U.S. patent application number 14/059744 was filed with the patent office on 2014-02-13 for electromagnetic scanning imager.
This patent application is currently assigned to Walleye Technologies, Inc.. The applicant listed for this patent is Walleye Technologies, Inc.. Invention is credited to Christopher P. Adams, David S. Holbrook.
Application Number | 20140043046 14/059744 |
Document ID | / |
Family ID | 42130608 |
Filed Date | 2014-02-13 |
United States Patent
Application |
20140043046 |
Kind Code |
A1 |
Adams; Christopher P. ; et
al. |
February 13, 2014 |
ELECTROMAGNETIC SCANNING IMAGER
Abstract
In one aspect, the present invention provides an imager,
preferably portable, that includes a source of electromagnetic
radiation capable of generating radiation with one or more
frequencies in a range of about 1 GHz to about 2000 GHz. An optical
system that is optically coupled to the source focuses radiation
received therefrom onto an object plane, and directs at least a
portion of the focused radiation propagating back from the object
plane onto an image plane. The imager further includes a scan
mechanism coupled to the optical system for controlling thereof so
as to move the focused radiation over the object plane. A detector
optically coupled to the lens at the image plane detects at least a
portion of the radiation propagating back from a plurality of
scanned locations in the object plane, thereby generating a
detection signal. A processor that is in communication with the
detector generates an image of at least a portion of the object
plane based on the detection signal.
Inventors: |
Adams; Christopher P.;
(Somerville, MA) ; Holbrook; David S.; (Lexington,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Walleye Technologies, Inc. |
Somerville |
MA |
US |
|
|
Assignee: |
Walleye Technologies, Inc.
Somerville
MA
|
Family ID: |
42130608 |
Appl. No.: |
14/059744 |
Filed: |
October 22, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12614745 |
Nov 9, 2009 |
8593157 |
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14059744 |
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11353882 |
Feb 14, 2006 |
7626400 |
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12614745 |
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60653228 |
Feb 15, 2005 |
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Current U.S.
Class: |
324/642 |
Current CPC
Class: |
G01S 13/89 20130101;
G01S 13/426 20130101; G01R 27/04 20130101; H01Q 19/065
20130101 |
Class at
Publication: |
324/642 |
International
Class: |
G01R 27/04 20060101
G01R027/04 |
Claims
1. An imaging system, comprising a housing adapted for positioning
on a surface, said housing comprising: a source of electromagnetic
radiation for generating radiation with one or more frequencies in
a range of about 2 GHz to about 100 GHz, an optical system
optically coupled to said source so as to focus radiation received
therefrom onto an object plane, said optical system directing at
least a portion of the focused radiation propagating back from the
object plane onto an image plane, a scan mechanism coupled to the
optical system for receiving radiation therefrom and/or directing
radiation thereto, said scan mechanism effecting the scanning of
the focused radiation over a two-dimensional portion of the object
plane, said housing, optical system, and scan mechanism remaining
translationally stationary relative to said surface as said scan
mechanism effects the scanning of the radiation, a detector
optically coupled to said optical system at the image plane to
detect at least a portion of the radiation propagating back from a
plurality of scanned locations in the object plane, thereby
generating a detection signal, and a processor in communication
with the detector to generate an image of at least a portion of the
object plane based on said detection signal.
2. The imaging system of claim 1, wherein said scan mechanism
comprises two rotatable reflective elements, said elements
comprising generally planar reflective surfaces, each of said
elements configured to rotate about a respective rotation axis,
said axes being substantially parallel to their respective planar
surfaces, the two axes being substantially orthogonal with respect
to each other.
3. The scan mechanism of claim 2, wherein the reflective elements
are disposed optically in series.
4. The scan mechanism of claim 2, wherein the reflective elements
comprise a two-dimensional raster scanner.
5. The scan mechanism of claim 2, wherein the reflective elements
comprise a two-dimensional spiral scanner.
6. The scan mechanism of claim 2, wherein the reflective elements
comprise a floret scanner.
7. The scan mechanism of claim 1, wherein said scan mechanism
comprises two rotatable reflective elements, said elements
comprising substantially planar surfaces, said elements configured
individually to rotate about rotation axes, said axes disposed at
an angle with respect to the normals to the planar surfaces.
8. The imaging system of claim 1, further comprising one or more
position sensors coupled to said scan mechanism for determining a
two-dimensional position thereof relative to a reference
position.
9. The imaging system of claim 1, further comprising a display in
communication with said processor for displaying said image.
10. The imaging system of claim 1, wherein said processor maps a
variation of said detection signal to said scanned locations so as
to generate said image.
11. The imaging system of claim 9, wherein for each image point
coordinate corresponding to one of the scanned locations, the
display presents a brightness proportional to a strength of the
detection signal corresponding to back-propagating radiation from
that scanned location.
12. The imaging system of claim 1, wherein said radiation source
and said detector are formed as a single transmit/receive module
operating in said frequency range of about 2 GHz to about 100
GHz.
13. The imaging system of claim 1, wherein said housing is a
portable housing.
14. The imaging system of claim 13, further comprising an
electronic processor and display module (EPDM) that includes said
processor.
15. The imaging system of claim 14, wherein said EPDM is integrated
within said portable housing.
16. The imaging system of claim 15, further comprising a second
housing, separate from the portable housing, for containing said
EPDM, said second housing being in data or signal communication
with said portable housing.
17. The imaging system of claim 14, wherein said EPDM comprises a
display module for displaying said image.
18. The imaging system of claim 1, further comprising a mechanism
coupled to the source for modulating frequency of radiation
generated by the source.
19. The imaging system of claim 18, wherein said mechanism
modulates the radiation frequency at a rate of about 100 kHz.
20. A method of imaging, comprising: providing a housing adapted
for positioning on a surface over an object plane and having a
source of electromagnetic radiation for generating radiation with
one or more frequency components in a range of about 2 GHz to about
100 GHz, providing a focusing element in said housing that is
optically coupled to said source, utilizing the focusing element to
focus radiation from the source onto the object plane, utilizing a
scan mechanism to effect scanning of the focused radiation over at
least a two-dimensional portion of the object plane so as to
illuminate a plurality of locations over a two-dimensional region
of the object plane, said scan mechanism receiving radiation from
said source and/or directing radiation to said source, said
housing, focusing element, and scan mechanism remaining
translationally stationary relative to said surface as said scan
mechanism effects the scanning of the radiation, detecting at least
a portion of the radiation propagating back from said illuminated
locations on the object plane thereby generating a time-dependent
detection signal, and analyzing said detection signal and said
signals generated by said at least one sensor to form an image of
the scanned portion of the object plane.
21. The method of claim 20, wherein the step of focusing radiation
further comprises directing the radiation to the object plane along
a direction forming a non-zero angle relative to a normal to the
object plane.
22. The method of claim 21, wherein said angle is about 7
degrees.
23. The imaging system of claim 1, wherein said source of
electromagnetic radiation is adapted to generate radiation with one
or more frequencies in a range of about 2 GHz to about 24 GHz.
24. The imaging system of claim 1, wherein said source of
electromagnetic radiation is adapted to generate radiation with one
or more frequencies in a range of about 24 GHz to about 100
GHz.
25. The method of claim 20, wherein said source of electromagnetic
radiation is adapted to generate frequency components in a range of
about 2 GHz to about 24 GHz.
26. The method of claim 20, wherein said source of electromagnetic
radiation is adapted to generate frequency components in a range of
about 24 GHz to about 100 GHz.
Description
RELATED APPLICATION
[0001] The present application claims priority as a
continuation-in-part (CIP) application to a co-pending patent
application entitled "Electromagnetic Scanning Imager" filed on
Feb. 14, 2006 and having an application Ser. No. 11/353,882, which
in turn claims priority to a provisional application entitled
"Electro-magnetic Scanning Imager," filed on Feb. 15, 2005 and
having a Ser. No. 60/653,228, which is herein incorporated by
reference. Both of these applications are herein incorporated by
reference in their entirety.
BACKGROUND
[0002] The present invention relates generally to imaging systems,
and methods of imaging, and more particularly, to such systems and
methods that can be utilized to acquire images of objects hidden
behind visibly opaque obstructions.
[0003] A variety of conventional systems are available for
obtaining images through visibly opaque materials. For example,
X-ray systems have been utilized to acquire images of objects that
are hidden from visual inspection by visibly opaque materials
(e.g., anatomical structures or objects within a luggage). X-ray
systems, however, have many disadvantages. By way of example, such
systems can be expensive and bulky, and can utilize ionizing
radiation that may pose health hazards to humans. Moreover, X-ray
systems typically detect a beam that has been transmitted through a
target sample, thus requiring access to both sides of the
target.
[0004] Ultrasound imaging systems, in turn, require the presence of
a continuous, high quality acoustic transmission path between a
transducer and a "hidden" object of interest. In many cases,
however, such acoustic transmission paths may be not be
available.
[0005] Millimeter-wave imaging systems have recently been developed
for securing screening applications. Such conventional
millimeter-wave systems are, however, complex, costly and
bulky.
[0006] Accordingly, there is a need for enhanced imaging systems
and associated image acquisition methods for obtaining images of
objects behind visibly opaque obstructions, e.g. images of
interiors of walls/floors/ceiling, boxes, suitcases and the like.
There is also a need for such imaging systems that are field
portable. Further, there is a need for such systems and methods
that can be utilized for screening luggage and other containers for
hazardous substances. e.g., explosive materials and devices.
SUMMARY
[0007] The present invention generally provides imaging systems
operating in a frequency range of about 1 GHz to about 2000 GHz
that allow acquiring, and displaying, images of objects, and
particularly of objects that are hidden from view by visibly opaque
materials.
