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?"
Photo courtesy of B.E.
Gyro-stabilized day/night binoculars
manufactured by B.E. Meyers
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
- 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
In this edition of HowStuffWorks,
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.
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 is a small part of the light
Infrared light can be split into three categories:
- 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
- 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
- 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.
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.
An atom has a nucleus and an electron
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
Anything that is alive uses energy, and so do many
inanimate items such as engines and
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
In night vision, thermal imaging takes advantage of this
infrared emission. In the next section, we'll see just how it
Here's how thermal imaging
- 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.
- The impulses are sent to a signal-processing unit, a
circuit board with a dedicated chip that translates the
information from the elements into data for the display.
- The signal-processing unit sends the information to the
display, where it appears as various colors depending on the
intensity of the infrared emission. The combination of all
the impulses from all of the elements creates the image.
Image courtesy of Infrared,
The basic components
of a thermal-imaging
Most thermal-imaging devices scan at a rate of 30 times per
second. They can sense temperatures ranging from -4 degrees
Fahrenheit (-20 degrees Celsius) to 3,600 F (2,000 C), and can
normally detect changes in temperature of about 0.4 F (0.2 C).
Image courtesy of Infrared,
It is quite easy to
see everything during the
Image courtesy of Infrared,
...but at night, you
can see very
There are two common types of thermal-imaging devices:
- Un-cooled - This is the most common type of
thermal-imaging device. The infrared-detector elements are
contained in a unit that operates at room temperature. This
type of system is completely quiet, activates immediately
and has the battery
built right in.
- Cryogenically cooled - More expensive and more
susceptible to damage from rugged use, these systems have
the elements sealed inside a container that cools them to
below 32 F (zero C). The advantage of such a system is the
incredible resolution and sensitivity that result from
cooling the elements. Cryogenically-cooled systems can "see"
a difference as small as 0.2 F (0.1 C) from more than 1,000
ft (300 m) away, which is enough to tell if a person is
holding a gun at that distance!
While thermal imaging is great for detecting people or
working in near-absolute darkness, most night-vision equipment
uses image-enhancement technology, which you will learn about
in the next section.
technology is what most people think of when you talk about
night vision. In fact, image-enhancement systems are normally
called night-vision devices (NVDs). NVDs rely on a
special tube, called an image-intensifier tube, to
collect and amplify infrared and visible light.
The image-intensifier tube changes photons to
electrons and back
Here's how image enhancement works:
- A conventional lens, called the objective
lens, captures ambient light and some near-infrared
- The gathered light is sent to the image-intensifier
tube. In most NVDs, the power supply for the
image-intensifier tube receives power from two N-Cell or two
The tube outputs a high voltage, about 5,000 volts, to the
- The image-intensifier tube has a photo cathode,
which is used to convert the photons of
light energy into electrons.
- As the electrons pass through the tube, similar
electrons are released from atoms in the tube, multiplying
the original number of electrons by a factor of thousands
through the use of a microchannel plate (MCP) in the
tube. An MCP is a tiny, glass disc that has millions of
microscopic holes (microchannels) in it, made using fiber-optic
technology. The MCP is contained in a vacuum and has
metal electrodes on either side of the disc. Each channel is
about 45 times longer than it is wide, and it works as an
When the electrons from the photo cathode hit the first
electrode of the MCP, they are accelerated into the glass
microchannels by the 5,000-V bursts being sent between the
electrode pair. As electrons pass through the microchannels,
they cause thousands of other electrons to be released in
each channel using a process called cascaded secondary
emission. Basically, the original electrons collide with
the side of the channel, exciting atoms and causing other
electrons to be released. These new electrons also collide
with other atoms, creating a chain reaction that results in
thousands of electrons leaving the channel where only a few
entered. An interesting fact is that the microchannels in
the MCP are created at a slight angle (about a 5-degree to
8-degree bias) to encourage electron collisions and reduce
both ion and direct-light feedback from the phosphors on the
- At the end of the image-intensifier tube, the electrons
hit a screen coated with phosphors.
These electrons maintain their position in relation to the
channel they passed through, which provides a perfect image
since the electrons stay in the same alignment as the
original photons. The energy of the electrons causes the
phosphors to reach an excited state and release photons.
These phosphors create the green image on the screen that
has come to characterize night vision.
- The green phosphor image is viewed through another lens,
called the ocular lens, which allows you to magnify
and focus the image. The NVD may be connected to an
electronic display, such as a monitor,
or the image may be viewed directly through the ocular lens.
NVDs have been around for more than 40 years. They are
categorized by generation. Each substantial change in
NVD technology establishes a new generation.
- Generation 0 - The original night-vision system
created by the United States Army and used in World War II
and the Korean War, these NVDs use active infrared.
