U.S. patent application number 11/434576 was filed with the patent office on 2007-11-15 for rapid 4-stokes parameter determination via stokes filter.
This patent application is currently assigned to The United States of America as Represented by the Army. Invention is credited to Grant R. Gerhart, Roy M. Matchko.
Application Number | 20070263218 11/434576 |
Document ID | / |
Family ID | 38664621 |
Filed Date | 2007-11-15 |
United States Patent
Application |
20070263218 |
Kind Code |
A1 |
Gerhart; Grant R. ; et
al. |
November 15, 2007 |
Rapid 4-Stokes parameter determination via stokes filter
Abstract
A system for determining polarization profiles of points in a
scene from video frames using Stokes parameters includes a scene
having a region that emits scene light rays that correspond to the
points in the scene, an optical chopper controller, a rotating
Stokes filter wheel that includes five trigger holes, three
rotating linear polarizers, a circular polarizer, and a reference
screen, a color filter, a video camera having a video frame, and a
computer system having a frame grabber apparatus. The scene light
rays are transmitted through the Stokes filter and the color filter
to the video camera. Images corresponding to the scene light rays
are projected onto respective pixels in the video frame and
recorded as two-dimensional (2-D) arrays, and the images
corresponding to the scene light rays from four unique images,
obtained from light transmitted consecutively through three linear
polarizers and a circular polarizer of the rotating Stokes filter
wheel, are used by programming in the computer system to calculate
respective Stokes parameters of the points in the scene.
Inventors: |
Gerhart; Grant R.;
(Bloomfield Hills, MI) ; Matchko; Roy M.; (Payson,
AZ) |
Correspondence
Address: |
U.S. ARMY TACOM;ATTN: AMSTA-LP/281
6501 E. 11 MILE RD.
WARREN
MI
48397-5000
US
|
Assignee: |
The United States of America as
Represented by the Army
Washington
DC
|
Family ID: |
38664621 |
Appl. No.: |
11/434576 |
Filed: |
May 10, 2006 |
Current U.S.
Class: |
356/364 |
Current CPC
Class: |
G01J 4/04 20130101 |
Class at
Publication: |
356/364 |
International
Class: |
G01J 4/00 20060101
G01J004/00 |
Goverment Interests
GOVERNMENT INTEREST
[0002] The invention described here may be made, used and licensed
by and for the U.S. Government for governmental purposes without
paying royalty to us.
Claims
1. A system for determining polarization profiles of points in a
scene from video frames using Stokes parameters, the system
comprising: a scene having a region that emits scene light rays
that correspond to the points in the scene; a motorized rotating
Stokes filter wheel having three linear polarizers, a circular
polarizer, and a reference screen consecutively located in
apertures on the wheel in a first radial band, and five trigger
holes located adjacent to respective ones of the polarizers and the
screen in a second radial band that is outward on the wheel from
the apertures; a sensor that generates a trigger signal when any
one of the trigger holes passes by the sensor; a narrowband color
filter; a video camera having a video frame; an optical chopper
controller; and a computer system containing programming, and
having a frame grabber apparatus, wherein the optical chopper
controller transmits a speed control signal to control rotational
speed of the motor, and the programming sends a mode control signal
to the video camera to establish mode of operation and exposure
time; the programming internally transmits to the frame grabber
apparatus a frame grab control signal that determines a selected
number of video frames for capture by the frame grabber apparatus,
and a starting time for the capture of the video frames; when the
frame grabber initiates capture, the chopper controller receives
the trigger signal from the sensor, and generates a capture control
signal in response to the trigger signal, and transmits the capture
control signal to the frame grabber apparatus, and the frame
grabber apparatus transmits the capture control signal to the video
camera activating exposure, and the video camera transmits the
captured video image to the frame grabber to be processed by the
programming; the scene light rays are transmitted consecutively
through the polarizers in the Stokes filter wheel and the color
filter to the video camera; images corresponding to the scene light
rays are projected onto respective pixels in the video frame and
recorded as respective two-dimensional (2-D) arrays in response to
the trigger signal; and the images corresponding to the scene light
rays from four unique images obtained from light transmitted
consecutively through the three linear polarizers and the circular
polarizer of the rotating Stokes filter wheel are formatted by the
frame grabber apparatus and are used by the programming to
calculate respective Stokes parameters of the points in the scene,
and the image corresponding to the reference screen indicates a
completed revolution of the rotating Stokes filter wheel.
2. The system of claim 1 further comprising a polarization standard
at a region in the scene separate from the region of the scene that
emits the scene light rays, wherein: the polarization standard
comprises a fourth linear polarizer that emits a linear
polarization standard light ray, and a second circular polarizer
that emit a circular polarization standard light ray, wherein the
linear polarization standard light ray, and the circular
polarization standard light ray are transmitted through the three
linear polarizers and the circular polarizer of the Stokes filter
wheel and the color filter to the video camera, images
corresponding to the linear polarization standard light ray and the
circular polarization standard light ray are projected onto
respective pixels in the video frame and recorded, the images
corresponding to the linear polarization standard light ray and the
circular polarization standard light ray from four unique images,
obtained from light transmitted consecutively through the three
linear polarizers and the circular polarizer of the rotating Stokes
filter wheel, that are used by the programming in the computer
system are used to solve for respective Stokes parameters of the
linear polarization standard light ray and the circular
polarization standard light ray, and data related to the scene is
rejected as invalid when the polarization parameters of the linear
polarization standard light ray and the circular polarization
standard light ray fail to correspond to respective known values,
wherein the respective known values are predetermined in a
laboratory under controlled lighting conditions.
3. The system of claim 1 wherein total time duration to obtain the
four unique images that are used to obtain the four Stokes
parameters for each pixel in the scene is regulated by the
rotational speed of the rotating Stokes filter wheel and the frame
rate of the video camera, and the system generates a sufficient
number of the unique scene images to generate the respective four
Stokes parameters in nearly real time.
4. The system of claim 1 wherein the circular polarizer comprises
at least one optically transparent, birefringent material.
