able to get to the root of the problem.