[0008] In one aspect, the present invention provides an imaging
system that includes a source of electromagnetic radiation that is
capable of generating radiation with one or more frequencies in a
range of about 1 GHz to about 2000 GHz (one or more wavelengths in
a range of about 0.015 mm to about 30 mm). The imaging system
further includes an optical system that is optically coupled to the
source so as to focus radiation received therefrom onto an object
plane. The optical system directs at least a portion of the focused
radiation propagating back from the object plane onto an image
plane. A scan mechanism is coupled to the optical system for
controlling thereof so as to move the focused radiation over the
object plane. A detector, which is optically coupled to the optical
system at the image plane, detects at least a portion of the
radiation propagating back from a plurality of scanned
(illuminated) locations in the object plane, thereby generating a
detection signal (typically a time-varying signal). And a
processor, which is in communication with the detector, generates
an image of at least a portion of the object plane based on the
detection signal.
[0009] The terms "object plane" and "image plane" are known in the
art. To the extent that any further explanation may be needed, the
term "object plane" can refer to plane--which can contain one or
more surfaces (or surface portions) of one or more objects--whose
image is desired. And the image plane can refer to a plane on which
the image of the object plane (or a portion thereof) is formed,
e.g., the surface of a detector. In some cases, the object and
image planes can have not only a two-dimensional extent but also a
depth, e.g., one associated with depth of focus of the radiation on
the object plane.
[0010] A variety of radiation sources and detectors can be
employed. Some examples of suitable radiation sources include,
without limitation, Gunn oscillators, magnetrons, IMPATT diodes,
Dielectric Resonator Oscillators (DROs), and MIMICs. Some examples
of suitable detectors include, without limitation, various types of
circuitry incorporating a non-linear device such as a schottky
diode. In some cases, the radiation source and the detector are
formed as a single transmit/receive module operating in the
frequency range of interest. By way of example, a Gunnplexer can
function as such a transmit/receive unit.
[0011] In a related aspect, the imager further includes a position
sensor coupled to the scan mechanism for determining a position
thereof relative to a reference position. The position sensor is in
communication with the processor to communicate the position of the
scan mechanism, and consequently the location of the focused
radiation on the object plane, to the processor. The processor, in
turn, maps variations of the detection signal to one or more
respective locations in the object plane from which the
back-propagating radiation giving rise to the detection signal
originates. The processor utilizes this mapping to generate an
image of at least a portion of the object plane illuminated by the
radiation.
[0012] In another aspect, the imager includes a display in
communication with the processor for displaying an image of at
least a portion of the object plane generated by the processor. By
way of example, the processor can apply image drive signals to the
display to cause display of a plurality of pixels, each
corresponding to a location on the object plane, where the
intensity of each pixel is proportional to the strength of the
back-propagating radiation originating from that location.
[0013] In a related aspect, the scan mechanism can be adapted to
cause the optical system to generate a variety of radiation scan
patterns on the object plane. Some examples of such patterns
include, without limitation, a generally elliptical (e.g.,
circular) pattern, a spiral pattern, a floret pattern or a raster
pattern.
[0014] In another aspect, the imaging system is adapted to be moved
by a user so as to scan the radiation, in combination with the scan
mechanism, in two dimensions in the object plane. In such a case,
the detection signal generated by the detector corresponds to
back-propagating radiation originating from the scanned locations
in the object plane. The processor maps variations of the detection
signal to those scanned locations so as to generate an image of a
region in the object plane, which includes those locations.
Further, for each image point coordinate corresponding to one of
the scanned locations, the display presents, in response to image
drive signals provided by the processor, a brightness proportional
to a strength of the detection signal corresponding to the
back-propagating radiation from that scanned location.
[0015] In a related aspect, the imaging system includes means for
generating data indicative of locations and orientations of the
system (e.g., relative to a reference location/orientation), as it
is moved by a user (e.g., over a wall surface) and for
communicating that data to the processor. Such means can include a
variety of sensors, such as tracking balls with orthogonal rotation
encoding devices and inertial sensors. The processor utilizes the
data for mapping the detection signal to the scanned locations over
the object plane.
[0016] In some cases, the processor correlates a plurality of
partially overlapping image frames to form an image of a portion of
the object plane. By way of example, the processor can utilize one
or more pixels in an overlap region between two image frames as
reference for determining relative positions of the other pixels in
the two image frames. This process can be repeated for other
overlapping regions so as to build an entire image.
[0017] In another aspect, the imaging system can comprise a
portable housing, preferably handheld, in which various components
of the system (e.g., radiation source, scanner, lens) are disposed.
The imaging system can comprise an electronic processing and
display module (EPDM) that includes the processor and/or the
display. The EPDM can be integrated within the portable housing
containing the other optical components, or can be contained within
a separate enclosure that is in communication with the portable
housing.
[0018] In another aspect, the imaging system comprises a focus
drive mechanism coupled to the optical system for varying an axial
distance between the optical system and the transmit/receive module
so as to focus the radiation at a plurality of axially separated
object planes. In some embodiments, the source is capable of being
frequency tuned (e.g., via a tuning mechanism) so as to allow
focusing the radiation via a longitudinal chromatic aberration of
the optical system onto axially separated object planes.
[0019] In a related aspect, in the above imaging system, a position
sensor is in communication with the focus drive mechanism for
determining an axial position of the optical system relative to a
reference position. The position sensor can communicate the optical
system's position to the processor, which can, in turn, utilize
this information to temporally correlate a detection signal at a
given time to a respective object plane.
[0020] In another aspect, the imaging system includes a mechanism
coupled to the source for modulating its radiation frequency. By
way of example, the radiation frequency can be modulated over a
range of about 24.1 GHz to about 24.2 GHz (e.g., about 100 MHz). By
way of example, the modulation frequency can have an amplitude
(i.e. excursion about a center frequency) that is about 1% of the
center frequency.
[0021] In other aspects, a portable imager is disclosed that
includes a source for generating electromagnetic radiation with one
or more frequency components in a range of about 1 GHz to about
2000 GHz. The imager further includes means for directing radiation
from the source to an object plane and for directing at least a
portion of the radiation propagating back from the object plane to
an image plane. Further, the imager includes means coupled to the
radiation-directing means for controlling thereof so as to scan the
radiation over at least a portion of the object plane, and means
for detecting radiation propagating back from a plurality of
scanned locations in the object plane and generating a detection
signal. The imager also includes means for analyzing the detection
signal so as to generate an image of at least a portion of the
object plane.
[0022] In another aspect, the invention provides an imaging system
that includes a handheld housing. The housing contains a source of
electromagnetic radiation, a focusing system, a scan mechanism, a
detector and a processor. The source is capable of generating
radiation with one or more frequency components in a range of about
1 GHz to about 2000 GHz. The focusing system directs radiation
generated by the source onto an object plane and directs at least a
portion of the radiation propagating back from the object plane
onto an image plane. The scan mechanism is coupled to the focusing
system for controlling thereof so as to provide a one-dimensional
scan of the radiation over the object plane. The detector is
optically coupled to the focusing system at the image plane to
detect at least a portion of the back-propagating radiation so as
to generate a detection signal (e.g., a time-varying detection
signal). The processor is coupled to the detector to receive the
detection signal, and to analyze that signal. The housing is
adapted for movement so as to scan the radiation, in combination
with the one-dimensional scan, over a plurality of locations in a
two-dimensional region of the object plane.
[0023] In a related aspect, in the above portable imager, the
processor generates an image of the two-dimensional region based on
the detection signals corresponding to the scanned locations. For
example, the processor maps the detection signals temporally to
respective scanned locations for generating the image. The imager
can further include a display, mounted to the housing or remotely
located, that is in communication with the processor for displaying
the image.
[0024] In another aspect, the invention provides an imaging system
that comprises a transmit/receive module capable of generating and
detecting electromagnetic radiation having one or more frequency
components in a range of about 1 GHz to about 2000 GHz. An optical
system is coupled to the transmit/receive module for focusing the
radiation onto a focal plane and for directing a portion of the
radiation propagating back from the focal plane to that module for
detection. The imaging system further includes a drive mechanism
coupled to the optical system for varying an axial distance of a
focusing element thereof relative to the transmit/receive module so
as to axially vary a position of the focal plane. The
transmit/receive module generates a detection signal corresponding
to the radiation propagating from the varying focal plane. A
processor is coupled to the transmit/receive module to analyze the
detection signal for generating an image of an axial region over
which the focal plane is varied.
[0025] In a related aspect, in the above imager, the position
sensor is coupled to the drive mechanism for determining an axial
position of the focusing element relative to a reference position.
The position sensor communicates the information regarding the
focusing element's axial position to the processor. The processor,
in turn, analyzes the detection signal temporally as a function of
the focusing element's axial position so as to map the detection
signal at a plurality of time intervals to the axial locations of
the focal plane. The imager can also include a scan mechanism
coupled to the focusing element for scanning the radiation over a
surface of the focal plane substantially perpendicular to the axial
direction, and a scan position sensor that can determine the
position of the scan mechanism relative to a reference. The
processor analyzes the detection signal temporally as a function of
the focusing element's axial position so as to map the detection
signal at a plurality of time intervals to axial locations of the
focal plane. Further, the processor can employ information from the
scan position sensor to correlate detection signal originating from
a focal plane to various coordinate points within that plane.