5. The system of claim 1 wherein the rotating Stokes filter wheel
is implemented as a precision motorized, rotating Stokes filter
wheel having precise rotational speed control such that the angular
velocity and angular acceleration of the rotating Stokes filter
wheel are controlled within predetermined limits.
6. The system of claim 1 wherein the video camera further comprises
a lens, and the video frame comprises an array of light sensitive
receptors that correspond to the pixels.
7. The system of claim 1 wherein the three linear polarizers have
transmission axes with three unique orientations with respect to
the horizontal.
8. The system of claim 7 wherein the three unique orientations are
vertical, horizontal, and 45 degrees.
9. A method of determining polarization profiles of points in a
scene from video frames using Stokes parameters, the method
comprising: emitting scene light rays from points in a region in a
scene; rotating a motorized Stokes filter wheel having three linear
polarizers, a circular polarizer, and a reference screen
consecutively located in apertures on the wheel in a first radial
band, and five trigger holes located adjacent to respective ones of
the polarizers and the screen in a second radial band that is
outward on the wheel from the apertures; generating a trigger
signal using a sensor when any one of the trigger holes passes by
the sensor; transmitting the scene light consecutively through the
polarizers and the screen in the Stokes filter wheel and through a
narrowband color filter to a video camera having a video frame;
presenting the trigger signal from the sensor to an optical chopper
controller that generates a control signal in response to the
trigger signal, and transmits the control signal to a frame grabber
apparatus, and the frame grabber apparatus transmits the control
signal to the video camera; projecting images corresponding to the
scene light rays onto respective pixels in the video frame and
recording the images as respective two-dimensional (2-D) arrays in
response to the trigger signal; and transmitting consecutively the
images corresponding to the scene light rays from four unique
images obtained from light through the three linear polarizers and
the circular polarizer of the rotating Stokes filter wheel to the
frame grabber apparatus, formatting the images using the frame
grabber apparatus, using programming in a computer system to
calculate respective Stokes parameters of the points in the scene,
and indicating a completed revolution of the rotating Stokes filter
wheel using the image corresponding to the reference screen.
10. The method of claim 9 further comprising implementing a
polarization standard at a region in the scene separate from the
region of the scene that emits the scene light rays, wherein: the
polarization standard comprises a fourth linear polarizer that
emits a linear polarization standard light ray, and a second
circular polarizer that emit a circular polarization standard light
ray, wherein the linear polarization standard light ray, and the
circular polarization standard light ray are transmitted through
the three linear polarizers and the circular polarizer of the
Stokes filter wheel and the color filter to the video camera,
images corresponding to the linear polarization standard light ray
and the circular polarization standard light ray are projected onto
respective pixels in the video frame and recorded, the images
corresponding to the linear polarization standard light ray and the
circular polarization standard light ray from four unique images,
obtained from light transmitted consecutively through the three
linear polarizers and the circular polarizer of the rotating Stokes
filter wheel, that are used by the programming in the computer
system are used to solve for respective Stokes parameters of the
linear polarization standard light ray and the circular
polarization standard light ray, and data related to the scene is
rejected as invalid when the polarization parameters of the linear
polarization standard light ray and the circular polarization
standard light ray fail to correspond to respective known values,
wherein the respective known values are predetermined in a
laboratory under controlled lighting conditions.
11. The method of claim 9 wherein total time duration to obtain the
four unique images that are used to obtain the four Stokes
parameters for each pixel in the scene is regulated by the
rotational speed of the rotating Stokes filter wheel and the frame
rate of the video camera, and the system generates a sufficient
number of the unique scene images to generate the respective four
Stokes parameters in nearly real time.
12. The method of claim 9 wherein the circular polarizer comprises
at least one optically transparent, birefringent material.
13. The method of claim 9 wherein the rotating Stokes filter wheel
is implemented as a precision motorized, rotating Stokes filter
wheel having precise rotational speed control such that the angular
velocity and angular acceleration of the rotating Stokes filter
wheel are controlled within predetermined limits.
14. The method of claim 9 wherein the video camera further
comprises a lens, and the video frame comprises an array of light
sensitive receptors that correspond to the pixels.
15. The method of claim 9 wherein the three linear polarizers have
transmission axes with three unique orientations with respect to
the horizontal.
16. The method of claim 15 wherein the unique orientations are
vertical, horizontal, and 45 degrees.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present invention may relate to applications having Ser.
No. 10/822,355, filed Apr. 13, 2004, and Ser. No. 11/158,357, filed
Jun. 20, 2005, which are hereby incorporated by reference in their
entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention generally relates to a system and
method for rapidly determining Stokes parameters.
[0005] 2. Background Art
[0006] The Stokes method provides a seminal method for the
determination of a state of polarization of a beam of light using
measurable quantities. The Stokes method includes the process of
measuring four intensities of the light beam. Each measurement
corresponds to the intensity of the light beam after the light beam
passes through each of four different filter system arrangements.
The four Stokes parameters, by convention, are generally designated
S.sub.0, S.sub.1, S.sub.2, and S.sub.3. The four Stokes parameters
are derived from the four measured light beam intensities, and form
a four-element column vector in four-dimensional mathematical
space.
[0007] Since the discovery of the Stokes method in 1852, many
conventional filter systems based on the Stokes method have been
developed. Typically, in conventional approaches four separate,
unique images are used to calculate the Stokes parameters for each
element in a scene. A manually rotated retarder and a linear
polarizer are used in conventional approaches to obtain the data
used for determining the Stokes parameters.
[0008] However, the conventional approaches of using the Stokes
parameters for acquiring polarization information from images have
the deficiencies of sometimes having errors in temporal
registration and in spatial registration. The temporal registration
errors occur because of the capture time differential to generate
separate images in conventional approaches. The capture time
differential can affect polarization measurements that are taken
outdoors when changing sun position, cloud position, and the like
change the intensity or the polarization state of the light
entering the filter system. Indoors (e.g., in a laboratory),
temperature, atmospheric pressure, and density or concentration of
variations associated with scene elements can change the
polarization state of the light entering the filter system during
the time duration used to record four separate images.
