There are two common types of
thermal FLIR 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. Image
Enhancement: Image-enhancement 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 again. A conventional lens,
called the objective lens, captures ambient light
and some near-infrared light. 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 "AA"
batteries. The tube outputs a high voltage, about
5,000 volts, to the image-tube components. 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 electron multiplier. 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 output side.
Night-vision images are known
for their eerie green tint. 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.Â
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 the device. 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.
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. T 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. 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. 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 needs.
