U.S. patent application number 09/988660 was filed with the patent office on 2003-06-05 for multiband, single element wide field of view infrared imaging system.
Invention is credited to Cole, Jeff, Falter, Peter, Myers, Mark.
Application Number | 20030102435 09/988660 |
Document ID | / |
Family ID | 25534374 |
Filed Date | 2003-06-05 |
United States Patent
Application |
20030102435 |
Kind Code |
A1 |
Myers, Mark ; et
al. |
June 5, 2003 |
Multiband, single element wide field of view infrared imaging
system
Abstract
A compact, wide field of view, infrared imaging system with two
Mid-Wave Infrared (MWIR) and, optionally, an additional one
Long-Wave Infrared (LWIR) band, has a single, color corrected lens
element embedded within the detector/dewar assembly. The lens
element has two aspherical surface profiles and utilizes a
holographic optical element to manipulate and detect bands of
energy that are harmonic components of each other. The infrared
imaging system simplifies and shrinks the MWIR/LWIR imager while
maintaining all of the required functionality. An exemplary
infrared imaging apparatus performs at an F-stop (F/#) of at least
1.4 with a square field of view of 90.times.90 degrees.
Inventors: |
Myers, Mark; (Orlando,
FL) ; Cole, Jeff; (Orlando, FL) ; Falter,
Peter; (Orlando, FL) |
Correspondence
Address: |
Patrick C. Keane, Esq.
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
25534374 |
Appl. No.: |
09/988660 |
Filed: |
November 20, 2001 |
Current U.S.
Class: |
250/352 ;
348/E5.09 |
Current CPC
Class: |
G01J 5/0803 20130101;
G01J 5/0806 20130101; H04N 5/33 20130101; G02B 5/32 20130101; G01J
5/0875 20130101; G01J 2005/0077 20130101; G01J 5/061 20130101; G02B
13/14 20130101; G02B 23/12 20130101; G01J 5/08 20130101 |
Class at
Publication: |
250/352 |
International
Class: |
G01J 005/02 |
Claims
What is claimed is:
1. An infrared imaging apparatus, comprising: a dewar, having an
internal volume that defines a cold space; an IR transmissive
window that seals the cold space to receive IR energy directly from
an IR source; a first lens located within the cold space to receive
IR energy directly from the IR transmissive window; an IR detector
located within the cold space in operational communication with the
first lens and positioned coincident to the focal plane of at least
a first and second wavelength of IR energy; and an optical stop
located within the cold space in front of the single lens.
2. The infrared imaging apparatus of claim 1, wherein the single
lens has a first aspheric profile on a first side and a second
aspheric profile on a second side, the first side parallel to the
second side and the second side facing the detector.
3. The infrared imaging apparatus of claim 2, wherein the second
aspheric profile has a holographic optical element.
4. The infrared imaging apparatus of claim 3, wherein the
holographic optical element color corrects at least one color band
of infrared energy.
5. The infrared imaging apparatus of claim 4, wherein the
holographic optical element color corrects a red MWIR band and a
blue MWIR band.
6. The infrared imaging apparatus of claim 1, wherein the detector
is a hyperspectral detector.
7. The infrared imaging apparatus of claim 1, wherein the detector
detects at least three wavelengths of IR energy including at least
one LWIR band of energy.
8. The infrared imaging apparatus of claim 1, wherein the LWIR band
of energy is preferably an indigo LWIR band.
9. The infrared imaging apparatus of claim 1, wherein the
holographic optical element coincidently focuses a MWIR band and a
LWIR band of IR energy at a common focal plane.
10. The infrared imaging apparatus of claim 1, wherein the second
wavelength of IR energy is a harmonic component of the first
wavelength.
11. The infrared imaging apparatus of claim 1, wherein the single
lens is made of germanium.
13. The infrared imaging apparatus of claim 1, wherein the single
lens is made of silicon.
14. The infrared imaging apparatus of claim 1, wherein the
apparatus performs at an F-stop (F/#) of at least 1.4 with a square
field of view of 90.times.90 degrees.
15. The infrared imaging apparatus of claim 1, wherein the detector
concurrently collects radiation from multiple, adjacent spectral
radiation bands.
