There are lots
of challenges in performing video surveillance at night. The optimal
solution for a particular application will depend on the requirements
for the specific application. For example, is daytime operation
required? Does the system need to be covert (so that no visible
illumination is used so that it is either undetectable or limits
light pollution)? What is the size and shape of the area to be
monitored? Is the goal of the surveillance to detect, recognize
or identify subjects in the field of view?
The "best" system design for a specific
video surveillance system that includes imaging at night will utilize
one of the three primary night vision technologies: low-light imaging,
thermal imaging and near-infrared illumination. In this article,
these three technologies are explained and the advantages of each
reviewed.
As shown in the figure below, each night vision technology
exploits a different range of the electromagnetic spectrum. Low-light
imaging devices basically work in the visible light range, normally
400-750nm. Near-infrared illumination operates in the non-visible
near-infrared spectrum, wavelengths slightly longer than visible
light. Thermal imaging operates in the infrared range of wavelengths,
normally 3-5μm or 8-12μm.
Electromagnetic Spectrum
|
Low-light
Imaging Range |
Near Infrared
Imaging Range |
Thermal
Imaging
Range |
Ultraviolet |
|
Near-infrared |
Infrared |
The table below summarizes some of the differences in the technologies.
Comparison of Capabilities for Different Night Vision Technologies
Capabilities |
Low-light
Imaging |
Thermal
Imaging |
Near-Infrared
Illumination |
24-hour surveillance |
+++ |
+++ |
+++ |
Detection |
+ |
+++ |
+ |
Recognition/identification |
+++ |
o |
++ |
Passive operation |
+++ |
+++ |
o |
Ambient light level independent |
o |
+++ |
++ |
Key: +++ - Best ++ - Good + - Marginal o - Poor
Low-Light Imaging
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FIGURE 1: Image from EMCCD
Camera showing high sensitivity without blooming |
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The
most popular technology for night vision is the image intensifier
which is commonly used in portable systems such as for night vision
photography and night-time electronic news gathering. However,
for the video monitoring applications that are considered here,
other technologies exist that produce results that are in many
ways superior. Specifically, electronic multiplication CCD detectors,
or EMCCD cameras, have become popularized for performing low-light
security and surveillance monitoring. Other enhanced CCD cameras,
integrating CCD cameras, are also available that perform image
processing tricks to enhance certain aspects of camera performance
at the expense of other performance specifications.
EMCCD
Cameras: Electron multiplication CCD or EMCCD cameras
incorporate a significant advancement to CCD detector technology
that results in performance better than intensified CCDs. At
the heart of the EMCCD is a highly sensitive CCD detector that
has its image signal amplified on the chip while it is still
in the charge domain. The multiplication gain (which takes place
after photons have been detected in the device’s active area
but before one of the detector’s primary noise sources) amplifies
signal electrons up to 1000 times so that they may be detected
above the CCD output and camera electronics noise.This allows
the sensor to operate in real time with sub-electron equivalent
readout noise, enabling very dim sources and very dark scenes
to be imaged. These sensors maintain full well capacity and high
quantum efficiency while minimizing blooming from strong light
sources in the object being imaged. As a result, EMCCDs are now
preferred in many low-light applications such as night-time surveillance
and astronomy. In addition, EMCCDs have the capability to operate
across the full range of light levels which results in it’s usefulness
for 24-hour surveillance.
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FIGURE 2: SunStar 800 EMCCD
Camera
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EMCCD cameras offer the ability to operate in full daylight as well as starlight.
Advantages of EMCCD Cameras:
-
High sensitivity to low-light
-
High spatial resolution & contrast
-
No halo from bright sources allowing visible detail in adjacent
pixels
-
Not susceptible to damage from bright lights
-
No scintillation giving improved image quality
-
Solid state giving lower lifecycle cost
-
Excellent for day and night operation
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FIGURE 3: SunStar 300 Integrating
CCD Camera
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Integrating CCD Cameras: 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).
By performing special processing of the signal inside the CCD,
superior performance over standard CCD cameras can be achieved.
Certain CCD cameras have the capability to automatically perform
binning (where the signal from adjacent pixels are averaged) and
frame averaging (where the pixel signal from sequential video images
are averaged) when light levels fall below acceptable values. These
image processing techniques decrease noise levels and result in
superior low-light performance up to 100 times lower than standard
CCDs. However, this technique does have compromised performance.
Binning will result in a decrease in resolution. Normally, binning
is performed in the vertical dimension first, retaining the level
of horizontal resolution normally important when imaging objects
at the horizon. Temporal integration causes the apparent sensitivity
to still objects increases while moving objects may become blurry.
