U.S. patent application number 17/001037 was filed with the patent office on 2022-08-18 for compact imaging system using a co-linear, high-intensity led illumination unit to minimize window reflections for background-oriented schlieren, shadowgraph, photogrammetry and machine vision measurements.
The applicant listed for this patent is United States Of America As Represented By The Administrator Of Nasa. Invention is credited to Brett F. Bathel, Stephen B. Jones, Joshua M. Weisberger.
Application Number | 20220263995 17/001037 |
Document ID | / |
Family ID | |
Filed Date | 2022-08-18 |
United States Patent
Application |
20220263995 |
Kind Code |
A9 |
Bathel; Brett F. ; et
al. |
August 18, 2022 |
COMPACT IMAGING SYSTEM USING A CO-LINEAR, HIGH-INTENSITY LED
ILLUMINATION UNIT TO MINIMIZE WINDOW REFLECTIONS FOR
BACKGROUND-ORIENTED SCHLIEREN, SHADOWGRAPH, PHOTOGRAMMETRY AND
MACHINE VISION MEASUREMENTS
Abstract
One aspect of the present disclosure is an imaging system
including an optical sensor defining an optical axis. The system
further includes a light source. The system may include an optical
beam splitter, and may also include an optional diffusing lens that
may be configured to diffuse and/or collimate light from the light
source and direct light exiting the diffusing lens to the optical
beam splitter. The optical beam splitter is configured to direct
light from the light source along the optical axis of the optical
sensor.
Inventors: |
Bathel; Brett F.; (Yorktown,
VA) ; Jones; Stephen B.; (Newport News, VA) ;
Weisberger; Joshua M.; (Newport News, VA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
United States Of America As Represented By The Administrator Of
Nasa |
Washington |
DC |
US |
|
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Prior
Publication: |
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Document Identifier |
Publication Date |
|
US 20210058541 A1 |
February 25, 2021 |
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|
Appl. No.: |
17/001037 |
Filed: |
August 24, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62890161 |
Aug 22, 2019 |
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62935516 |
Nov 14, 2019 |
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63024958 |
May 14, 2020 |
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International
Class: |
H04N 5/235 20060101
H04N005/235; G02B 27/14 20060101 G02B027/14; G03B 15/05 20060101
G03B015/05; H04N 5/225 20060101 H04N005/225; H04N 5/247 20060101
H04N005/247; G06T 11/00 20060101 G06T011/00; H04N 7/18 20060101
H04N007/18 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Joint Government, Large Business, Small business or
Nonprofit Organization Invention: The invention described herein
was made in the performance of work under NASA contracts and by an
employee/employees of the United States Government and is subject
to the provisions of the National Aeronautics and Space Act, Public
Law 111-314, .sctn. 3 (124 Stat. 3330, 51 U.S.C. Chapter 201) and
35 U.S.C. .sctn. 202, and may be manufactured and used by or for
the Government for governmental purposes without the payment of any
royalties thereon or therefore. In accordance with 35 U.S.C. .sctn.
202, the contractor elected not to retain title.
Claims
1. An imaging system comprising: an imaging lens; an optical sensor
defining an optical axis; a light source; an optical beam splitter;
and a diffusing lens configured to perform at least one of
diffusion and collimation of light from the light source and direct
light exiting the diffusing lens to the optical beam splitter;
wherein the optical beam splitter is configured to direct light
from the diffusing lens along the optical axis of the optical
sensor.
2. The imaging system of claim 1, wherein: the light directed along
the optical axis by the optical beam splitter has a minimum
cross-sectional size that is about equal to a size of the diffusing
lens.
3. The imaging system of claim 2, wherein: the optical sensor
comprises a digital camera including an imaging lens defining a
lens diameter; and the light directed along the optical axis has a
minimum diameter that is at least as large as the lens
diameter.
4. The imaging system of claim 1, wherein: The optical beam
splitter comprises a 50/50 beam-splitting cube.
5. The imaging system of claim 1, wherein: the light source
comprises one or more LEDs that are configured to produce a
short-duration, high-intensity illumination pulse.
6. The imaging system of claim 5, wherein: the LED light source is
configured to produce an illumination pulse of less than about 10
microseconds.
7. The imaging system of claim 1, wherein: the optical sensor, the
light source, the optical beam splitter, and the diffusing lens are
rigidly interconnected to form an imaging unit.
8. The imaging system of claim 7, wherein the imaging unit is a
plurality of imaging units, each imaging unit of the plurality of
imaging units including an optical sensor, a light source, and an
optical beam splitter, wherein the optical axes of the imaging
units are radially spaced about a test region having a fluid
disposed therein; and the system further comprising: at least one
background pattern aligned with each optical axis, whereby at least
some light from the light source of each imaging unit is reflected
back to the optical sensor of each imaging unit, whereby the
optical sensors capture images of the background patterns, wherein
the images from each imaging unit comprises a 2-dimensional BOS
image, whereby the synchronous images can be processed to provide a
tomographic reconstruction.
9. The imaging system of claim 8, including: a controller
configured to simultaneously actuate the optical sensors and light
sources of each imaging unit.
10. The imaging system of claim 9, wherein: the optical sensors
comprise digital cameras; the light sources comprise LED light
sources; and the controller comprises electrical circuitry that is
configured to generate a camera actuation signal to the digital
camera followed by an actuation signal to the LED light sources
whereby the electrical circuitry compensates for an actuation time
delay of the digital camera relative to the LED light sources and
causes the LED light sources to generate a pulse of light that is
reflected back to the digital cameras when the digital cameras are
actuated.
11. The imaging system of claim 8, wherein: the optical sensors
comprise digital cameras; the light sources comprise LED light
sources; each imaging unit includes a housing, and the digital
camera, LED light source, and optical beam splitter of each imaging
unit are supported by the respective housing; and each imaging unit
further including an adjustable bracket having a first part
connected to the housing, and a base, wherein the first part is
adjustably connected to the base, whereby the first part can be
rotated and translated relative to the base about three axes to a
selected position.
12. The imaging system of claim 8, wherein the test region is a
test region of a wind tunnel having side walls disposed about an
interior space that includes the test region, the side walls
including light-transmittal material forming windows, wherein the
optical axis of each imaging unit is aligned with a window, and
wherein the imaging units are disposed outside of the wind tunnel
to capture images of material in the test region.