[0009] Spatial registration errors can occur when conventional
approaches are used whenever the scene is imaged onto the image
plane of the camera from different positions. For example, when
four adjacent lenses are used to simultaneously image the scene on
the image plane of the camera, each lens will obtain an image of
the scene from a slightly different perspective. As such, the four
images will be different, and spatial errors can occur.
[0010] Thus, there exists a need and an opportunity for an improved
system and an improved method for the determination of polarization
profiles of points in a scene from video frames. Such an improved
system and method may overcome one or more of the deficiencies of
the conventional approaches.
SUMMARY OF THE INVENTION
[0011] Accordingly, the present invention may provide an improved
system and an improved method for the determination of polarization
profiles of points in a scene from video frames using rapidly
determined Stokes parameters. The present invention may
significantly reduce temporal errors and eliminate spatial errors
when compared to conventional approaches for determining Stokes
parameters. The present invention generally comprises an optical
chopper controller, a motorized rotating Stokes filter wheel, a
narrowband color filter, a high speed video camera, and a computer
system including a frame grabber, and software to coordinate
operation of the optical chopper controller, the frame grabber and
the video camera, and a polarization calibration standard.
[0012] Light from points in a scene are generally transmitted
through the system and exit from the system having respective
intensities that may be uniquely attenuated for each wavelength of
the light. A narrowband color filter is generally used to select a
particular wavelength. The respective attenuated intensities in
each of four scene-images are generally used to calculate the
respective Stokes parameters for selected points in the scene for
the selected wavelength.
[0013] The system and method of the present invention may express
the Stokes parameters explicitly as a function of wavelength. As
such, the present invention may overcome the deficiency of the
different phase differential for each individual wavelength that is
typically introduced by the retarder that is implemented in
conventional approaches.
[0014] A polarization standard that generally comprises a linear
polarizer and a circular polarizer of known azimuth and angle of
ellipticity may be included in the scene to provide testing for the
introduction of pseudo polarization parameters. The Stokes
parameters may be calculated for each point in the scene.
[0015] The present invention generally provides for obtaining the
four Stokes parameters more rapidly when compared with conventional
approaches. The present invention may provide for the generation of
video images having changing polarization, and may reduce, minimize
or eliminate spatial and temporal registration errors when compared
to conventional approaches.
[0016] According to the present invention, a system for determining
polarization profiles of points in a scene from video frames using
Stokes parameters is provided. The present invention generally
comprises an optical chopper controller, a motorized rotating
Stokes filter wheel, a narrowband color filter, a high speed video
camera, a computer system including a frame grabber, and software
to coordinate the operation of the optical chopper controller, the
frame grabber and the video camera, and a polarization calibration
standard.
[0017] Images corresponding to the scene light rays may be
projected onto respective pixels in the video frame and recorded as
two-dimensional (2-D) arrays. The images corresponding to the scene
light rays from four unique images obtained from light transmitted
consecutively and rapidly through three linear polarizers and a
circular polarizer of the rotating Stokes filter wheel are
generally used by programming in the computer system to calculate
respective Stokes parameters of the points in the scene.
[0018] Further, according to the present invention, a method of
reducing spatial and temporal errors in polarization profiles of
points in a scene from video frames using Stokes parameters that
are obtained via the system described above is provided.
[0019] The above features, and other features and advantages of the
present invention are readily apparent from the following detailed
descriptions thereof when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a diagram of a system for determining Stokes
parameters according to the present invention;
[0021] FIG. 2 is a diagram of the transmitted light from the scene
and the polarization standard through the system according to the
present invention; and
[0022] FIG. 3. is a diagram of a trigger sensor that may be
implemented in connection with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0023] The present application generally describes a system and a
method of determination of Stokes parameters for a given example.
However, one of ordinary skill in the art would understand that the
system and method described herein may be implemented to meet the
design criteria or to optimize the Stokes parameter determination
of any particular application using the more general formulae as
presented, for example, in U.S. patent application Ser. No.
11/158,357, filed Jun. 20, 2005 which, as noted previously, is
incorporated by reference in its entirety.
[0024] With reference to FIGS. 1-3, the preferred embodiments of
the present invention will now be described in detail. Generally,
the present invention provides an improved system and an improved
method for the determination of polarization profiles of points in
a scene from video frames using Stokes parameters. In particular,
the present invention may advantageously generate the Stokes
parameters more rapidly and accurately than conventional
approaches. The present invention generally includes an optical
chopper controller that is implemented in connection with a
mechanical device (e.g., a wheel rotated by a precision motor, the
wheel having apertures through which light rays pass) to form an
optical chopper.
[0025] The optical chopper controller generally performs at least
two operations. The optical chopper controller may be configured to
set (i.e., determine, establish, etc.) the rotation frequency of
the Stokes filter wheel (e.g., via a control signal, ROT) and may
also be configured to send pulsed trigger signals (e.g., TTL 5 volt
signals, TRIG) to a frame grabber apparatus and a video camera to
activate a shutter in the video camera. During each rotation of the
Stokes filter wheel, five circular trigger holes that are located
around the perimeter of the Stokes filter wheel generally pass
through a sensor.
[0026] The sensor may, in one example (see, FIG. 3), comprise an
infrared transmitter and an infrared receiver. As a trigger hole in
the Stokes filter wheel passes through 'the sensor, the trigger (or
synchronization) signal (or command) TRIG may be generated by the
sensor (i.e., the infrared receiver). The trigger signal may be
sent (transmitted) to (and received by) the video camera via the
optical chopper controller and the frame grabber. The voltage
associated with the synchronization/trigger signal generally
activates the shutter of the video camera to provide
synchronization of the video camera and the Stokes filter
wheel.
[0027] The trigger holes around the perimeter of the Stokes filter
wheel are generally substantially equally spaced and are positioned
such that a polarization element (e.g., a polarizer) that is
inlayed (i.e., inset, fixed, positioned, mounted, disposed, etc.)
in the Stokes filter wheel is present in the field of view of the
video camera when the shutter of the video camera is open. That is,
the light rays that are emitted from points in the scene are
optically chopped.
