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How does night vision work?

beach setting viewed at night

What does night vision mean?

Night vision, also called scotopic vision, describes our natural ability to see in the dark. It happens when a few different components, in particular our pupils and retinas, work together inside our eyes — and it’s pretty handy when we need it.

Seeing in the dark

Have you ever caught a glimpse of your eyes immediately after you’ve turned on a bright light in a dark room? If you’re fast enough, you’ll catch your pupils quickly shrinking down.

Pupils get wider in low light because our eyes are trying to gather as much light as possible to help us see. When there’s ample lighting, our pupils shrink because they don’t need any additional light to see comfortably.

This natural process is one reason we have night vision, but it’s not the whole picture. For that, we have to look at the microscopic cells in the back of our eyes.

There are millions of rod-shaped and cone-shaped cells in every eye’s retina. These rods and cones are critical to our eyesight, and each type is better at receiving a different form of light.

While cones are great at registering bright lights, colors and fine details, they’re not so great at helping you see in the dark — and that’s where your rods come in.

Rods excel in peripheral vision and are much more sensitive to light photons, which makes them extremely helpful during low-light situations. Without rods scattered across our retinas, any sort of darkness would be virtually blinding.

The only catch is, rods can’t process color. This is why our night vision usually appears in black and white. Look around next time you’re lying in a darkened bedroom; your view probably won’t be very colorful.

CONCERNED ABOUT YOUR NIGHT VISION? Find an eye doctor near you and make an appointment today.

Do humans have good night vision?

Last time you got up in the middle of the night, did you find yourself frustrated by how little you could actually see?

Unfortunately, one of the downsides to being human is that our night vision isn’t great compared to much of the animal kingdom.

And adjusting to the darkness certainly isn’t a quick process for us either. In fact, reaching our maximum level of night vision is almost painfully slow.

According to "Webvision: The Organization of the Retina and Visual System," our eyes start adjusting to low light in about five to eight minutes — but they don’t reach maximum adjustment until we’ve been in the dark for about 40 minutes.

It’s safe to say that 2 a.m. stumble to the bathroom didn’t quite qualify.

But there's a good reason for all of this: We’re primarily daytime creatures. What we lack in night vision we make up for in vibrant colors and contrasts during the day.

Animals that are nocturnal or spend more time in the dark tend to have better night vision. Cats and dogs, along with cows, horses and deer, have an iridescent coating behind the retina that boosts their night vision, a feature called tapetum lucidum. It’s what gives their eyes that signature reflective glare at night.

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Difference between night vision in a human (top) and a cat (below).

Can your eyes see in total darkness?

If you've ever experienced a darkness so sweeping that you can't see your hand in front of your face, then you know the answer is no.

Our eyes’ rod cells may be hypersensitive, but they aren’t so sensitive that they can make something out of nothing.

Our night vision works outdoors and in our homes because it’s almost never completely dark. There’s always light creeping in from the moon, stars, street lamp or alarm clock. Even the smallest amount of light can give our eyes enough to work with.

Walk far enough into an underground cavern, and you can experience true, absolute darkness. Without a functioning light source, all forms of shape or motion will disappear, rendering your eyes all but useless. We perceive this absence of light as total blackness.

Artificial night vision

The visible light we see takes up a tiny portion of the electromagnetic spectrum, a scale that includes every type of radiation. Thermal, or infrared, cameras focus on radiation just past our visible scale. Anything we feel as heat usually qualifies as infrared radiation.

Thermal cameras can function in total darkness, but only in the presence of objects containing or emitting heat. They’re commonly used for seeing people or animals at night, since the visible heat the body creates draws a stark contrast to a much cooler, darker background.

Since our eyes can’t see infrared radiation, images or videos captured by thermal cameras have to be processed so we can see them. Once the camera converts these infrared images, we’re able to see objects that appear to be glowing.

The more traditional “green” night vision requires at least a small amount of light, similar to our own eyes. This technology slightly intensifies the dark image so we can see it better, and in greater detail. The green color lets us see the final product more distinctly than a black-and-white image would.

How can I test my night vision?

Nighttime activities like driving involve much lower-contrast surroundings than they would during the day. The Pelli-Robson contrast sensitivity test is the most common method of testing your eyes’ ability to distinguish between high- and low-contrast characters.

The test is usually reserved for people with disorders that could affect their low-contrast vision, but you can also request it if you suspect your night vision could be impaired.

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The Pelli Robson contrast sensitivity chart tests your ability to detect letters that are gradually less contrasted with the white background as your eyes move down the chart.

You also can test your night vision at home. Emily Trudeau at Sciencing proposes a solution that requires little more than a bit of patience.

After letting your eyes adjust to darkness for about 20 minutes, she suggests focusing directly on a small object and picking up as many details about the object as you can.

Here’s where it gets interesting. Remember how the rod cells in our eyes specialize in low light and peripheral vision?

Look slightly to the side of the object you were focusing on for a moment. There’s a good chance the object will now appear more clearly, since the rod cells are going to work in your low-light periphery.

When to see an eye doctor

If you’re experiencing problems with your night vision, it’s important to schedule an appointment with an optometrist or ophthalmologist. If your eye doctor sees any optical disorders present, they can work with you to develop a prompt treatment plan.

READ NEXT: Tips to improve your driving at night and Night driving glasses: Help or hoax?

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