[0026] In another aspect, an imaging system is disclosed that
includes a housing adapted for movement over a surface located at a
distance from an object plane. A transmit/receive module (e.g., a
Gunnplexer) disposed in the housing generates and detects
electromagnetic radiation. The imaging system further includes an
optical system that is optically coupled to the transmit/receive
module to focus radiation received therefrom onto the object plane,
and to direct at least a portion of the radiation propagating back
from the object plane onto the transmit/receive module. A scanning
system coupled to the optical system rotates a focusing element
thereof about a rotation axis, thereby moving the focused radiation
over a region of the object plane. The imaging system further
includes a position sensor in communication with the scanning
mechanism to generate signals indicative of the position of the
focusing element relative to a reference position. And a subsystem
disposed in the housing generates signals indicative of location of
the housing on the surface as the housing is moved over that
surface. By way of example, the subsystem can comprise optical or
inertial sensors. The imaging system further includes a processor
in communication with the transmit/receive unit, the position
sensor and the location-determining subsystem, which generates an
image of at least a portion of the object plane based on the
detected back-propagating radiation and signals generated by the
position sensor and the subsystem.
[0027] In a related aspect, the imaging system can further include
a display that is in communication with the processor for
displaying the image. In some cases, the optical system and the
scanning mechanism, as well as the processor and the display, are
disposed in the same housing. In other cases, the processor and/or
the display can be disposed in a separate housing.
[0028] In a related aspect, the processor generates a set of image
point coordinates in a coordinate space of the object plane based
on signals received from the position sensor coupled to the
focusing element and the location-determining subsystem. Further,
the processor applies a plurality of image drive signals to the
display to effect the display of an image corresponding to those
image point coordinates. The processor generates each image drive
signal based on a strength of the detected back-propagating
radiation originating from a location on the object plane
corresponding to one of the image point coordinates. Each image
drive signal, in turn, causes the presentation of an image point on
the display, where the image point exhibits a brightness
corresponding to the strength of the detected signal associated
with a respective image point coordinate. The location of a
displayed image point in the display is based on the position of
the focusing element and the location of the housing at a time when
back-propagating radiation originating from an object plane
location corresponding to that image point is detected.
[0029] In a related aspect, the focusing element in the above
imaging system comprises a diffractive optical element. In some
cases, the diffractive optical element can comprise diffractive
zones that are disposed about an optical axis that is offset from
the element's rotational axis, which can be substantially centered
relative to the emitting aperture of the transmit/receive module. A
variety of diffractive optical elements can be employed, such as
amplitude zone plates and phase zone plates. While in some cases,
the optical and rotational axes of the focusing element are
substantially parallel, in other cases, they can intersect at the
aperture of the transmit/receive module.
[0030] In a related aspect, the emitting aperture of the
transmit/receive module, the focusing element, and the focused
radiation in the object plane generated by the focusing element are
disposed in a confocal configuration.
[0031] In another aspect, the transmit/receive module comprises a
frequency-modulating subsystem for modulating the frequency of the
emitted radiation in response to a control signal.
[0032] In other aspects, an imaging system is disclosed that
includes a transmit/receive module for generating and detecting
electromagnetic radiation. The system further includes a
telecentric lens system that receives and focuses the radiation
onto an object plane. The lens system can comprise an eccentric
rotatable lens disposed at a selected distance from a fixed lens.
Further, the lens system directs radiation propagating back from
the object plane onto the transmit/receive module for detection. A
scanning mechanism is coupled to the rotatable lens for rotating
the lens about a rotation axis, thereby moving the focused
radiation in the object plane. The imaging system further includes
a processor coupled to the transmit/receive module for generating
an image of at least a portion of the object plane, which is
illuminated by the radiation, based on the detected
back-propagating radiation.
[0033] In a related aspect, the eccentric lens is disposed at a
distance substantially equal to its focal length from the emitting
aperture of the transmit/receive module. The eccentric lens
collimates radiation received from the transmit/receive module and
directs the collimated radiation along an off-axis direction to the
fixed lens. The fixed lens, in turn, focuses the off-axis radiation
onto the object plane.
[0034] In another aspect, the invention provides a method of
imaging that includes providing a source of electromagnetic
radiation that is capable of generating radiation with one or more
frequency components in a range of about 1 GHz to about 2000 GHz.
The radiation is focused from the source onto an object plane, and
the focused radiation is scanned over at least a portion of the
object plane. At least a portion of the radiation propagating back
from the object plane is detected, thereby generating a detection
signal, which is typically a time-varying signal. The detection
signal is analyzed to form an image of the scanned portion of the
object plane. The image can then be displayed.
[0035] Further understanding of various aspects of the invention
can be obtained by reference to the following detailed description,
in conjunction with the associated drawings, which are described
briefly below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 schematically depicts an imaging system according to
one embodiment of the invention,
[0037] FIG. 2 schematically illustrates a wire grid polarizer
suitable for use in the imaging system of FIG. 1 and other
embodiments of the invention,
[0038] FIG. 3 schematically depicts a Fresnel lens having a
plurality of diffractive zones,
[0039] FIG. 4 is a top schematic view of a Binary Fresnel Lens
(BFL) suitable for use in the imaging system of FIG. 1 as well as
other embodiments,
[0040] FIG. 5 is a perspective schematic view of a quarter-wave
plate (QWP) suitable for use in the imaging system of FIG. 1 as
well as other embodiments,
[0041] FIG. 6 schematically depicts a scanning mechanism suitable
for rotating the lens of the imaging system of FIG. 1 as well as
lenses utilized in other embodiments,
[0042] FIG. 7 schematically illustrates a circular scan pattern of
radiation directed by an embodiment of an imaging system according
to the invention onto an object plane,
[0043] FIG. 8 schematically depicts a swath over an object plane
illuminated by radiation from an imager according to an embodiment
of the invention via rotation of a lens and concurrent translation
of the imager over a surface substantially parallel to the object
plane,
[0044] FIG. 9 schematically illustrates that in some embodiments an
imager according to teachings of the invention is operated at a
selected tilt angle relative to a scanning plane over which it is
translated,
[0045] FIG. 10 schematically depicts a method of correlating pixels
from a plurality of image frames acquired by an imager of the
invention to build up an entire image,
[0046] FIG. 11 schematically depicts a front view of an imager
according to an embodiment of the invention having a display in
which an image acquired by the imager is presented,
[0047] FIG. 12 schematically depicts an imager according to another
embodiment of the invention,
[0048] FIG. 13A schematically depicts a Gunnplexer suitable for use
as a transmit/receive unit in an imager of the invention,
[0049] FIG. 13B schematically depicts further details of the
Gunnplexer shown in FIG. 13A,
[0050] FIG. 14 schematically depicts the transmit/receive unit as
well as the lens of the embodiment of FIG. 12, illustrating that
the rotation axis of the lens is offset relative to its optical
axis,
[0051] FIG. 15 is a schematic view of an imager according to an
embodiment of the invention operating in a snapshot mode,
[0052] FIG. 16A schematically illustrates a spiral radiation
pattern generated by some embodiments of the invention for
illuminating an object plane in order to acquire a two-dimensional
image thereof,
[0053] FIG. 16B schematically depicts a floret radiation pattern
generated by some embodiments of the invention for illuminating an
object plane in order to acquire a two-dimensional image
thereof,
[0054] FIG. 16C schematically depicts a raster radiation pattern
generated by some embodiments of the invention for illuminating an
object plane in order to acquire a two-dimensional image
thereof,
[0055] FIG. 17A schematically depicts a mechanism for
two-dimensional scanning of a radiation beam, which is suitable for
use in some embodiments of the invention,
[0056] FIG. 17B schematically depicts another radiation scan
mechanism 200 optically coupled to the source for providing a
two-dimensional radiation scan,
[0057] FIG. 17C schematically depicts another exemplary embodiment
of a scan mechanism for generating a two-dimensional radiation
scan, which utilizes two rotating transmissive prisms having axes
of rotation that are generally perpendicular to the bisector of
prisms' vertex angles.
[0058] FIG. 18 schematically depicts an imager according to another
embodiment of the invention capable of generating images of a
plurality of axially separated object planes,
[0059] FIG. 19 schematically illustrates a lens focus-drive
mechanism suitable for use in the embodiment of FIG. 18,
[0060] FIG. 20 schematically depicts an imager according to another
embodiment of the invention that utilizes chromatic aberration of a
lens for focusing radiation on a plurality of axially separated
object planes.
[0061] FIG. 21 schematically illustrates an imager according to
another embodiment of the invention that employs a telecentric
arrangement of optical components for acquiring an imager, and
[0062] FIGS. 22A-22E show test images obtained by utilizing a
prototype image constructed in accordance with the teachings of the
invention.
DETAILED DESCRIPTION
[0063] FIG. 1 schematically depicts an imager 10 (also referred to
herein as a camera) according to one embodiment of the invention
having a source 12 and a detector 14 for generating and detecting
radiation, respectively. In this exemplary embodiment, the
radiation source 12 is capable of generating radiation having one
or more frequencies in a range of about 1 to about 2000 GHz, and
preferably in a range of about 2 GHz to about 100 GHz. In many
embodiments, one or more radiation frequencies are selected such
that the radiation can penetrate an obstruction (e.g., a wall) that
is generally opaque to visible radiation. Some examples of suitable
radiation sources include, without limitation. Gunn oscillators,
magnetrons, IMPATT diodes, Dielectric Resonator Oscillators (DROs),
MIMICs, or other suitable radiation sources. A horn antenna 16 is
coupled to the source so as to facilitate coupling the radiation
generated by the source into free space (e.g., by providing a
better impedance match) for propagation to an object to be imaged,
as discussed further below. In this embodiment, the source, in
conjunction with the horn, generates a diverging cone of radiation
beam 18 disposed about a propagation axis 20 (herein also referred
to as a rotational axis). Similar to the source, a horn 22 is
coupled to the detector to facilitate coupling of radiation into
the detector. In general, the combination of the detector 14 and
its associated horn 22 is capable of receiving radiation beams
disposed about a detector axis 24 with a given angular distribution
that depends at least in part on the horn geometry.