16. The infrared imaging apparatus of claim 3, wherein the first
aspheric surface has the following prescription: radius=-0.94467;
k=28.345216; a=-2.13952; b=-69.5274; c=2342.04; d=-56841.9; and
first surface thickness=0.548467.
17. The infrared imaging apparatus of claim 16, wherein the second
aspheric surface has the following prescription: radius=-0.61281;
k=0.1399; a=0.033459; b=-2.3598; c=10.889; d=-36.331; and second
surface thickness=0.462731.
18. The infrared imaging apparatus of claim 17, wherein the
holographic optical element has the following prescription:
-0.0051393, -0.10212, 0.91035, -2.3946.
19. The infrared imaging apparatus of claim 3, wherein the first
aspheric surface has the following prescription: radius=-1.23508;
k=36.049455; a=-1.69104; b=-98.6413; c=5589.83; d=-162359; and
first surface thickness=0.761661.
20. The infrared imaging apparatus of claim 19, wherein the second
aspheric surface has the following prescription: radius=-0.81270;
k=-0.10748; a=0.054475; b=-0.72423; c=2.9155; d=-7.8939; and second
surface thickness=0.480234.
21. The infrared imaging apparatus of claim 20, wherein the
holographic optical element has the following prescription:
-0.017112, -0.038991, 0.55069, -1.6405.
Description
BACKGROUND
[0001] 1. Field of Invention
[0002] The present device relates generally to an infrared imaging
system. More specifically, the device relates to wide field of
view, infrared imaging systems with mid-wave infrared bands and,
optionally long-wave infrared bands.
[0003] 2. Background Information
[0004] Infrared electromagnetic radiation refers to the region of
the electromagnetic spectrum between wavelengths of approximately
0.7 and 1000 .mu.m, which is between the upper limit of the visible
radiation region and the lower limit of the microwave region.
Infrared radiation is sometimes broken into three sub-regions:
near-infrared radiation with wavelengths between 0.7-1.5 .mu.m,
intermediate-infrared radiation with wavelengths between 1.5-20
.mu.m, and far-infrared radiation with wavelengths between 20-1000
.mu.m. The intermediate-infrared radiation region is often further
broken into the mid-wave (MWIR) region with wavelength limits of
3-5 .mu.m and the long-wave (LWIR) region with wavelength limits of
8-12 .mu.m.
[0005] Infrared radiation is produced principally by
electromagnetic emissions from solid materials as a result of
thermal excitation. The detection of the presence, distribution,
and direction of infrared radiation requires techniques which are
unique to this spectral region. The wavelengths of infrared
radiation are such that optical methods may be used to collect,
filter, and direct the infrared radiation. Photosensitive devices
convert heat, or infrared electromagnetic radiation, into
electrical energy and are often used as infrared sensitive
elements. Such photosensitive devices respond in proportion to the
number of infrared photons within a certain range of wavelengths to
provide electrical energy.
[0006] Generally, an imaging infrared sensor includes a plurality
of infrared sensitive elements in order to provide suitable
resolution of the field of view which is to be monitored. In
addition to the plurality of infrared sensitive elements, an
infrared sensor includes other components for complete processing
of the information provided by incident infrared electromagnetic
radiation. Optical filters and apertures are used to define and
focus the radiation directed at the infrared sensitive elements.
Electronics are necessary for controlling the data collection and
processing the collected data from the infrared sensitive elements.
Cooling apparatus is necessary to maintain the operation of the
infrared sensitive elements as well as the electronics. One
approach for processing the electrical energy provided by the
plurality of infrared sensitive elements is to use multiplexers to
provide a single signal having a serial data stream since it is
simpler to process the single resulting serial signal than the
plurality of signals which correspond to the plurality of infrared
sensitive elements. The serial signal is generally further
processed by any number of techniques known in the art to provide
interpretable, useful information regarding objects in the field of
view of the infrared sensor.
