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"Night Vision" as
referenced here is that technology that
provides us with the miracle of vision
in total darkness and the improvement of vision in
low light environments.
This
technology is an amalgam of several different methods
each having its own advantages and disadvantages. The
most common methods as described below are Low-Light
Imaging, Thermal Imaging and Near-infrared
Illumination. The most common applications include
night driving or flying, night security and surveillance,
wildlife observation, sleep lab monitoring and search
and rescue. A wide range of night
vision products are available to suit the various
requirements that may exist for these applications:
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Low-Light Imaging
Today, the most popular and well known method of
performing night vision is based on the use of image intensifiers. are commonly used in night
vision goggles and night
scopes. More recently, on-chip gain multiplication CCD
cameras have become popularized for performing low-light security,
surveillance and astronomical observation. |
| Image intensifiers |
| How they work: This method of night
vision amplifies the available light to achieve
better vision. An objective lens focuses available
light (photons) on the photocathode of an image
intensifier. The light energy causes electrons
to be released from the cathode which are accelerated by an
electric field to increase their speed (energy level). These
electrons enter holes in a microchannel plate and bounce off
the internal specially-coated walls which generate more electrons
as the electrons bounce through. This creates a denser "cloud"
of electrons representing an intensified version of
the original image. |
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The final stage of the image intensifier involves electrons
hitting a phosphor screen. The energy of the electrons
makes the phosphor glow. The visual light shows the desired
view to the user or to an attached photographic camera
or video device. A green phosphor is used in these applications
because the human eye can differentiate more shades of
green than any other color, allowing for greater differentiation
of objects in the picture. |
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| All image intensifiers operate in the above fashion.
Technological differences over the past 40 years have resulted
in substantial improvement to the performance of these devices.
The different paradigms of technology have been commonly identified
by distinct . Intensified camera systems
usually incorporate an image intensifier to create a brighter
image of the low-light scene which is then viewed by a traditional
camera. |
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Excellent low-light level sensitivity
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Enhanced visible imaging yields the best possible recognition and
identification performance.
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High resolution
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Low power and cost
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Ability to identify people
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Because they are based on amplification methods, some light is required. This
method is not useful when there is essentially no light.
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Inferior daytime performance when compared to daylight-only methods
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Possibility of blooming and damage when observing bright sources under
low-light conditions.
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| On-chip
Gain Multiplication Cameras |
| How they work: In order to overcome some of the disadvantages
of image intensifiers, CCD image detector manufacturers
have substantially improved the sensitivity of
certain CCD detectors by incorporating an on-chip
multiplication gain technology to multiply photon-generated
charge above the detector's noise levels. The multiplication
gain takes place after photons have been detected
in the device's active area but before one of the
detector's
primary noise sources (e.g. readout noise). In
a new multiplication register, electrons are accelerated
from pixel-to-pixel by applying high CCD clock
voltages. As a result, secondary electrons are
generated via an impact-ionization process. Gain can be controlled
by varying the clock voltages. |
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| Because the signal boost occurs before the
charge reaches the on-chip readout amplifier and gets added
to the primary noise source, the signal-to-noise ratio for
this device is significantly improved over standard CCD cameras
and yields low-light imaging performance far superior than
traditional CCD cameras. However, since the CCD temperature
also affects the on-chip gain multiplication (lower temperatures
yield higher gain) and because other noise sources exist that
occur before the multiplication (i.e. dark noise), it is prudent
in these systems to temperature stabilize these detectors at
temperatures about of below room temperature. |
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| Another method for improving a CCD camera's
sensitivity is to perform averaging to reduce noise
either temporally (where sequential video frames
are averaged) or spatially (where neighboring pixels
are "binned" or
added together). |
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High sensitivity in low-light.
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Reduced likelihood of damage to the imaging detector due to viewing bright
sources.
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High speed imaging capability.
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Good daytime imaging performance.
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High power dissipation due to the necessity to have a temperature stabilizer.
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Blooming when viewing bright sources in dark scenes.
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Thermal
Imaging
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Different from low-light imaging methods of night vision
(which require some ambient light in order to produce an
image), thermal imaging night vision methods do not require
any ambient light at all. They operate on the principal that
all objects emit infrared energy as a function of their temperature.
