U.S. patent application number 11/109022 was filed with the patent office on 2007-03-08 for sensor having differential polarization capability and a network comprised of several such sensors.
Invention is credited to William C. Pittman.
Application Number | 20070051890 11/109022 |
Document ID | / |
Family ID | 37829206 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070051890 |
Kind Code |
A1 |
Pittman; William C. |
March 8, 2007 |
Sensor having differential polarization capability and a network
comprised of several such sensors
Abstract
Polarimetric processing capability is added to a typical sensor
so that a target object can be more clearly distinguished from the
background clutter in a given scenery. A polarizer with several
segments of different polarization orientations is used to improve
the typical sensor. The segments are sequentially advanced to pass
therethrough infrared radiation images of pre-selected polarization
orientations which are then collected by respective polarized frame
grabbers. Image processing circuit processes these images to yield
the polarization difference between any given pair of orthogonal
polarizations. In a surveillance network, the polarization
differences are subsequently used in the control center, to which
such sensors are connected, to enhance the distinction of the
observed objects against the background clutter suspended in the
propagation medium.
Inventors: |
Pittman; William C.;
(Huntsville, AL) |
Correspondence
Address: |
AMSAM-L-G-I (Ms. Anne Lanteigne);US Army Aviation and Missile Command
Legal Office, 4th Floor
Building 5300
Redstone Arsenal
AL
35898-5000
US
|
Family ID: |
37829206 |
Appl. No.: |
11/109022 |
Filed: |
April 8, 2005 |
Current U.S.
Class: |
250/332 ;
257/E27.151; 348/E5.028; 348/E5.09 |
Current CPC
Class: |
G01J 4/00 20130101; H01L
27/14806 20130101; H01L 27/14625 20130101; H04N 5/33 20130101; G01N
21/359 20130101; H04N 5/23206 20130101; G01N 21/21 20130101 |
Class at
Publication: |
250/332 |
International
Class: |
H01L 25/00 20060101
H01L025/00; G01J 4/00 20060101 G01J004/00; G02F 1/01 20060101
G02F001/01 |
Goverment Interests
DEDICATORY CLAUSE
[0001] The invention described herein may be manufactured, used and
licensed by or for the Government for governmental purposes without
the payment to me of any royalties thereon.
Claims
1. In a system for viewing a pre-selected scenery in the
near-infrared spectrum, the system having an iris lens assembly for
collecting incoming radiation, a charge-coupled device camera; a
relay lens for focusing the collected radiation onto the CCD
camera; a microchannel image intensifier positioned between the
iris lens assembly and the relay lens, an optical path length
compensator selectively-placeable between the iris lens and the
image intensifier, the intensifier amplifying the incorning
radiation level prior to transmitting the radiation to the relay
lens; and a photocathode coupled to the image intensifier, AN
IMPROVEMENT for providing a polarimetric processing capability to
the system so as to enable distinction of various features of the
pre-selected scenery, said IMPROVEMENT comprising: a polarizer
placeable between the iris lens and the photocathode, said
polarizer being composed of several discrete segments of different
polarization orientations and being translatable along the length
of the image intensifier in discrete steps so as to bring
sequentially said discrete polarization segments to be engaged
between the iris lens and the photocathode, each of said segments
being of sufficient size to cover all the pixels of the image
intensifier; a plurality of polarized frame grabbers coupled to the
CCD camera, each polarized frame grabber receiving radiation of a
particular polarization orientation and outputting an image frame
of that particular polarization orientation; and a logic circuit
for performing image processing on said image frame outputs from
said polarized frame grabbers and producing therefrom polarization
difference of any given pair of orthogonal polarization
orientations, said polarization difference being used to enhance
the visual perception of the diverse features of the scenery.
2. (canceled)
3. An IMPROVEMENT as set forth in claim 2, wherein said improvement
further comprises: a means to translate said polarizer in said
sequential manner along the length of the image intensifier.
4. An IMPROVEMENT as set forth in claim 3, wherein each of said
several discrete segments of said polarizer covers all of the pixel
elements in the microchannel image intensifier.
5. An IMPROVEMENT as set forth in claim 4, wherein said improvement
still further comprises: a first switch coupled between the CCD
camera and said polarized frame grabbers, said first switch
selectively connecting, from time to time, said CCD camera with one
of said polarized frame grabbers.