[0028] The Stokes filter wheel generally further includes five oval
shaped filter/reference apertures. The oval shaped filter/reference
apertures are generally disposed adjacent to a respective circular
trigger hole, and towards the center of the Stokes filter wheel.
The oval shaped filter/reference wheels are "large" (i.e., at least
three times larger in total area than the respective circular
holes).
[0029] The five large filter/reference apertures on the Stokes
filter wheel generally include, respectively, three linear
polarizers, one circular polarizer, and a reference screen. The
reference screen is generally implemented to identify a completed
revolution of the rotating Stokes filter wheel. The linear and
circular polarizers assembled in the Stokes filter wheel may have
any orientation; however, to simplify calculations and to optimize
results, when one of the linear polarizers enters the field of view
of the video camera, the orientation of the transmission axis of
the linear polarizer may be one of horizontal, vertical, and 45
degrees from the horizontal. When the circular polarizer appears in
the field of view of the video camera, the fast axis of the
retarder element of the circular polarizer may be vertical.
[0030] The large filter/reference apertures of the Stokes filter
wheel may be substantially oval shaped. The oval shaped
filter/reference apertures may reduce or eliminate the possibility
of a reduction in the field of view of the video camera during
exposure, as can occur when circular apertures are implemented to
hold the respective filters.
[0031] Each closure of the shutter of the video camera generally
sends (transmits) the recorded image (e.g., an image, VIDIM) to the
frame grabber apparatus. The frame grabber may format the image
according to preset (i.e., predetermined, pre-selected, chosen,
etc.) instructions in coordinating software and store the image in
memory of a computer.
[0032] The present invention may reduce temporal registration
errors when compared to conventional approaches for using Stokes
parameters by implementing a precision motor for the rotation of
the Stokes filter wheel. The degree of precision that is
implemented is generally selected to provide registration error
reduction to a predetermined level.
[0033] Because positive ellipticity is generally introduced into
the imagery when the light increases between exposures, and
negative ellipticity is generally introduced into the imagery when
the light decreases between exposures, the present invention may
include a polarization standard in the scene to determine the
validity of the data. The polarization standard generally comprises
a linear polarizer and a circular polarizer.
[0034] The present invention may reduce or eliminate spatial
registration errors when compared to conventional approaches by
implementing a single lens for imaging onto the image plane of the
video camera.
[0035] The system and method of the present invention generally
provide for determining polarization profiles of a scene via near
real time calculation of the respective Stokes parameters. The
present invention generally comprises an optical chopper
controller, the polarization calibration standard, the motorized
rotating Stokes filter wheel, a narrowband color filter, a high
speed video camera having a video frame, and a computer system
containing the frame grabber and respective software to coordinate
operation of the optical chopper controller, the frame grabber and
the video camera. The polarization calibration standard generally
comprises a calibrated linear polarizer and a calibrated circular
polarizer.
[0036] The motorized rotating Stokes filter wheel generally
comprises five trigger holes and five respective unique
filter/reference apertures with inlayed polarization elements. The
Stokes filter wheel is generally attached to a precision high speed
motor. An optical chopper controller generally supplies (i.e.,
provides, generates, etc.) a variable frequency control signal
(i.e., the signal ROT) to the Stokes filter wheel motor. Three of
the filters in the filter/reference apertures generally comprise
linear polarizers, each having a different (e.g., horizontal,
vertical, and 45 degrees) orientation with respect to light rays
received from the scene. A fourth filter in one of the
filter/reference apertures generally comprises a circular
polarizer, and the fifth filter/reference aperture generally
includes the reference screen which is used to indicate a completed
revolution of the rotating Stokes filter wheel.
[0037] The present invention generally calculates the four Stokes
parameters (i.e., the parameters S.sub.0, S.sub.1, S.sub.2, and
S.sub.3) for each individual image pixel using four two-dimensional
(2-D) images from the video camera (i.e., four video images, VIDIM)
and appropriate polarization determination equations (described in
detail below).
[0038] Light from the scene may be transmitted, generally as rays,
through the motorized rotating Stokes filter wheel. The rotating
Stokes filter wheel generally comprises three linear polarizers and
a circular polarizer that correspond to the four video images VIDIM
that are used to calculate the four Stokes parameters, and a blank
reference (i.e., a video image VIDIM that indicates a completed
revolution of the Stokes filter wheel). The light that exits the
Stokes filter wheel has generally been attenuated and may be
recorded using the video camera and computer system. The video
camera generally includes an array (generally a 2-D array) of light
receptors that operate to form respective pixels of a video frame
(i.e., the video frame that corresponds to the video image
VIDIM).
[0039] The high speed video camera generally determines the
intensity (i.e., level, amount, etc.) of the attenuation to the
light transmitted through the system of the present invention. The
present invention may generate uniquely attenuated images of the
light from the scene on each video frame.
[0040] Also, according to the present invention, a computer (e.g.,
processor, microprocessor, controller, etc.) may be implemented
with appropriate programming (e.g., software, firmware, and the
like) stored in suitable memory (e.g., RAM, PROM, EPROM, EEPROM,
flash memory, and the like ) to perform processes (i.e.,
instructions, routines, algorithms, steps, blocks, methods,
operations, equations, etc.) of the present invention including a
cropping (e.g., editing) process for the selection of corresponding
elements from each frame containing images of the scene. A
calibration equation may also be implemented via the computer
programming to convert respective pixel values to optical
densities, and the optical densities may be converted to relative
intensities.
[0041] Each scene image generally comprises a rectangular array of
pixel values (i.e., a matrix) that corresponds to the attenuated
intensities of the light that is transmitted through the motorized
rotating Stokes filter wheel. The calibration equation generally
converts respective pixel values to optical densities and to
relative intensities.