13. The imaging system of claim 1, wherein: the optical sensor
comprises a CMOS device; the light source comprises a green or red
LED; and the optical beam splitter comprises a 50/50 beam-splitting
plate.
14. An imaging system comprising: a plurality of imaging units,
each imaging unit including: a digital camera defining an optical
axis; a light source configured to generate light traveling
transverse relative to the optical axis of the digital camera; an
optical beam splitter configured to couple light from the light
source and direct a coaxial beam of light along the optical axis of
the digital camera; a substantially rigid structure interconnecting
the digital camera, the light source, and the optical beam
splitter; and a controller configured to actuate the digital camera
and the light source of the units in a substantially simultaneous
manner; wherein the imaging units are disposed about a test region
with the optical axes of the digital cameras extending through the
test region.
15. The imaging system of claim 14, wherein: each imaging unit
includes an aperture positioned between the light source and the
optical beam splitter to block a portion of the light from the
light source whereby light traveling through the aperture reaches
the optical beam splitter and the coaxial beam of light is suitable
for producing a shadowgraph.
16. The imaging system of claim 15, wherein: each imaging unit
includes a diffusing lens positioned between the light source and
the optical beam splitter whereby light from the light source
passes through the aperture and the diffusing lens.
17. The imaging system of claim 14, wherein: a substantially rigid
structure of each imaging unit comprises a housing and an
adjustable mount that permits three-axis rotation and three-axis
translation of the digital camera relative to a base.
18. A method of generating images, the method comprising: providing
a plurality of imaging units, each imaging unit including a digital
camera defining an optical axis, a light source, and an optical
beam splitter; using the optical beam splitter to cause light from
each light source to be coupled onto the optical axis of each
digital camera in the form of a coaxial beam; positioning the
imaging units about a test space; causing the coaxial beams of the
imaging units to pass through a substance in the test space;
causing the coaxial beams to reflect back to the digital cameras
from background patterns; and processing data from the digital
cameras to generate tomographic background-oriented schlieren
images having features corresponding to pressure gradients of the
substance.
19. The method of claim 18, including: positioning the imaging
units around a wind tunnel having a plurality of windows comprising
light transmitting material; and causing the coaxial beams to pass
through the windows.
20. The method of claim 18, including: activating the light sources
of each imaging unit at substantially the same time to provide
simultaneous pulses of light; and actuating the digital cameras of
each imaging unit at substantially the same time, whereby the
digital cameras capture light reflected back from the background
patterns.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATION(S)
[0001] This patent application claims the benefit under 35 U.S.C.
.sctn. 119(e) to U.S. Provisional Patent Application No.
63/024,958, filed May 14, 2020, entitled "COMPACT IMAGING SYSTEM
USING A CO-LINEAR, HIGH-INTENSITY LED ILLUMINATION UNIT TO MINIMIZE
WINDOW REFLECTIONS FOR BACKGROUND-ORIENTED SCHLIEREN AND MACHINE
VISION MEASUREMENTS," which is incorporated by reference in its
entirety for any and all non-limiting purposes.
BACKGROUND
[0003] Various imaging techniques have been developed to produce
images showing variations in density of a gas or other material
having density gradients. These imaging techniques may be used in
wind tunnels or other creations involving flow of air and/or other
materials. Known imaging techniques include schlieren and
shadowgraph imaging techniques. In known schlieren techniques, a
direct line-of-sight through the test section, a set of high
quality lenses or mirrors, and a point illumination source are all
typically required for a measurement to be performed.
Background-oriented schlieren techniques (BOS) have also been
developed. BOS techniques may be sensitive to gradients in density.
This sensitivity comes from measurement of small distortions in an
image background pattern captured with and without the density
gradients present in the test section (where the test section
generally refers to anything between the camera and the background
pattern). The relatively straight-forward equations that govern the
apparent distortion absorbed in the image-background pattern make
the BOS technique capable of providing quantitative information
pertaining to the visualized density field. BOS differs from
conventional schlieren techniques, where the imaged intensity
variations are related to the density gradients in the test
section. The use of a background pattern and camera permit the BOS
technique to be scaled to an arbitrary field-of-view. Although this
scalability may be achieved in some cases using direct shadowgraph,
the shadowgraph method is primarily a qualitative tool.
Multi-camera schlieren tomographic systems for wind tunnel
measurements have also been developed. However, known imaging
systems and techniques may suffer from various drawbacks.
BRIEF SUMMARY
[0004] One aspect of the present disclosure is an imaging system
that may be utilized for BOS imaging applications, shadowgraph,
photogrammetry, machine vision applications, and other
applications. The system includes an optical sensor, such as a
digital camera defining an optical axis. The system further
includes a light source. The light source may comprise an LED or
other suitable device. The system further includes an optical beam
splitter, and may include an optional diffusing lens that is
configured to diffuse and concentrate light from the LED light
source and direct light exiting the diffusing lens to the optical
beam splitter. The optical beam splitter is configured to direct
light from the light source along the optical axis of the digital
camera.
[0005] Light coupled onto the optical axis by the optical beam
splitter may have a minimum cross-sectional size that is about
equal to a size of the diffusing lens. The digital camera may
include an imaging lens defining a lens diameter, and the light
coupled onto the optical axis may have a minimum diameter that is
at least as large as the lens diameter. This aspect of the design
minimizes the presence of shadows projected onto the image plane,
which may arise from refractive index gradients in the measurement
region. The optical beam splitter may comprise a 50/50
beam-splitting plate, cube, or other suitable beam-splitting
device. The light source may be configured to produce a
short-duration, high-intensity illumination pulse. The duration may
be less than about 10 microseconds, less than about 5 microseconds,
or less than about 1 microsecond in certain embodiments, however,
those of ordinary skill in the art will appreciate that other
durations are within the scope of this disclosure. The duration and
intensity of light may be set (adjusted) as required or beneficial
for a particular application, and in certain embodiments the
intensity may be substantially continuous. In general, shorter
light pulses may be utilized, if required or beneficial, for higher
flow velocities in high speed wind tunnels or other
applications.