[0007] As is known in the art, military and space applications
employ infrared electromagnetic radiation detection for such
functions as tracking and searching. These applications require the
detection of low-level radiation in the intermediate infrared
radiation range. One example of an application for infrared
detection is in radar systems, where greater angular resolution and
obstacle penetration capabilities improve overall platform imaging
capabilities while the inclusion of infrared detection,
particularly the detection of more than one band, or range, of
infrared radiation, makes the system more difficult to jam, or
disable.
[0008] Electro-optical sensor assemblies, such as infrared imaging
systems, use optical components to route and focus received
radiation onto a detector. However, the size and weight of
electro-optical sensor assemblies have always been a significant
design consideration. For example, in an airborne application, the
size of the sensor assembly dictates the size of the required
gimbal, which in turn affects the overall system size and weight.
Since the sensor assembly and gimbal may both be in the airstream,
the size of each can affect the overall aircraft drag. In another
example, such as in ground applications, a head mirror may be used
for elevation pointing. The number of optical apertures and the
size of these apertures dictate the head mirror size, which, in
turn, affects the size and weight of the surrounding armor
plate.
SUMMARY
[0009] The present invention is generally directed to an infrared
imaging system. The infrared imaging apparatus with a dewar has an
internal volume defining a cold space. An IR transmissive window
seals the cold space and receives IR energy directly from an IR
source. Within the cold space, an optical stop located in front of
a first lens, a first lens with aspherical surface profiles on both
the first and the second surface, and an IR detector are positioned
and aligned in operational communication to receive IR energy
directly from the IR transmissive window and direct the IR energy
to the detector coincidently positioned at the focal plane of at
least a first and second wavelength of IR energy.
[0010] The second aspherical surface profile has a holographic
optical element that color corrects at least one color band of
infrared energy. The holographic optical element may detect a
second or subsequent wavelength of IR energy that is a harmonic
component of the first wavelength. Preferably, the holographic
optical element color corrects a red MWIR band and a blue MWIR
band. The holographic optical element also coincidently focuses a
MWIR band and a LWIR band of IR energy at a common focal plane. The
detector detects and manipulates at least three wavelengths of IR
energy including at least one LWIR band of energy, preferably an
indigo LWIR band.
[0011] An exemplary infrared imaging apparatus has lens made from
germanium or silicon and performs at an F-stop (F/#) of at least
1.4 with a square field of view of 90.times.90 degrees.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0012] Objects and advantages of the invention will become apparent
from the following detailed description of preferred embodiments in
connection with the accompanying drawings, in which like numerals
designate like elements and in which:
[0013] FIG. 1 is a perspective view of the imaging system; and
[0014] FIG. 2 is a schematic representation of the line trace of
energy in a first embodiment of optical components.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] FIG. 1 is a perspective view of an infrared imaging system
100. The infrared imaging system 100 has a compressor housing 102
and an optical housing 104. The optical housing 104 further has a
cryogenic subassembly 106, an optical subassembly 108, and an
electronics subassembly 110.
[0016] The compressor housing 102 contains suitable support
components required to maintain cooling of the optical housing 104.
Specifically, the compressor housing 102 can contain a compressor
for circulating a cooling medium through the compression and
expansion cycle used for cooling.
[0017] The cryogenic subassembly 106 has a cavity 114,
alternatively called a dewar, which defines a cold space 116.
Infrared energy detectors generally require cooling to improve
performance in converting incident energy into an electrical
signal. The cryogenic subassembly 106 provides the required
cryogenic cooling capability. The cavity 114 is in fluid
communication with the compressor housing 102 through the transfer
line 112. The cooling medium, such as liquid nitrogen (LN2), is
circulated in a closed loop from the compressor, through the
transfer line 112, and through the cavity 114. The cold space 116
is sealed by an IR transmissive window 120 and is evacuated to
<50 mTorr, preferably less than 5 mTorr.
[0018] The optical subassembly 108 is positioned within the cold
space 116 at the receiving end 118 of the optical housing 104.
Elements of the optical subassembly 108, including a lens 122 and
an IR detector 124, are housed in the cold space 116. Elements of
the optical subassembly 108 are maintained at a suitable cryogenic
temperature by the cryogenic subassembly 106, typically 150 to
180.degree. K.