[0042] Four unique scene images VIDIM that are related to
(correspond to) the four filters in the oval shaped filter
apertures of the Stokes filter wheel are generally selected as the
basis for calculation of the Stokes parameters for each pixel in
the scene. Polarization parameters including the degree of
polarization, polarization azimuth angle, and polarization
ellipticity angle may be calculated for each pixel using the Stokes
parameters associated with each pixel value.
[0043] The present invention generally provides for the association
of red-green-blue (RGB) values with normalized values of the Stokes
parameters. The present invention generally provides for close to
(i.e., nearly, substantially, about, etc.) real time generation of
video images having changing polarization values. The total time
duration to acquire four unique images of a scene is generally
governed by (i.e., corresponds to, is related to, etc.) the frame
rate of the video camera in association with the Stokes filter
wheel rotational speed as determined by the control signal ROT. The
frame rate of the video camera is generally synchronized to
rotating speed (angular velocity) of the motorized rotating Stokes
filter wheel via the synchronization (or trigger) signal TRIG that
is generated by the optical chopper controller in connection with
the circular trigger holes in the Stokes filter wheel (i.e., the
signal ROT may be generated by the optical chopper controller, at
least in part, in response to the signal TRIG). When the calculated
polarization parameters that are obtained from the polarization
standard are determined to fail to match the respective known
values that have been predetermined in a laboratory under
controlled lighting conditions, temporal registration errors or
improper conditioning may have occurred and the data is generally
considered invalid.
[0044] The system and method of the present invention may be
advantageously implemented in connection with any appropriate
wavelength of light (e.g., visible, infrared, ultraviolet, etc.).
The system and method of the present invention may be
advantageously implemented in connection with any appropriate video
recording protocol or format (e.g., analog, digital, etc.). The
computer programming of the present invention may be configured to
provide for processing that includes cropping (editing), curve
fitting, mathematical calculations, manipulation of video frames,
pseudo-coloring of polarization parameters, and conversion of
images into at least one standard format.
[0045] Referring generally to FIGS. 1-3, in combination, a system
12 of the present invention is shown. The system 12 generally
comprises a computer system 10 having a frame grabber apparatus 11,
a high speed video camera 9, an optical chopper controller 7, a
sensor 25, a motor 42, and a rotating Stokes filter wheel 8. The
computer system 10 may be internally electrically coupled to the
frame grabber 11, the frame grabber apparatus 11 may be
electrically coupled to the optical chopper controller 7 and to the
video camera 9, and the chopper controller 7 may also be
electrically coupled to the sensor 25 and to the motor 42. The
computer system 10 generally contains appropriate programming to
perform the operations (i.e., processes, calculations, steps,
blocks, algorithms, etc.) of the present invention.
[0046] The system 12 may be implemented as a system for the
determination of polarization profiles of points in a scene 13 from
video frames 28 using Stokes parameters (i.e., the parameters
S.sub.0, S.sub.1, S.sub.2, and S.sub.3). The system 12 of the
present invention generally provides at least one system and method
for obtaining polarization profiles from the video camera 9. The
system 12 of the present invention generally produces (i.e.,
generates, provides, etc.) a sufficient number of unique scene
images 29 to obtain (i.e., generate, calculated, determine, etc.)
the respective four Stokes parameters in nearly (i.e., with very
insignificant time delay, substantially, almost, about,
approximately, etc.) real time.
[0047] The total time duration to obtain the four images generally
used to obtain the four Stokes parameters S.sub.0, S.sub.1,
S.sub.2, and S.sub.3 for each pixel in the scene is generally
regulated by the rotational speed (e.g., revolutions per minute,
RPM) of a motorized rotating Stokes filter wheel 8 and the frame
rate (e.g., frames per second, FPS) of the video camera 9. The
Stokes filter wheel 8 is generally mechanically coupled to and
rotated by the motor 42. The motor 42 is generally implemented as a
precision motor (i.e., a motor having rotational frequency
stability about 250 ppm/deg. C., and RMS phase jitter of less than
0.5% over all normal frequency ranges of operation). The rotational
speed of the motor 42 is generally determined (i.e., established,
set, chosen, etc.) in response to the control signal ROT.
[0048] The system 12 generally includes the optical chopper
controller 7, the motorized rotating Stokes filter wheel 8, the
high speed video camera 9, and the computer system 10 generally
includes the frame grabber apparatus 11. The system 12 generally is
used in connection with the scene 13. The scene 13 is generally
implemented having a horizontal axis, x, and a vertical axis, y,
that is perpendicular to the x axis. A z axis is generally mutually
perpendicular to the x and y axes. The system 12 generally further
comprises a polarization standard 30 that is implemented in
connection with (e.g., within a region of) the scene 13.
[0049] A user (e.g., operator, etc.) of the system 12 generally
desires to obtain polarization information regarding elements of
the scene 13. An optical chopper is generally implemented as a
mechanical or electromechanical device for passing and then
interrupting a beam of light for a known brief interval of time.
The optical chopper controller 7 and the rotating Stokes filter
wheel 8 generally comprise an implementation of an optical chopper.
The control signal ROT may be generated by the optical chopper
controller 7 in response to the frame rate of the video camera 9
and the synchronization trigger signal TRIG.
[0050] A frame grabber apparatus (i.e., subsystem, circuit, device,
etc.) is generally a component of a computer system that is
implemented to perform digitizing analog video signals. A typical
implementation of a frame grabber comprises a circuit to recover
the horizontal and vertical synchronization pulses from the input
signal, an analog-to-digital converter, a NTSC/SECAM/PAL color
decoder circuit (which may be implemented in software), a memory
for storing the acquired image (e.g., a frame buffer), a bus
interface to the main processor in the computer system for
acquisition control and data access. Frame grabbers such as the
apparatus 11 generally store and also compress video frames in
substantially real time using algorithms such as MPEG, JPG, TIF,
and the like, or any other appropriate format to meet the design
criteria of a particular application.
[0051] The frequency (i.e., rate of revolution, angular velocity,
etc.) of the rotating Stokes filter wheel 8 is generally selected
(e.g., chosen, determined, varied, controlled, adjusted, etc.) via
the signal ROT that is generated by the optical chopper controller
7 (in connection with the computer system 10) and sent to the
Stokes filter wheel 8 via an electrical connection 1 (e.g.,
electrical coupling, interface, wiring, cable, interconnect, etc.).