[0064] In this embodiment and some that follow, without any loss of
generality, the functioning of the imagers according to the
teachings of the invention are discussed by considering acquiring
images within a depth of a wall (or other obstructions) that is
opaque to visible radiation. Such imagers can, however, be also
utilized to acquire images of other objects. For example, the
imaging systems of the invention can be utilized to image objects
within containers.
[0065] The source 12 and the detector 14 are disposed on opposite
sides of a beam splitter 23 such that the propagation axis 20
associated with the source and the detection axis 24 associated
with the detector typically intersect at an approximately 90-degree
angle. The beam splitter 23 is perpendicular to a plane formed by
the propagation and the detection axes and is oriented such that a
normal to its surface bisects the angle between those axes, e.g.,
it typically forms a 45-degree angle with each of those axes. The
radiation emitted by the source passes through the beam splitter to
be directed by other optical components onto a region of interest,
as discussed below.
[0066] By way of example, the beam splitter 23 can be preferably
implemented as a polarizing beam splitter having a polarization
axis that is preferably oriented either parallel or perpendicular
to a plane defined by the propagation and detection axes. In some
embodiments, a so-called wire grid polarizer (WGP) is employed,
which can be made, e.g. of a one-dimensional array or grid of very
fine parallel electrically conductive elements disposed upon a
suitable transparent base material or, e.g., by a grid of fine
parallel wires strung on a frame. By way of example, FIG. 2
schematically depicts a wire grid polarizer 26 formed of a
plurality of aluminum strips 28 disposed over a dielectric
substrate 30 (e.g., a Teflon.TM. substrate). The spacing between
adjacent parallel conductive elements is selected to be
considerably less than the radiation wavelength generated by the
source 12. This allows the component of the radiation having an
electric field vector parallel to the grid elements to be reflected
by the polarizer and the component having an electric field vector
perpendicular to the grid elements to be transmitted through the
polarizer. In this exemplary embodiment, each strip can have a
width in a range of about 1/100.sup.th wavelength to about 4
wavelength, e.g., about 2 millimeters, and can be separated from an
adjacent strip by a spacing in a range of about 1/100.sup.th
wavelength to about 1/4 wavelength, e.g., about 2 millimeters.
[0067] Referring again to FIG. 1, the imager 10 further includes a
lens 32 that receives the radiation emitted by the source 12 after
its passage through the polarizer 23. The lens 32 can have, e.g., a
receiving cone of the order of f/1 or narrower. By way of example,
the lens 32 can be a decentered optical element configured to
operate, e.g., at approximately unity magnification. In some
embodiments of the invention, the lens can be a diffractive lens
having a diffractive pattern whose symmetry axis (optical axis) is
laterally offset from its physical symmetry axis. For example, the
lens can be a Binary Fresnel Lens (BFL) whose optical axis is
offset laterally from its physical symmetry axis. As shown
schematically in FIG. 3, an exemplary Fresnel lens 34 can include a
plurality of diffractive zones 34a separated from one another by
steps at each of which a phase discontinuity is generated in a
manner that results in coherent interference of radiation passing
through the different zones at a focal plane of the lens. In other
words, the surface shape of a Fresnel lens can be viewed as a
piece-wise approximation to the surface shape of a conventional
refractive lens, e.g., such as that depicted by dashed lines.
Further, a BFL can be viewed as a Fresnel lens in which each
section of the approximation is further approximated by a staircase
profile.
[0068] In some embodiments of the invention, the BFL includes four
steps each providing a 1/4-wave phase delay. By way of example,
FIG. 4 schematically depicts a BFL 35 suitable for use in various
embodiments of the invention having a plurality of diffractive
zones 35a disposed about an optical axis that is laterally offset
from its physical center. In other words, an axis of symmetry of
the diffractive pattern (perpendicular to the plane of the drawing
at point B) is separated by a selected offset D from a physical
symmetry axis of the lens (perpendicular to the plane of drawing at
point A). In some embodiments, the BFL includes a diameter R (e.g.,
about 8 inches) and a focal length in a range of about 2 inches
(about 5 cm) to about 10 inches (about 25 cm). e.g., about 8 inches
(about 20.3 cm), with an optical axis offset in a range of about 1
inch (about 2.5 cm) to about 3.5 inches (about 8.9 cm), and more
preferably about 2 inches (about 5 cm). As discussed in more detail
below, the offset between the physical and the optical axis of the
BFL 35 allows scanning the focal point of the lens about a
generally elliptical path (e.g. a circular path) by rotating the
lens about its physical axis.
[0069] Referring again to FIG. 1, in this exemplary embodiment, the
imager 10 further includes a 1/4-wave plate (QWP) 36 that is
adapted to operate in a frequency range of interest (e.g., in a
frequency range of about 1 GHz to about 2000 GHz). The QWP 36 is
disposed in the path of the radiation between the polarizing beam
splitter and an object to be imaged. While in this embodiment the
QWP is placed between the lens and a wall 56, in other embodiments,
the QWP can be disposed between the polarizing beam splitter and
the lens, typically closer to the beam splitter at a location where
the radiation beam diameter is relatively small.
[0070] In some embodiments, the QWP 36 can be implemented as a
grooved dielectric plate, such as that schematically depicted in
FIG. 5. For example, the QWP 36 can be fabricated by machining a
plurality of grooves on a dielectric plate (e.g., a Teflon.TM.
plate). The plate can have a thickness in a range of about 5
millimeters to about 80 millimeters, and the grooves can have a
depth in a range of about 4 millimeters to about 70 millimeters and
a width in a range of about 1/100.sup.th wavelength to about %
wavelength of the radiation. Further, the center-to-center spacing
between adjacent grooves can be in a range of about 0.1 millimeters
to about 3 millimeters. The theory of operation of QWPs suitable
for use in the frequency range of interest and typical techniques
for their fabrication are described in an article entitled "A high
precision quasi-optical polarizer for Zeeman splitting
observation," by J. W. Lamb, M. Carter, and F. Mattiocco, published
in Int. J. Infrared and Millimeter Waves, vol. 22, No. 5 (May
2001), incorporated herein by reference.
[0071] In this exemplary embodiment, the QWP 36 is disposed
perpendicular to the propagation axis of radiation from the source
12 with its fast axis preferably oriented at +/-45 degrees from the
plane of polarization of the outgoing radiation. As is well known
in the art, linearly polarized radiation passing through a QWP
oriented in this manner emerges from the QWP as substantially
circularly polarized.
[0072] The imager 10 further includes a scan mechanism 38 coupled
to the lens 32 for rotating the lens about its rotation axis
(herein also referred to as the lens's physical axis). The lens is
preferably disposed relative to the source such that its rotation
axis is substantially coincident with the propagation axis of the
outgoing radiation. As noted above, an optical axis 40 of the lens
is displaced from its rotation axis by a predetermined distance,
e.g., by about 1/2 of the radius of the lens. The optical axis of
the lens can be parallel to its rotation axis, or alternatively, it
can intersect the rotation axis at the emitting aperture of the
source. As discussed in more detail below, the rotation of the lens
32 allows scanning the radiation at the focal plane of the lens
over a path in an object plane.
[0073] A variety of scanning mechanisms can be utilized in the
practice of the invention. For example, referring to FIG. 6, a
rotating shaft 33, which is driven by a motor 33a, can be coupled
to the lens to cause the lens to rotate. Referring again to FIG. 1,
in many embodiments, the imager further includes a lens scan
position sensor 42 for indicating an angular rotation of the lens
about the rotation axis relative to a predetermined reference. A
variety of commercially available sensors can be employed. For
example, optical sensors can be utilized to determine the angular
position of the lens. An example of a suitable sensor is a shaft
encoder that can provide information regarding angular position of
a rotating shaft (such as the rotating shaft depicted in FIG. 6) to
which the lens is coupled. In some embodiments, the lens scan
mechanism 38 causes the lens to rotate at a substantially constant
angular velocity with the scan position sensor 42 sensing the
lens's angular position once, or more, per revolution using. e.g. a
magnet on the edge of the lens and a fixed Hall effect sensor.
[0074] In this exemplary embodiment, various components of the
imager, such as those discussed above, are disposed in a portable,
preferably handheld housing 44. An optional window 46 (e.g., formed
of a material transparent at the operating wavelength) is coupled
to the housing 44 through which the radiation generated by the
source can be transmitted to illuminate interior portions of the
wall, as discussed further below. In other embodiments, no window
is provided.
[0075] In operation, the lens 32 directs radiation generated by the
source 12, after its passage through the beam splitter 23, via the
QWP 36 and the window 46 into the interior of a wall (or other
obstruction, or a region behind such an obstruction) to illuminate
portions thereof, such as the object region 48. Preferably, the
lens 32 forms an image of the source so as to create an
illuminating focused point (e.g., an area of maximal radiation
intensity) at a distance from the lens that is less than infinity
and more than one focal length of the lens. In many embodiments,
the radiation from the imager is focused onto an object plane
(e.g., object plane 50) within the wall, and the radiation
returning from that object plane is detected and analyzed to form
an image thereof, as discussed in more detail below. In general,
the object plane 50 has an axial extension (a depth) corresponding
to the axial extension of the focal volume, as schematically
illustrated by volume 48, which represents a portion of the object
plane.