6. An IMPROVEMENT as set forth in claim 5, wherein said first
switch is selectively connected to said polarized frame grabbers in
synchronization with said translating means such that, at any given
moment in) time, said engaged polarization segment and said
connected polarized frame grabber are of the same polarization
orientation.
7-11. (canceled)
12. A surveillance network for surveilling multiple sceneries in
the near-infrared spectrum, said network comprising: a plurality of
ground-based sensors positioned for optically sensing objects in
the various sceneries; and a control center, said control center
being coupled to each of said ground-based sensors, said center
receiving image inputs from said sensors relative to their
respective sensed objects and processing, using appropriate
algorithms, said inputs to identify and track said sensed
objects.
13. A surveillance network as set forth in claim 12, wherein said
network further comprises: a plurality of air-borne sensors for
sensing objects in the sceneries, said air-borne sensors being
coupled to said control center and inputting to said center their
respective image information.
14. A surveillance network as set forth in claim 13, wherein said
center is coupled to each of said ground-based and air-borne
sensors via a wireless link or optical fiber link.
15. A surveillance network as set forth in claim 14, wherein each
of said sensors is capable of polarimetric processing of
near-infrared radiation that emanates from said sceneries and is
incident on said sensors to produce polarization difference of any
given pair of orthogonal polarization orientations, said
polarization differences being input to said control center,
thereby enabling said control center to distinguish target objects
from background clutter in the sceneries.
16. A surveillance network as set forth in claim 15, wherein each
of said sensors comprises: an iris lens assembly for collecting
incoming radiation, a charge-coupled device camera; a relay lens
for focusing the collected radiation onto said CCD camera; a
microchannel image intensifier positioned between said iris lens
assembly and said relay lens, an optical path length compensator
selectively-placeable between said iris lens and said image
intensifier, said intensifier amplifying the incoming radiation
level prior to transmitting the radiation to said relay lens; a
photocathode coupled to said image intensifier; a polarizer
composed of several discrete segments embodying different
polarization orientations, said polarizer being placeable between
said iris lens and said photocathode, said placement of said
polarizer alternating selectively with the placement of said
optical path length compensator in the same position; a plurality
of polarized frame grabbers coupled to said CCD camera, each
polarized frame grabber receiving radiation of a particular
polarization orientation and producing an image frame of that
particular polarization orientation; and a logic circuit for
performing image processing on said image frame outputs from said
polarized frame grabbers and producing therefrom polarization
difference of any given pair of orthogonal polarization
orientations, said polarization difference being transmitted to
said control center to be used thereby to enhance the visual
distinction of target objects against background clutter.
17. A surveillance network as set forth in claim 16, wherein said
polarizer in each sensor is disposed to be translatable linearly
along the length of said image intensifier in discrete steps so as
to bring sequentially said discrete polarization segments to be
engaged between said iris lens and said photocathode and said each
sensor contains therein, an actuator for translating said polarizer
in said sequential manner.
18. A surveillance network as set forth in claim 17, wherein each
said sensor further comprises: a switch coupled between said CCD
camera and said polarized frame grabbers, said switch selectively
connecting, from time to time, said CCD camera with one of said
polarized frame grabbers, said selective connecting of said switch
being synchronized with said translating means such that at any
given moment in time, said engaged polarization segment and said
connected polarized frame grabber are of the same polarization
orientation.
19. A surveillance network as set forth in claim 18, wherein each
said sensor still further comprises: an unpolarized frame grabber,
said unpolarized frame grabber being activated only when said
optical path length compensator is placed between said iris lens
and said image intensifier.
Description
BACKGROUND OF THE INVENTION
[0002] In these times of heightened security, there are many
instances when particular geographic areas may need to be placed
under surveillance to protect potential targets from terrorist
attacks or as an adjunct to military operations.
[0003] When an object is imbedded in a medium such as rain or fog,
the presence of scatterers in the medium causes the image of the
object to appear indistinct or blurred, making object detection
difficult. One aspect of man-made objects that may assist in their
detection, however, is that they emit and reflect near-infrared
radiation that is more highly polarized than does natural
background that may include trees, brush grass or terrain.
[0004] There are two broad categories of infrared systems designed
for military use; namely, scanning system and staring system.
