US010254166B2(12) United States Patent(10) Patent No.: US 10 ,254, 166 B2(45) Date of Patent:

Kester et al.

(58) Field of Classification Search(54 ) DIVIDED - APERTURE INFRA -RED

SPECTRAL IMAGING SYSTEM FOR

CPC ...... .... GO1J 3 /36 ; G01J 30232 ; G01J 3 /2823
See application file for complete search history.
(56 )References Cited
U .S . PATENT DOCUMENTSCHEMICAL DETECTION(71) Applicant: Rebellion Photonics, Inc., Houston , TX
(US)(72) Inventors : Robert T . Kester , Pearland , TX (US );Nathan A . Hagen , Utsunomiya (JP )3 ,841,763 A * 10 /1974 Lewis ..... ......... GOIN 33 /0027
3 , 849 ,005 A
4 , 134 ,683 A(73 ) Assignee: Rebellion Photonics, Inc ., Houston , TX(US)( * ) Notice :patent is extended or adjusted under 35
U . S . C . 154 (b ) by 0 days .

CA

Related U . S . Application Data
(63 ) Continuation of application No. 14 / 543,692, filed on
application

PCT/US2013/041278, filed on May 16 , 2013 .

& Bear LLP

filed on May 18 , 2012 .
(51) Int. Cl.
GOIJ 3 /28( 2006 .01)
GO1J 5 / 00

(2006 .01)

GO1J 3 /36( 2006 .01)

ABSTRACT(57)
A divided -aperture infrared spectral imaging (DAISI) sys
tem that is structured to provide identification of target
chemical content in a single imaging shot based on spec
trally -multiplexed operation . The system is devoid of spec

( 2006 .01)

tral scanning acquisition of infrared ( IR ) spectral signaturesof target content with an IR detector and does not require

GO1J 3 /36 (2013 .01); G01J 3 /0232(2013.01); GOIJ 3 / 2823 (2013 .01); G01J

content .

16 Claims, 11 Drawing Sheets

2003 /2826 ( 2013 .01 )400

7 /2011(Continued )Primary Examiner - Michael C Bryant(74 ) Attorney, Agent, or Firm — Knobbe Martens Olson

No .

(60 ) Provisional application No . 61/764 ,776 , filed on Feb .
14 , 2013, provisional application No. 61/688,630 ,CPC .... .

9 / 2000

size -weight-power/.

Nov. 17 , 2014 , now Pat. No. 9,625 ,318 , which is a(52) U . S . CI.

2 787 303

OTHER PUBLICATIONS
Adams, et al., “ Advances in Detectors: Hot IR sensors improve IR
camera size , weight, and power” , Laser Focus World , vol. 50 , Issue
01, Jan . 17 , 2014 , 6 pages. Also available at http ://www . ircameras .
com /articles/advances -detectors-hot-ir -sensors - improve - ir -camera

US 2017/0356802 A1 Dec. 14, 2017

GO1J 3/02

356 /219(Continued )Prior Publication Dataof

2 365 866

CACA(21) Appl. No.: 15/471,398(22) Filed : Mar. 28, 2017

continuation

11/ 1974 Girard et al.
1/ 1979 Goetz et al.

(Continued )FOREIGN PATENT DOCUMENTS

Subject to any disclaimer, the term of this(65 )Apr. 9 , 2019

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DIVIDED - APERTURE INFRA - RED
SPECTRAL IMAGING SYSTEM FOR

sponding optical channel from the at least two spatially and

CHEMICAL DETECTION

of a longpass optical filter and a shoilpass optical filter (withor without a combination with another filter such as a notch

spectrally differentoptical channels and includes at least one

5 filter, for example ). The optical system may further include
at least two reimaging lenses, each reimaging lens disposedto transmit IR radiation in one embodiment — between
This application is a continuation of U .S . patent applica
about 1 micron and about 20 microns), that has been
CROSS -REFERENCE TO RELATEDAPPLICATIONS

tion Ser. No . 14 /543,692 , filed on Nov. 17 , 2014 , which is atransmitted through a corresponding optical filter towards
continuation of International Application No. PCT/US2013 / 10 the optical FPA unit. In one embodiment, the optical FPA
041278 , filed on May 16 , 2013 which claims benefit of and unit is positioned to receive IR radiation from the object
priority from the U . S . Provisional Application Nos . 61/688 , through the at least two reimaging lenses to form respec
630 filed on May 18 , 2012 and titled “ Divided Aperture tively -corresponding two or more sets of imaging data
Infrared Spectral Imager (DAISI) for Chemical Detection " , 15 representing the object and the processor is configured toand 61/ 764,776 filed on Feb . 14 , 2013 and titled “ Divided 15 acquire said optical data from the two or more sets ofAperture Infrared Spectral Imager for Chemical Detection ” .

data .
The disclosure of each of the above -mentioned applications imaging
Embodimentsof the present invention additionally pro
is incorporated by reference herein in its entirety .
vide a method for operating an infrared (IR ) imaging system .
20 The method includes receiving IR radiation from an object
TECHNICAL FIELD
along atleast two optical channels defined by components of

The present invention generally relates to a system and an optical system of the IR imaging system , which at least
method for gas cloud detection and, in particular, to a system
two optical channels arc spatially and spectrally different
and method of detection of spectral signatures of chemical from one another. The method further includes transmitting
compositions in a mid - and long -wave infrared spectral 25 the received IR radiation towards an optical focal plane
region with a use of systemic compensation for parallax
array (FPA ) unit that is not being cooled in the course of

normal operation , and removably positioning at least one ofinduced and motion - induced imaging artifacts.
at least two temperature - controlled shutters in front of the
BACKGROUND
opticalsystem to block IR radiation incidentonto the optical
30 system from the object.
Most of the existing IR spectral imaging systems require
focal plane detector arrays (FPAs) that have to be highly
BRIEF DESCRIPTION OF THE DRAWINGS

sensitive and cooled in order to compensate , during theoptical detection, for the reduction of the photon flux caused