[0076] In this exemplary embodiment, the lens 32 is placed at a
distance from the source substantially equal to twice its focal
length, thereby forming an image of the source at a distance of
approximately two focal lengths from the lens. Accordingly, the
image is displaced radially from the rotation axis by twice the
displacement of the lens's optical axis from the rotation axis. As
shown schematically in FIG. 7, in this embodiment, as the scanner
rotates the lens about its rotation axis, the illuminating point of
radiation 52 sweeps out a circular path 54 around the rotational
axis in the object plane 50 (FIG. 1). As the imager is translated
laterally along an external wall surface 56 (see FIG. 1), e.g., by
a user or an automated mechanism, the scan pattern of the
illuminating radiation, generated by combined rotation of the lens
and translation of the imager, covers a swath 58 on the object
plane, as shown in FIG. 8.
[0077] In some embodiments, it is preferable to operate the imager
with a small tilt angle (e.g., approximately 7 degrees) between a
scanning plane (e.g., a plane perpendicular to the lens's rotation
axis) and a translation plane (i.e., the plane over which the
imager is translated to build up an image of an area). For example,
as shown schematically in FIG. 9, the imager 10 can be tilted by an
angle (0) relative to the wall surface 56 as it is translated along
the wall. The tilt angle can be in or against direction of the
translation. The use of such a tilt can increase the number of
"look angles" the scanner produces tin the tilt plane), thereby
increasing the probability of receiving a strong specular
reflection from an illuminated object. In some embodiments, such a
tilt is built in the imager, e.g. by disposing the imager's optical
components at a selected tilt angle relative to its housing.
[0078] Referring again to FIG. 1, in many embodiments, the imager
10 further includes a plurality of sensors 60 coupled to its
housing 44 for indicating the displacement of the housing relative
to the wall surface 56 on which the imager is translated so as to
obtain an image of an interior portion of the wall. Preferably, the
imager can include at least two such location sensors physically
separated such that each sensor would measure displacement of the
housing relative to a starting reference along one of two
orthogonal coordinate axes (e.g. two orthogonal Cartesian
coordinates). In general, the displacement of the imager can be
obtained by determining three degrees of freedom (e.g., two
translational and a rotational degree of freedom). Each sensor can
provide independent data regarding two degrees of freedom, and
hence the combined data from two sensors is sufficient for
determining three degrees of freedom associated with the
translation of the imager over the wall. A variety of location
sensors can be employed. By way of example, in some embodiments,
each location sensor can be implemented as a tracking ball with
orthogonal rotation encoding devices similar in design and
construction to a computer mouse. In other embodiments, inertial
sensors can be utilized. It should be understood that location
sensors suitable for use in the practice of the invention are not
limited to these examples, and other sensors (e.g., a variety of
optical sensors) known in the art can also be employed.
[0079] As noted above, the combined rotation of the lens and the
translation of the imager over the wall surface results in
illuminating various locations within the interior of the wall. As
the illuminating radiation impinges on an object that is not
transparent to the radiation, e.g., a metal pipe and/or electrical
wiring within the wall, at least a portion of the radiation is
reflected or scattered. In the frequency range of about 1 to about
2000 GHz, most objects specularly reflect, rather than scatter, the
radiation. Hence, at least some of the radiation incident on such
objects within the wall is reflected back towards the imager, e.g.
depending on the "look angle" of the illumination and a normal to
the reflecting surface at the point of illumination. The lens
collects this back-propagating radiation (or at least a portion
thereof), after its passage through the QWP 36, and directs that
radiation, as a converging radiation beam, to the beam splitter 23.
As is known in the art, the passage of the returning radiation,
which is circularly polarized (or at least substantially circularly
polarized, as the reflection of the incident radiation may have
cause some change in the polarization) through the QWP results in
conversion of its polarization to linear polarization with a
polarization axis normal to that of the linearly polarized
radiation generated by the source. As such, the beam splitter
directs this linearly polarized back-propagating radiation to the
detector 14. In this embodiment, the detector 14 operates in
heterodyne mode, that is, it mixes the returning radiation with
radiation from a local oscillator 62 to generate an intermediate
frequency (IF) electrical signal whose strength is proportional to
the intensity of the returning radiation and whose lower frequency
can be more readily processed by electronics circuitry. A variety
of detectors and local oscillators can be employed. For example, in
some embodiments, a receive diode of a Gunnplexer can be employed
as the detector. In some other embodiments, a small portion of the
transmit oscillator power can act as an oscillator for the
receiver. In such a case, a single oscillator can be used for
microwave emission as well as detection.
[0080] The detector 14 transmits the electrical signal generated in
response to detection of the returning radiation to a digital data
processor 64. The digital data processor is also in communication
with the scan position sensor 42 and the location sensors 60 to
receive information regarding, respectively, the angular rotation
of the lens (herein also referred to as A(t)) and the location of
the imager on the wall surface (herein also referred to as
P.sub.1(t) and P.sub.2(t), where P.sub.1(t) denotes the information
from the first location sensor 60 and P.sub.2(t) denotes the
information from the second location sensor 60). The digital data
processor employs the received data to map the time-varying
detection signal to a plurality of respective locations on the
object plane from which the back-propagating radiation originates.
More specifically, the electrical signal, herein also referred to
as I(t), is typically a time-varying signal whose strength at any
instant is proportional to the intensity of the returning radiation
detected by the detector 14 at that time. The intensity is related
to the reflecting properties of the object that is at that time at
the location where the illuminating radiation is directed. It will
be understood by those familiar with the art of scanning image
sensing that signal I(t) varies as a function of time because the
lens is scanning the radiation in time over the object space. That
is,
I(t)=I[x(t),y(t)].
where x(t), and y(t) define the instantaneous position of the
illuminating radiation in the object plane. In the remaining
equations, the time dependence is dropped for convenience.
[0081] Digital data processor 64 transforms/inverts, and combines,
the measurement A, P.sub.1(x,y), and P.sub.2(x,y) to generate x and
y. In the exemplary embodiment of FIG. 1, location sensors 60
measure the rigid body x- and y-displacement of the imager body
relative to an arbitrary starting location and the scan position
sensor measures the angular position of the lens, known to be
offset from the axis of rotation by a fixed distance (e.g. d)
relative to the frame of reference of the imager housing. This can
be expressed as:
X.sub.r=d cos (A), and
Y.sub.r=d sin (A)
where X.sub.r and Y.sub.r are the x and y coordinates of the lens
relative to the axis of rotation in the frame of reference of the
imager housing, and d is the off-axis distance to the imager
spot.
[0082] Similarly, X.sub.h, Y.sub.h, and .theta..sub.z coordinates
of the imager housing can be calculated from P.sub.1 and P.sub.2,
where it is understood each of P.sub.1 and P.sub.2 comprises an x-
and a y-measurement (x.sub.1, y.sub.1; x.sub.2, y.sub.2). For
example, if the location sensor P.sub.1 is selected as the housing
reference point, then
X h = x 1 , Y h = y 1 , and ##EQU00001## .theta. z = arctan [ y 2 -
y 1 x 2 - x 1 ] - .theta. 0 ##EQU00001.2##
where .theta..sub.0 is the initial angle relative to the x-axis
passing through P.sub.1 of the line connecting P.sub.1 with
P.sub.2.
[0083] Finally, the position of the imaging spot can be calculated
by adding the following three vectors: (1) a vector representing
the rigid body displacement of the housing, (2) the position of the
axis of rotation relative to the housing reference point, and (3)
the displacement of the image point due to the angular rotation of
the lens. More specifically, x and y can be obtained by employing
the following relations:
x=x.sub.1+D[cos .theta..sub.z cos .theta..sub.0-sin .theta..sub.z
sin .theta..sub.0]+d cos A, and
y=y.sub.1+D[cos .theta..sub.z sin .theta..sub.0+sin .theta..sub.z
cos .theta..sub.0]+d sin A,
where D is the distance between P.sub.1 and the axis of
rotation.
[0084] The processor 64 is also adapted to generate image position
drive signals suitable for application to an image display 66. The
image position drive signals cause the display of a plurality of
image points, each having a brightness that corresponds to the
detected signal strength from a respective coordinate point in the
object plane. In operation, a complete image is built up on the
imaging display 66 as the imager's housing in moved over the wall
surface (or in proximity thereof) while the imager's lens is
rotated to scan the beam over an interior swath of the wall.
[0085] In some embodiments, relative locations of the pixels in an
image obtained by combined rotation of the lens and translation of
the imager are determined by acquiring a plurality of partially
overlapping image frames, and tracking one or more pixels in the
overlap regions. By way of example, FIG. 10 schematically depicts
two image frames 11 and 13 that include an overlap region 15. At
least one of the pixels in the overlap region (e.g., pixel A first
observed in the image frame 11 and then repeated in a subsequent
image frame 13) can then be utilized as a reference pixel to
determine the relative locations of other pixels in the image frame
11 (e.g., pixels C, D and E) to those in image frame 13 (e.g.
pixels F, G, H and I). The same process can be repeated for another
image frame (not shown) that partially overlaps the image frame 13.
In this manner, the relative locations of pixels in an image
constructed by assembling a plurality of image frames can be
determined.
[0086] By way of example and only for illustrative purposes, FIG.
11 schematically depicts an exemplary image 68 of an interior
portion of the wall scanned by the imager 10, which is presented in
the imager's display 66. This exemplary image depicts a pipe 70 and
a plurality of electrical wires 72 disposed within a depth of the
wall. The imager can also provide images of other objects within
the wall, such as, termites, water leaks, etc. In fact, any object
that is sufficiently reflective (or scattering) at the
interrogating radiation wavelengths can be imaged.