[0006] The digital camera, light source, optical beam splitter, and
diffusing lens may be rigidly interconnected to form an imaging
unit. A plurality of imaging units, each including a digital
camera, a light source (e.g. LED), and an optical beam splitter may
be utilized if required or beneficial for a particular application.
The optical axes of the imaging units may be radially spaced about
a test region. A system may include at least one background pattern
aligned with each optical axis, whereby at least some light from
the light source of each imaging unit is reflected back to the
digital camera of each imaging unit, whereby the digital cameras
capture images of the background patterns. The images include
variations due to density gradients in a fluid in the test region,
whereby the images can be processed to provide background-oriented
schlieren (BOS) images. Each camera may be configured to provide a
2-dimensional BOS image, and synchronous images may be inputted to
a tomographic algorithm to back out a tomographic
reconstruction.
[0007] The system may include a controller, such as an electrical
circuit programmable controller, or other suitable device. The
controller may be configured to simultaneously actuate the digital
cameras of each imaging unit, and to simultaneously actuate the
light sources of each imaging unit. The controller may, optionally,
be configured to generate a camera actuation signal to the digital
cameras followed by an actuation signal to the light sources,
whereby the electrical circuitry compensates for a time delay of
the digital cameras relative to the light sources, and causes the
light sources to generate a pulse of light in a manner that is
synchronized with actuation of the cameras.
[0008] Each imaging unit may include a support structure, such as a
housing, and the digital camera, light source, and optical beam
splitter may be supported by the housing. Each imaging unit may be
supported by an adjustable bracket having a first part and a base
part. In accordance with one embodiment, the first part of the
adjustable bracket may be rigidly connected to the housing. The
first part of the adjustable bracket may also be adjustably
connected to the base part, such that the first part can be rotated
and translated relative to the base about three axes to a selected
position. The imaging units may be utilized in a wind tunnel having
sidewalls disposed about the test region. The sidewalls of the wind
tunnel may include light-transmitting material forming windows, and
the optical axis of each imaging unit may be aligned with an
opening. The imaging units may be disposed outside of the wind
tunnel with the optical axis of each camera passing through a
window of the wind tunnel. The digital camera may optionally
comprise a CMOS or CCD device, the light source may optionally
comprise a blue, green, or red LED, and the optical beam splitter
may comprise a 50/50 beam-splitting plate or cube. Alternatively,
the light source may comprise a U.V. light source (e.g., LED). It
will be understood that virtually any suitable light source may be
utilized as required or beneficial for a particular
application.
[0009] Another aspect of the present disclosure is an imaging
system including a digital camera defining an optical axis and a
light source that is configured to generate light traveling
transverse relative to the optical axis of the digital camera. The
imaging system further includes an optical beam splitter that is
configured to couple light from the light source and direct a
coaxial beam of light along the optical axis of the digital
camera.
[0010] The imaging system may include an aperture positioned
between the light source and the optical beam splitter to block a
portion of the light from the light source such that only light
traveling through the aperture reaches the optical beam splitter.
The coaxial beam of light may be suitable for producing
shadowgraphs.
[0011] Optionally, a diffusing lens may be positioned between the
light source and the optical beam splitter, whereby light from the
light source passes through the aperture and through the diffusing
lens.
[0012] The imaging system may optionally include a substantially
rigid support structure which may be in the form of a module,
bracket, or housing. The digital camera, the light source, and the
optical beam splitter may be secured to the substantially rigid
housing to form an imaging unit.
[0013] Another aspect of the present disclosure is a method of
generating images. The method includes providing a plurality of
imaging units, each imaging unit including a digital camera
defining an optical axis, a light source, and an optical beam
splitter. The beam splitter is used to cause light from each light
source to be coupled onto the optical axis of each digital camera
in the form of a coaxial beam. The imaging units are positioned
about a test space, and the coaxial beams of imaging units pass
through a substance in the test space. The coaxial beams are
reflected back to the digital cameras from background patterns.
Data from the digital cameras may be processed to generate
2-dimensional or 3-dimensional tomographic background-oriented
schlieren (BOS) images having features corresponding to pressure
gradients of the substance. Alternatively, the data may be
processed to provide 2-dimensional shadowgraphs or photogrammetry
images.
[0014] The method may include positioning imaging units around a
wind tunnel having a plurality of windows comprising
light-transmitting material. The coaxial beams may pass through the
windows into a gas or other substance in the test space.
[0015] The method may include activating the light sources of each
imaging unit at substantially the same time to provide simultaneous
pulses of light. The method may also include actuating the digital
cameras of each imaging unit at substantially the same time,
whereby the digital cameras capture light reflected back from the
background patterns.
[0016] One or more embodiments of the present disclosure may
optionally provide one or more of the following benefits or
advantages: [0017] Co-axial illumination minimizes reflections from
windows in a wind tunnel. [0018] Co-axial illumination minimizes
the presence of shadows cast by a wind tunnel model on background
panels. [0019] Co-axial illumination provides for a high amount of
returned light to the camera when using a retro-reflective
background. [0020] Use of a condensing lens with a diffuser
enlarges the apparent size of the light source such that it is
generally of the same scale as the lens entrance aperture to
minimize shadowgraph effects if required. [0021] The compact nature
of the imaging units allows for measurements in wind tunnel
facilities with severe working space restrictions.
[0022] These and other features of various embodiments will be
further understood and appreciated by those skilled in the art by
reference to the following specification, claims, and appended
drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0023] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0024] FIG. 1 is a partially schematic isometric view of a wind
tunnel;
[0025] FIG. 1A is a fragmentary, schematic top plan view of the
wind tunnel of FIG. 1;
[0026] FIG. 1B is an isometric schematic of a camera
field-of-view;
[0027] FIG. 2A is a perspective view of an imaging unit according
to one aspect of the present disclosure;
[0028] FIG. 2B is an isometric view of the imaging unit of FIG.
2A;
[0029] FIG. 2C is an isometric view of the imaging unit of FIG.