invention will be more fully understood by referringby spectrum -scanning operation . There remains a need , 35 toto The
the following Detailed Description in conjunction withtherefore , in a system enabling an optical data acquisitionmode that does not require the cooling of the usedthe Drawings , of which :FIG . 1 shows an embodiment of the system of thein the IR but yet well fit for continuous monitoring appli
stem 4040 pupil
spectrally and re-imaged onto an infrared FPA .
cations. There also remains a need in an IR imaging system
P FIG divided. 2 shows an embodiment with a divided front objec
the operation of which is substantially not susceptible tomotion artifacts (which is a common problem with spec tive lens and an array of infrared sensing FPAs.
FIG . 3A represents an embodiment employing an array of
trally - scanning systems causing errors in either the spectral
front objective lenses operably matched with the re -imaging
data , spatial data , or both ).
45 lens array .
FIG . 3B illustrates a two -dimensional array of optical
SUMMARY
components corresponding to the embodiment of FIG . 3A .
Embodiments of the present invention provide an infrared
FIG . 4 is a diagram of the embodiment employing an
detector ( s ), which detectors can be less sensitive to photons

invention utilizing a common front objective lens that has a(IR ) imaging system for determining a concentration of aarray of field references and an array of respectively corre

is configured to receive IR radiation from the object along atto spectrally divide an optical wavefront imaged with antarget species in an object. The imaging system includes (i) 50 sponding relay lenses .
an optical system , having an optical focal plane array (FPA )FIG . 5A is a diagram of a 4 -by -3 pupil array of circular
unit that is devoid of a cooling means , which optical system
optical filters (and IR blocking material among them ) used

least two optical channels defined by components of the embodiment of the invention .
optical system , said at least two optical channels being 55 FIG . 5B is a diagram of a 4 -by -3 pupil array of rectangularspatially and spectrally different from one another; (ii) first optical filters (and IR blocking material among them ) usedand second temperature -controlled shutters removably positioned to block IR radiation incident onto the optical system

to spectrally divide an optical wavefront imaged with anembodiment of the invention .

multispectral optical data representing said target species 60 teristics of a combination of band -pass filters used with anfrom the received IR radiation in a single occurrence of dataacquisition . The optical system may include an opticalaperture (a boundary of which is defined to circumscribe,

embodiment of the invention .

FIG . 6B depicts theoretical plots of transmission characteristics of spectrally multiplexed notch -pass filter combi

encompass said at least two spatially distinct optical chan - nation used in an embodiment of the invention .
nels ) and at least two spectrally -multiplexed optical filters . 65 FIG . 6C shows theoretical plots of transmission charac
Each of these optical filters is positioned to transmit ateristics of spectrally multiplexed long -pass filter combina
portion of the IR radiation received in a respectively corre - tion used in an embodiment of the invention .

US 10 ,254 , 166 B2
FIG . 6D shows theoretical plots of transmission charac

In the drawings like numbers are used to represent theteristics of spectrally multiplexed short-pass filter combina

same or similar elements wherever possible . The depictedtion used in an embodiment of the invention .

structural elements are generally not to scale , and certain

FIG . 7 is a set of video - frames illustrating operability of components are enlarged relative to the other components
an embodiment of the invention used for gas detection . 5 for purposes of emphasis and understanding. It is to be
FIGS. 8A , 8B are plots illustrating results of dynamic
understood that no single drawing is intended to support acomplete description of all features ofthe invention. In othercalibration of an embodiment of the invention .
words, a given drawing is generally descriptive of only
DETAILED DESCRIPTION

some, and generally not all, features of the invention . A
10 given drawing and an associated portion of the disclosurea description referencing such drawing do not,Embodiments of the present invention illustrate a divided containing
generally,
containelements of a particular view or all
aperture infrared spectral imaging (DAISI) system that is features that can beallpresented
is this view , for purposes ofstructured and adapted to provide identification of target simplifying the given drawing anddiscussion , and to direct
chemical contents of the imaged scene based on spectrally 15 thediscussion
toparticularelements
that are featured in thismultiplexed operation and single- shot (also referred to as drawing . A skilled artisan will recognize
the invention
snapshot), that is devoid of spectral and spatial scanning may possibly be practiced without onethat
or more of theacquisition of infrared (IR ) spectral signatures of the target specific features , elements , components, structures , details ,chemical contents with an TR detector ( such as, for example, or characteristics, or with the use of other methods, com
infrared focal plane array or FPA ) to form a spectral cube of 20 ponents, materials , and so forth . Therefore , although aimaging data . In contradistinction to commonly used IR
particular detail of an embodiment of the invention may not
imaging systems, the DAISI system does not require cooling.
Implementations of the present invention provide several

be necessarily shown in each and every drawing describing

such embodiment , the presence of this detail in the drawing

may be implied unless the context of the description requires

operational advantages over existing IR spectral imaging 25 otherwise . In other instances , well known structures, details ,systems, most if not all of which require FPAs that have tomaterials , or operations may be not shown in a given

be highly sensitive and cooled in order to compensate ,during the optical detection , for the reduction of the photon
flux caused by spectrum -scanning operation . The highly

drawing or described in detail to avoid obscuring aspects ofan embodiment of the invention that are being discussed .
Furthermore , the described single features, structures, orsensitive and cooled FPA systems are expensive and require 30 characteristics of the invention may be combined in any

a great deal of maintenance . As an embodiment of theinvention is configured to operate in single-shot acquisitionmode , the instrument receives photons from every point of
the object substantially simultaneously, during the single

suitable manner in one or more further embodiments .
Moreover, if the schematic flow chart diagram is included ,it is generally set forth as a logical flow - chart diagram . As
such , the depicted order and labeled steps of the logical flow

reading . In comparison with a system of related art , this 35 are indicative of one embodiment of the presented method .

feature enables an embodiment to collect a substantiallygreater amount of optical power from the imaged scene ( forexample , an order ofmagnitudemore photons ) at any given

Other steps and methodsmay be conceived that are equiva
lent in function , logic , or effect to one or more steps, or
portions thereof, of the illustrated method . Additionally, themoment in time. Consequently, an embodiment is enabled toformat and symbols employed are provided to explain theoperate using uncooled detector (s ) ( for example , FPA such 40 logical steps of the method and are understood not to limit
as an array of microbolometers ) that are less sensitive tothe scope of the method . Although various arrow types andphotons in the IR but are well fit for continuous monitoring line types may be employed in the flow -chart diagrams, they
applications since they are capable of operating in extreme

are understood not to limit the scope of the corresponding

weather conditions, require less power, can operate both day

method . Indeed , some arrows or other connectors may be

and night, and are less expensive . On the other hand , 45 used to indicate only the logical flow of the method . For

embodiments of the invention are advantageous in that theirinstance , an arrow may indicate a waiting or monitoring

operation is substantially immune to motion artifacts (which
is a common problem with spectrally - scanning systems

period of unspecified duration between enumerated steps ofthe depicted method . Without loss of generality , the order incausing errors in either the spectral data , spatial data , orwhich processing steps or particular methods occur may orboth ). Moreover , present embodiments are structured to 50 may not strictly adhere to the order of the corresponding

acquire spectrally -multiplexed datacubes during a single shot acquisition which , when combined with the detector

steps shown .
The invention as recited in claims appended to thisnoise limited performance of the FPA 's , result in increase of

disclosure is intended to be assessed in light of the disclosurelevel of the detected signal by a factor of 2 to 10 times, as as a whole , including features disclosed in prior art to which
compared with the systems of related art.
55 reference is made .