As such, the controller 7 may control the rotation of the Stokes
filter wheel 8 in response to the signal ROT which is generally a
variable frequency control signal. The optical chopper controller 7
and the computer system 10 may generate the control signal ROT such
that the angular velocity and angular acceleration of the rotating
Stokes filter wheel 8 are controlled within predetermined limits.
The rotational speed of the motor is generally set once, prior to
data acquisition, to meet the criteria of a particular
application.
[0052] The computer system 10 may be used to select a mode of
operation (e.g., internal triggering, external triggering, etc.)
provided as desired by the user, the region of interest (ROI, the
pixel area to be used in the exposure), and the exposure time of
the video camera 9 via a control or selection signal (e.g., a
control signal, MODE). The computer system 10 may be electrically
coupled to the frame grabber subsystem 11. The respective
selections MODE are generally transmitted to the frame grabber 11
via an internal electrical connection, and the frame grabber 11 may
transmit the respective selections via the control signal MODE to
the video camera 9 via an electrical connection 5.
[0053] The rotating Stokes filter wheel 8 generally comprises five
trigger holes (e.g., holes 15, 17, 19, 21, and 23), three linear
polarizers (e.g., polarizers 18, 20, and 22). that are generally
sequentially positioned about the Stokes filter wheel 8, a circular
polarizer (e.g., polarizer 16), and a reference screen (e.g., the
reference screen 24). When one of the trigger hole 15, 17, 19, 21,
and 23 passes between an infrared transmitter 37 and an infrared
receiver 38 that are implemented in connection with the sensor 25
(see FIG. 3), respective trigger signals TRIG may be generated and
sent to the optical chopper controller 7 via an electrical
connection 3.
[0054] The three linear polarizers 18, 20, and 22, the circular
polarizer 16, and the reference screen 24 are generally
consecutively mounted (e.g., inset, inlayed, fixed, etc.) into oval
(ellipsoidal) shaped filter/reference apertures in the Stokes
filter wheel 8. The oval shaped filter/reference apertures
generally have a smaller axis (e.g., axis SA) that is radially
disposed on the Stokes filter wheel 8, and a larger axis (e.g.,
axis LA) that is disposed in a circular band (e.g., a band centered
on a radius, RI) on the Stokes filter wheel 8.
[0055] The apertures that correspond to the filters/reference
screen 18, 20, 22, 16, and 24 are generally disposed radially
internally (towards the center) with respect to corresponding
(respective) trigger holes on the Stokes filter wheel 8. That is,
the circular polarizer 16 may be radially internal to the trigger
hole 15, the linear polarizer 18 may be radially internal to the
trigger hole 17, the linear polarizer 20 may be radially internal
to the trigger hole 19, the linear polarizer 22 may be radially
internal to the trigger hole 21, and the reference screen 24 may be
radially internal to the trigger hole 23. The trigger holes 15, 17,
19, 21, and 23 are generally disposed sequentially (consecutively)
on the Stokes filter wheel 8 in a circular band (e.g., a band
centered on a radius, RO) that is radially outward from the
circular band, RI, in which the filter/reference apertures 18, 20,
22, 16, and 24 are disposed sequentially (consecutively). The
trigger holes 15, 17, 19, 21, and 23 and the filter/reference
apertures 18, 20, 22, 16, and 24 are generally substantially
equally spaced around the Stokes filter wheel 8.
[0056] The optical chopper controller 7 generally sends a capture
control signal (e.g., CAPT) (e.g., a 5-volt TTL voltage, or,
alternatively any other appropriate signal to meet the design
criteria of a particular application) to the frame grabber 11 via
an electrical connection 4 when the optical chopper controller 7
receives the trigger signal TRIG from the sensor 25.
[0057] The computer program in the system 10 internally transmits
to the frame grabber apparatus 11 a frame grab control signal
(e.g., GRAB) that determines a selected number of video frames
VIDIM for capture by the frame grabber apparatus 11, and a starting
time for the capture of the video frames VIDIM.
[0058] The frame grabber apparatus 11 generally sends the capture
control signal CAPT to the video camera 9 via the electrical
connection 5 when the optical chopper controller 7 receives the
trigger signal TRIG from the sensor 25. The video camera 9
generally sends a captured video frame 28 (i.e., the image VIDIM)
to the frame grabber 11 via an electrical connection 6 when the
shutter of the video camera 9 closes. The electrical couplings 1,
3, 4, 5, and 6 generally transmit/receive the respective trigger,
control, and video image signals (e.g., the signals CAPT, MODE,
TRIG, ROT, and VIDIM). The frame grabber apparatus 11 generally
formats (e.g., generates an appropriate image file in a protocol
such as BMP, JPG, TIF, MPEG, etc.) the raw image frame data 28
(VIDIM) from the video camera 9 and saves the formatted image file
to a hard drive (not shown) in the computer system 10. The saved
(i.e., recorded, captured, held, stored, etc.) image file VIDIM is
generally processed via the computer system 10 to generate the
respective Stokes parameters, as detailed below.
[0059] Referring specifically to FIG. 2, the video camera 9
generally comprises a lens 26 and the video frame 28 that generally
includes the scene image 29. The video frame 28 generally comprises
an array of light sensitive receptors (e.g., CCD elements) that may
correspond to pixels in the scene 13. However, the video frame 28
may be implemented using any appropriate receptors to meet the
design criteria of a particular application. The polarization
standard 30 generally comprises a linear polarizer 31 and a
circular polarizer 32. The polarization standard 30 is generally
implemented in a region of the scene 13 that is peripheral to
points for which the user desires to generate corresponding
polarization profiles.