2A;
[0030] FIG. 2D is an isometric view of a portion of the imaging
unit of FIG. 2A;
[0031] FIG. 2E is a cross sectional isometric view of a portion of
the imaging unit of FIG. 2D;
[0032] FIG. 3 is a partially fragmentary isometric view of a
plurality of imaging units in a test configuration;
[0033] FIG. 4 is a partially fragmentary isometric view of an
imaging unit;
[0034] FIG. 5 is a partially fragmentary isometric view of an
imaging unit;
[0035] FIG. 6 is a partially fragmentary isometric view of an
imaging unit;
[0036] FIG. 7 is a partially fragmentary isometric view of an
imaging unit;
[0037] FIG. 8 is a partially fragmentary isometric view of an
imaging unit;
[0038] FIG. 9 is a partially fragmentary isometric view of an
imaging unit with an acrylic window positioned in front of the
imaging unit;
[0039] FIG. 10 is a resulting image with the acrylic window of FIG.
9 in place;
[0040] FIG. 11 is a resulting image with the acrylic window of FIG.
9 in place when the camera horizontal angle is 0 and the pitch
angle is approximately 8 degrees;
[0041] FIG. 12 is a resulting image with the acrylic window of FIG.
9 in place when the camera is aligned approximately normal to the
surface of the acrylic window of FIG. 9;
[0042] FIG. 12A is an image taken with a camera wherein the optical
axis is oriented at 0 degrees incidence relative to an acrylic
sheet;
[0043] FIG. 12B is an image taken with a camera wherein the optical
axis is oriented at 4 degrees off normal incidence relative to an
acrylic sheet;
[0044] FIG. 12C is an image taken with a camera wherein the optical
axis is oriented at 10 degrees off normal incidence relative to an
acrylic sheet;
[0045] FIG. 13 is a partially fragmentary isometric view of a
camera and heat gun configured to provide 2-dimensional BOS
images;
[0046] FIG. 14 is a shadowgraph image taken using the setup of FIG.
13;
[0047] FIG. 15 is a 2-dimensional BOS image formed utilizing the
setup of FIG. 13;
[0048] FIG. 16 is a camera image of a model in a wind tunnel taken
by a first camera;
[0049] FIG. 17 is a camera image of a model in a wind tunnel taken
by a second camera;
[0050] FIG. 18 is plot showing model position in the wind tunnel Y
and Z coordinates in time based on measurements made with the first
camera; and
[0051] FIG. 19 is plot showing model position in the wind tunnel Y
and Z coordinates in time based on measurements made with the
second camera.
DETAILED DESCRIPTION OF THE INVENTION
[0052] For purposes of description herein, the terms "upper,"
"lower," "right," "left," "rear," "front," "vertical,"
"horizontal," and derivatives thereof shall relate to the invention
as oriented in FIG. 1. However, it is to be understood that the
invention may assume various alternative orientations and step
sequences, except where expressly specified to the contrary. It is
also to be understood that the specific devices and processes
illustrated in the attached drawings, and described in the
following specification, are simply exemplary embodiments of the
inventive concepts defined in the appended claims. Hence, specific
dimensions and other physical characteristics relating to the
embodiments disclosed herein are not to be considered as limiting,
unless the claims expressly state otherwise. Further, although
reference is made to commercially-available devices, such
references are made solely to provide the reader with context of
certain embodiments, and such reference is not to be implied as an
endorsement or promotion of any commercial product, service, or
activity.
[0053] A wind tunnel 1 (FIGS. 1 and 1A) may include sidewalls 2A,
2B, a sidewall or ceiling 3, and a sidewall or floor 4 positioned
around a test region or space 5. FIG. 1 is a view looking
downstream along the centerline (CL) of the wind tunnel 1. In use,
fluid (e.g., air) flows through the wind tunnel 1 in a known
manner. For example, the wind tunnel 1 may comprise an existing 11
foot.times.11 foot transonic wind tunnel. Alternatively, wind
tunnel 1 may comprise a hypersonic wind tunnel for testing
hypersonic flow. It will be understood, however, that the present
disclosure is not limited to wind tunnels, or to a specific type of
wind tunnel.
[0054] Example wind tunnel 1 includes a pair of side windows 6 and
a plurality of round windows in the form of portholes 7. Wind
tunnel 1 may also include a window 6H in ceiling 3 and a window 6A
in floor 4. The side windows 6 and portholes 7 may include
light-transmitting material (e.g., acrylic polymer, glass, etc.)
covering openings of the windows 6 and portholes 7 to thereby
prevent flow of air from the test space 5 to an exterior space such
as plenum 8 (see, e.g., FIG. 1A). As discussed in more detail
below, one or more imaging units 10A-10H including cameras 20A-20H,
respectively, may be mounted in plenum 8. Plenum 8 may be formed
between an outer barrier 9 and the sidewalls 2A, 2B, ceiling 3 and
floor 4 of wind tunnel 1. The plenum 8 of some wind tunnels may be
relatively small, such that there is limited space available for
mounting of imaging units 10 in the plenum 8. The plenum 8 may
experience low pressures and high temperatures during operation of
the wind tunnel 1. The sidewalls, windows, portholes, and plenum
may comprise features of existing wind tunnels. However, it will be
understood that not all wind tunnels include a plenum 8, and the
operating conditions in the space around the interior test space 5
of a given wind tunnel may vary depending upon the wind tunnel
design. Thus, the present disclosure is not limited to any specific
wind tunnel configuration or design.
[0055] Imaging units 10A-10H may be positioned in plenum 8 adjacent
one or more windows and/or portholes to collect image data of
gasses or other material in test space 5 during operation of wind
tunnel 1. Each imaging unit 10A-10H may comprise on imaging unit 10
(FIGS. 2A, 2B). Imaging units 10A-10H may (optionally) be
configured to provide a tomographic background-oriented schlieren
(BOS) system defining a tomographic reconstruction
region-of-interest (ROI) 15 in test space 5. The ROI 15 may be
generally spherical or other shape. It will be understood that the
spherical shape of an ROI is used to illustrate the general concept
of an ROI, but the ROI does not necessarily have a precisely
defined size or shape. As discussed below in connection with FIGS.
13-15, a single imaging unit 10 may be utilized to provide
conventional 2-dimensional BOS or shadowgraph capabilities. Also,
as discussed below in connection with FIGS. 16-19, one or two
imaging units 10 may be used to provide tracking of a target on a
wind tunnel model (photogrammetry).