In general, the hotter an object is, the more radiation it
emits. A thermal imager is a product that collects the infrared
radiation from objects in the scene and creates an electronic
image. Since they do not rely on reflected ambient light,
thermal imagers are entirely ambient light-level independent.
In addition, they also are able to penetrate obscurants such
as smoke, fog and haze. There are two types of thermal imaging
detectors: cooled and uncooled. require cryogenic cooling
to very cold temperatures (below 200K). are normally either
temperature stabilized (at room temperatures) or entirely
unstabilized.
Thermal images are normally black and white in nature, where
black objects are cold and white objects are hot. Some thermal
cameras show images in color. This false color is an excellent
way of better distinguishing between objects at different
temperatures.
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 Cooled-detector Infrared
Cameras
How they work: Cooled infrared detectors
are typically housed in a vacuum-sealed case and cryogenically
cooled. The detector designs are similar to other more common
imaging detectors and use semiconductor materials. However,
it is the effect of absorbed infrared energy that causes
changes to detector carrier concentrations which in turn
affect the detector's
electrical properties. Cooling the detectors
(typically to temperatures below 110K, a value
much lower than the temperature of objects being detected)
greatly increases their sensitivity. Without cooling, the
detectors would be flooded by their own self-radiation.
Materials used for infrared detection include a wide range
of narrow gap semiconductor devices, where mercury cadmium
telluride (HgCdTe) and indium antimonide (InSb) are the most
common.
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- The highest possible thermal sensitivity.
- Able to detect people and vehicles at great distances.
- Not affected by bright light sources.
- Able to perform high speed infrared imaging.
- Able to perform multi-spectral infrared imaging.
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- Expensive to purchase and to operate.
- Limited cooler operating lifetime.
- May require several minutes to cool down upon initiation.
- Bulky
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Uncooled-detector Cameras
How they work: Unlike the cryogenically cooled detectors
described above, uncooled infrared detectors operate at or
near room temperature rather than being cooled to extremely
low temperatures by bulky and expensive cryogenic coolers.
When infrared radiation from night-time scenes are focused
onto uncooled detectors, the heat absorbed causes changes
to the electrical properties of the detector material. These
changes are then compared to baseline values and a thermal
image is created. Despite lower image quality than cooled
detectors, uncooled detector technology makes infrared cameras
smaller and less costly and opens many viable commercial
applications.
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Uncooled detectors are mostly based on materials that change
their electrical properties due to pyroelectric (capacitive)
effects or microbolometer (resistive) effects.
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- Relatively inexpensive compared
to other thermal imaging technologies.
- High contrast in most night-time scenarios.
- Easily detects people and vehicles.
- Not affected by bright light sources.
- Higher reliability than cooled detector
thermal imagers.
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- Less sensitive than cooled detector
thermal imagers.
- Cannot be used for multispectral or high-speed
infrared applications.
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Near
Infrared Illumination
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A popular and sometimes inexpensive method for performing
night vision is by near infrared illumination.
In this method, a device that is sensitive to invisible near
infrared radiation is used in conjunction with an infrared
illuminator. The Sony
Night Shot camcorder popularized this method. Because
of the IR sensitivity of the camcorder's CCD
detector and since Sony installed an infrared light source
in the camcorder, infrared illumination was available to
augment otherwise low-light video scenes and produce reasonable
image quality in low-light situations.
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The method of near-infrared illumination has been used in
a variety of night vision applications including perimeter
protection where, by integrating with video motion detection
and intelligent scene analysis devices, a reliable low-light
video security system can be developed.
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IR
Illumination
How they work: Several different near infrared
illumination devices are available today, including:
- Filtered incandescent lamps: A standard high power lamp
that is covered by an infrared filter designed
to pass the lamp's near infrared radiation and block the
visible light component. These devices typically
need good heat transfer properties since the intense visible
light is internally absorbed and dissipated as heat.
- LED type illuminators: These illuminators utilize an
array of standard infrared emitting LEDs.
- Laser type: The most efficient infrared illuminator,
these devices are based on an infrared laser diode that
emits near infrared energy.