This means that a projection unit, called an IR
Illuminator, is attached to the NVD. The unit projects a
beam of near-infrared light, similar to the beam of a normal
flashlight. Invisible to the naked eye, this beam reflects
off objects and bounces back to the lens of the NVD. These
systems use an anode in
conjunction with the cathode to accelerate the electrons.
The problem with that approach is that the acceleration of
the electrons distorts the image and greatly decreases the
life of the tube. Another major problem with this technology
in its original military use was that it was quickly
duplicated by hostile nations, which allowed enemy soldiers
to use their own NVDs to see the infrared beam projected by
- Generation 1 - The next generation of NVDs moved
away from active infrared, using passive infrared
instead. Once dubbed Starlight by the U.S. Army,
these NVDs use ambient light provided by the moon and stars to
augment the normal amounts of reflected infrared in the
environment. This means that they did not require a source
of projected infrared light. This also means that they do
not work very well on cloudy or moonless nights.
Generation-1 NVDs use the same image-intensifier tube
technology as Generation 0, with both cathode and anode, so
image distortion and short tube life are still a problem.
- Generation 2 - Major improvements in
image-intensifier tubes resulted in Generation-2 NVDs. They
offer improved resolution and performance over Generation-1
devices, and are considerably more reliable. The biggest
gain in Generation 2 is the ability to see in extremely low
light conditions, such as a moonless night. This increased
sensitivity is due to the addition of the microchannel plate
to the image-intensifier tube. Since the MCP actually
increases the number of electrons instead of just
accelerating the original ones, the images are significantly
less distorted and brighter than earlier-generation NVDs.
- Generation 3 - The latest and greatest NVD
technology, Generation 3 is currently used by the U.S.
military. While there are no substantial changes in the
underlying technology from Generation 2, these NVDs have
even better resolution and sensitivity. This is because the
photo cathode is made using gallium arsenide, which
is very efficient at converting photons to electrons.
Additionally, the MCP is coated with an ion barrier, which
dramatically increases the life of the tube. Generation-3
NVDs are considered so state-of-the-art that they cannot be
exported from the United States without a license from the
U.S. Department of State that details the recipient and the
purpose it will be used for.
Many of the so-called "bargain" night-vision scopes use
Generation-0 or Generation-1 technology, and may be
disappointing if you expect the sensitivity of the devices
used by professionals. Generation-2 and Generation-3 NVDs are
typically very expensive to purchase, but they will last a
lifetime if properly cared for. Also, any NVD can benefit from
the use of an IR Illuminator in very dark areas where there is
almost no ambient light to collect.
Photo courtesy of B.E.
in a variety of styles, including ones that can be
A cool thing to note is that every single image-intensifier
tube is put through rigorous tests to see if it meets the
requirements set forth by the military. Tubes that do are
classified as MILSPEC. Tubes that fail to meet military
requirements in even a single category are classified as
Night-vision equipment can be
split into three broad categories:
- Scopes - Normally handheld or mounted on a
weapon, scopes are monocular (one eye-piece). Since
scopes are are handheld, not worn like goggles, they are
good for when you want to get a better look at a specific
object and then return to normal viewing conditions.
- Goggles - While goggles can be handheld, they are
most often worn on the head. Goggles are binocular
(two eye-pieces) and may have a single lens or stereo lens,
depending on the model. Goggles are excellent for constant
viewing, such as moving around in a dark building.
- Cameras - Cameras
with night-vision technology can send the image to a monitor
for display or to a VCR for
recording. When night-vision capability is desired in a
permanent location, such as on a building or as part of the
equipment in a helicopter,
cameras are used. Many of the newer camcorders
have night vision built right in.
applications for night vision include:
- Law enforcement
- Wildlife observation
- Hidden-object detection
The original purpose of night vision was to locate enemy
targets at night. It is still used extensively by the military
for that purpose, as well as for navigation, surveillance and
targeting. Police and security often use both thermal-imaging
and image-enhancement technology, particularly for
surveillance. Hunters and nature enthusiasts use NVDs to
maneuver through the woods at night.
Detectives and private investigators use night vision to
watch people they are assigned to track. Many businesses have
permanently-mounted cameras equipped with night vision to
monitor the surroundings.
A really amazing ability of thermal-imaging is that it
reveals whether an area has been disturbed -- it can show that
the ground has been dug up to bury something, even if there is
no obvious sign to the naked eye. Law enforcement has used
this to discover items that have been hidden by criminals,
including money, drugs and bodies. Also, recent changes to
areas such as walls can be seen using thermal imaging, which
has provided important clues in several cases.
Photo courtesy of B.E.
are a fast-growing segment
of the night-vision
Many people are beginning to discover the unique world that
can be found after darkness falls. If you're out camping or
hunting a lot, chances are that night-vision devices can be
useful to you -- just be sure to get the right type for your
For more information on night vision and related topics,
check out the links on the next page.
Lots More Information!
More Great Links