Night vision goggles
Last updated: April 26, 2007.
Photo: The US Navy on a helicopter
mission in Iraq
in 2002,
seen through night vision goggles.
Public domain photo courtesy of
US
Navy.
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?
Animals of 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 coloured 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 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 animal's retina so the animal has twice 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 shine 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!
Seeing with electricity
Imagine your job is to invent a pair of glasses that will help
people see at
night. It's obvious what you have to do. Light rays will travel into
the glasses at the front, so you must capture them somehow, boost
them in strength, and then fire them into the person's eyes. But how
can you capture and boost light? Binoculars,
telescopes, and
even ordinary glasses will bring light to a focus, but they don't
make it any brighter. It's easy to invent a pair of glasses that make
things dimmer: you just coat the lenses with something that absorbs
some of the light—and that's how sunglasses work. But glasses
that make things brighter are a tall order.
Electricity, on the other hand, is
very easy to boost in
strength. People have
invented all sorts of electrical devices that take in a small
electric current (flow of electricity) at one end and produce a
bigger flow at the other. Something that does this is called an
amplifier. A hearing aid, for example, uses a
tiny electronic
component called a transistor to
amplify sounds (increase
their volume) so that someone hard of hearing can listen to them more
easily. An electric guitar uses a much more powerful amplifier to
turn the plucking sounds the strings make into sounds that can fill a
stadium.
So 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 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.
How night vision goggles work
Photo: A Land Warrior PVS-14 night vision device
used by the US Army.
Photo by courtesy of US Army.
Night vision goggles boost a dim, dark scene in a series of simple
steps.
Dim light from a night scene enters the lenses at the front. The
light is actually made of photons (particles
of light) and, as
they
enter the goggles, they strike a light-sensitive surface called a
photocathode. 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). The electrons are amplified and
then fired at a screen coated with phosphor
chemicals, like the
screen on an old-fashioned television. As the electrons hit the
phosphor, they create tiny flashes of light. Since there are many
more photons than originally entered the goggles, the screen makes a
much brighter version of the original scene.
So why does everything look green through night vision goggles? The
incoming
photons carry light of all colours. But when they are converted to
electrons, there's no way to preserve that information. Effectively,
the incoming, coloured 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).
That's why night vision goggles have their characteristic, eerie
green glow.
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