However, infrared sensors for extant missile seekers and
forward-looking infrared (FLIR) sensors lack the capability for
detecting the polarization orientation of the incident radiation,
i.e. they respond to any polarization orientation vector of the
incident radiation. Studies have shown that the capability to
detect and analyze the polarization orientation of near-infrared
radiation emanating or reflecting from targets and background
scenery can provide a potential means for improving the detection
and discrimination of the targets in military systems. Infrared
instrumentation having this feature would also have potential
commercial applications in areas such as pattern recognition,
materials characterization and analysis.
[0005] Thus, one way to improve the performance of target detection
system is to utilize polarized infrared signatures of a given
scenery. What renders this possible is that polarization properties
are independent of target-background contrast (i.e. polarization
properties are distinct even when there is no temperature
difference between the target and the background) and of normal
infrared target and clutter temperature fluctuations (i.e. standard
deviation of temperature). As is well-known, the formal
characterization of an electromagnetic wave that may be linear,
elliptical or circular is accomplished with the derivation of the
four Stokes parameters.
SUMMARY OF THE INVENTION
[0006] This invention relates to a system for detecting and
processing passive and active polarized near-infrared radiation for
applications in devices such as research, instrumentation,
surveillance systems, infrared missile sensors or seekers, search
and acquisition systems and guidance systems.
[0007] Polarimetric processing capability is added to a typical
sensor so that a target object can be more clearly distinguished
from the background clutter in a given scenery. An embodiment to
accomplish this involves using a polarizer with several segments of
different polarization orientations in conjunction with the
microchannel image intensifier tube as taught by David B. Johnson,
et al in U.S. Pat. No. 5,373,320 (Dec. 13, 1994), resulting in
improved sensors. The polarization segments are sequentially
advanced to pass therethrough infrared radiation of pre-selected
polarization orientations, which radiation, then, impinges on the
charge coupled device (CCD) camera. Multiple polarized frame
grabbers coupled to the camera produce image frames of the
respective pre-selected polarization orientations and image
processing circuit then processes these image frames to yield the
polarization difference between any given pair of orthogonal
polarizations. The polarization differences are subsequently used
to enhance the distinction of the objects against the background
clutter suspended in the propagation path.
[0008] In a surveillance system, many such sensors placed to
observe diverse geographical locations can be connected to a
control center that controls and coordinates the functions of the
individual sensors. Also, the control center receives relevant
polarization information from the sensors and correlates the
information for a more effective surveillance of the diverse
locations.
DESCRIPTION OF THE DRAWING
[0009] FIG. 1 shows a diagram of a preferred embodiment of the
improved sensor, the parts enclosed in the dashed lines indicating
the improvement over the Johnson system.
[0010] FIG. 2 illustrates the several polarization segments of the
polarizer 101.
[0011] FIG. 3 depicts a surveillance network comprising a plurality
of the sensors.
[0012] FIG. 4 is a flowchart of a typical algorithm that may be
used by control center 300 to process the inputs from individual
sensors 100.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] This invention is an improvement that may be used with the
camera attachment that converts a standard daylight video camera
into a day/night-vision video camera as taught by Johnson in the
U.S. Pat. No. 5,373,320 (Dec. 13, 1994). Therefore, the disclosure
of the Johnson patent is incorporated herein in its entirety.
[0014] Referring now to the drawing wherein like numbers represent
like parts in each of the several figures, the improvement to
impart differential polarization capability to sensor 100 is
explained in detail. Any and all of the numerical dimensions and
values that follow should be taken as nominal values rather than
absolutes or as a limitation on the scope of the invention.
[0015] In FIG. 1, near-infrared radiation 59 emanating or
reflecting from the observed scenery (not shown in the figure)
enters sensor 100 through iris lens assembly 60 which may be an
auto iris lens. At a pre-selected low level of ambient illumination
(usually night-time or a very dark day), image intensifier tube 61
having coupled thereto photocathode 117, and polarizer 101 swing
into place to be in the path of the incoming radiation 59. The
radiation then impinges on polarizer 101, a particular segment of
the polarizer having a specific polarization orientation being in
the path of the incoming radiation. The particular segment is one
of several segments of the polarizer, each segment having a
distinct polarization orientation and transmitting only the portion
of the incoming radiation that has the same polarization
orientation as that of the particular segment in the radiation
path. As illustrated in FIG. 2, the several segments of polarizer
101 comprise four at 0.degree., 45.degree., 90.degree., and
135.degree. orientations, respectively. Actuator 103, powered by
power source 62, translates the polarizer segments sequentially in
discrete steps along the length of image intensifier tube 61 so
that they are sequentially engaged to be in the radiation path
between iris lens assembly 60 and the image intensifier tube, which
may be a two-dimensional microchannel plate.