[0060] A region of the scene 13 generally presents (i.e., emanates,
sends, projects, emits, etc.) rays (i.e., beam, light, etc.) 14
that may be received by (i.e., transmitted to) the Stokes filter
wheel 8. The ray 14 may be a scene ray. The ray 14 is generally
emitted in the direction of the z axis. The user of the system 12
generally desires the polarization parameters related to
(corresponding to) the region of the scene 13 that presents the
rays 14.
[0061] The Stokes filter wheel 8 may present a respective ray 39 to
a narrowband color filter 40. The narrowband color filter 40 may
present a respective ray 41 to the lens 26 of the video camera 9.
The lens 26 of the video camera 9 may present a respective ray 27
to the video frame 28 as the respective scene image 29. Light rays
(e.g., the rays 14, 39, 41, and 27) are generally presented along
the optical axis of the system 12 (i.e., along the z axis).
[0062] With continued reference to FIG. 2, in connection with the
polarization standard 30, the linear polarizer (i.e., the
polarization standard linear polarizer) 31 may present a ray 33,
and the circular polarizer (i.e., the polarization standard
circular polarizer) 32 may present a ray 34. The rays 33, 34 are
generally a polarization standard linear polarizer ray and a
polarization standard circular polarizer ray, respectively. The
rays 33, 34 are generally transmitted through the Stokes filter
wheel 8 (i.e., through one of the polarizers 16, 18, 20, and 22),
through the color filter 40 and the video camera 9 (i.e., through
the lens 26) to the video frame 28, and may form respective
polarization standard images 35, 36 on the video frame 28. The
image 35 generally corresponds to the ray 33 and the image 36
generally corresponds to the ray 34. The images 35, 36 may be
integral to a peripheral region of the scene image 29.
[0063] Because the four unique images VIDIM that are used to
determine the Stokes parameters are not generally acquired
instantaneously (i.e., not substantially simultaneously), the
polarization standard 30 may be implemented (e.g., placed in the
scene 13) to provide for testing the validity of recorded data
(e.g., recorded versions of the video frame 28, VIDIM). Data is
generally rejected as invalid when the polarization parameters of
the standard (e.g., polarization parameters that correspond to the
polarization standard images 35, 36) such as degree of
polarization, azimuth angles, and ellipticity angles, fail to
correspond to respective known (e.g., predetermined, expected,
etc.) values (e.g., amounts, levels, etc.). The respective known
values are generally obtained (i.e., measured, predetermined, etc.)
in a laboratory under controlled lighting and other relevant
parameter conditions.
[0064] The linear and circular polarizers (e.g., the polarizers 18,
20, 22, and 16) assembled in the Stokes filter wheel 8 may have any
orientation when appearing in the field of view of video camera 9.
However, in one example, to simplify calculations and to optimize
results, the transmission axes of the linear polarizers and the
fast axis of the circular polarizer may have the following
orientations: (i) linear polarizer 22 may be horizontally oriented
when the trigger hole 19 is at the sensor 25, (ii) the transmission
axis of linear polarizer 20 may be vertically oriented when trigger
hole 17 is at the sensor 25, (iii) the transmission axis of linear
polarizer 18 may be oriented at an angle of 45 degrees to the
horizontal when trigger hole 15 is at the sensor 25, and (iv) the
fast axis of the circular polarizer may be vertically oriented when
trigger hole 23 is at the sensor 25.
[0065] With continued reference to FIG. 2, light from the scene 13
(e.g., the ray 14) is generally incident upon the Stokes filter
wheel 8. The ray 14 may be transmitted through one of the three
linear polarizers (i.e., the polarizers 18, 20, and 22), the
circular polarizer 16, or, alternatively, may be incident on the
opaque reference screen 24. The ray 14 is generally transmitted
through the linear polarizer 22, with the respective transmission
axis horizontally oriented, when the trigger hole 19 is at the
sensor 25. The ray 14 is generally transmitted through the linear
polarizer 20, with the respective transmission axis vertically
oriented, when trigger hole 17 is at the sensor 25. The ray 14 is
generally transmitted through the linear polarizer 18, with the
respective transmission axis oriented at an angle of 45 degree to
the horizontal, when the trigger hole 15 is at the sensor 25. The
ray 14 is generally transmitted through the circular polarizer 16,
with the respective fast axis horizontally oriented, when the
trigger hole 23 is at the sensor 25. As the reference screen 24 is
opaque, optical transmission through the reference screen 24 is
generally impossible. The respective blank video image VIDIM
related to the ray 14 that is generally incident on the reference
screen 24 may identify (indicate) a completed revolution of the
rotating Stokes filter wheel 8.
[0066] The light that exits from the rotating Stokes filter wheel
(e.g., the ray 39) may be transmitted through the narrowband color
filter 40. The light that exits from the narrowband color filter 40
(e.g., the ray 41) may be transmitted through the lens 26 of the
video camera 9. The light that exits from the lens of the video
camera 26 (e.g., the ray 27) is generally focused (or projected)
onto the video frame 28 to form (i.e., generate, produce, etc.) the
image 29 (and the respective video image VDIM) of the scene 13.
[0067] All of the video frames (e.g., the video frame 28) are
generally downloaded (as the respective video images VIDIM) into
the computer system 10 via the connection 6 and the frame grabber
11. The images 29 (i.e., the images VIDIM) may be stored (e.g.,
written to memory, held, etc.) for processing by the computer
(e.g., controller, processor, etc.) system 10 using appropriate
programming. Computer programming is generally stored on
appropriate media (i.e., memory such as RAM, ROM, PROM, etc., not
shown) in the computer system 10, and, in one example, implemented
to crop (e.g., edit, trim, etc.) selected (e.g., chosen, picked,
etc.) corresponding picture elements (pixels) from each 2-D image
29. The pixel values of each video frame 28 (e.g., the image 29)
VIDIM may form a matrix, M. The computer programming generally
identifies the pixel values of the video image VIDIM that
corresponds to the reference screen 24 as an indicator that the
rotating Stokes filter wheel 8 has completed a revolution and that
a new set of video images VIDIM are to be recorded.