[0056] Referring again to FIG. 1, in the illustrated example, the
ROI 15 is offset from the centerline (CL) of interior test space 5
to accommodate an aerodynamic model 34 (see FIG. 1A) positioned on
centerline CL of wind tunnel 1. However, the imaging units 10A-10H
may be configured to utilize an ROI 15 at different locations
within the interior space 5 as required or beneficial for a
particular application. As discussed in more detail below, the
imaging units 10A-10H include optical sensors such as digital
(e.g., CMOS) cameras 20A-20H, respectively, having fields-of-view
12A-12H (FIG. 1B). The fields-of-view 12A-12H and corresponding
optical axes 13A-13H of each camera 20A-20H are generally disposed
about center 16 of ROI 15, whereby the optical axes 13A-13H are
radially disposed about the center 16 of ROI 15. Fields-of-view
12A-12H may have a rectangular shape as shown in FIG. 1B. As
discussed in more detail below, backgrounds, such as panels
17A-17H, may be positioned in test space 5. Panels 17A-17H may be
aligned with corresponding optical axes 13A-13H, respectively. As
discussed in more detail below, surfaces 18A-18H of panels 17A-17H,
respectively, may comprise optical patterns that are utilized to
generate BOS images. Images captured by the cameras 20A-20H of
imaging units 10A-10H of panels 17A-17H, respectively, vary due to
density gradients in fluid (e.g., air) flowing through ROI 15. It
will be understood that variations result from effects of density
gradients over the complete distance between the camera and
background, and the variations captured by the imaging units 10 are
not limited to a small measurement volume (e.g., spherical ROI 15).
These images may be processed utilizing known BOS software to
provide graphic reconstruction images corresponding to the pressure
gradients of the fluid in the ROI 15 during operation of wind
tunnel 1.
[0057] With reference to FIGS. 2A-2E, each imaging unit 10 may
include a camera 20 (any image capturing device. In one embodiment,
a Balser acA1920-40 gm, commercially available from Basler AG with
a 25 mm focal length lens 21 (e.g., M118FM25, commercially
available from Tamron USA, Inc. or other suitable lens). Smaller or
larger focal lengths (e.g., 50 mm, or more) may be utilized as
required or beneficial for a particular application. The lens 21
may be optically aligned with a beam-splitting device 22. The
beam-splitting device 22 may comprise a 50/50 beam splitter cube
(e.g., CCM1-BS013, commercially available from Thorlabs Inc.), beam
splitting plate, or other suitable beam splitting device. A light
source 24 and condenser-diffuser lens 25 are disposed inside a
housing 23 above beam-splitter 22 as shown in FIGS. 2D and 2E.
Support structure or housing 23 may include a first component 23A
that is secured to a second component 23B by threaded fasteners 47A
(FIG. 2C) to retain LED 24, Printed Circuit Board ("PCB") 24A, and
heat sink 27. PCB 24A may include metal conductors 24B that are
configured to electrically connect PCB 24A and LED 24 to a driver
circuit 30 (FIGS. 2A-2C). Bosses/fastening features 63 threadably
receive standoffs 29 (FIGS. 2A, 2B) to retain cover 28 and driver
circuit 30. Housing 23 may also include a third component 23C that
is secured to second component 23B by threaded fasteners 47B (FIGS.
2D and 2E). Third component 23C may be configured to retain lens 25
in optical alignment with LED 24. Openings 64 in third component
23C are configured to receive connectors (e.g., threaded fasteners)
to secure housing 23 to beam splitter 22. It will be understood
that support structure/housing 23 may have virtually any suitable
configuration, and the present disclosure is not limited to any
specific embodiment or arrangement.
[0058] Light source 24 preferably comprises one LED in accordance
with a particular embodiment disclosed herein. However, a plurality
of LEDs may also be utilized. As discussed in more detail below,
light from light source 24 may be an LED light source and is
coupled onto optical axis 13 of camera 20 to produce a beam of
light 26 that is generally coaxial with optical axis 13 of camera
20 to thereby provide light for optical imaging without requiring
lights that are offset from optical axis 13. In general, if a light
source is offset from optical axis 13, the light may reflect from
the light-transmitting material covering the windows 6 and
portholes 7, which interferes with the imaging process. A
condenser-diffuser lens 25 may be utilized, and comprise virtually
any suitable lens. In a non-limiting embodiment, lens 25 may
comprise a 1500 grit aspheric lens (e.g., Thorlabs,
ACL2520U-DG15-A). Light source 24 may comprise virtually any device
(e.g., an LED) that is capable of producing a high intensity pulse
of light for a short period of time (e.g., CBT-120-R-C11-HK101,
commercially available from Luminous Devices, Inc.). It will be
understood that the present disclosure is not limited to those
specific components. For example, lens 25 may comprise a fine grit
diffuser and aspheric condenser lens or other suitable components.
Although light source 24 may comprise an LED, any suitable light
source may be utilized.
[0059] The imaging unit 10 further includes a controller, such as
PCB-based LED driver circuit 30 which may be mounted to the housing
23. Imaging unit 10 may also include a protective cover 28 that is
secured to the housing 23 by standoffs 29. Driver circuit 30 may be
configured to account for differences in actuation lag time of
camera 20 relative to LED light source 24. For example, camera 20
may have a lag time upon time receiving an actuation signal that is
greater than a lag time of the LED light source 24, and driver
circuit 30 may be configured to generate an actuation signal to the
camera 20 before generating an actuation signal to the LED light
source 24 such that the camera is actuated to generate an image
during a light pulse from LED light source 24. The PCB 24A and LED
light source 24 may (optionally) be directly connected to the
driver circuit 30 with spade connectors to minimize the path length
between the LED light source 24 and the power supply of driver
circuit 30. This may facilitate over-driving LED light source 24
with high electrical current and short pulse duration as may be
required in some applications. Light from the LED light source 24
is diffused by the condenser-diffuser lens 25 to minimize the
appearance of patterns that may be present on the emitting surface
of the LED light source 24. The condenser-diffuser lens 25 also
increases the apparent size of the LED light source 24 to minimize
any potential shadowgraph effects. However, it will be understood
that the condenser-diffuser lens 25 is optional. The light 26 is
coupled onto the optical axis 13 of the camera 20, and the light
reflects off the background pattern (e.g., surfaces 18 of panels
17, FIG. 1) and straight back into the camera 20 for image
acquisition. An optional angled neutral density filter 31 (e.g.,
Thorlabs SM1L03T) and optional flat black cap 32 (Thorlabs SM1CP2)
may be used to prevent unwanted internal reflections if required or
desirable.
