Why do cats’ eyes glow in the dark?
On a foggy night in 1933, a businessman named Percy Shaw was driving home from the pub in Yorkshire, England. The road was winding and difficult to see. Suddenly, two bright spots appeared to him from the side of the road. Percy braked suddenly.
The glowing spots belonged to a cat and they probably saved its life. If he had continued driving, he could have gone straight off the road.
That’s when Percy had his eureka moment. What if roads could “illuminate” drivers in the same way as cats’ eyes? Within a year, he had invented Catseye® reflectors, these studs that we still see embedded in the roads today. They reflect your own headlights back at you, helping you see where you’re going in the dark.
But why do cats’ eyes shine like that?
The ancient Egyptians believed that a cat’s eyes held the sun after sunset, giving them their glowing nocturnal appearance. The real reason was discovered less than a hundred years ago, in 1929, and it was even later that scientists understood how this characteristic affected vision.
Why do cats’ eyes glow in the dark?
All eyes, including yours, have a retina. The retina is a thin layer at the back of your eye that absorbs light and turns it into signals your brain can understand. Think of it as a transparent screen that captures light.
Behind a cat’s retina is a mirror-like layer called the tapetum lucidum. Humans don’t have that. When light enters the human eye and is not absorbed by the retina, it is not used.
But in a cat’s eye, light that passes through the retina without being absorbed reaches the tapetum lucidum, which reflects it back through the retina. This gives the retina a second chance to capture light. The glow you see in cats’ eyes at night is actually light bouncing off the tapetum lucidum.
Integrated night vision goggles
Thanks to this characteristic, cats have significantly better light sensitivity than humans, meaning their eyes can capture even the smallest amount of light. They can see in light so dim that it appears black to us – a very useful ability for animals that hunt much of the night.
Cats aren’t the only animals to have a tapetum lucidum. Cows, sheep, goats and horses have them, perhaps to spot predators in the dark. Fish, dolphins and whales depend on it to see in dark, murky waters.
In contrast, squirrels, pigs, and most primates, including humans, do not have it because they are active during the day and do not need night vision.
(Interestingly, most dogs have a tapetum lucidum, but it is sometimes absent in blue-eyed dogs and small domestic breeds.)
New technology inspired by cat eyes
Much like Percy Shaw in the 1930s, modern scientists borrow ideas from cats’ eyes.
Dr. Young Min Song, a professor of electrical engineering at Korea Advanced Institute of Science and Technology, recently designed a camera inspired by feline eyes.
“Cat eyes have fascinated me for a long time,” says Song Popular science“particularly their ability to see clearly in extremely wide lighting conditions, from daylight to near darkness.”
It was the tapetum lucidum that initially gave Song the idea to design a camera with a reflective layer to enhance photos and videos taken in low-light conditions. This is “a beautifully simple but powerful strategy for improving light sensitivity without additional energy consumption,” he says.
As Song expected, the camera showed increased sensitivity in low-light conditions thanks to the artificial reflector inspired by the tapetum lucidum.
He then decided to add another feature: a slit-like opening that mimics a cat’s vertically elongated pupil.
Why do cats have slit eyes?
In broad daylight, cats’ pupils constrict into thin vertical slits to block extra light and protect the sensitive retina. The vertical shape also refines the contours of objects, helping cats judge distance with incredible accuracy, an essential skill for tracking and ambushing prey.
While Song expected the reflective layer inspired by the cat’s tapetum lucidum to help the camera in low-light conditions, he was surprised to see how much applying a cat’s slit-shaped eyes also helped the camera.
“What surprised us most, however, was how effectively the vertical pupil-inspired aperture broke camouflage in complex scenes,” he says. In other words, it was excellent at spotting objects that would normally blend into the background when viewed through traditional cameras.
Song thinks cats’ eyes could inspire more innovation.
“Beyond cameras, similar principles could be applied to robotic vision, autonomous systems, and even wearable or surveillance devices that need to operate reliably in bright and dark environments.”
Percy Shaw’s road cleats have helped save countless lives. Who knows what future cat-inspired technology might do?
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