Everyone has found themselves in a dark room, at one point or another, whether it be during childhood, due to a power outage, or just waking up in the middle of the night. Gradually, the things in the room begin become visible. This remarkable process is ''dark adaptation''.
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 all this operates. Every eye has, in addition to other cells, rod cells and cone cells, which are found on the retina at the back of the eye. Together they form the sensory layer. This is the part that enables the eye to pick up light and color. These cells are found throughout your 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 creating a focused image. As you may know, the details and colors we see are sensed by cone cells, while rod cells allow us to see black and white, and are light sensitive.
Considering these facts, if you want to see something in the dark, like a distant star in the night sky, instead of looking directly at it, try to use your peripheral vision. Since there no rods in the fovea, you'll see better if you avoid using it when it's dim.
Also, the pupils dilate in response to darkness. It requires less than a minute for your pupil to completely enlarge; however, dark adaptation keeps enhacing your ability to see over approximately 30 minutes and, as everyone has experienced, during this time, your ability to see in the dark will increase enormously.
You'll experience dark adaptation when you first enter a dark movie theatre from a well-lit lobby and have trouble finding somewhere to sit. But soon enough, your eyes get used to the dark and before you know it, you can see. This same thing occurs when you're looking at the stars in the sky. Initially, you probably won't be able to actually see that many. If you keep staring, your eyes will dark adapt and millions of stars will gradually appear. It takes a few noticeable moments until your eyes fully adjust to normal indoor light, but if you go back into the brightness, that dark adaptation will be lost in the blink of an eye.
This is actually why a lot people have trouble driving at night. If you look right at the ''brights'' of an approaching car, you are briefly blinded, until you pass them and you readjust to the night light. To prevent this, try not to look directly at the car's lights, and instead, use peripheral vision in those situations.
There are a number of things that could be the cause of difficulty seeing in the dark. Here are some possibilities: a nutritional deficiency, macular degeneration, cataracts, glaucoma, and others. If you suspect problems with night vision, book an appointment with one of our eye care professionals who will be able to locate the source of the problem.