It is a popular belief that if one eats carrots, one will be able to see wonderfully at night just like nocturnal animals. If there were actually any truth in this, then 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 do night vision goggles work?
In theory
Night vision goggles boost a dim, dark scene in a series of simple steps:
1. Dim light from a night scene enters the lens at the front. The light is made of photons (particles of light) of all colors.
2. As the photons enter the goggles, they strike a light-sensitive surface called a photo-cathode. It's a bit like a very precise solar panel: it's job is to convert photons into electrons (the tiny, subatomic particles that carry electricity round a circuit).
3. The electrons are amplified by a photo-multiplier, a kind of photoelectric cell. Each electron entering the photo-multiplier results in many more electrons leaving it.
4. The electrons leaving the photo-multiplier hit a phosphor screen, similar to the screen in an old-fashioned television. As the electrons hit the phosphor, they create tiny flashes of light.
5. Since there are many more photons than originally entered the goggles, the screen makes a much brighter version of the original scene.
Why does everything look green through night vision goggles?
Cleaning the image intensification tube on night vision goggles
Even at night, the photons that hit the lens at the front of night vision goggles are carrying light of all colors. But when they are converted to electrons, there's no way to preserve that information. Effectively, the incoming, colored light is turned into black and white. Why, then, don't night vision goggles look black and white? The phosphors on their screens are deliberately chosen to make green pictures because our eyes are more sensitive to green light. It's also easier to look at green screens for long periods than to look at black and white ones (that's why early computer screens tended to be green).
What if there really is no light?
Night vision goggles like the ones described above are sometimes called image intensifiers, because they take the tiny amount of light that's available in near darkness and boost it enough for our eyes to see. But sometimes there just isn't enough light to do this—and image intensifier goggles simply don't work. Suppose, for example, you're a firefighter trying to see if there's anyone trapped inside a smoke-filled building, An image intensifier would be as useless as your own eyes.
The alternative is to use what's called thermal imaging. Instead of looking for the light that objects reflect, we look for the heat they give off instead. Generally, living things moving around in the darkness are going to be hotter than their surroundings; that goes for vehicles and machines too. Hot objects give off infrared radiation, which is a similar kind of energy to light but with a slightly longer wavelength (lower frequency). It's relatively easy to make a camera that picks up infrared radiation and converts it into visible light: it works like a digital camera except that the image detector chip (either a charge-coupled device (CCD) or a CMOS image sensor) responds to infrared instead of visible light; it still produces a visible image on a screen the same way as an ordinary digital camera. Other types of thermal imaging cameras use different colors to indicate objects of different temperature—and they're commonly used to show things like the heat loss from badly insulated buildings.