References throughout this specification to " one embodi-

ment," " an embodiment," " a related embodiment,” or similar language mean that a particular feature , structure , or

FIG . 1 provides a diagram schematically illustrating aspatial and spectral division of incoming lightby an embodi
ment 100 of the system of the invention (also referred to as

characteristic described in connection with the referred to DAISI system ) that is enabled to image an object 110“ embodiment” is included in at least one embodiment of the 60 possessing IR spectral signature(s ). An aperture of thepresent invention . Thus, appearances of the phrases " in one

system (associated with a front objective lens system 124 ) is

embodiment,” “ in an embodiment,” and similar language

spatially and spectrally divided . The spatial and spectral

throughout this specification may, but do not necessarily , alldivision of the aperture into distinct aperture portions cor

refer to the same embodiment. It is to be understood that noresponding to separate channels 120 (in object space and /or
portion of disclosure , taken on its own and in possible 65 image space ) along which light propagates through theconnection with a figure , is intended to provide a complete
description of all features of the invention .

aperture is enabled with the use of an array 128 of reimaging lenses 128a and an array of spectral filters 130 ,US 10 ,254 , 166 B2
which respectively correspond to the distinct channels 120 .
In one implementation , the distinct channels 120 may
include optical channels that are separated in angular space .
The array of spectral filters 130 may additionally include a

The front objective lens element of the system can gen
across the TR wavefront emitted by the objectbeing imaged

erally include an array of front objective lenses configured

with the DAISI system such as to divide such wavefront

filter-holding aperture mask ( containing, for example , IR 5 spatially in a non -overlapping fashion . To this end, FIG . 2

light-blocking materials such as ceramic , metal, or plastic ).

illustrates a related embodiment 200 , in which a frontoptical
portioncontributing to the spatial division of the aperture ofexample ), the optical properties of which in the IR arethesystemis defined by a multiplicity of objective lenses
described by a unique absorption , reflection and / or emission
224
configured
a two -dimensional (2D ) array of lenses .
spectrum , is received by the aperture of the system through 10 FIG . 2 presents asa generalview of the system 200 and, ineach of the channels 120 and is further imaged onto an
figureinsert,aportion202
of it in greater detail, including
optical detector component 136 (which may include at least a field reference (aperture stop
) 204 . The configuration 200
one FPA ). Each of the re-imaging lenses 128a is spatially has an operational advantage overembodiment 100 of FIG .
aligned with a respectively -corresponding region of thedivided aperture and , therefore , with respectively - corre - 15 1 in that the overall size and /or weight and / or cost of
sponding spatial channel 120 to form , on the FPA compo manufacture of the embodiment 200 is critically reduced

Light from the object 110 (such as a cloud of gas, fornent 136 a single sub - image of the object 110 . Generally, while the associated parallax (the change in the FOVs of
two or more sub - images of the object can be characterized individual lenses 224 of the lens -array disposed across andby close or substantially equal spectral signatures . The FPA
substantially perpendicularly to a general optical axis 226 ofcomponent 136 is further operably connected with a pro - 20 the embodiment 200 ;marked as 228 ) is substantially small.
cessor 150 (not shown ) specifically programmed to aggre gate the data acquired with the system 100 into a spectral

datacube representing , in spatial (x , y ) and spectral (a )As the distance between the portion 202 and the object 110
increases, the overlapping region 230 between the FOVs of

the individual lenses 224 increases while the amount of

coordinates an overall spectral image of the object 110
parallax 228 remains approximately the same, thereby
within the spectral region defined by the combination of the 25 reducing its effect on the system 200 . When the ratio of thefilters 130 . Additionally, the processor 150 may be option - parallax -to - object- distance is substantially equal to theally and specifically programmed to determine the unique pixel- size -to - system - focal length ratio then the parallax
absorption characteristic of the object 110 and , alternatively
effectmay be considered to be negligible and, for practicalor in addition , map the overall image datacube into a cube purposes , no longer distinguishable . While the lenses 224
of data representing spatial distribution of concentrations c 30 are shown to be disposed substantially in the same plane ,of targeted chemical components within the field of view
optionally the array of front objective lenses such as lenses

224 can be defined in more than one plane. For example ,In order to facilitate the operational performance of the some of the individual lenses 224 can be displaced withembodiment 100 , an optional moveable temperature -conrespect to some other individual lenses 224 along the axis
trolled reference target 160 ( including , for example , a shut- 35 226 (not shown ). It is noted that when multiple detectors 236
ter system containing two reference shutters maintained at are employed with the embodiment 200 , the embodiment isdifferent temperatures ) is removably and , in one implemen - preferably complemented with field reference 204 to operatetation , periodically inserted into an optical path of light properly, as discussed below .
traversing the system 100 from the object 110 to the FPA
In one implementation , the front objective lens systemcomponent 130 along at least one of the channels 120 to 40 such as the array of lenses 224 is configured as an array of

associated with the object 110 .

block such optical path and to provide a reference IR
spectrum required to recalibrate the operation of the system

100 in real time. The configuration of the moveable refer-lenses integrated or molded in association with a monolithic
substrate , thereby reducing the costs and complexity otherwise accompanying the optical adjustment of individual
lenses within the system . An individual lens 224 can option
In the embodiment 100 , the front objective lens system 45 ally include a lens with varying magnification. As one

ence (s ) 160 is further discussed below .

