How Does a Dog See at Night?by Betty Lewis
Though cats are known for their keen night vision, dogs don't do so bad either. Though your cat probably sees in the dark better than Skip, he sees well enough to navigate better than you do in dim light. Your pup's vision is one tool in his survival toolbox.
Seeing His Way
If you've ever had to find your way to a bathroom in the middle of the night, you've likely been grateful for a nightlight -- or any stray light -- that helped navigate your way. While Skip's sees just fine in bright light, he also can find his way in dim light. In fact, scientists believe dogs can see in light five times dimmer than what human eyes need to see.
The Mechanics of Seeing at Night
Skip's eyes are built for working in dim light. He has a larger pupil to provide more light and the center of his retina has more light and motion sensitive cells, known as rods. Rods help him distinguish between shadow and light. His eye's lens is closer to the retina, making whatever he's looking at brighter on the retina.
His Secret Weapon: The Tapetum
In addition to his large pupil and additional rods, Skip has an extra helper when he prowls around the house at night: his tapetum lucidum. The tapetum is a membrane under the retina that reflects light the rods didn't register back to the retina, allowing the retina to absorb even more light. Essentially, it's a second opportunity for the retina to register light. When you notice his eyes eerily glowing at you on your evening walks, it's the light bouncing off his tapetum's mirror-like surface. The downside of the tapetum is that it also scatters reflected light, which means Skip's visual acuity isn't as good as yours.
Seeing What He Needs to See
Skip doesn't need to see to read books or work on the computer. Though he doesn't hunt for his dinner anymore, his eyes are useful for helping him survive in the wild, particularly for catching prey. Not only do the rods in his eyes give him superior night vision, they also help him to detect movement. Despite what some believe, a dog can see in more than black and white. Color perception is accomplished by cone photoreceptors in the retina. About 20 percent of a dog's retina's photoreceptors are cones, giving him limited color vision. However, color vision isn't critical to Skip's survival, so don't feel bad if he can't tell red from green.
No Glasses for Skip
Skip doesn't see color as well as you do, nor does he have a human's visual acuity, which is the ability to see objects as distinct entities. His eyes are positioned a bit different from yours, giving him a larger visual field. Depending on the distance of the object, Skip may see a blurry outline instead of a car because he can't focus on detail as well as you do. His vision is approximately 20/75, meaning what you see at 75 feet away, he sees at 20 feet. However, don't fret. Your pup doesn't need glasses or an assistance animal to help him along; his whiskers, great sense of smell and sharp hearing make up for whatever vision shortcomings he may have.
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