[0060] The imaging unit 10 may form a compact rigid structure that
can be adjustably connected to a base 36 by a support assembly 35.
Support assembly 35 includes first and second shafts 37 and 38, and
clamping brackets 39 and 40 that can be clamped to the shafts 37
and 38 by tightening threaded connectors 41 and 42. Specifically,
clamping bracket 39 permits translation along axis "A1" of second
shaft 38, and also permits rotation "R1" about axis A1, and second
clamping bracket 40 permits longitudinal adjustment along axis "A2"
and rotational adjustment "R2" about axis "A2" of first shaft 37.
An adapter plate 43 (see also FIG. 2C) is fixed to the body of
camera 20 and beam-splitting device 22 (e.g., secured with threaded
fasteners 47), and the adapter plate 43 is secured to clamp bracket
39. The adapter plate 43 includes an arcuate slot 45 that receives
a threaded fastener 44 that threadably engages a plate 46 that is
fixed to clamp 39. Threaded fastener 44 can be loosened, and the
imaging unit 10 can be rotated about an axis "A3" as shown by the
arrow "R3." The support assembly 35 permits the imaging unit 10 to
be mounted in a confined space, such as plenum 8 (FIG. 1A), and
permits adjustment of the position and angle of imaging unit 10 and
camera 20 to permit the optical axis 13 to extend through a window
or port of a wind tunnel 1 (FIG. 1). Clamping brackets 39 and 40
may comprise commercially available components.
[0061] In the depicted embodiment, to enable focus adjustments of
the lens, the screw for the beam splitter device (e.g., cube 22) is
loosened and slides along a slot 43A (FIGS. 2A, 2C) as the focus
ring is adjusted. To lock the focus in place, both the focus ring
setscrew and the beam-splitter cube screw are tightened.
[0062] Each camera 20 may have two connections, namely, an Ethernet
cable 51 (FIG. 2B) and a trigger cable 52. The Ethernet cable 51
connects to a network switch 53, which is connected to a computing
device, such as a laptop computer 50, for data acquisition. It will
be understood that network switch 53 is not required if the system
does not include multiple cameras 20 configured to operate
simultaneously. For example, if only one camera 20 is being used,
it can be connected directly to an Ethernet card on a computer.
Additionally, a USB-based camera 20 can be used with connections to
USB ports on a computer or to a USB hub if multiple cameras 20 are
being used.
[0063] Commercially available software (e.g., Basler Pylon Viewer)
may be used to acquire data synchronously for all cameras 20 in the
system. Trigger cables 52 from each camera 20 are connected to a
trigger/power board 54, which acts as a central hub for all
triggering signals. A function generator 55 is also connected to
the trigger board 54, as well as to a BNC trigger input for the
driver circuit 30. Function generator 55 triggers the LED light
sources 24, controls the pulse duration of the LED light sources 24
and controls the phase delay between the camera trigger and the LED
trigger. As noted above, a delay between the LED and camera trigger
may be required if there is an inherent delay in the camera
exposure start time after receiving a trigger (actuation signal)
whereas the response of the LED light source 24 may be
substantially instantaneous. A single trigger coaxial cable from
function generator 55 may be connected to a first camera of a
system (e.g., FIG. 1), and the other cameras may be connected to
each other with coaxial cables using BNC T connectors. According to
a non-limiting embodiment, a 12-volt DC power supply 56 may include
a 120-volt input line 57 and a 12-volt line 58 that is operably
connected to the camera 20 and LED light source 24 to provide power
from a conventional 120-volt outlet 59. However, virtually any
suitable power supply may be utilized. Also, it will be understood
that multiple imaging units 10 may be interconnected to a single
trigger board 54 and computer 50 utilizing cables 51 and 52 or
other suitable arrangement.
[0064] The imaging units 10 (FIGS. 2A and 2B) can be positioned in
a plenum 8 (FIG. 1A) with the axis 13 of each imaging unit 10
aligned with a window 6 or porthole 7 about the tomographic region
of interest (ROI) 15. As shown in FIG. 1, the ROI 15 may be offset
from centerline CL of interior test space 5 of wind tunnel 1.
However, the imaging units 10 may be arranged to provide an ROI 15
at different locations within the interior test space 5 of wind
tunnel 1 as required for a particular application. As shown in FIG.
1, the imaging units 10A-10H may be arranged around approximately
one-half (1/2) of the tunnel circumference (i.e., approximately 180
degrees coverage). However, the imaging units 10 may be positioned
at different locations as required for a particular application.
Also, the number of imaging units 10 may also vary as required for
a particular application. Although eight imaging units 10A-10H are
shown, fewer imaging units 10 could be utilized (e.g., 4 or 5), or
the number of imaging units 10 could be greater. In the illustrated
example, the imaging units 10A-10H are generally positioned with
the optical axes 13 of imaging units 10A-10H arranged in a plane
"P" (FIG. 1A). However, the imaging units 10 could be positioned
such that the optical axes 13 are positioned in two or more
planes.