124 is shown to include a single front objective lens posi-

example , a pair of thin and large diameter Alvarez plates cantioned to establish a common field - of -view ( FOV ) for thereimaging lenses 128a and to define an aperture stop for thebe used to define at least a portion of the front objective lenssystem .

whole system (which , in this specific case , substantially

In further reference to FIG . 1 , the FPA component con

spatially coincides with limiting apertures corresponding to 50 figured to receive the optical data representing spectral
different optical channels 120 ). As a result , the positions for
signature ( s ) of the imaged object can be configured as aspectral encoding of the different optical channels 120
single FPA 136 adapted to acquire more than one sub -image

coincide with the position of the aperture stop of the whole ( formed along more than one optical channel 120 ) simulta
system , which is defined as a surface between the lens neously. Alternatively , the detector component may include
system 124 and the array 128 of the reimaging lenses 128a. 55 a set of optical FPAs at least one ofwhich can be configured

Generally , however, the field apertures corresponding toto acquired more than one spectrally distinct sub -image ofdifferent optical channels may be located in different planes. the imaged object (For example , as shown in the embodi
In one implementation the field apertures corresponding to ment 200 of FIG . 2 , an array of optical FPAs can include
different optical channels are located in different planes , FPAs 236 the number of which may correspond to thewhich planes are optical conjugates of one another ( as 60 number of the front objective lenses 224 ) . In one implemen

defined by the whole optical system ). Similarly, while all oftation of the system , an array of optical FPAs includes anlie in one plane, generally spectral filters corresponding todifferent optical filters can be associated with different

e ters advantageously allows for an inexpensive way toincrease the total number of detection elements ( i. e . pixels )the spectral filters 130 of the embodiment 100 are shown toarray of microbolometers . The use of multiple microbolom

planes . In one implementation , different spectral filters 130 65 for recording of the datacube in one snapshot. An array ofare situated in different planes are that are optically conju - microbolometers more efficiently utilizes the detector pixels
for each FPA as the number of unused pixels is minimized
gate to one another.

US 10 ,254 , 166 B2
and / or eliminated between the sub - images that may exist
FIG . 3A illustrates schematically a related embodiment
300 of the imaging system of the invention, in which thewhen using a single microbolometer.

460a, 460b , and then onto the detector module 456 (aThe embodiment 400 commissions several operational
advantages. It is configured to provide a spectrally known

microbolometer array or other IR FPA ).

number of the front objective lenses 324a in the lens array 5 object within every sub - image and for every snapshot acqui

324, the number of re - imaging lenses 128a in the lens array

128 , and the number of FPAs 336 are the same. So config -sition which can be calibrated against. (Such spectral cer

tainty is expedient when using an array of IR FPAs likeured , each combination of respectively corresponding front
microbolometers the detection characteristics of which canobjective lens 324 , re -imaging lens 128a , and FPA 336
change from one imaging frame to the next due to , in part,defines an individual imaging channel associated with acqui- 10 changes in the scene being imaged as well as the thermal

sition of the IR light transmitted from the object 110 throughan individual optical filter component 130 . A field reference

effects caused by neighboring FPAs. ) In addition , the fieldreference array 438 of the embodiment 400 is preferably —338 of the system 300 is configured to have a uniform

but not necessarily - disposed within the Rayleigh range

temperature across its surface and be characterized by a(„ the depth of focus ) associated with the front objective
predetermined spectral curve of radiation emanating there - 15 lenses 424 , thereby removing unusable blurred pixels due tofrom . The filed reference 338 is used for dynamicallyadjusting the data output from each FPA 336 after acquisition of light from the object 110 to ensure that outputof eachhaving the field reference outside of this range . Moreover,the embodiment 400 is more compact then , for example , theconfiguration 300 of FIG . 3A (which requires the employedof the FPAs 336 represents correct acquired data , withfield reference 338 to be separated from the lens array 324
respect to the other FPAs 336 for analysis , as discussed 20 by a distance greater than several ( for example , five ) focal
below in more detail. In one implementation , when a 4x3 lengths to minimize blur contributed by the field reference toarray 340 of optical components (lenses 324a, 128a ; detec an image formed at a detector plane.

tor elements 336 ), shown schematically in FIG . 3B , is usedbehind the temperature controlled reference target 160 , thein another related embodiment (not shown in FIGS. 1 , 2 ,3A , and 4 ) , the multi - optical FPA unit of the IR imaging

field reference 338 is adapted to obscure and/ or block a 25 system of the invention additionally includes an FPA con
peripheral portion of the bundle of light propagating fromfigured to operate in a visible portion of the spectrum . In

the object 110 towards the detector(s ) 336 . As a result , the reference to FIG . 1 , for example, an image of the scene offield reference 338 obscures and /or blocks the border or interest formed by such visible-light FPA may be used as aperipheral portion ( s) of the images of the object 110 formed background to form a composite image by overlappingon the FPA elements located along the perimeter 346 of the 30 (whether virtually , with the use of a processor and specifi
detector system . Generally , two detector elements will becally -designed computer program product enabling suchproducing substantially equal values of digital counts whendata processing , or actually, by a viewer ) an IR image ( thatthey are used to observe the sameportion of the scene in thesame spectral region using the same optical train . If any ofis created based on the image data acquired by the individual

FPAs 130 ) with the visible -light image. The so -formed

these input parameters ( scene to be observed , spectral con - 35 composite image facilitates the identification of the precise

tent of light from the scene , or optical elements delivering

spatial location of the target species the spectral signatures

light from the scene to the two detector elements ) differ, theofwhich the system of the invention is enabled to detect andcounts associated with the detectors will differ as well.

Accordingly , and as an example , in a case when the two

FPAs 336 (such as those denoted as # 6 and # 7 in FIG . 3B ) 40

remain substantially un -obscured by the field reference 338 ,the outputs from these FPAs can be -dynamically adjusted tothe output from one of the FPAs located along border (suchrecognize .

Optical Filters .

It is appreciated that the optical filters, used with anembodiment of the system , that define spectrally - distinct IR
sub -images of the object can employ absorption filters ,interference filters , and Fabry -Perot etalon based filters , toas, for example, the FPA element # 2 ) that processes spec - name just a few . When interference filters are used , the45 image acquisition through an individual imaging channel
trally similar light.