Thermal Imaging
Different
from low-light imaging methods of night vision (which require some
ambient light in order to produce an image), thermal imaging does
not require any ambient light at all. Thermal infrared cameras
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. An infrared camera is a product
that collects the infrared radiation from objects in the field-of-view
and creates an electronic image based on the level of radiation
detected. Since they do not rely on reflected ambient light, infrared
cameras 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. Cooled
detector infrared cameras require cryogenic cooling to very
cold temperatures (below 200K). Uncooled
detector infrared cameras are normally either temperature stabilized
(at room temperatures) or entirely unstabilized.
Thermal infrared cameras operate on the principal that all objects emit infrared energy as a function of their temperature. |
Thermal images are normally black and white in nature,
where black objects represent cold objects and white objects represent
hot objects. 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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FIGURE 4: Epsilon Engine Cooled-detector
Infrared Camera
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Cooled-detector Infrared Cameras: 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 110 K, 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. Cameras based on cooled HgCdTe and InSb detectors have similar performance, however HgCdTe detectors are often preferred because they exhibit higher quantum efficiency and wavelength tunability (including performance in shortwave, midwave and longwave bands as well as dual-band).
Advantages of Cooled-detector Infrared Cameras:
-
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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FIGURE 5: ATOM 1024 Uncooled
Infrared Camera
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Uncooled-detector
Infrared Cameras: Unlike the cooled detector cameras 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 resistance properties of the detector material.
These changes are then compared to baseline values and a thermal
image is created. Despite lower sensitivity than realized in cameras
using cooled detectors, uncooled detector infrared cameras are
smaller, much less costly and have higher operating reliability
over time. As a result, these new technology infrared cameras open
many viable commercial applications.
Advantages of Uncooled Infrared Cameras:
-
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
Near Infrared Illumination
A
popular and sometimes inexpensive method for performing night vision
is by near infrared illumination. In this method, a camera that
uses a detector 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 without the use
of visible light sources.
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.
Several
different near infrared illumination devices are available today,
including:
-
Filtered incandescent lamps: A standard high power lamp covered
by a filter designed to pass the lamp’s near infrared radiation
and block the visible.
-
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.
Near
infrared illuminators are typically 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 improves
the quality of solid state cameras that have the ability to convert
near infrared images to visible.
Providing supplemental infrared illumination is covert, eliminates the variability of available ambient light, and allows observers to illuminate only specific areas of interest. |
Advantages:
-
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.
Summary
Clearly, the optimal solution for a particular night-time video
surveillance application will depend on the requirements for that
application. The “best” design will utilize one of the three primary
night vision technologies: low-light imaging, thermal imaging and
near-infrared illumination.
The table below summarizes some of the differences in the technologies.
Comparison of Capabilities for Different Night Vision Technologies
Capabilities |
Low-light
Imaging |
Thermal
Imaging |
Near-Infrared
Illumination |
24-hour surveillance |
+++ |
+++ |
+++ |
Detection |
+ |
+++ |
+ |
Recognition/identification |
+++ |
o |
++ |
Passive operation |
+++ |
+++ |
o |
Ambient light level independent |
o |
+++ |
++ |
Key: +++ - Best ++ - Good + - Marginal o - Poor
The following refer to each capability in the table:
24-hour surveillance – All three technologies permit (to some capacity) 24 hour surveillance.
Detection – Thermal imaging is considered the superior technology when detection is required. Most objects under surveillance, whether they are people or vehicles in motion, have thermal signatures that are distinct from the background. As a result, these objects normally show high contrast with respect to the background and are easy to spot.
Recognition/identification – For
recognition/identification applications, thermal imaging is not
the preferred technology since thermal signatures are in many cases
difficult to identify and distinguish. Low-light imaging is best
since it generates an amplified version of the visible light signature.
Near-infrared illumination is also useful, but it is important
to realize that reflectivity in this spectral range may differ
from the reflectivity in the visible range. As a result, identifying
marks on vehicles may be difficult to read, hair color and complexion
will also differ when imaging in these wavelengths.
Passive operation
- Low-light and thermal imaging are both passive technologies since
neither surveillance method is easily detected. Near-infrared illumination,
on the other hand, is considered active since it emits a beam of
near infrared light that can be detected with other near-infrared
sensitive devices.
Ambient light level independent –Thermal imaging and near infrared illumination are somewhat light level independent. The presence of sunlight will cause solar heating that will impact thermal images, creating shadows and hot spots that are different than night-time. Near-infrared systems are often considered independent of ambient light since they emit their own illumination, but quite often these systems take advantage of the sun’s near-infrared radiation and may disable the separate illuminators. Image quality for low-light imagers will depend greatly on the ambient conditions.
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