2019-05-17

The former Navy SEAL who wrote about the mission to kill Osama bin Laden is in trouble with the Pentagon for writing about the night vision goggles the elite teams use, but they’re available to buy online（www.mh-elec.com） — for $2800 or less.

How many times has someone said to you: "If you eat all your carrots, you'll be able to see in the dark." If there were any truth in that, soldiers, sailors, and pilots wouldn't go anywhere without a carrot stuffed in their pocket. Much more useful on the battleground are night vision goggles: electronic eyes that boost weak night-time vision into something many times more powerful. If you want to fight wars at night or watch wildlife in the twilight, night vision goggles are the way to go—but how exactly do these clever bits of kit turn darkness into light? Let's take a closer look!

How animals see in the dark

Humans are built for living in the daytime and sleeping in the dark. The retina (the light-sensitive part of our eyes) has cells called cones (for seeing colored light) and rods (for detecting movement and dim light). We have 20 times more rods than cones (120 million rods and only 6 million cones), yet we're still not very good at seeing in the dark.

Other animals are built differently. Creatures that live in the dark tend to have much bigger pupils (holes in front of their eyes) to let in more light. Tarsiers, for example, have enormous eyes relative to their body size. Like other nocturnal creatures, their retinas contain many more rods than the human eye. Cats, which also spend much of their time hunting at night, are among creatures whose eyes contain a tapetum. This is a natural mirror that reflects light back out of the eye. Its job is to bounce the incoming light twice through the retina so the animal has double the chance to see things. That's why cats are so good at seeing in the dark—and why, when you shine at torch at them, their eyes reflect light straight back like mirrors.

Humans can't use any of these tricks. Our pupils open wider in dim light, but not wide enough to help us that much at night. Our eyes don't have enough rods—and we don't have a tapetum. So what can we do to see at night? We can reach for technology!

How to invent night vision goggles

Here's a way to invent goggles that boost light. What if we turn the light into electricity, boost the electricity, and then turn the boosted electricity back into the light? That should make the light much brighter so we can see even at night. This unlikely sounding trick really does work—and it's how night vision goggles help us to see.

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