FIG . 4 illustrates schematically a portion of anotherembodiment 400 that contains an array 424 of front objec tive lenses 424a adapted to receive light from the object 110
that relay the received light to the array 128 of re -imaging

defined by an individual reimaging lens ( such as a lens 128aof FIGS. 1 , 2 , 3 , and 4 ) may be carried out in a single
spectral bandwidth or multiple spectral bandwidths. Referring again to the embodiments 100 , 200 , 300 , 400 of FIGS.

lenses 128a through an array 438 of field references ( field 50 1 through 4 , and in further reference to FIG . 3B , examples
stops ) 438a the spectral characteristics of which are known ,of a 4 -by - 3 array of spectral filters 130 is shown in FIGS .

and through an array 440 of the relay lenses . The field

5A , 5B , where individual filters 1 through 12 are juxtaposed

references 438a are disposed at corresponding intermediatewith a supporting opto -mechanical element (not shown ) toimage planes defined , with respect to the object 110 , by define a filter-array plane that is oriented , in operation ,respectively corresponding front objective lenses 424a . 55 substantially perpendicularly to the general optical axis 226(When refractive characteristics of all of the front objective of the embodiment.
lenses 424a are substantially the same, all of the field
The optical filtering configuration of one present embodi
references 438a are disposed in the same plane ). A field

ment advantageously differs from a common approach usedreference 438a of the array 438 obscures ( casts a shadow on )to measure spectra with an array of FPAs, where a bandpass

the detector plane 444 through a respectively corresponding

example , any of the filters Oa through 4a the transmission

spatial imaging channel 450 of the system 400 prior to such

curves of which are shown in FIG .6A ) is placed in front of

sub - image being spectrally processed by the processor 150 .
The array 440 of relay lenses then transmits light along eachthe optical FPA (generally, between the optical FPA and theobject ). In particular, and in further reference to FIGS. 1, 2of the imaging channels 450 through different spectral filters 65 3, and 4 when optical detector (s) 136 , 236 , 336 , 456 of an
454a of the filter array 454, past the two - point calibration
apparatus that includes two temperature controlled shuttersembodiment include ( s ) microbolometers , the predominant
contribution to noise associated with image acquisition isUS 10 ,254 , 166 B2
10

due detector noise. To compensate and / or reduce the noise ,that in practice an embodiment of the invention can employ

an embodiment of the invention utilizes spectrally -multi

any number of filters ). For illustration , the case of M = 7 isteristics of spectrally -multiplexed filters Ob through 4b forlocation in each of sub - images formed by differing imaging

plexed filters . An example of spectral transmission charac -considered . Analysis presented below relates to one spatial

use with an embodiment of the invention is depicted in FIG . 5 channels defined by the system . As similar analysis can be

6B . (Filters of FIG . 6B are so -called long -wavelength pass , performed for each point at a sub -image , the analysis can be
LP filters . An LP filter generally attenuates shorter wave
appropriately extended as required .
lengths and transmits (passes ) longer wavelengths over the
The unknown amount of light within each of the M
active range of the target IR portion of the spectrum . It is
spectral channels ( corresponding to these M filters ) isappreciated that, in a related embodiment, short -wave - 10 denoted with si, 32, 33, . . . , f and readings from corre

length -pass filters , SP, may also be used . An SP filter

sponding detector elements receiving light transmitted bygenerally attenuates longer wavelengths and transmits

each filter is denoted as g1 , 92 , 93 . . . gy , while measurement(passes ) shorter wavelengths over the active range of the errors are represented by ni, ng, nz, . . . ny . Then , thetarget IR portion of the spectrum .)readings at the seven FPA pixels each of which is optically
The related art appears to be silent with respect to an IR 15 filtered by a corresponding bandpass filter of FIG . 6A can be
imaging system , adapted for detection of spectral signatures represented by :of chemical species that combines the use of the spectrally
81 = fi +N 1
multiplexed filters with a snap - shot image acquisition . The
lack of such teaching can probably be explained by the fact
82 = 42 + n2,
that related imaging systems require the use of highly 20
sensitive and, for that reason , expensive cooled FPAs with
83 =f3+N3,
reduced noise characteristics . Accordingly , the systems of
the related art are commonly employing bandpass filters
84 = f4 +14)
instead , to take full advantage of spectral sensitivity of theused FPAs. Simply put, the use of spectrally multiplexed 25
85= fs+15,
filters such as notched , LP , and SP filters would be coun
terproductive in a system of the related art, and would at
8646 +16)

least reduce an otherwise achievable SNR thereby degrading

the performance of the related art system for the intended
purpose . In contradistinction with the systems of the related 30

art, however, and at least in part due to the snap - shot/nonscanningmode of operation , an embodiment of the imaging
system of the invention is enabled to use less sensitive

87 = f7 +N7,m
These
readings (pixelmeasurements ) g ; arc estimates ofthe spectral intensities f;. The estimates g; are not equal to thecorresponding f; values because of the measurement errors
nj. However, if the measurement noise distribution has zero

microbolometers without compromising the SNR . The use mean , then the ensemble mean of each individualmeasureof microbolometers , as detector -noise - limited devices, in 35
can be considered to be equal to the true value , i.e .
turn not only benefits from the use of spectrally multiplexed ment(g :) = = f;. Here , the angle brackets indicate the operation offilters, but also does not require cooling of the imaging calculating
the ensemble mean of a stochastic variable . The
system during normal operation .
variance of the measurement can , therefore , be represented

Referring again to FIGS. 6A , 6B , each of the filters(Ob . . . 4b ) transmits light in a substantially wider region of 40 as.

spectrum as compared to those of the filters ( 0a . . . 4a ) .
Accordingly, when the spectrally -multiplexed set of filters( 0b . . . 0d) is used with an embodiment of the invention , the{(82- 87)2} = (n;}) = 0
In an alternative design utilizing spectrally -multiplexedfilters and in comparison with the design utilizing bandpass
filters , the amount of radiant energy transmitted by each of
236 , 336 ) is larger than would be received when using the 45 thespectrally -multiplexed LP or SP filters towards a given
bandpass filters (0a . . . 4a ). This “ added ” transmission of detector
element can exceed that transmitted through alight defined by the use of the spectrally -multiplexed LP (orspectralbandof a bandpass filter. IN this case , the intensities
SP ) filters facilitates increase of the signal on the FPAS of light corresponding
to the independent spectral bands canabove the level of the detector noise. Additionally , by using ,overall amount of light received by the FPAs (for example ,reconstructed by computational means . ( Such design isin an embodiment of the invention , filters the spectra of 50 bereferred to as a “ multiplex design ” ).

