Night vision has been used on navy ships for fire control and missile
detection for about 30 years. The superiority obtained by NATO
forces in the Gulf war led to a greatly increased interest in night
vision generally and increased purchases by military users since then
have resulted in development of better cameras at lower prices so
bringing night vision into the realm of commercial users. Initially
Vistar manufactured hand held systems for security use and in 1981
introduced night vision equipment for use on commercial ships probably
becoming the first supplier for this market.
Two technologies are in common use, image intensifiers and thermal
imagers.
Image Intensifiers
Image Intensifiers (II or I2) electronically amplify
available light by
as much as 200,000 times so that an image is projected onto a CCD
camera. This is the older technology but is still being developed and
improved. II cameras cannot operate with no light but are extremely
sensitive and are designed to operate in starlight alone that equates
to a level of about 1 millilux.
For navigation purposes the
target contrast and thus the performance can be enhanced using
illuminators (searchlights) that operate at a wavelength that is
visible to image intensifiers but invisible to the human eye so that
they do not interfere with the natural night sight ability of ship’s
officers. In this way target acquisition ranges of up to a kilometre
can be achieved using wide beam illuminators of only 150 watts. This is
particularly useful for non military applications and is the principle
behind the Vistar 223 system.
Image intensifier systems offer good performance and long working life
but performance can be affected by a blooming when looking
towards
bright lights. An advantage is that very weak lights such as a yacht
using battery powered navigation lights can be detected at great
distances.
Thermal Imagers
Thermal Imagers (TI), sometimes called Infra Red (IR) cameras, detect
minute temperature differences between objects so enabling an image
to be constructed from the outputs of thousands of individual
sensors. The camera sensor has to be cooled to achieve the
required sensitivity using mechanical or electronic cooling engines.
The most sensitive sensors are cooled to about -200°C and so-called
uncooled (solid state electronic) sensors are cooled to about -20°C
with corresponding reduced sensitivity.
The camera can be switched to
show white hot or black hot images on the display. Most
TV
viewers will have seen helicopter pictures showing white images
of persons being followed or rescued because they are much hotter than
the background due to body heat.
Modern cooled sensors can detect
temperature differences of as little as 0.2°C and have range
performances of several kilometres but cost much more than
uncooled thermal imagers or image intensifiers. Due to atmospheric
attenuation there are three frequency windows that allow
detection of small temperature differences at a distance; 8-12 micron,
3-5 micron
and 1.2-1.5 micron. The best performance at sea is currently offered by
sensors in the 3-5 micron wavelength. Better performance can be
obtained in the 1.5 micron wavelength when pulsed lasers are used to
illuminate targets and improve contrast but this technology is strictly
controlled and is not yet available for commercial use.
Whilst thermal
sensors are completely unaffected by light and blooming often
found on image intensifiers does not occur, the longer wavelengths will
not
pass through glass making navigation lights on ships and buoys
invisible.
Cooled imagers rely on mechanical compressors for
cooling and these have to be replaced after service times of 5,000 to
10,000 hours. These repairs usually require return to the manufacturer
with consequent cost and are subject to export control procedures
similar to new items that can lead to turnaround times of several
months.
Vistar
use both types of night vision camera depending on the
application and customer requirement. We manufacture our own
image intensified cameras using tubes purchased from military
suppliers.
We are able to integrate most thermal imaging cameras, made by
specialist manufacturers, into Vistar surveillance systems.
The image intensifier receives light via the objective lens,
which is converted to electrons by the photocathode and accelerated
by an applied electric field.
These electrons, entering the micro-channel
plate strike the micro-channels' wall and are multiplied thousands of times.
The resulting electron stream strikes the phosphor screen and is presented to
the fibre-optic viewing plate.
The resultant image at the output is typically
250,000 times brighter than that received at the input.