[0068] The chopper controller 11 transmits the speed control signal
ROT to the motor 42 to control rotational speed of the motor 42,
and a computer program in the system 10 sends mode control signals
MODE to the video camera 9 to establish the mode of operation and
exposure time. The computer program in the computer 10 internally
transmits the frame grab control signal GRAB to the frame grabber
apparatus 11. The frame grab control signal GRAB determines a
selected number of video frames VIDIM for capture by the frame
grabber apparatus 11, and a starting time for the capture of the
video frames.
[0069] When the frame grabber 11 initiates video image capture via
the capture control signal CAPT, the chopper controller 7 receives
the trigger signal TRIG from the sensor 25, and generates the
capture control signal CAPT in response to the trigger signal TRIG,
and transmits the capture control signal CAPT to the frame grabber
apparatus 11, and the frame grabber apparatus 11 transmits the
capture control signal CAPT to the video camera 9 to activate
exposure, and the video camera 9 transmits the captured video image
VIDIM to the frame grabber 11 to be processed by the computer
program in the system 10.
[0070] Because the Stokes parameters generally use intensity, I,
measurements, and the light sensor array in the video camera 9
generally records RGB values, x, a relationship between x and I is
generally obtained for the light sensor array of all recorded
frames 28 (i.e., the images VIDIM). In one example, a calibration
method for obtaining the relationship between x and I is to pass a
beam (or ray) of light through neutral density filters of different
optical densities, y, and to record (or otherwise determine) an
average x value for each respective y value. One of ordinary skill
in the art would understand that the x and y of the intensity, I,
relationship (e.g., Equations 1-3 below) is generally different
from the relative Cartesian coordinates x and y. Curve-fitting
(e.g., performed using programming via the computer system 10)
generally yields y as a function of x as follows. y=f(x) (Equation
1)
[0071] Because some light sensor arrays may be implemented as
multi-channel arrays, a relationship between RGB values, x, and
optical densities, y, is generally obtained for each channel. The
optical density, y, is generally related to the intensity, I, using
the relationship as follows. I=10.sup.-y (Equation 2)
[0072] Equation 1 may be substituted into the Equation 2 to yield a
calibration relationship between I and x as follows: I=10.sup.-f(x)
(Equation 3)
[0073] Using Equation 3, each pixel value, x, in each of the scene
matrices M.sub.1, M.sub.2, M.sub.3 . . . M.sub.n can be converted
to a respective intensity value in new intensity matrices I.sub.1,
I.sub.2, I.sub.3 . . . I.sub.n.
[0074] Four consecutive intensity matrices (e.g., I.sub.1, I.sub.2,
I.sub.3, I.sub.4) may be used to obtain the Stokes parameters for
each point in the scene 13. Equation 4 may be used to calculate the
Stokes parameters for each point in scene 13 as follows:
S.sub.0=I.sub.2+I.sub.2 S.sub.1(.delta./sin .delta.)[(sin
.delta.+cos .delta.)I.sub.1+(sin .delta.-cos .delta.)I.sub.2-2 sin
.delta.I.sub.3] S.sub.2=(.delta./sin .delta.)[(sin .delta.-cos
.delta.)I.sub.1-(sin .delta.+cos .delta.)I.sub.2+2 cos
.delta.I.sub.3] S.sub.3=(S.sub.0+S.sub.2 cos
.epsilon.-2I.sub.4)/sin .epsilon. (Equation 4) where .delta. is the
angular rotation of the Stokes filter wheel during exposure.
.delta. is a function of the frame rate (F) of the Stokes filter
wheel and the exposure time (t) according to .delta.=72F.
t(degrees).
[0075] As .delta. approaches zero, sin .delta. approaches .delta.,
.delta./sin .delta. approaches 1, and S.sub.1=I.sub.1-I.sub.2
S.sub.2=2I.sub.3-I.sub.1-I.sub.2=2I.sub.3-S.sub.0
[0076] The equation for the fourth Stokes parameter, S.sub.3, as
given in Equation 4, is based on a left circular polarizer (i.e.,
the transmitted electric field vectors rotate counter clock-wise).
As circular polarizers such as the polarizer 16 are generally
implemented using birefringent materials (e.g., at least one
quarter-wave retarder), a phase difference, .epsilon., which is
wavelength, .lamda., dependent may occur between light traveling
along the fast and slow axes. The relationship between .epsilon.
and .lamda. is generally determined before the system 12 is used to
accurately determine the fourth Stokes parameter. A birefringent
material may be implemented as an optically anisotropic material
such as calcite and quartz that generally provides for the
splitting of a light wave into two transmitted waves having
different velocities.
[0077] Each of the corresponding elements in the matrices S.sub.0,
S.sub.1, S.sub.2, S.sub.3 (e.g., elements s.sup.(0).sub.11,
s.sup.(1).sub.11, s.sup.(2).sub.11, s.sup.(3).sub.11 are generally
associated with a point (x, y) (i.e., a point having Cartesian
coordinates with respective horizontal and vertical values x and y)
in the scene 13. Thus, the polarization state of points (x, y) in
the scene 13 can be determined (e.g., calculated using programming
that may be stored in a medium in the computer system 10) using the
relationships as follows. sin
2.sub..chi.=S.sub.3/(S.sub.1.sup.2+S.sub.2.sup.2
+S.sub.3.sup.2).sup.1/2 tan 2.psi.=S.sub.2/S.sub.1
P=(S.sub.1.sup.2+S.sub.2.sup.2+S.sub.3.sup.2).sup.1/2/S.sub.0,
(Equations 5)
[0078] Where .chi. is the polarization ellipticity angle, .psi. is
the polarization azimuth angle, and P is the degree of
polarization.
[0079] As is apparent then from the above detailed description, the
present invention may provide an improved system and an improved
method for acquiring sufficient data to rapidly measure the four
Stokes parameters, when compared to conventional approaches. The
present invention may reduce or eliminate temporal errors and
spatial errors that can be generated when conventional approaches
are implemented.
[0080] Various alterations and modifications will become apparent
to those skilled in the art without departing from the scope and
spirit of this invention and it is understood this invention is
limited only by the following claims.
* * * * *