[0016] The radiation transmitted through the engaged polarizer
segment is then brought to a focus on photocathode 117 that covers
all the pixel elements of the microchannel plate (image intensifier
tube). It is noted here that the particular segment of the
polarizer engaged at any given moment in time should be
sufficiently large to cover all the pixel elements of the
microchannel plate. For each pixel element of the radiated image,
electrons are emitted from the rear of the photocathode. The
electrons thusly emitted from each pixel position enter the
corresponding channels of the microchannel plate and are multiplied
in the channels of the microchannel plate, subsequently exiting as
output light from the microchannel plate and being relayed by relay
lens 63 to be incident on CCD camera 64.
[0017] A plurality of polarized frame grabbers 105, 107, 109 and
111, each with a different polarization orientation, are
selectively connected to the CCD camera via first switch 119 and
second switch 121. The operation of the switches is synchronized
with the operation of actuator 103 such that the polarization
orientation of the engaged polarizer segment is the same as that of
the connected polarized frame grabber. In exemplar operation of the
above-described process, the image at 0.degree. is read into frame
grabber 105; then the actuator translates the polarizer to engage
the 90.degree.-segment, resulting in the image at 90.degree. being
read into frame grabber 107. Then the logic circuitry at the output
of the two polarized frame grabbers forms both the sum, V.sub.0
+V.sub.90, and the difference, V.sub.0 -V.sub.90, of the two
orthogonal polarizations. From this, the normalized ratio is
formed: Polarization .times. .times. Difference = V 0 - V 90 V 0 +
V 90 ##EQU1## This information is transmitted to control center 300
for further processing as will be explained below. Collecting two
orthogonal polarizations of an object image and differentiating
between the two as illustrated above improves the detectability of
the object. This process is known as polarization difference
imaging and its general principles are explained in U.S. Pat. No.
5,975,702 (Nov. 2, 1999). The process of summation and differencing
and obtaining the ratio can be performed with any other pair of
orthogonal polarizations.
[0018] Since the polarizer functions with the microchannel plate
that amplifies the low light level, a condition that is prevalent
in night-time, the polarizer is not operational during daylight
operation of the sensor, but is removed from the path of the
incoming radiation. Thus, at a selected higher level of ambient
illumination, under the control of control center 300 the image
intensifier tube (microchannel plate) and the polarizer-are rotated
out of position and, in their place, optical path length
compensator 67 is rotated into position and second switch 121
connects the CCD camera with unpolarized frame grabber 125. With
the compensator in place, unpolarized imagery may now be collected
by the unpolarized frame grabber and further processed by the
control center.
[0019] A plurality of ground sensors 100 (perhaps placed along a
defensive perimeter to detect hostile intrusion by man or vehicles)
may be connected via wireless links or fiber optic links 301 to
control center 300 as depicted in FIG. 3 to form a surveillance
network. Additional sensors (having the same or different
capabilities as the ground sensors) on air-borne platforms may be
connected to the control center as well for greater versatility of
the network. In such a network, the control center performs
correlation of the imagery inputs from the ground sensors with each
other as well as with imagery inputs from air-borne sensors and
other sources, such as satellites, to identify and track a target
object against background clutter. For example, the satellite or
air-borne sensors, having the capability to view a large area from
their elevated positions, can alert the ground sensors to hostile
movements toward the protected perimeter. The ground sensors, on
the other hand, having visibility underneath vegetation canopy, may
be able to alert the elevated sensors to movements hidden from
their view. In addition to transmitting data from the sensors to
the control center, links 301 also allows the control center to
adjust the settings at each of the sensors: for example, the frame
readout rates of each sensor, the selective connections of switches
119 and 121, the occasional activation of power source 62 and the
level of incoming radiation through iris lens assembly 60.