[0087] Although in the above embodiment, the processor 64 and the
display 66 are housed within a single housing with the other
components of the imager, in other embodiments, the processor
and/or the display can be disposed in a separate housing. In
embodiments in which the processor/display are disposed in a
separate enclosure, one or more communications channels can be
provided to allow the processor and/or display to communicate with
one or more imager components disposed in another enclosure. In
some cases, the communications channels employ wireless channels
that utilize known wireless protocols.
[0088] The implementation of an imager according to the teachings
of the invention is not limited to the embodiment described above.
In fact, such as imager can be implemented in a variety of
different ways. For example, FIG. 12 schematically depicts an
electromagnetic scanning imager 74 according to another embodiment
of the invention that includes a head 76 containing a source of
electromagnetic radiation 78 capable of generating radiation with
one or more frequency components in a range of about 1 GHz to about
2000 GHz. In this embodiment, the source is combined with a
detector of electromagnetic radiation capable of detecting
radiation having the same frequency as that generated by the
source. Such combined source/detector units are known the art, and
are commonly referred to as transmit/receive (or transceiver)
units. As shown schematically in FIGS. 13A and 13B, an example of a
suitable transmit/receive unit is a Gunnplexer 80, which includes a
Gunn diode 82 for generating radiation (it functions as the
transmitter of the unit) and a receiver diode 84 for detecting the
radiation (it functions as the receiver of the unit). A circulator
86 (e.g., a ferrite circulator) isolates the transmitter and
receiver functions. A horn 88 facilitates coupling of radiation out
of and into the Gunnplexer. The Gunnplexer can further include a
tuning varactor diode 90, typically mounted close to the Gunn
diode, for electrically varying (tuning) the outgoing radiation
frequency. For example, the varactor can deviate the fundamental
frequency (e.g. by about 60 MHz) when a proper tuning voltage is
applied thereto. Gunn diode oscillators, which convert dc signals
to RF energy, are available, e.g., at specific preset operating
frequencies. The Gunnplexer can also include a Schottky mixer (not
shown). The Gunn diode can function simultaneously as a transmitter
and a local oscillator with a portion of its energy--in one
configuration approximately 0.5 mW--being coupled to the mixer. The
mixer can provide an intermediate frequency (IF) output that can be
amplified by an IF pre-amplifier 81 and an IF amplifier 83.
[0089] Referring again to FIG. 12 as well as FIG. 14, similar to
the previous embodiment, the imager 74 further includes a lens 92
that is optically coupled to the transmit/receive unit 78 to form
an image of the radiation generated by that unit in the general
vicinity of an object region 94 of interest, which is typically
located behind a visibly opaque material 96, e.g., the surface of a
wall, floor or a ceiling. The lens 92 can be a transmissive,
diffractive element (e.g., a zone plate) disposed relative to the
source at a distance, e.g., greater than--but typically less than
three times--its focal length. Generally, the lens 92 forms an
image of the transmit/receive unit (an image of radiation generated
by that unit) at a distance of less than infinity and more than one
focal length from the lens, on the side of the lens away from the
source. Alternatively, the lens can be reflective zone plate, in
which case the image of source generated by the lens and the source
itself are on the same side of the lens. As known in the art, a
zone plate can be implemented as a magnitude zone plate (i.e.,
alternating regions of high transmission/reflection and regions of
low transmission/reflection), or as a phase zone plate (e.g.,
alternating regions imparting zero or 180 degrees of relative phase
shift to incident radiation).
[0090] The lens 92 is rotated about a rotation axis (illustrated as
RA) by a scan mechanism 108, such as those discussed above in
connection with the previous embodiment. Similar to the previous
embodiment, an optical axis (OA) of the lens 92 is displaced
relative to its rotation axis by a selected distance, e.g., about
one-half the lens's radius. The rotation axis is generally centered
on the emitting aperture of the transmit/receive unit 78 parallel
to general direction of propagation of the radiation (parallel to
the central ray of a cone-like bundle of rays). The optical axis
can be parallel to the rotation axis, or may form a non-zero angle
with the rotation axis so as to intersect that axis at the emitting
aperture of the transmit/receive unit. The rotation of the lens
causes the image of the source, generated by the lens, to scan a
selected path (e.g. a generally circular path) over an object
plane, in a manner similar to that discussed above in connection
with the previous embodiment.
[0091] In some embodiments, the emitting aperture of the
transmit/receive unit 78, the lens 92, and the image of the
emitting aperture are preferably disposed in a confocal
configuration. That is, the illuminating radiation is focused onto
a small region in a plane of interest (e.g., the object plane), and
the reflected (or scattered) radiation reaching the detector (the
transmit/receive module in this ease) is limited to those rays that
originate from the illuminated region. In some embodiments, such a
confocal imaging system is employed to reject stray light by
utilizing, for example, two strategies: (1) by illuminating a
single point (small area) at any given time with a focused beam
such that the focused intensity drops off rapidly at axial
locations away from that plane of focus (e.g., in front or behind
that plane), and (2) by utilizing a blocking or a pinhole aperture,
or a point detector, in a conjugate receiver plane so that light
reflected (or scattered) from the illuminated object region is
blocked from reaching the detector.
[0092] With continued reference to FIGS. 12 and 14, the combined
rotation of the lens and translation of the imager's head over a
surface of the obstruction 96 (e.g., a wall) can result in
illuminating a region of interest behind the surface (e.g., an
interior region of a wall). At least a portion of the illuminating
radiation is reflected (or scattered) back toward the lens. The
lens collects the radiation propagating back (e.g., via reflection)
from the illuminated region and focuses the collected radiation
onto the transmit/receive unit 78, which, functioning as a
detector, converts the back-propagating radiation into an output
electrical signal.
[0093] The output electrical signal is communicated, e.g., via a
communication channel 100, to an electronic processor 102 (e.g. a
digital data processor), disposed in an electronic processing and
display module (EPDM) 104. While in this embodiment the EPDM is
contained in a separate housing, in other embodiments, it can be
integrated with the head 76 within a single housing. The processor
102 includes a signal processing module that is adapted to convert
the output signal generated by the transmit/receive unit 78 into
image strength drive signals suitable for application to an image
display 106.
[0094] In addition to communicating with the detector, the
processor 102 is also electrically coupled to a scan position
sensor 110, e.g., via a communications channel 112, that can sense
the position of the scan mechanism, and thereby that of the lens
92, relative to a predetermined reference position. A variety of
scan position sensors, such as those discussed above, can be
employed. The position sensor communicates the information
regarding the position of the lens to the processor.
[0095] Similar to the previous embodiment, the imager 74 further
includes a body location-determining subsystem 114 for determining
the rigid body location of the head 76 on a surface (e.g., wall
surface) over which it is moved to build up an image of a region
behind the surface. The subsystem 114 can be in optical and/or
mechanical communication with a surface over which the imager is
translated. Typically, the subsystem 114 estimates the location and
orientation of the head 76 via three parameters "X.sub.h",
"Y.sub.h" and ".theta..sub.Z", where X, Y and Z denote orthogonal
Cartesian coordinates. The X and Y denote coordinates in a plane
(e.g. a planar surface of a wall over which the head is translated)
and .theta..sub.Z denotes an angle about the Z-axis that is
perpendicular to that plane. By way of example, the origin of the
coordinates can be established as the location and orientation of
the imager upon its initial placement on the plane. This can be
done automatically or by a user-issued command (which can also be
employed to reset the location of the origin, if desired). The
location-determining subsystem can then determine subsequent
locations and orientations of the imager relative to the origin. A
number of location-determining subsystems can be utilized. For
example, in some embodiments, the subsystem can comprise two
computer-mouse sensing mechanisms, separated by a known base line.
Alternatively, the subsystem can be implemented by employing a
plurality of inertial sensors.
[0096] The location-determining subsystem 114 transmits signals
indicative of the location of the imager's head to the processor
102, e.g., via a communications channel 116. The processor utilizes
these signals, together with those transmitted by the lens position
sensor, to generate a set of image point coordinates in the
coordinate space of the object region. The processor further
correlates these image coordinates to the time-variation of the
signal received from the detector to generate a reflectance image
of the illuminated portion. In addition, the processor derives
image position drive signals, based on the image coordinates and
intensity of reflected signals originating from those coordinates,
for application to the display 106. The image drive signals cause
the display to present an image in which the brightness of an image
point corresponds to the intensity of the detected reflected
radiation originating from a coordinate point (e.g., an area or
voxel in vicinity of that point) mapped to that image point.
[0097] In some embodiments, the frequency of the radiation
generated by the source (e.g., the above transmit/receive unit 78)
is modulated by a control signal. For example, in the above
Gunnplexer 80 (FIG. 13A), the varactor 90 can be employed to cause
such a frequency modulation. For example, the frequency can be
modulated rapidly (e.g., at a rate of 100 (kHz) over a relatively
narrow range (e.g., 1% of the center frequency) to suppress
coherent echoes, interference and/or speckle noise. Typically, the
focal distance of the imager can represent about 100 waves and the
reflected intensity is acquired at a rate of, e.g. 5 kHz. In such a
case, the frequency of the radiation generated by the Gunnplexer
can be modulated by about 1% at a rate of about 100 kHz to
introduce a one-wave phase shift as each image pixel is acquired.
Intensity integration during this phase shift can significantly
reduce coherent artifact noise. It should be appreciated that other
frequency modulation parameters can also be utilized based on the
requirements of a particular application.
Snapshot Embodiments
[0098] In some embodiments, the imager can provide an image of a
two-dimensional area while the imager (e.g. imager housing) remains
stationary (i.e., without the need to physically move the imager).
