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Millimeter-Wave and Submillimeter-Wave Imaging for Security and Surveillance
I N V I T E D
P A P E R
Millimeter-Wave and
Submillimeter-Wave Imaging
for Security and Surveillance
Explosives hidden under clothing can be imaged by submillimeter waves,
but millimeter waves are better suited for guiding helicopter navigation
in poor weather.
By Roger Appleby,
Member IEEE
, a n d R u p e r t N . A n d e r t o n
ABSTRACT
|
Passive equipments operating in the 30300 GHz
(millimeter wave) band are compared to those in the 300 GHz
3 THz (submillimeter band). Equipments operating in the
submillimeter band can measure distance and also spectral
information and have been used to address new opportunities
in security. Solid state spectral information is available in the
submillimeter region making it possible to identify materials,
whereas in millimeter region bulk optical properties determine
the image contrast. The optical properties in the region from
30 GHz to 3 THz are discussed for some typical inorganic and
organic solids. In the millimeter-wave region of the spectrum,
obscurants such as poor weather, dust, and smoke can be
penetrated and useful imagery generated for surveillance. In
the 30 GHz3 THz region dielectrics such as plastic and cloth
are also transparent and the detection of contraband hidden
under clothing is possible. A passive millimeter-wave imaging
concept based on a folded Schmidt camera has been devel-
oped and applied to poor weather navigation and security. The
optical design uses a rotating mirror and is folded using
polarization techniques. The design is very well corrected over
a wide field of view making it ideal for surveillance and
security. This produces a relatively compact imager which
minimizes the receiver count.
KEYWORDS
|
Millimeter-wave imaging; security; submillimeter-
wave imaging; surveillance; terahertz; 35 GHz; 94 GHz
I .
I N T R O D U C T I O N
In the region of the spectrum below 30 GHz, conventional
imaging is impractical due to the large apertures required.
It has, however, been known since the 1940s that
operating equipment in this part of the spectrum provides
excellent penetration of the atmosphere and other obscur-
ants. This has given rise to air defense and surveillance
radar systems capable of standoff ranges of hundreds of
kilometers. These systems make use of apertures of several
meters which are either real or synthetic. Synthetic
aperture radar (SAR) uses the motion of the platform on
which the radar is mounted to synthesize the aperture.
Radars operating at frequencies less than 30 GHz use very
mature technology and there are powerful transmitters
and well established techniques for data collection and
display [1]. In this paper we are primarily concerned
with systems which operate at frequencies of greater
than 30 GHz and which have smaller apertures.
Above 30 GHz the transmission of the atmosphere
varies more strongly as a function of frequency. This
variation is primarily caused by the absorption due to
water vapor and oxygen and generally increases with
frequency. There are relatively transparent windows in the
atmosphere which occur at 35, 94, 140, and 220 GHz, and
equipments typically operate at these frequencies. The
scene being observed will also have optical properties
which are frequency dependent and will give rise to
contrast in an image. The origin of this contrast is
discussed in Section II.
In the millimeter-wave (MMW) region from 30 to
300 GHz (1 cm to 1 mm), imaging systems have been
developed. In this waveband both passive and active
imaging systems have been demonstrated. They are able to
penetrate poor weather for surveillance and also see
Manuscript received October 12, 2005; revised March 19, 2007. This work was
supported by the U.K. Ministry of Defence.
The authors are with QinetiQ Ltd, Malvern WR14 3PS, U.K.
(e-mail: rappleby@QinetiQ.com; rnanderton@QinetiQ.com).
Digital Object Identifier: 10.1109/JPROC.2007.898832
Vol. 95, No. 8, August 2007 |
P r o c e e d i n g s o f t h e I E E E
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2 0 0 7 I E E E through materials such as cloth and polymers for security
applications. One example of an active imaging system
would be the 94 GHz FMCW radars produced for aircraft
landing [2]. The development of passive imaging sensors
has lagged behind the development of radar, but over the
last ten years the development of low noise receivers using
III-V semiconductors such as gallium arsenide and indium
phosphide has given rise to systems with good thermal
sensitivity. This has made it possible to image in real time
with mechanically scanned systems; these are described in
Section III-B.