[0065] The imaging units 10 may be configured to provide low-speed
acquisition (on the order of a few Hz), if time resolution is not a
priority. The imaging units 10 are preferably configured to be able
to withstand low pressures and high temperatures in plenum 8 during
pump-down and operation of wind tunnel 1. The imaging units 10 may
be configured to provide short exposure to "freeze" fluid flow up
to Mach 1.4 or higher Mach numbers (e.g., Mach 6-10). A controller,
such as driver circuit 30 (FIGS. 2A and 2B), may be configured to
synchronously (or substantially synchronously) trigger all cameras
20A-20H remotely from a control room 33 (FIG. 1A). The driver
circuit 30 may be configured to maximize LED output intensity and
minimize pulse width. Also, the on-axis lighting (i.e., beam of
light 26) maximizes intensity return from surfaces 18A-18H of
background panels 17A-17H, respectively. The optical axes 13 of the
imaging units 8-10 are preferably at least about 10 degrees from
perpendicular of the transparent material of windows 6 and
portholes 7 to prevent light from reflecting directly back into the
cameras 20. The angle required to avoid reflection may vary
somewhat depending on the distance "D" (FIG. 1A) of the camera 20
from the surface 14 of the transparent material of side windows 6
and portholes 7. In general, if the distance "D" is relatively
small, the angle of the optical axis 13 relative to perpendicular
of surface 14 must be larger. However, the angle from perpendicular
can be smaller if the distance "D" is larger. In the non-limiting
embodiment of FIG. 1, the approximate angle of the imaging units 10
relative to the window perpendiculars is 11.1 degrees for imaging
units 10A and 10H, 31.5 degrees for imaging units 10B and 10G, 46.3
degrees for imaging units 10C and 10F, and 28.0 degrees for imaging
units 10D and 10E. These angles may result from positioning the
imaging units 10A-10H radially about ROI 15, with the axes 13A-13H
of imaging units 10A-10H, respectively, generally passing through
center 16 of ROI 15.
[0066] During operation of wind tunnel 1, air or other fluid 11
flows through interior test space 5 around a model 34 (FIG. 1A).
Model 34 may comprise, for example, a scale model of an aircraft
that is positioned on centerline CL of interior test space 5. In
the illustrated example, the ROI 15 is offset from centerline CL to
provide imaging of flow around a side portion of model 34. As noted
above, the axes 13A-13H of imaging units 10A-10H, respectively, may
be coplanar and lie in a plane "P" (FIG. 1A). However, one or more
of the imaging units 10 may be positioned fore or aft of plane "P"
if required for a particular application. As discussed in more
detail below, small retro-reflective targets (i.e., small dots) may
be placed on the model 34 and the imaging system may be used to
track the position of these dots for photogrammetry purposes.
[0067] Surfaces 18A-18H of background panels 17A-17H, respectfully,
may include a uniform density and distribution of retroreflective
background dot patterns based on the field of view 15 of the
cameras 20 of imaging units 10, and the distance "Dl" (FIG. 1A) of
the cameras 20 from the background 17. The dot pattern may be
printed or otherwise formed directly on the background material 17,
and the background panels 17 may preferably comprise a material
that is suitable for attaching the walls to ceiling 3 and floor 4
of wind tunnel 1 and remain attached during operations at, for
example, supersonic Mach numbers. The surfaces 18 of the
backgrounds 17 are preferably configured to provide a high
intensity light return. For example, the surfaces 18 may comprise
high gain reflective sheeting. It will be understood that this is
merely an example of one suitable material, and other materials may
be utilized for the surfaces 18A-18H.
[0068] According to a non-limiting embodiment, a dot pattern was
formed on surfaces 18A-18H by spraying flat-black spray paint onto
the surface. In particular, the nozzle of the spray paint can was
enlarged from its original diameter, the background panels 17A-17H
were placed on the floor, and the can was held upright with the
nozzle depressed slightly until a stream of paint particles were
ejected and fell onto the background material. This combination of
an enlarged nozzle diameter and the light nozzle pressure resulted
in generally repeated background patterns with some variation in
dot size and distribution period. In the illustrated example, the
sprayed backgrounds were then applied to flat panels, utilizing the
adhesive backing of the high gain reflective shielding, which
panels were cut to the appropriate size for each background
17A-17H. It will be understood that this is merely one example of
suitable background configuration, and a wide range of backgrounds
may be utilized.
[0069] Prior to use, the imaging units 10 may be calibrated
utilizing a calibration plate (not shown). Calibration of BOS
systems is generally known, and a detailed description of the
calibration process is, therefore, not believed to be required. In
use, as fluid 11 flows through the interior test space 5, the
imaging units 10 are actuated to capture images, and the images can
be processed utilizing known software to provide tomographic
reconstruction. An example of software that may be utilized to
process the image data from the imaging units 10 is DaVis Version
10.0.5 (or 10.1.1) software commercially available from LaVision of
Ypsilanti, Mich. It will be understood that virtually any suitable
program may be utilized.
[0070] With further reference to FIG. 3, an imaging system 100
according to another aspect of the present disclosure includes
imaging units 110A-110J disposed about a tomographic region of
interest ROI 115. Imaging system 100 comprises a test system with
the imaging units 110A-110J mounted to a test table 102. The
imaging units 110A-110J operate in substantially the same manner as
the imaging units 10A-10H, described in more detail above, and
include support assemblies 135 that adjustably support the imaging
units 110A-110J on test table 102. The imaging units 110A-110J are
mounted radially about ROI 115 with the optical axis of imaging
units 110A-110J being aligned with backgrounds 117A-117J. Surfaces
118A-118J of backgrounds 117A-117J, respectively, may comprise
reflective surfaces with dots or other suitable patterns. A candle
135 is positioned in ROI 115 to generate pressure gradients for
test purposes.
[0071] With reference to FIGS. 4-7, imaging units 110 include a
digital camera 120, which may be substantially similar to the
camera 20 described in more detail above. An LED light source 124
produces light that is coupled onto optical axis 113 by a
beam-splitting device 122 after passing through an aspheric
condensing lens with a diffuser 125, which may be similar to the
condenser aspheric lens 25 with diffuser shown in FIG. 2D. The
light forms a beam 126 that is coaxial with optical axis 113 of
camera 120. An optional heat sink 127 provides the coupling of the
LED light source 124. Adapter plate 143 is an arcuate slot 145 that
provides for rotational adjustment of imaging unit 110 in
substantially the same manner as discussed in more detail above in
connection with the imaging units 10. The imaging unit 110 may
optionally include a blackened cap 132 (FIG. 6) to prevent light
from escaping beam splitter 122. A neutral density filter may be
utilized instead of blackened cap 132. With reference to FIG. 7,
the imaging unit 110 may, optionally include an angled neutral
density filter as an alternative to the cap 132.