One such exemplary embodiment 118 shown in FIG. 15 includes a
transmit/receive unit 120 for generating and detecting radiation,
which preferably operates in a frequency range of about 1 GHz to
about 2000 GHz. A lens 122 optically coupled to the
transmit/receive unit receives the radiation and focuses it into an
object region 124 whose image is desired. In some cases, the object
region can be visibly hidden from view by an opaque obstruction,
e.g., a wall portion.
[0099] A scan mechanism 126 scans the radiation, which is directed
by the lens to the region 124, over a plurality of locations in
that region. Preferably, although not required, the radiation is
collimated or nearly collimated before entering scan mechanism 126.
The lens and the scan mechanism can be configured to produce a
plurality of radiation scan patterns to cover (illuminate) at least
a portion, e.g., an object plane 124a, within the region 124. The
scan mechanism typically moves the radiation within a plane (e.g. a
plane perpendicular to the lens's optical axis) so as to generate a
desired radiation scan pattern. By way of example. FIGS. 16A, 168,
16C depict, respectively, a spiral, a floret and a raster scan
pattern suitable for use in the practice of the invention. The
spiral and the floret patterns can be generated, for example, by a
combined rotation and radial motion (i.e., motion perpendicular to
the optical axis of the lens) of the radiation. The raster pattern
can be formed, in turn, by combination of two linear and orthogonal
motions of the radiation. In each case, the focused beam size
(e.g., shown schematically by dashed lines in FIGS. 16A-16C) can be
selected such that the radiation scan causes illumination of a
two-dimensional area of interest.
[0100] By way of example, FIG. 17A schematically depicts a
radiation scan mechanism 200A optically coupled to the source for
providing a two-dimensional radiation scan. The mechanism 200A
includes two angularly rotatable reflective elements 201A and 202A
that are configured to rotate about two orthogonal axes (depicted
schematically as axes A and B, where axis A is perpendicular to the
plane of the figure whilest axis B is in the plane of the figure).
The reflective element 201 receives the radiation from the source
and directs that radiation to the reflective element 202, which, in
turn, directs the radiation to the lens (not shown). Rotation about
axis A causes reflective element 201A to scan the radiation in the
plane of the figure as the radiation propagates toward reflective
element 202A. Similarly, rotation about axis B causes reflective
element 202A to scan the radiation out of the plane of the figure
as the radiation propagates to the lens. The relative rotational
rates, scan angles, and timing of the rotations of the two elements
can be adjusted so as to obtain a variety of scan patterns, such as
those discussed above. In this exemplary embodiment of a
two-dimensional beam scanner the most natural area scan pattern is
a raster scan similar to that illustrated in FIG. 16C, though other
scan patterns can also be utilized,
[0101] As another example, FIG. 17B schematically depicts a second
radiation scan mechanism 200B optically coupled to the source for
providing a two-dimensional radiation scan. This exemplary
mechanism 200B is known as a "swash plate" scanner. Swash plate
scanner 200B includes two rotatable, generally planar, reflective
elements 201B and 202B that each of which is configured to rotate
about axes (indicated by "C" and "D" respectively in FIG. 17B) that
are at a slight angle, .alpha., relative to the normal, N, of the
respective reflective surface, as shown in the inset in FIG. 178
for reflective element 201B. Generally the rotation axis angle,
.alpha., is displaced less than about 10 degrees from the normal to
the reflective surface. The selection of angular displacement is a
design choice based on the optical parameters desired for the
scanner.
[0102] The two reflective elements 201B and 2028 are typically
disposed in a periscope arrangement: that is, as shown in FIG. 17B,
incident radiation impinges on element 201B with an approximate 45
degree angle of incidence and is reflected toward element 202B,
where it also impinges with an approximate angle of incidence of 45
degrees. Radiation reflected from element 202B in this exemplary
embodiment propagates generally parallel to the incident radiation,
that is, towards lens 122.
[0103] As reflective element 201B rotates about axis C, the normal
to the surface sweeps out a cone in space, where the vertex angle
of this cone is twice the angular displacement, .alpha., between
the normal and axis C. As the normal to the surface sweeps out this
cone, the angle of incidence of the incident radiation and angle of
reflection (and therefore the direction of the reflected radiation)
varies in accordance with the laws of basic geometry. Circular
arrow 210 notionally depicts the angular sweep of the beam
reflected from reflective element 201B.
[0104] A similar process occurs at reflective element 202B.
Convoluted arrow 212 notionally depicts the complex angular sweep
of the beam propagating toward lens 122.
[0105] As with the previously described two-dimensional scanner,
disposing the two reflective elements in series, coupled with
appropriate design and mirror drive commands, permits any scan
pattern to be achieved. In this exemplary embodiment of a
two-dimensional beam scanner the most natural area scan pattern is
a floret scan similar to that illustrated in FIG. 16B, though other
scan patterns can also be utilized.
[0106] FIG. 17C shows a third exemplary embodiment of scan
mechanism 200C, two rotating transmissive, prisms 201C and 202C are
used in place of the two reflective elements 201B and 202B, in
which embodiment the axes of rotation for the prisms are generally
perpendicular to the bisector of prisms' vertex angles. As shown
schematically in FIG. 17C, radiation propagates through the prisms
and then onto the lens. As illustrated, the radiation propagating
through the prisms is deflected laterally in a direction determined
by the rotation angles of the prisms. Although each individual
prism can only deflect the beam into a direction on the surface of
a cone, combining the two prisms in series permits the beam to be
deflected anywhere within the volume of a cone, in this exemplary
embodiment of a two-dimensional beam scanner the most natural area
scan pattern is also a floret scan similar to that illustrated in
FIG. 16C, though other scan patterns can also be utilized.
[0107] In some embodiments of the invention operating at the
wavelengths of interest, refractive prisms can be replaced by their
diffractive counterparts, diffraction gratings.
[0108] Finally, it will be understood by one of skill in the art
that the embodiment of the Binary Fresnel Lens illustrated in FIG.
4 already combines the focusing function of a lens with the
deflecting function of a prism (or diffraction grating). Therefore,
two-dimensional scan mechanism 200 may comprise one diffraction
grating and one BFL, wherein each of these elements is
independently rotatable.
[0109] In each embodiment of two-dimensional scan mechanism
discussed herein, the two scanning elements can be individually
equipped with sensing devices (illustrated collectively as item 132
in FIG. 15) used to communicate the instantaneous position of the
scanning elements to processor 128.
[0110] With reference to FIG. 15 as well as FIG. 17, one or more
objects within the illuminated portion of the object region can
reflect (and/or scatter) at least a portion of the incident
radiation back towards the lens. For example, by way of
illustration, a surface of an object 124b illuminated by the
incident radiation can reflect a portion of that radiation back to
the lens 122. The lens 122, in turn, directs the back-propagating
radiation to the transmit/receive unit 120 for detection. Similar
to the previous embodiment, the detector module of the
transmit/receive unit generates an electrical signal (typically a
time-varying signal) in response to detection of the
hack-propagating radiation, and communicates this signal to a
processor 128, e.g. via a communications channel 130. The magnitude
of the generated detection signal at a given time is proportional
to the intensity of the reflected radiation detected at that time.
The processor also receives information from a radiation scan
sensor 132, e.g., via a communications channel 134, regarding the
position of the illuminating spot on the object plane as a function
of time. For example, the scan sensor can provide the processor
with information regarding the instantaneous orientation and
rotation rate of each of reflective elements 201 and 202. The
processor can convert this information to the position of the
illuminating spot on the object plane at any given time. The
processor further utilizes this information, e.g., in a manner
discussed above, to generate a plurality of image point
coordinates, each corresponding to a cordinate in the object plane.
The processor further assigns a brightness value to each image
point coordinate in proportion to the magnitude of the detected
signal associated with an object point corresponding to that image
point. In this manner, the processor calculates an image
corresponding to the illuminated portion.
[0111] Further, the processor generates a plurality of image drive
signals for application to a display 136 for displaying the
calculated image. In this embodiment, the processor and the display
are disposed in separate enclosures with communication channels
coupling the processor to the transmit/receive unit as well as the
lens position sensor. In other embodiments, the various components
of the imager can be housed in a single, preferably handheld,
enclosure.
[0112] In some embodiments, an imager according to the teachings of
invention is capable of acquiring images of a plurality of object
planes located at different axial locations (e.g., at difference
depths within an obstruction, such as a wall). For example, FIG. 18
schematically depicts an imager 138 according to another embodiment
of the invention in which a lens 140 (e.g., a diffractive element
whose optical axis is offset relative to its rotation axis) can be
axially moved (e.g. along the lens's rotation axis RA) so as to
focus radiation generated by a transmit/receive unit 142 at
different axial locations within an object region 144. For example,
a rotational scan mechanism 146 can rotate the lens, and a
focus-drive mechanism 148 can move the lens back-and-forth along
its rotational axis so as to focus radiation onto different object
planes (e.g. exemplary planes 144a and 144b) within the object
region 144. A plurality of focus-drive mechanisms can be employed.
By way of example, as shown schematically in FIG. 19, the focus
drive mechanism 300 can include a movable stage 301 on which the
lens is mounted. In some embodiments, the lens is coupled to a
shaft mechanism that rotates and simultaneously axially translates
the lens. In such an embodiment, for each axial position of the
lens, the rotation of the lens, together with the translation of
the imager, can result in scanning the radiation over a path within
an object plane. In other embodiments, a room lens system can be
employed for focusing the radiation on axially separated object
planes.