[0019] The electronics subassembly 110 receives inputs from the IR
detector 124 and transmits signals to a processing unit (not
shown).
[0020] FIG. 2 shows a plan cross-section of a first embodiment of
an optical subassembly 200. The optical subassembly 200 has an IR
transmissive window 202, an optical stop 204, a lens 206, and an IR
detector 208. The optical stop 204, lens 206, and IR detector 208
are positioned inside the cold space 210 of the cryogenic
subassembly 106 shown in FIG. 1. An example of an IR transmissive
window 202 is optical grade germanium or an amorphous solid, such
as zinc selenide. The IR transmissive window 202 has a 120.degree.
circular field of view and receives incident IR energy directly
from an IR source. In an alternative embodiment, there may be an
additional filter placed before the IR transmissive window 202 that
discriminates a desired wavelength of energy or wavelengths of
energy.
[0021] The optical stop 204 is positioned in the cold space 210 at
the limiting aperture in the transmission path where the incident
energy has a first crossover point A. The position of the optical
stop 204 may be abutting the lens 206 or it may abut the IR
transmissive window 202 and will be determined by the wavelengths
of energy to be detected and the characteristics of the other
optical components. In a preferred embodiment, the optical stop 204
may be from 20/1000th to 60/1000th from the lens 206; more
preferably the optical stop 204 is 40/1000th from the lens 206. The
optical stop 204 has an opening 212 circularly symmetric about axis
X-X', the radius of which is the size of the cross-section of the
caustic at the first crossover point A. The caustic is the envelope
curve of the transmitted beam. The optical stop 204 helps to
prevent stray energy from traveling down the transmission path
toward the lens 206 and thus improves optical performance. By
placing the optical stop 204 within the cold space 210 and in front
of the lens 206, design requirements are simplified while still
maintaining the required cold shield efficiency.
[0022] A first surface 214 of the lens 206 is oriented toward the
IR transmissive window 202 and a second surface 216 that is
oriented toward the detector 208. IR energy 218 is directly
received by the lens 206 from the IR transmissive window 202. The
first and second surfaces 214, 216 of the lens 206 are aspherical
over at least a portion of the lens 206 and such that the
aspherical surfaces 220 are aligned radially symmetric in the
transmission path about axis X-X'. Alternatively, the entire first
or second surface 214, 216 may be aspherical. However, the
cross-section of the caustic at the points B, C is no greater than
the surface area of the first or second surface 214, 216 and is
such that the transmission path may propagate through the
aspherical surfaces 220.
[0023] The second surface 216 of the lens 206 is also a holographic
optical element (HOE) 222, alternatively called a binary surface or
a diffractive grating on a curved surface. The HOE 222 uses
principles of harmonics to discriminate and propagate a plurality
of wavelengths. Preferably, the HOE 222 discriminates and
propagates at least two wavelengths. For example, a first
wavelength is manipulated by the HOE 222, a second wavelength must
be a harmonic component of the first wavelength for the HOE 222 to
manipulate it. The requirement applies to all subsequent
wavelengths to be manipulated by the HOE 222.
[0024] A detector 208 is positioned in alignment with the other
components of the optical subassembly 200 about the axis X-X' at a
focal length distance d from the second surface 216 of the lens
206, at a coincident focal plane to at least 2 wavelengths
manipulated and transmitted by the lens 206 and the HOE 222. The
detector 222 can discriminate at least two, and preferably
multiple, wavelengths of incident energy in the IR spectrum, and
more preferably wavelengths at 3-12 .mu.m. The detector 208
processes the wavelengths to produce multiple waveband detection
capability within a single detector. In one embodiment, the
detector 208 concurrently collects radiation from multiple,
adjacent spectral radiation bands. This type of detector may be
used in "hyperspectral imaging." An example of such a detector is
disclosed in co-assigned U.S. Pat. No. 6,180,990 B1, issued to
Claiborne et al., the disclosure of which is incorporated herein by
reference.
[0025] In an another embodiment, the detector 208 may manipulate
multiple wavelengths of incident energy resulting in at least two
MWIR and one LWIR band being detected by the infrared imaging
system 100. A detector capable of hyperspectral imaging is suitable
for this application.