[0072] With further reference to FIG. 8, a cooling fan 127A may be
utilized to cool LED light source 124 as an alternative to the heat
sink 127.
[0073] With further reference to FIG. 9, an imaging unit 110 (or
imaging unit 10) may be positioned with optical axis 113 passing
through a sheet 106 of transparent material. Optical axis 113 of
imaging unit 110 may be positioned at an angle relative to a normal
direction of surface 108 of transparent sheet 106. As discussed
above, the angle prevents reflected light (i.e., from beam of light
126) from reflecting directly back into camera 120. FIG. 9 shows a
test configuration that may be utilized to determine an amount of
light reflected from surface 108 of sheet 106 into camera 120 (or
20). The sheet 106 may comprise, for example, an acrylic polymer
material that is about 0.5 inches thick. FIGS. 10-12 comprise
images taken utilizing camera 120 adjacent acrylic sheet 106 with a
background pattern 117 opposite the acrylic sheet 106. FIG. 10 is
an image 150A taken with a horizontal angle of approximately 14
degrees and a pitch angle of approximately 8 degrees relative to a
normal direction of surface 108 of sheet 106. FIG. 11 comprises an
image 150B resulting from a horizontal camera angle of
approximately 0 degrees and a pitch angle of approximately 8
degrees, and FIG. 12 is an image 150C resulting from optical axis
113 being parallel to a normal direction of surface 108 of sheet
106. In general, the optical axis 113 of camera 120 is preferably
at an angle of at least about 8 degrees relative to a normal
direction of surface 108 to prevent reflection of light from
surface 108 directly back into camera 120, which may form a bright
spot 152, as shown in FIG. 12.
[0074] FIGS. 12A, 12B, and 12C show images taken with a camera 20
of an imaging unit 10 (FIGS. 2A, 2B) at 0 degrees incidence
relative to an acrylic sheet 106 (i.e., with the camera axis normal
to surface 108 of acrylic sheet 106). The images of FIGS. 12A-12C
were generated using an imaging unit 10 (FIGS. 2A and 2B) utilizing
a test setup that is substantially similar to the arrangement shown
in FIG. 9. FIG. 12B is an image taken with the axis of the camera
at 4 degrees off normal incidence, and FIG. 12C is an image taken
with the camera axis 10 degrees off normal incidence. FIG. 12C
shows that the reflection from the light source can be eliminated
if the camera axis is sufficiently off a normal incidence relative
to the acrylic sheet 106 (FIG. 9). Significantly, the imaging units
may be positioned at, for example, 10 degrees off normal when the
imaging units are aligned with windows 6 or portholes 7 (FIG. 1) to
eliminate reflections that could otherwise occur. It will be
understood that the same principles apply to sheets (windows) made
of other materials (e.g., glass), and the acrylic sheet 106 is
merely an example of a light-transmitting material.
[0075] The cameras 20 and/or 120 may optionally comprise small CMOS
cameras fitted with C-mount lenses, and light from a high-intensity
light source (e.g., LEDs 24, 124) may pass through a lens 25, 125
that diffuses and collimates the LED output. This light is coupled
onto the camera's optical axis 13, 113 using a suitable
beam-splitting device, such as a 50/50 beam-splitting prism. The
illumination components, beam-splitter, and camera/imaging
components optionally comprise a single, rigid imaging unit. The
use of a collimating/diffusing lens to condition the LED light
output provides for an illumination source that is of similar
diameter to the imaging lens of the camera 20, 120. This reduces or
eliminates shadows that could otherwise be projected onto the
subject plane as a result of refractive index variations in the
imaged volume. By coupling the light from the LED unit 24, 124 onto
the camera's optical axis 13, 113, reflections from windows (which
are often present in wind tunnel facilities and allow direct views
of the test section) can be minimized or eliminated if the camera
is placed at an angle (e.g., 8-10 degrees) of incidence relative to
the surface of the window.
[0076] With further reference to FIG. 13, a single imaging unit 10
may be positioned to generate images resulting from a heat gun 60
utilizing a retroreflective background panel 62. This setup can be
utilized to provide a shadowgraph image (FIG. 14) or a
2-dimensional BOS image (FIG. 15). The images of FIGS. 14 and 15
were developed utilizing the setup of FIG. 13, and data from
imaging unit 10 was processed utilizing LaVision's DaVis 10
software.
[0077] FIGS. 16 and 17 are camera images of a wind tunnel model
having fiducial dot markers. FIG. 16 is an image taken utilizing a
first camera, and FIG. 17 is an image of the same model taken using
a second camera at a different location. The images of FIGS. 16 and
17 may be generated utilizing one or more imaging units 10 (FIGS.
2A and 2B) with the cameras aligned with windows 6 or portholes 7
(FIG. 1). FIGS. 18 and 19 are plots showing the model position in
tunnel Y- and Z-coordinates in time based on the measurements made
with the two camera systems.
[0078] The imaging system of the present disclosure may comprise a
compact unit with a built-in illumination source that can be
positioned in a plenum or other space behind a viewing port/window
of a typical wind tunnel. The camera may comprise a compact CMOS
camera that acquires images with sufficient resolution and
bit-depth to provide high-quality BOS data for a 3-D tomographic
BOS reconstruction. It will be understood that the camera data may
also be utilized for 2-dimensional BOS flow visualization
photogrammetry, machine vision, shadowgraphs, or other
applications. The support structure (housing) of the imaging units
is preferably constructed so that the output from a small LED light
source may be conditioned using a diffusing lens to increase the
apparent size of the light source such that it is similar in size
to the diameter of the main lens of the camera or to minimize
shadow effects while also concentrating the light onto the subject.
The output of the light source is preferably co-linear with the
optical axis of the camera due the beam-splitting device. Because
the light from the light source is co-linear with the optical axis
of the camera, the amount of light returned to the camera is
increased when imaging a retroreflective background target in the
image plane, while limiting the appearance of hard shadows formed
by objects (e.g., wind tunnel models) that occlude the object
plane.
[0079] It is to be understood that variations and modifications can
be made on the aforementioned structure without departing from the
concepts of the present disclosure, and further it is to be
understood that such concepts are intended to be covered by the
following claims unless these claims by their language expressly
state otherwise.
* * * * *