[0113] In some embodiments, both transmit/receive unit 142 and lens
are axially translated, while preferably maintaining the separation
between the transmit/receive unit and the lens, to focus the
radiation on planes at different axial locations.
[0114] The radiation reflected from each object plane can be
detected by the transmit/receive unit, which generates an
electrical signal in response to such detection and transmits the
signal to a processor 150 for analysis. The imager further includes
at least one lens position sensor 152 coupled to the rotational
scanner and the focus-drive mechanism for determining the axial
position as well as the rotational orientation of the lens (in some
embodiments, the functionality of the position sensor 152 can be
provided by two separate sensors, one for determining the lens's
axial position and the other for determining the lens's rotational
orientation). By way of example, the lens position sensor can be
implemented as a shaft encoder. The sensor 152 transmits the
information regarding the lens's axial position and rotational
orientation to the processor 150. The processor employs this
information to temporally correlate the detection signal generated
by the detector to different object planes, and for each object
plane, to a plurality of coordinate positions in that plane. In
this manner, the processor can build up a plurality of images, each
corresponding to a different depth within the object region. The
processor can further generate image drive signals for application
to a display 154 for displaying these images, e.g., as a
three-dimensional image. In some cases, the processor can cause the
display to present selected ones of these images, or present them
in a selected sequence, or in any other desired manner.
[0115] Although a transmit/unit is employed in the imager 138, in
other embodiments, separate source and detector can be employed to
generate and detect the radiation, for example, in a manner shown
in the above imager 10 (FIG. 1). Further, the imager 118 discussed
above, which provides a two-dimensional image of an area of
interest while stationary, can be similarly modified to provide a
plurality of two-dimensional images at different depths in a region
of interest.
[0116] In another embodiment, the longitudinal chromatic aberration
of the lens can be employed to focus radiation from a source at a
plurality of at different depths (e.g., onto a plurality of object
planes located at different axial distances from the lens). For
example, the frequency of the radiation generated by a source can
be varied (tuned) such that the chromatic aberration exhibited by
the lens would result in focusing different frequencies at
different axial locations from the lens.
[0117] By way of example, FIG. 20 schematically depicts such an
embodiment 156 in which a varactor diode 158 is employed to tune
the frequency of a radiation source 160 of a Gunnplexer 162. A
processor 164 in communication with the Gunnplexer can receive
information regarding the frequency of radiation as a function of
time. The processor employs this information to temporally
correlate detection signals corresponding to the detected
back-propagating radiation with different axial locations from the
lens. Images of different portions (e.g. different object planes
such as exemplary object planes 166 and 168) can then be built up
and displayed, e.g., in a manner similar to that discussed above,
in a display 170.
Telecentric Embodiments
[0118] FIG. 21 schematically illustrates an imager (camera) 172 in
accordance with another embodiment of the invention that employs a
telecentric arrangement of optical elements to acquire an image.
The imager comprises a two-element lens system 174 composed of a
rotating eccentric pupil lens 176 and a fixed centered lens 178.
Fixed lens 178 is illustrated as embedded in a square-shaped
surround 180 for clarity only; the surround emphasizes that lens
178 is fixed. The function of the eccentric pupil lens 176 can be
understood as being similar to a combination of a collimating lens
and a wedge prism. By way of example, a lens portion cut from an
edge of a spherical lens or a zone plate lens can be utilized as
the lens eccentric lens 176. Other ways of forming the lens 176 are
also known to those having ordinary skill in the art.
[0119] In the exemplary imager 172, the lens 176 is disposed
relative to an emitting aperture of a radiation source 182 at a
distance equal to one of its focal lengths. The lens 176 converts
an expanding cone of radiation generated by the source into a
generally collimated radiation beam, and directs the collimated
beam in a predetermined off-axis direction, as shown schematically
in FIG. 21. The fixed lens 178 is, in turn, disposed at a selected
distance from the lens 176 so as to focus the collimated beam
generated by the lens 176 into a point image 184 in a plane at one
of its focal lengths in the vicinity of an object region 184a. A
scan radius R.sub.s (i.e. a radial distance between
optical/rotation axis [RA,OA] and the image 184) can be estimated
as the product of the angle of propagation of the collimated beam
and the focal length of the fixed lens 178. R.sub.s is independent
of the distance between the rotating lens 176 and the fixed lens
178. In some embodiments, the fixed lens 178 can be axially moved,
e.g., by utilizing mechanisms such as those discussed above in
connection with some of the previous embodiments, so as to focus
the radiation at different axial locations relative to the imager
(e.g., at different depths of a wall or other obstruction). For
example, the imager can be held at a generally fixed distance from
a planar surface while the lens 178 is axially moved to focus the
radiation at different axial distances from that surface.
[0120] Preferably, the separation between the lenses 176 and 178 is
substantially equal to the focal length of the fixed lens 178. In
such a case, the fixed lens 178 forms the image 184 with an imaging
cone of radiation whose chief ray is parallel to the optical axis.
When the lens 178 is axially moved, the separation between the two
lenses can deviate from this preferred value, although in some
embodiments, both lenses can be moved so as to provide a depth of
scan of the radiation while maintaining the separation between the
lenses substantially equal to the preferred value.
Example
[0121] A prototype imager made based on the above teachings of the
invention is discussed in the following example for further
illustration of various aspects of the invention. It should,
however, be understood that this is intended only for illustrative
purposes, and not for indicating optimal performance of imagers
according to the teachings of the invention, or to suggest that the
specific arrangement of the various optical components and other
design parameters utilized in the prototype are in any way meant to
limit the scope of the invention.
[0122] The prototype imaging system based on the teachings of the
invention was fabricated by utilizing a Gunn oscillator operating
at a frequency of 24.15 GHz (a wavelength of about 12.4
millimeters) as the emitting source. The Gunn oscillator was
coupled to a 10 dB feedhorn, with an exit aperture having
dimensions of 15 by 11 mm, so as to output a cone of linearly
polarized radiation at a power of 5 mW with an angular spread of
+/-57 degrees.
[0123] After passage through a 45-degree wire grid polarizer
(composed of 30 gauge wires with 0.8 mm center spacing disposed on
an Acrylic frame), the radiation from the oscillator was focused to
a focal point by an F/0.6 quarter-wave focusing lens, formed of Low
Density Polyethylene (LDPE). The lens was configured to image the
radiation at a focal spot approximately 100) mm off the lens's
optical axis. The distance of the source from the lens (about 125
mm) was substantially equal to that of the image from the lens,
thus resulting in a magnification of about 1.
[0124] A 32 mm thick birefringent quarter-wave plate, composed of
an array of 2.5 mm wide slots cut into a LPDE sheet, was placed
between the lens and the focal point. The slots of the quarter-wave
plate were oriented at 45 degrees relative to the polarization axis
of the incident beam, thus converting the beam's linear
polarization to circular polarization. Upon reflection from an
object at the focal point and a second passage through the
quarter-wave plate, the beam's circular polarization was converted
back to linear polarization, albeit with a 90-degree rotation
relative to the polarization axis of the incident beam. The
back-propagating radiation was then transmitted by the wire grid
polarizer to a second Gunn oscillator having an integrated mixer
(receiver). The optical system effectively operated in a confocal
mode, where the diffracted radiation spot served to illuminate an
object and radiation reflected (or scattered) from the object was
focused back through a small aperture (the feedhorn entrance) to
the mixer detector.
[0125] The emitter and receiver Gunn oscillators were tuned to have
a frequency mismatch of approximately 2 MHz. This frequency
mismatch causes radiation reflected by an object at the focal point
and relayed to the receiver to generate a 2 MHz beat frequency
signal. The beat frequency was amplified, low-pass filtered
(frequency cutoff was about 500 Hz) and rectified. The rectified
signal was, in turn, fed to a computer data acquisition system.
[0126] By rotating the lens at 300 revolutions-per-minute (rpm), a
circularly scanned "probe spot" was generated. A magnet and a Hall
effect sensor were utilized to measure the rotational position of
the lens. Object imaging was accomplished by moving objects
transversely through the scanning focused spot. A sheet of gypsum
wallboard having a thickness of about 5/8 inches (15.9 cm) was
placed between the lens and the focal plane of the probe spot.
Radiation passing through the wallboard interacted with various
test objects (e.g., wires, pipes, human skin, etc). A software
program was utilized to use the rotational position of the lens so
as to determine the Cartesian Coordinates of locations on the focal
plane from which the detected reflected radiation originated. This
information was utilized, in a manner discussed in detail above, to
construct images of objects that were moved transversely through
the scanned field.
[0127] FIGS. 22A-22F show some illustrative images obtained by the
above prototype, illustrating that objects formed from a variety of
different materials (e.g., wood, metal, plastic) with features as
small as 6 mm can be imaged behind a gypsum wallboard with contrast
ratios as high as 10:1.
[0128] As noted above, the above prototype was discussed only for
illustrative purposes. The particular selections and arrangements
of the optical components (e.g. source, lens and receiver) were
made only by way of example. Alternative components and
arrangements can also be utilized. For example, sources operating
at other wavelengths can be employed.
[0129] It should be understood that various modifications can be
made to the above illustrative embodiments without departing from
the scope of the invention. For example, a variety of different
lenses can be utilized. The lenses can be fabricated, e.g., as zone
plates, parallel metal plates, dielectric materials. Further, the
optics can be designed as confocal, near confocal, telecentric, or
dark field. The scanning of the radiation can be one or
two-dimensional (radial, tangential, raster, or a combination
thereof). The camera body location-determining subsystem can be
internal or external to the camera body. Further, the location
sensing technology can be mechanical, optical, RF or any suitable
mode.
* * * * *