The terahertz region (11000 THz) occupies the part
of the spectrum between 300 and 0.3
m and a section
of this from 300 GHz3 THz (1 mm to 100
m ) is the
submillimeter-wave (SMMW) region. In this region it has
until recently been difficult to develop high-power sources
and sensitive detectors. Hu and Nuss [3] reported the first
imaging system based on optoelectronic terahertz time-
domain spectroscopy. A 100 fs pulse generated from a
Tisapphire laser incident on semi-insulating gallium
arsenide was used to generate terahertz radiation. The
radiation after collimation and focusing on to the sample
was relayed onto an optically gated dipole detector. This
technique is a coherent method enabling images to be
generated at different depths in the sample and spectral
information to be measured. The atmosphere is strongly
absorbing in the SMMW region but short-range applica-
tions such as medical imaging, security, and nondestruc-
tive evaluation have been reported [4].
A comparison of imaging technology for the MMW and
the SMMW regions is constrained to some degree by the
difference in their maturity. Ditchfield and England [5]
reported the first MMW imaging systems in the United
Kingdom, which was 40 years before Hu and Nuss [3]
reported a similar demonstration in the terahertz region.
The history of MMW technology can be traced back as far
as the 1890s, but the first significant activities in this field
were conducted in the 1930s [6]. Since then, this
technology has continued to develop, with the most rapid
advances occurring in recent years.
Prototype passive MMW imaging systems have been
developed for poor weather navigation and security
scanning. Prototype terahertz systems have also been
developed for security screening and are discussed in
Section III. Section IV summarizes the comparison of
imaging in the SMMW and MMW regions.
I I .
C O N T R A S T I N T H E S C E N E
In an imaging system it is important to understand the
origin of the contrast in the scene. In the visible part of the
spectrum, where our eyes are most sensitive and the atmo-
sphere is virtually transparent, contrast is derived pri-
marily from the differences in reflectivity between objects
and their backgrounds. These objects are surrounded by a
hemisphere of sky which illuminates them. Objects have
different reflectivities in different parts of the visible
spectrum and so take on different colors. The reflectivity
can also be influenced by their surface properties which
can be either smooth giving rise to specular reflection, or
rough giving rise to diffuse reflection.
The contrast in the scene in any part of the spectrum is
a function of the optical properties of the object being
imaged and its background. The apparent temperature of
an object T
o
in the scene is defined by (1). T is the physical
temperature of the object, " its emissivity, T
S
is the
temperature of the background which is reflected by
the object with reflectivity r, T
B
is the temperature of the
background immediately behind the object, and t is the
objects transmission
T" þ T
S
r þ T
B
t ¼ T
o
:
(1)
Imagery can be generated in at least two ways. The first
is by receiving natural radiation which has been emitted
and reflected from the scene and is known as passive
imaging. The second is by transmitting radiation at the
scene which after reflection is collected, and is known as
active imaging (radar).
Passive MMW images can look similar to visible
pictures given an imager with a similar spatial resolution
[7], as the reflected component is often large. This is due
to two factors: firstly many surfaces are smooth on a
wavelength scale and act as specular reflectors and
secondly the sky has a low radiation temperature. The sky
radiation temperature is typically 100 K on a clear day at
the zenith. The cosmic background (5 K) is increased to
100 K by atmospheric emission. When reflectivity ap-
proaches zero, as is the case with grass (see Table 2),
the apparent temperature will be a function of its
physical temperature and will be similar to the ambient
temperature.
In active imaging the signature is dominated by
reflection, and only the parts of the target which provide
a return signal will be detected. A surface which has this
property could be normal to the direction of illumination
or be a structure such as a corner cube; these are often
referred to as scattering centers. Active images are often
dominated by speckle and multipath effects but these can
be reduced by maximizin