which are wider than those of conventionally used band - passfilters, the uncooled FPAs of the embodiment experience

One matrix of such “ multiplexed filter” measurements

includes a Hadamard matrix (requiring “ negative” filters thatless heating due radiation incident thereon from the imaged
may
be necessarily appropriate for the optical embodi
scene and from radiation emanation form the FPA in ques mentsnotdisclosed
herein ) An S -matrix approach (which istion itself, due to a reduction in the back -reflected thermal 55

restricted to having a number of filters equal to an integer
emission ( s) coming from the FPA and reflecting off of the thatis multiple of four minus one ) or a row - doubled Had
filter from the non band -pass regions. As the transmission amard(requiring a number of filters to be equal to an
region of the multiplexed LP (or SP ) filters is wider, such integermatrix
multiple of eight) present alternative methodologies .
parasitic effects are reduced thereby improving the overall Here, possible numbers of filters using an S -matrix setup are
60
performance of the FPA unit .
3 , 7 , 11 , etc and , if a row -doubled Hadamard matrix setup is
In one implementation, the LP and SP filters can be used
, then 8 , 16 , 24 , etc . For example , the goal of the
combined, in a spectrally -multiplexed fashion as described , measurement
may be to measure seven spectral band f
in order to maximize the spectral extent of the transmission intensities using
seven measurements', as follows:
region of the filter system of the embodiment.
The advantage of using spectrally multiplexed filters is 65
81=fi+O + f3 +0 + fs +O +f=+n1,
appreciated based on the following derivation , in which asystem of M filters is examined ( although it is understood

82 = 01f2\/31010 polfz|n2

US 10 ,254 , 166 B2
83= f2+ f2 + 0 + 0 + f3 + 0 +f7+ n3
84 +0 + 0 + 0 + f4 + fs +f7+ f3 +n4

11

85 =f1+ 0 + f3 + f4 + 0 +f6+0 +ng
86= 0 + f3 + f3 +f4 + f3 +0 + 0+ n6
87=fi+f2+0 +f4 +0 + 0 + f4 + 17
Optical transmission characteristics of the filters

in FIG . 3A ), or an array of field reference components (438in FIG . 4 ), to enable dynamic calibration for spectral acquisition of every datacube, a spectrally -neutral camera -tocamera combination to enable dynamic compensation of
5 parallax artifacts , and a visible and / or IR camera for com

pensation of motion artifacts. The use of the temperature

controlled reference unit (for example , temperature-con

trolled shutter system 160 ) and field -reference component (s )facilitates maintenance of proper calibration of each of thedescribed above are depicted in FIG . 6B . Here , we no longer 10 FPAs individually and the entire FPA unit as a whole.
have a direct estimate of the f; through a relationship similar
In particular , and in further reference to FIGS. 1, 2 , 3 , andto ( g :) - f;. Instead , if a “ hat” notation is used to denote an
estimate of a given value , then a linear combination of themeasurements can be used such as , for example,

4 , the temperature - controlled unit generally employs a sys
tem having first and second temperature zones maintained at
first and second different temperatures . For example , shutter

15 system of each of the embodiments 100 , 200 , 300 and 400=( + 81 – 82 + 83 - 84 + 85 – 86 + 87 ) ,=wN

00 1 + 82 + 83 - 84 - 85 + 86 + 87 ) ,=( + 81 + 82 – 83 - 84 + 95 + 86 - 87 ),=to- 82 - 83 + 84 + 95 + 86 + 87 ) ,== 7 ( + 81 – 82 + 83 + 84 - 95 + 86 - 87 ),fo= 5 - 81 +82+ 83 + 84 + 85 – 86 – 87),=

( + 81 + 82 – 83 + 84 - 85 - 86 + 87 ).

can employ not one but at least two temperature - controlled
shutters that are substantially parallel to one another andtransverse to the general optical axis 226 of theembodiment(s ) 100 , 200 , 300 , 400 . Referring , for example ,20 to FIG . 4 , in which such multi-shutter structure is alreadyindicated , the use of multiple shutters enables the user tocreate a known reference temperature difference perceived ,by the FPAS 456 through the IR radiation emitted by the
shutter (s ) 160a, 160b when these shutters are positioned to

25 block the radiation from the object 110 . As a result, not only
the offset values corresponding to each of the individual
FPAs pixels can be adjusted but also the gain values of these
FPAs. In an alternative embodiment, the system having firstand second temperature zones may include a single or

30 multi -portion piece ( such as a plate , for example ) mechani

cally -movable across the optical axis with the use of appro

These f; are unbiased estimates when the n ; are zero mean

stochastic variables, so that (f;- f;) = 0 . The measurement

variance corresponding to ith measurement ispriate guides and having a first portion at a first temperature

and a second portion at a second temperature .

Indeed , the process of calibration of an embodiment of the

35 invention starts with estimating gain and offset ( that varyfrom detector pixel to detector pixel) by performing mea

10 :- 1 )= Too
Therefore , by employing spectrally -multiplexed system
the signal-to - noise ratio (SNR ) of a measurement has been
improved by a factor of V16 /7 = 1.51.

surements of radiation emanating , independently , from atleast two temperature - controlled shutters of known anddifferent radiances . Specifically , first the response of the
40 detector unit 456 to radiation emanating from one shutter(for example, shutter 160a that is blocking the FOV of thedetectors 456 and the temperature T , of which is measured
directly and independently with thermistors ) is carried out.
For N channels , the SNR improvement achieved with a
Following such initial measurement, the shutter 160a is
spectrally -multiplexed system can be expressed as 45 removed from the optical path of light traversing the( N + 1 )/( 2VN ) . For example , in an embodiment employing 12
embodiment and another shutter ( for example, 160b ) is
spectral channels is characterized by SNR improvement, inserted in its place across the optical axis 226 to prevent theover a non -spectrally -multiplexed system , by a factor of up propagation of light through the system . The temperature of
the second shutter 160b is T2 + T? is also independently
Two additional examples of related spectrally -multi - 50 measured with thermistors placed in contact with this shut

to 1 .88 .

plexed filter arrangements 0c through 4c, Od through 4dfrom the use of which an embodiment of the invention canter, and the detector response to radiation emanating fromthe shutter 160b is also recorded . Denoting operational

benefit when such embodiment includes an uncoolcd FPA

response of FPA pixels ( expressed in digital numbers , or

( such as a microbolometer ) arc shown in FIGS . 6C and 6D .

