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Introduction
Night vision technology, by definition, literally allows one to see in the dark. Originally
developed for military use, it has provided the United States with a strategic military
advantage, the value of which can be measured in lives. Federal and state agencies now
routinely utilize the technology for site security, surveillance as well as search and
rescue. Night vision equipment has evolved from bulky optical instruments in lightweight
goggles through the advancement of image intensification technology.
The first thing you probably think of when you see the words night vision is a spy or
action movie you've seen, in which someone straps on a pair of night-vision goggles to
find someone else in a dark building on a moonless night. And you may have wondered
"Do those things really work? Can you actually see in the dark?"
The answer is most definitely yes. With the proper night-vision equipment, you can see a
person standing over 200 yards (183 m) away on a moonless, cloudy night! Night vision
can work in two very different ways, depending on the technology used.
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Image enhancement - This works by collecting the tiny amounts of light,
including the lower portion of the infrared light spectrum, that are present but
may be imperceptible to our eyes, and amplifying it to the point that we can
easily observe the image.
Thermal imaging - This technology operates by capturing the upper portion
of the infrared light spectrum, which is emitted as heat by objects instead of
simply reflected as light. Hotter objects, such as warm bodies, emit more of this
light than cooler objects like trees or buildings.
In this article, you will learn about the two major night-vision technologies. We'll also
discuss the various types of night-vision equipment and applications. But first, let's talk
about infrared light.
The Basics
In order to understand night vision, it is important to understand something about light.
The amount of energy in a light wave is related to its wavelength: Shorter wavelengths
have higher energy. Of visible light, violet has the most energy, and red has the least. Just
next to the visible light spectrum is the infrared spectrum.
Infrared light can be split into three categories:
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Near-infrared (near-IR) - Closest to visible light, near-IR has wavelengths
that range from 0.7 to 1.3 microns, or 700 billionths to 1,300 billionths of a
meter.
Mid-infrared (mid-IR) - Mid-IR has wavelengths ranging from 1.3 to 3
microns. Both near-IR and mid-IR are used by a variety of electronic devices,
including remote controls.
Thermal-infrared (thermal-IR) - Occupying the largest part of the infrared
spectrum, thermal-IR has wavelengths ranging from 3 microns to over 30
microns.
The key difference between thermal-IR and the other two is that thermal-IR is emitted by
an object instead of reflected off it. Infrared light is emitted by an object because of what
is happening at the atomic level.
Atoms
Atoms are constantly in motion. They continuously vibrate, move and rotate. Even the
atoms that make up the chairs that we sit in are moving around. Solids are actually in
motion! Atoms can be in different states of excitation. In other words, they can have
different energies. If we apply a lot of energy to an atom, it can leave what is called the
ground-state energy level and move to an excited level. The level of excitation depends
on the amount of energy applied to the atom via heat, light or electricity.
An atom consists of a nucleus (containing the protons and neutrons) and an electron
cloud. Think of the electrons in this cloud as circling the nucleus in many different
orbits. Although more modern views of the atom do not depict discrete orbits for the
electrons, it can be useful to think of these orbits as the different energy levels of the
atom. In other words, if we apply some heat to an atom, we might expect that some of the
electrons in the lower energy orbitals would transition to higher energy orbitals, moving
farther from the nucleus.
Once an electron moves to a higher-energy orbit, it eventually wants to return to the
ground state. When it does, it releases its energy as a photon -- a particle of light. You
see atoms releasing energy as photons all the time. For example, when the heating
element in a toaster turns bright red, the red color is caused by atoms excited by heat,
releasing red photons. An excited electron has more energy than a relaxed electron, and
just as the electron absorbed some amount of energy to reach this excited level, it can
release this energy to return to the ground state. This emitted energy is in the form of
photons (light energy). The photon emitted has a very specific wavelength (color) that
depends on the state of the electron's energy when the photon is released.
Anything that is alive uses energy, and so do many inanimate items such as engines and
rockets. Energy consumption generates heat. In turn, heat causes the atoms in an object to
fire off photons in the thermal-infrared spectrum. The hotter the object, the shorter the
wavelength of the infrared photon it releases. An object that is very hot will even begin to
emit photons in the visible spectrum, glowing red and then moving up through orange,
yellow, blue and eventually white. Be sure to read How Light Bulbs Work, How Lasers
Work and How Light Works for more detailed information on light and photon emission.
In night vision, thermal imaging takes advantage of this infrared emission. In the next
section, we'll see just how it does this.
Thermal Imaging and Image Enhancement
Here's how thermal imaging works:
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A special lens focuses the infrared light emitted by all of the objects in view.
The focused light is scanned by a phased array of infrared-detector
elements. The detector elements create a very detailed temperature pattern
called a thermogram. It only takes about one-thirtieth of a second for the
detector array to obtain the temperature information to make the
thermogram. This information is obtained from several thousand points in
the field of view of the detector array.
The thermogram created by the detector elements is translated into electric
impulses.