[0026] The first and second aspherical surfaces 214,216 and the HOE
222 combine to manipulate infrared energy from at least two
wavebands in the infrared spectrum. In one embodiment, a first
waveband is a mid-wave infrared (MWIR) waveband with wavelength of
3-5 .mu.m, preferably 4-4.5 .mu.m, and a second waveband is a
mid-wave infrared (MWIR) waveband with wavelength of 3-5 .mu.m,
preferably 4-4.5 .mu.m. In a second embodiment, the first and
second aspherical surfaces 214,216, the HOE 222, and the detector
208 combine to manipulate infrared energy from at least two
wavebands in the infrared spectrum. In this embodiment, a first and
second waveband similar to the first embodiment is detected. A
detector 208, as described above, can be a detector suitable for
hyperspectral imaging and can manipulate and discriminate a third
coincident and coregistered waveband. This third waveband may be a
LWIR waveband with wavelength of 8-12 .mu.m, preferably 8.5-9.5
.mu.m.
[0027] An aspherical surface may be mathematically defined by: 1 H
( x ) = rx 2 1 + 1 - r 2 ( k + 1 ) x 2 + ax 4 + bx 6 + cx 8 + dx 10
Eq . 1
[0028] where r=radius of curvature, k=conic coefficient, and a, b,
c, and d are aspheric coefficients.
[0029] There is a correspondence between the conic coefficient of
Eq. 1 and the geometric surface profile. Table 1 illustrates this
correspondence.
1TABLE 1 Correspondence between k and the type of profile Value of
k Type of Profile >0 ellipse =0 sphere -1 < k < 0 ellipse
=-1 parabola <-1 hyperbola
[0030] In practice, one skilled in the art could utilize
commercially available lens design software to obtain suitable
values for the coefficients of Eq. 1, including the aspherical
coefficients. An example of one such lens design software package
is "CODE V.COPYRGT." available from Optical Research Associates of
Pasadena, Calif. One skilled in the art could input information
including, for example, image size, focal distance, energy
distribution across the detector and determine the optimized values
for the coefficients of Equation 1. Examples of suitable
coefficients for use in an infrared imaging detector in keeping
with this invention are shown in Table 2 and 3.
[0031] Table 2 is a first embodiment of an optical prescription for
the lens 206 of the single element wide field of view infrared
imaging system 100. This example is a prescription for a dual band
lens.
2TABLE 2 Prescription of Dual Band Lens # Description Radius k
Thickness a b c d 1 First Surface -0.94467 28.345216 0.548467
-2.13952 -69.5274 2342.04 -56841.9 2 Second Surface -0.61281 0.1399
0.462731 0.033459 -2.3598 10.889 -36.331 HOE Coefficients (Radial)
-0.0051393 -0.10212 0.91035 -2.3946 3 Focal Plane
[0032] Table 3 is a second embodiment of an optical prescription
for the lens 206 of the single element wide field of view infrared
imaging system 100. This example is a prescription for a three band
lens.
3TABLE 3 Prescription of Three Band Lens # Description Radius k
Thickness a b c d 1 First Surface -1.23508 36.049455 0.761661
-1.69104 -98.6413 5589.83 -162359 2 Second Surface -0.81270
-0.10748 0.480234 0.054475 -0.72423 2.9155 -7.8939 HOE
Coefficients( Radial) -0.017112 -0.038991 0.55069 -1.6405 3 Focal
Plane
[0033] In Tables 2 and 3, "Radius" is the radius of curvature (r),
k is the conic coefficient, and a, b, c, and d are the aspherical
coefficients. The "thickness of the first surface" is the thickness
of the lens 206. The "thickness of the second surface" is the back
focal distance, which is the distance from the second surface 216
of the lens 206 to the detector 208, or focal distance d.
[0034] The optical performance of an infrared imaging system 100 in
keeping with the embodiments described may have an optical F/#=1.4
with a square field of review of 90.times.90 degrees. Additionally,
the infrared imaging system 100 has a wide field of view (field of
view greater than 60.degree.).
