[Note to reader: topic sentences are in green;
remaining weakness in red.]
Night Vision technologies encompass techniques of image
conversion, image intensification and thermal imaging. Each has
distinct characteristics and capabilities. The current
image intensification technology -- utilizing photocathodes, phosphor
screens and microchannel plates -- illustratres the technology of
light amplification and image resolution.
[No topic sentence in this introduction. Suggests for
jazzing it up appear later.] 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
Types of Night Vision
Night Vision technology is a term that presently encompasses three
distinct technologies. The first night vision equipment was developed
during the Korean War. Now sometimes referred to as generation-zero,
this equipment employed image converter technology to transform infrared
to visible light. The equipment operated in an active mode in that it
required the subject of observation to be illuminated with an infrared
light source. Lasers and filtered flashlights were utilized for this
purpose. Resolution of the detector was constrained by the available
processing technology at the time, limiting practical military
engagement distances to within a few hundred meters. This active system
also had the drawback of being easily detected by others with viewing
The subsequent principal technological development for night
vision is thermal imaging. Thermal imagers are passive systems that
respond to available infrared light at wavelengths in the 8-12
micrometer range. These wavelengths are readily emitted by all
blooded animals as well as soil and plant life, warmed during the
daytime hours. Infrared light is thus generated and available
continuously, during both the day and night, so that these viewers have
the distinction of performing equally well in both environmental lighting
The vast majority of night vision equipment and what most people think
of when the are referring to ``Night Vision'' are devices that utilize
image intensification technology. These devices are presently in their
third development phase. They are passive devices that operate using
naturally available light. Incoming light is converted to electrons,
which are amplified and converted back into visible light.
Image Intensifier Principles of Operation
Figure 1 illustrates first-generation night vision.
[Not a great topic sentence but it does has the advanage
of calling attention to the figure.] Incoming light is
collimated by fiber optic plates before impacting a photocathode t which
releases electrons, which in turn impact a phosphor screen. The excited
screen emits green light into a second fiber optic plate, and the process
is repeated. The complete process is repeated three times providing an
overall gain of 10,000.
Second-generation image intensification significantly increased gain and
resolution by employing a microchannel plate. Figure 2 depicts the
basic configuration. [These two sentences could
have been combined: "Figure2 depicts how second-generation image ...
plate."] The microchannel plate is composed of several
million microscopic hollow glass channels fused into a disk. Each
channel, approximately 0.0125 mm in diameter, is coated with a special
semiconductor which easily liberates electrons. A single electron
entering a channel initiates an avalanche process of secondary emission,
under influence of an applied voltage, freeing hundreds of electrons.
These electrons, effectively collimated by the channel, increase the
resolution of the device. With additional electron optics, details as
fine as 0.025 mm can be realized (half the diameter of a human hair).
Figure 1. First-generation image
intensifiers employ three stages of photocathode adn phosphor screen
Current image intensifiers incorporate their predecessor's resolution
with additional light amplification. The multialkali photocathode is
replaced with a gallium arsenide photocathode; this extends the
wavelength sensitivity of the detector into the near infrared. The moon
and stars provide light in these wavelengths, which boosts the
effectively available light by approximately 30%, bringing the total
gain of the system to around 30,000.
Figure 2. In second-generation night
vision the addition of the microchannel plate (MCP) collimated electron
flow and increased the light-amplification gain.
[No topic sentence. Indeed one might have moved this
material to the front in a more dramatic way, perhaps by calling
attention to the movie `Silence of the Lambs.'] slight green
tint similar to some sunglasses. The apparent lighting of the
landscape on a dark night is comparable to what the unaided eye would
see on a clear winter night with fresh snow on the ground and a full
Although the term ``night vision'' currently encompasses three distinct
technologies, it is the evolution of image intensification technology
that first made devices practical and widely used. Their success was
the result of advancements in light amplification and resolution
 Figure from ``Scoping Out Night Vision,' National Law Enforcement
and Corrections Technology Center (March 1996)
 L.S. Barrell (No date). Night Vision -- An Overview [Online].
[1997, April 24].
 [No date]. About Night Vision [Online]. Available:
http://www.keyinfo.com/cat/jdh/jdh~00012.html [1997,March 10].
 [No date]. Microchannel Plates Principles of Operation [Online].
Available: http://hea-www.harvard.edu/HRC/mcp/mcp.html [1997,
Your comments and
suggestions are appreciated.
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Edited by: firstname.lastname@example.org [September 1997]