" counts” ) as g to a source of radiance Li, the readings

FIG . 6C illustrates a set of spectrally -multiplexed long - 55 corresponding to the measurements of the two shutters canwavelength pass (LP ) filters is used in the system . An LP be expressed as:
filter generally attenuates shorter wavelengths and transmits
g1= YL ( T1)+ goffset(passes ) longer wavelengths over the active range of thetarget IR portion of the spectrum . A single spectral channel
g2yL2 ( T2) + goffset
having a transmission characteristic corresponding to the 60

difference between the spectral transmission curved of atleast two of these LP filters can be used to procure imaging

Here, & offset is the pixel offset value (in units of counts ),and y is the pixel gain value (in units of counts per radiancedata for the datacube with an embodiment of the invention .

unit ). The solutions of these two equations with respect to160 ; 160a , 160b ), employ a field reference component ( 338from the known temperatures T , and T2 together with the

As alluded to above , an embodiment may optionally, andthe two unknowns goffrer and y can be obtained if the values
in addition to temperature -controlled reference unit ( for 65 of g , and g , and the radiance values L , and L , are available
example temperature controlled shutters such as shutters(either measured by a reference instrument or calculatedUS 10 , 254 , 166 B2

13

14

any subsequent measurement , one can then invert the equa

Examples of Methodology of Measurements
Prior to optical data acquisition with an embodiment ofknown spectral response of the optical system and FPA ). Fortion (s ) above in order to estimate the radiance value of theobject from the detector measurement, and this can be done

the IR imaging system of the invention , it is preferred tofor each pixel in each FPA array within the system .
5 calibrate all the FPAs of the system (such as FPAS 336 eachAs already discussed , and in reference to FIGS . 1 throughof which forms an image of the object in light delivered in

4 the field -reference apertures may be disposed in an object
space or image space of the optical system , and dimensioned

a corresponding optical channel defined by the combinationof the corresponding front objective and re -imaging lenses

to block a particular portion of the IR radiation received

324 , 128a , in reference to FIG . 3 ) . The calibration isfrom the object that, in absence of the field - reference aper - 10 necessitated by a need to form individual images in equiva
ture, would transmit through the optical system without lent units ( so that, for example , the reading from each of thetraversing at least two spectrally -multiplexed optical filters. FPA pixels can be re - calculated in units of temperature or
For example , the field - reference aperture the opening ofradiance units ). Moreover, while it is often neglected inwhich is substantially similar, in shape, to the boundary ofpractice, each of the FPAs should be spatially co -registered

the filter array ( for example , and in reference to a filter array 15 with one another so that a given pixel of a particular FPA canof FIGS. 3B , 5B - rectangular ), can be placed in front of thebe confidently optically re -mapped through the optical sys

objective lens ( 124 , 224 , 324 , 424 ) at a distance that is at tem of the embodiment to the same location at the object as
least several times in one implementation — at least five the corresponding pixel at another FPA .
times ) larger than the focal length of the lens in order to
To achieve at least some of these goals, a so -called
minimize the image blur that would occur in absence of such 20 spectral differencing method may be employed , which
field -reference. In the embodiment 400 of FIG . 4 the field
employs forming a difference image from various combi
reference aperture can be placed within the depth of focus of nations of the images registered by two or more different
an image conjugate plane formed by the front objective lens FPAs. If the optical filter 130 corresponding to a particular
424 . The field reference , generally , effectuates and/ or

FPA 336 transmits light from the object including a cloud of

enables dynamic compensation in the system by providing a 25 gas, for example , with a certain spectrum that contains the
spectrally known and temporally - stable object within every
gas absorption peak or a gas emission peak while anotherscene to reference and stabilize the output from the different
FPAs in the array .

filter 130 corresponding to another FPA 336 does not transmit such spectrum , then the difference between the images

Because each FPA 's offset value is generally adjusted formed by the two FPAs at issue will highlight the presencefrom each frame to the next frame by the hardware, com - 30 of gas in the difference image.

paring the outputs of one FPA with another can have an errorthat is not compensated for by the calibration parameters

A shortcoming of the spectral differencing method is that
contributions of some auxiliary features associated withgoffset and y . In order to ensure that FPAs operate in radio metric agreement, it is necessary for a portion of each

imaging ( not just the target species such as gas itself ) can
also be highlighted in and contribute to the difference image .

source spectrum is known a priori ( such as a blackbody
source at a known temperature ), one can measure themagnification differences between the two or more opticalchannels , and differences in rotational positioning and ori

response of each FPA to the reference source in order toentation between the FPAs. While magnification -related

detector array to view a reference source (such as the field 35 The so contributing effects include , to name just a few ,reference 338 in FIG . 3A , for example ). Since the reference parallax -induced imaging of edges of the object, influence of

estimate changes to the pixel offset value . An example 40 errors and FPA - rotation - caused errors can be compensatedcalculation of the dynamic offset proceeds as follows.
Among the FPA elements in an array of FPAs in a given

for by increasing the accuracy of the instrument construction
as well as by post- processing of the acquired imaging ,embodiment of the invention , we select one FPA to be theparallax is scene - induced and is not so easily correctable . In

" reference FPA ” . We will attempt to make all of the FPAS

addition , while it is not widely recognized , the spectral

agree with this one about the field reference temperature . 45 differencing method is vulnerable to radiance calibration
The image measured in each FPA contains a set of pixels
errors . Specifically , if one FPA registers radiance of light

obscured by the field reference 338 . Using the previously

obtained calibration parameters offer and y (the pixel offset
and gain ), we estimate the effective blackbody temperaturefrom a given feature of the object as that having a tempera

ture of 40° C ., for example , while the data from another FPA
represents the temperature of the same object feature as

T of the field reference as measured by each FPA i. That is, 50 being 39° C ., then such feature of the object will beenhanced or highlighted in the difference image ( formed at
T =mean {(g +48i+ offse /Y}=mean {(8 -8offset){Y} AT;
least in part based on the images provided by these two
Here , the mean value is procured over all pixels that arc FPAs
) due to such radiance- calibration error.
obscured by the field reference , and Ag, is the difference in
One solution to some of the problems introduced by theoffset value of the current frame from Agoffset obtained 55 spectral differencing is to normalize the resulting image data
during the calibration step . For the reference FPA , Agi is by the data corresponding to a temporal reference image .
simply set to zero . Then , using the temperature differences

measured by each FPA , one obtains
Ty- Tref =mean {(g + Agi+ goffset(Y} + AT;
mean {(8 - 8offset )/Y} = AT ;Once At; for each FPA is measured , its value can be
agreement between such FPA and the reference FPA . While