[0035] In operation, incident infrared energy 218 travels through
the limiting aperture of the optical stop 204 and is incident to
the aspherical portion 220 of the first surface 214 of the lens
206. The infrared energy 218 then is translated by the aspherical
surface of the second surface 216 of the lens 206 and the HOE 222
and is focused onto the detector 208.
[0036] In an embodiment of an optical layout in keeping with the
invention, the incident infrared energy is color corrected to
realize at least one band of energy on the detector surface. For
example, in the first embodiment the incident infrared energy is
color corrected across both the red and blue MWIR bands. In the
alternative optical layout, the incident infrared energy is color
corrected across the red MWIR, blue MWIR, and indigo LWIR
bands.
[0037] The single lens 206 is made of silicon and has aspheric
surface profiles on both sides. Alternatively, the single lens 206
may be made of germanium. The HOE 222 helps to achieve the required
color correction across both the red and blue MWIR bands. The color
correction across the indigo LWIR bands is provided by the detector
208 in conjunction with the HOE 222. The optic performs at an F/#
of 1.4 with a square field of view of 90 by 90 degrees.
[0038] The use of a single, color corrected element in the dewar
provides an optical subassembly 200 that is shorter and provides
for a better form factor and lower part count for the entire
infrared imaging system 100. Also, by enclosing the single lens 206
within the detector dewar, the optical subassembly 200, including
the optical stop 204, lens 206 and detector 208, are all located
within a single enclosure. Previously, tight alignment tolerances
had to be maintained across the detector-to-dewar mount, the
dewar-to-optical housing mount and the optical housing-to-optics
mount. By eliminating the multiple interfaces the total tolerance
budget can be applied on the single interface, reducing the
required manufacturing and assembly tolerances and reducing the
requirement for precision alignment across multiple interfaces.
[0039] Another advantage of being able to place the single, color
corrected lens 206 in the cryogenic subassembly 106 is that it
places the optical subassembly 200 in a controlled temperature
environment. By maintaining the lens 206 at a nearly constant
temperature, the need for a passive or active athermalization
system to correct the thermally induced focus variations may be
eliminated. While this could be accomplished previously by heating
or cooling the optics with a separate device, this approach makes
use of the cooling capabilities that are already present in the
system.
[0040] The alignment of the optical components is important so that
a detector is located at the focal plane for the lens system. In
previous multi-lens imaging systems, it was difficult to ensure
alignment of the optical components because the thermal coefficient
of expansion resulted in disparate movement of the individual
optical components. A unitary structure housed within the cold
space essentially eliminates thermal transients amongst the
components once a temperature equilibrium has been achieved by the
cryogenic housing and compressor, thereby overcoming the alignment
problems.
[0041] Also, enclosing the optical subassembly 200 in the cryogenic
subassembly 106 places the optics in a sealed, evacuated
environment, protecting it against dust or other contamination.
While this could be accomplished in a separate enclosure, this
approach makes use of capabilities already present in the optical
housing 104.
[0042] Lastly, all of these qualities permit the design of a lower
cost system with the same performance capabilities of current, more
expensive ones.
[0043] In one exemplary application, the use of wide field of view
(greater than 100.degree.) MWIR imaging systems on military
platforms provides the capability of performing missile warning,
defensive infrared search and track, navigation and situational
awareness functions. Adding a second wave band to the sensor helps
to discriminate between natural and manmade objects and increases
the effectiveness of the sensor in these tasks. Additionally,
adding a third LWIR band to the sensor further improves the imaging
system's ability to discriminate between natural and manmade
objects and increases the effectiveness of the sensor in these
tasks. Since providing complete spherical coverage around an object
requires a maximum of six sensors, the cost, size and complexity of
current systems can prohibit their large scale employment.
[0044] This invention has direct application to other wide field of
view multiband uses, including but not limited to dual band
navigation, advanced missile seekers and chemical agent
detection.
[0045] Although the present invention has been described in
connection with preferred embodiments thereof, it will be
appreciated by those skilled in the art that additions, deletions,
modifications, and substitutions not specifically described may be
made without department from the spirit and scope of the invention
as defined in the appended claims.
* * * * *