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available in a range of wavelengths (e.g. 730nm, 830nm, 920nm).
Providing supplemental infrared illumination of an appropriate
wavelength not only eliminates the variability of available
ambient light, but also allows the observer to illuminate only
specific areas of interest while eliminating shadows and enhancing
image contrast. The supplemental near infrared lighting not
only improves the quality of image intensifier devices (which
have both a visible and a near-infrared response), but also
permits the use of solid state cameras, which also have the
ability to convert near infrared images to visible. |
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- Lowest cost compared to other
night vision technologies.
- Eliminate shadows and reveal identifying lettering,
numbers and objects. Can also be used to perform
facial identification.
- Able to perform high-speed video capture (such
as reading license plates of moving vehicles).
- IR illuminators can see through night-time
fog, mist, rain and snowfall as well as windows.
- Eliminates the variability of ambient
light.
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- Users of infrared illuminators
can be detected by others that have near-infrared
viewing devices.
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| IR Illumination products: |
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Glossary of Night
Vision Terms
Atmospheric transmission
Absorption of the infrared energy by the atmosphere.
High transmission ranges are known as “atmospheric
windows” through which infrared imaging over very
long distances can be performed.
Electromagnetic spectrum
The electromagnetic spectrum divides up the regions
of electromagnetic radiation into different ranges having
unique characteristics. This radiation is divided up rather
arbitrarily into a number of regions based on wavelength:
Gamma <10 nanometers, Ultraviolet radiation, Visible
light 0.4 to 0.7 micrometers, Infrared Radiation, Microwaves,
Radio waves. The following is a sub-categorization for
the infrared range relevant for night vision:
- Short
wave infrared range (SWIR): Also known as the
Near infrared range, that portion of the infrared spectrum
from 750nm to 2500nm.
- Mid
wave infrared range (MWIR): That portion of
the infrared spectrum from about 3 microns to 5 microns.
- Long
wave infrared range (LWIR): That portion of
the infrared spectrum from about 8 microns to 12
microns.
Generations
of image intensifiers
The different paradigms of image intensifier
technology have been identified by
“generations” of technology (also known as “Gen”).
Generation 0 technology first developed in the
1950s depended on near infrared illumination
to produce reasonable night vision images. After
the light was converted to electrons, these electrons
were accelerated so they hit a phosphor screen
with greater energy, creating a visible image.
Unfortunately, the accelerated electrons were
somewhat distorted and vision with this method
was impaired. Generation 1 image intensifiers
were then developed that used a photocathode
material that was better than Gen 0 in converting
light to electrons. These units were able to
operate at lower light levels than the Gen 0 and, became
known as "starlight
scopes" since near infrared illumination was
not required. When three tubes were cascaded
together, the sensitivity was sufficient for most night vision
applications, but distortion existed. Generation
2 image intensifiers marked the development of
a microchannel plate which multiplies the number
of electrons by the thousands. A single unit
of a Generation 2 image intensifier produced the same sensitivity
as a 3-tube cascaded Generation 1 device but in
a much small package and without distortion.
Generation 3 is the most sophisticated night
vision technology available today. The image intensifier's
photocathode is coated with sensitive gallium
arsenide, which allows for a more efficient conversion
of light to electrical energy at extremely low
levels of light. Generation 3 provides the clearest,
sharpest night vision image available.
Image intensifier tube
An electro-optical device which converts photons
to electrons, amplifies them, then converts them back to
photons so the user can see at light levels that are normally
too low.
Infrared
The range of electromagnetic radiation having
a wavelength longer than that of visible light
and shorter than that of microwave radiation.
The name “infrared”
translates to "below red", where red is the color of visible
light of longest wavelength. Infrared radiation
spans the wavelengths between approximately 750nm
(0.75 microns) and 1mm (1000 microns).
Microbolometer
An infrared detector that absorbs the IR radiation
and warms slightly; the electrical resistance across the
bolometer changes as a function of temperature, which can
be measured and made into a thermal image. See also our White
Paper about Uncooled Microbolometers.
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| To see our line of Night Vision products
please click
here |
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