[0020] It is assumed that the control center is under the
supervision of an expert image analyst, as indicated in FIG. 4, who
is familiar with the topography and understands the physics of the
diurnal and long term changes in the scenes observed, even though
the image processing operations may be performed autonomously or
semi-autonomously. The terrain features being observed by the
sensors in the surveillance system may include natural and man-made
features both of which change over time. The scenes being observed
may include roads (a target area for emplacement of roadside
bombs), building structures that may conceal snipers, a forest area
that might conceal military movement or grassy area where land
mines may be buried. In all these applications, the scene images
are obtained at different points in time that may extend over a
period of an hour to a day, or a period of a day to weeks or to
months. This requires that a signal model of each of the emplaced
sensors be developed so that variations can be identified, and
imperfections in time be obtained to compensate for those
variations in each sensor. A suitable calibration procedure is
described by Mehrdad Soumekh et al in "Time Series Processing of
FLIR Imagery for MTI and Change Detection" in the IEEE's
Proceedings of the International Conference on Acoustics, Speech
and Signal Processing, Vol. 5, pages 21-24 (2003). The calibration
procedure is performed under the supervision of the control center
and its result becomes a part of the site model as depicted in FIG.
4.
[0021] Initially, to build the site model, an aerial image of a
site (a given scene) is obtained as a reference. Then subsequent
images of the site from emplaced ground sensors or air-borne
sensors are used to update and verify the site model, utilizing the
principle that when two or more images of a given scene are
available and the geometric relationship between the images and the
geo-spatial coordinates are known with some degree of accuracy, the
collected images can be coordinated. FIG. 4 illustrates a
representative software architecture for implementing signal
processing operation between the multiple images obtained from the
several ground emplaced sensors or aerial sensors. It is noted that
the aerial sensors and the ground sensors may not be of the same
type; it is envisioned that the aerial sensors would include
all-weather sensors, such as synthetic aperture radar (SAR), but
the ground sensors would not. The aerial sensors may also be
hyperspectral sensors whose several spectral bands can be directly
coordinated with the ground sensors.
[0022] The main operation of control center 300 is achieving the
match between images from the same or different sensor types taken
at different times and different resolutions. A common approach is
to designate one image, an aerial image for example, as a reference
and perform a rotation or warping of the second image to bring it
into alignment with the reference image. Many algorithms are
already known in the art that can be used to coordinate multiple
images, depending on the nature of the particular images. For
example, when comparing infrared images at wavelengths shorter than
4-5 microns and those at longer infrared wavelengths, a suitable
algorithm used to achieve a match between the day-time images and
night-time images is the Multivariate Mutual Information Algorithm
as is explained in "Registration of Image Cubes Using Multivariate
Mutual Information," by Jeffrey P. Kern et al in IEEE's
Thirty-Seventh ASILOMAR Conference: Signals, Systems and Computers,
Vol. 2, pages 1645-1649 (2003). Another example is when the
air-borne sensor includes a SAR. In such a case, the fusion of the
SAR image with near-infrared image will be the required signal
processing. Yet a third example is when the targets are far from
the sensors and thus appear in the image as points against the
background clutter. In this case, the necessary signal processing
may include a multi-scale decomposition based on wavelets that
allows the detection and characterization of the dynamic elements
in the scene. An algorithm to accomplish such decomposition is
explained in "Multiple Single Pixel Dim Target Detection in
Infrared Image Sequence,"by Mukesh A. Zaveri et al in IEEE's
Proceedings of International Conference on Circuits and Systems,
Vol. 2, pages II-380 to II-383 (2000).
[0023] Although a particular embodiment and form of this invention
has been illustrated, it is apparent that various modifications and
embodiments of the invention may be made by those skilled in the
art without departing from the scope and spirit of the foregoing
disclosure. For example, to obtain two frames of orthogonal
polarizations at the same instant in a given sensor, one can use a
stereo camera with the two orthogonal polarizers fixed to the two
separate sets of optics so the two frames of orthogonal
polarizations are obtained at the same instant. This may avoid the
image smearing that can happen if motion occurs during the
transition of the polarizer from one polarization segment to
another but would correspondingly require additional intensifier,
relay lens and switching mechanism, rendering the system bulkier
and more complicated. Another modification is to use sensors that
are capable of switching between daylight and night vision
operational modes. To provide differential polarization during
daylight operations, a second polarizer-actuator assembly would
need to be integrated with the optical elements of the sensor.
Accordingly, the scope of the invention should be limited only by
the claims appended hereto.
* * * * *