History of Night Vision :-
Night vision is the ability to see
in low light conditions. Whether by biological or technological means, night
vision is made possible by a combination of two approaches: sufficient spectral
range, and sufficient intensity range. Humans have poor night vision compared
to many animals, in part because the human eye lacks .
Types of ranges
Night-useful
spectral range techniques can sense radiation that is invisible to a human
observer. Human vision is confined to a small portion of the electromagnetic spectrum called visible light. Enhanced spectral range allows the
viewer to take advantage of non-visible sources of electromagnetic radiation
(such as near-infrared or ultraviolet
radiation). Some animals can see using much more of the infrared and/or
ultraviolet spectrum than humans.
Sufficient
intensity range is simply the ability to see with very small quantities of
light. Although the human visual system can, in theory, detect single photons under ideal conditions, the neurological noise
filters limit sensitivity to a few tens of photons, even in ideal conditions.
Many
animals have better night vision than humans do, the result of one or more
differences in the morphology and anatomy of their eyes. These include having a
larger eyeball, a larger lens, a larger optical aperture
(the pupils may expand to the physical limit of the eyelids), more rods than
cones (or rods exclusively) in the retina, a tapetum
lucidum.
Enhanced
intensity range is achieved via technological means through the use of an image
intensifier, gain multiplication CCD, or other
very low-noise and high-sensitivity array of photo detectors.
Night vision technologies:-
Night
vision technologies can be broadly divided into three main categories:
Image intensification
technologies work on the principle of magnifying the amount of received photons
from various natural sources such as starlight
or moonlight. Examples of such technologies include night glasses and low light
cameras.
Active illumination
technologies work on the principle of coupling imaging intensification
technology with an active source of illumination in the near infrared (NIR) or shortwave infrared (SWIR)
band. Examples of such technologies include low light cameras.
Thermal imaging
technologies work by detecting the temperature difference between the
background and the foreground objects.
Night glasses:-
Night
glasses are telescopes or binoculars
with a large diameter objective. Large lenses can gather and concentrate light,
thus intensifying light with purely optical means and enabling the user to see
better in the dark than with the open eye alone. Often night glasses also have
a fairly large exit pupil of 7 mm or more to let all
gathered light into the user's eye. However, many people can't take advantage
of this because of the limited dilation of the human pupil.
To overcome this, soldiers were sometimes issued atropine
eye drops to dilate pupils. Before the introduction of image intensifiers,
night glasses were the only method of night vision, and thus were widely
utilized, especially at sea. Second World War era night glasses usually had a
lens diameter of 56 mm or more with magnification of seven or eight. Major
drawbacks of night glasses are their large size and weight.
Active infrared:-
Imaging
results with and without active-infrared.
Active
infrared night vision combines infrared illumination of spectral range
700-1000nm (just below the visible spectrum of the human eye) with CCD cameras
sensitive to this light. The resulting scene, which is apparently dark to a
human observer, appears as a monochrome image on a normal display device.
Because
active infrared night vision systems can incorporate illuminators that produce
high levels of infrared light, the resulting images are typically higher
resolution than other night vision technologies. Active infrared night vision is now
commonly found in commercial, residential and government security applications,
where it enables effective night time imaging under low light conditions.
However, since active infrared light can be detected by night vision goggles,
there can be a risk of giving away position in tactical military operations.
Laser range gated imaging :-
Laser
range gated imaging is another form of active night vision which utilizes a
high powered pulsed light source for illumination and imaging. Range gating is
a technique which controls the laser pulses in conjunction with the shutter
speed of the camera's detectors. Gated imaging technology can be divided
into single shot, where the detector captures the image from a single
light pulse to multi-shot, where the detector integrates the light
pulses from multiple shots to form an image.
One
of the key advantages of this technique is the ability to perform target
recognition as opposed to detection with thermal imaging.
Thermal vision:-
Thermal
imaging cameras are excellent tools for night vision. They detect thermal
radiation and do not need a source of illumination. They produce an
image in the darkest of nights and can see through light fog, rain and smoke.
Thermal imaging cameras make small temperature differences visible. Thermal
imaging cameras are widely used to complement new or existing security
networks.
Image intensifier:-
The
image intensifier is a vacuum-tube based device that converts visible light
from an image so that a dimly lit scene can be viewed by a camera or the open
eye. While many believe the light is "amplified," it is not. When
light strikes a charged photo cathode plate, electrons are emitted through
a vacuum tube that strike the micro channel plate that cause the image screen to
illuminate with a picture in the same pattern as the light that strikes the
photo cathode, and is on a frequency that the human eye can see. This is much
like a CRT television,
but instead of color guns the photo cathode does the emitting.
The
image is said to become "intensified" because the output visible
light is brighter than the incoming IR light, and this effect directly relates
to the difference in passive and active night vision goggles.
Currently, the most popular image intensifier is the drop-in ANVIS module,
though many other models and sizes are available at the market.
Night vision devices:-
A night vision device (NVD) is a device comprising an image intensifier
tube in a rigid casing, commonly used by military forces. Lately night vision technology
has become more widely available for civilian use, for example, EVS, or
enhanced vision systems, which are included in the latest avionics packages in
cirrus and Cessna planes to help pilots with situational awareness and avoid
accidents. eVS is also available for rotary wing operators.
A
specific type of NVD, the night vision goggle (or NVG) is a night vision device
with dual eyepieces; the device can utilize either one intensifier tube with
the same image sent to both eyes, or a separate image intensifier tube for each
eye. Night vision goggle combined with magnification lenses constitutes night
vision binoculars. Other types include monocular night vision devices with only
one eyepiece which may be mounted to firearms as night sights. NVG and EVS
technologies are becoming standard operating products on helicopter operations
to improve safety. The NTSB is considering EVS as recommended equipment for
safety features. | 