This is referred to , for the purposes of this disclosure , as atemporal differencing algorithm or method . A temporalreference image may be formed , for example, by creating a
60 difference image from the two or more images registered by

the two ormore FPAs at a single instance in time. It does notinvention is that a prior knowledge of whether the object or

subtracted from each image in order to force operational

matter whether corollary of the use of the algorithm of thethe calibration procedure has been discussed above in ref-scene contains a target species (such as gas of interest ) does

methodology of calibration with respect to radiance value

can be implemented .

changes in the scene characteristics . Then , a spectral differ

ence image can be calculated as discussed above based on a

US 10 ,254 , 166 B2
15

16

snap -shot image acquisition at any later time and subtracted
from the temporal reference image to form a normalized
difference image . The difference between the two highlights

using images created by individual re -imaging lenses , 128a ,when using a single FPA or multiple FPA ' s as discussed
above .

the target species ( gas ) within the normalized difference

Another capability of the embodiment of the invention isreference frame. If necessary , more than two FPAs can beused both for registering the temporal reference image andcloud volumetric estimation . This can be accomplished byusing ( instead of compensating or negating) the parallax

image , since this species was not present in the temporal 5 the ability to perform the volumetric estimation of a gas

a later- acquired difference image to obtain a better SNR induced effects described above . In this case , the measured
parallax between two or more similar spectral response
While the temporal differencing method can be used to 10 images can be used to estimate a distance between the

figure of merit.

reduce or eliminate some of the shortcomings of the spectral
differencing , it can introduce unwanted problems of its own .

imaging system and the gas cloud or between the imaging
system and an object in the field of view of the system . The

For example , temporal differencing of imaging data is lessparallax - induced transverse image shift d between two

sensitive to calibration and parallax induced errors than theimages is related to the distance z between the cloud orspectral differencing of imaging data . However, any change 15 object and the imaging system according to z = - sz'/d , wherein the imaged scene which is not related to the target species
s is the separation between two similar spectral responseof interest (such as particular gas, for example) is high - images, and z' is the distance to the image plane from thelighted in a temporally -differenced image, and thus may be back lens (z' is typically approximately equal to the focal
erroneously perceived as a location of the target species length fof the lens of the imaging system ). Once the distancetriggering, therefore , an error in detection of target species. 20 z between the cloud and the imaging system is calculated ,For example , if the temperature of the scenic backgroundagainst which the gas is being detected changes ( due tothe size of the gas cloud can be determined based on themagnification equation , m = f/ z , where each image pixel on

natural cooling down as the day progresses, or increases due

the gas cloud, Ax ', corresponds to a physical size in object

to a person or animal or another object passing through thespace Ax= Ax '/m . To estimate the volume of the gas cloud , a
FOV of the IR imaging system ), then such temperature 25 particular symmetry in the thickness of the cloud based onchange produces a signal difference as compared to thethe physical size of the cloud can be assumed . For example ,measurement taken earlier in time. Accordingly , the cause of

the cloud image can be rotated about a central axis running

the scenic temperature change the cooling object, the per -through the cloud image to create a three dimensionalson walking, etc .) may appear as the detected target species

volume estimate of the gas cloud size. It is worth noting that( such as gas ). It follows, therefore , that an attempt to 30 only a single imaging system of the invention is required forcompensate for operational differences among the individual
FPAs of a multi- FPA IR imaging system with the use of

methods that turn on spectral or temporal differencing cause

additional problems leading to false detection of target

such volume estimation , in contradistinction with carryingout such estimate with a spectral imaging system of relatedart (in which case at least two imaging systems would benecessary ). Indeed , due to the fact that the information about

species . Among these problems are scene -motion - induced 35 the angle at which the gas cloud is seen by the system is

detection errors and parallax - caused errors that are notreadily correctable and/ or compensatable as of to -date .
Accordingly, there is an unfulfilled need to compensate forimage data acquisition and processing errors caused bydecoded in the parallax - effect, the image data includes theinformation about the imaged scene viewed by the system inassociation with at least two angles.
When the temporal differencing algorithm is used formotion of elements within the scene being imaged . Embodi- 40 processing the acquired imaging data , a change in the scene

ments of data processing algorithms of the present invention
address and fulfill the need to compensate for the motion
induced and parallax -induced image detection errors .

In particular, to minimize parallax - induced differencesthat is caused not by the target species is highlighted in the
resulting image . According to an embodiment of the invention , compensation of this error makes use of the temporal
differencing between two FPAs that are substantially equally

between the images produced with two or more predeter - 45 spectrally sensitive to the target species . In this case , themined FPAs, another difference image is used that is formed

temporal difference image will highlight those pixels thefrom the images of at least two different FPAs to estimate
parallax effects . For example, the spectral differencing of thespectra of which have changed in time. Subtracting the data
corresponding to these pixels at both FPAs to form theimage data is being performed with the use of the difference

resulting image , therefore , excludes the contribution of the

between the images collected by the outermost two cameras 50 target species to the resulting image . The differentiationin the array (such as , for example, the FPAs corresponding

between (i) changes in the scene due to the presence of target

a difference image referred to as a “ difference image 2 - 3 ” . In

the background not associated with the target species is ,to filters 2 and 3 of the array of filters of FIG . 5A ), forming

species and ( ii ) changes in the scene caused by changes inthis case , the alternative “ difference image 1 -4 ” is addition , therefore , enabled . It should be noted that, quite unexpect
ally formed from the image data acquired by, for example , 55 edly, the data acquired with the visible - light FPA (whenthe alternative FPAs corresponding to filters 1 and 4 of FIG . present as part of the otherwise IR imaging system ) can also

5A . Assuming or ensuring that both of these two alternative be used to facilitate such differentiation and compensation of
FPAs have approximately the same spectral sensitivity to thethe motion - caused imaging errors . Visible cameras generally
target species, the alternative “ difference image 1 -4 ” will have much lower noise figure than IR cameras (at least
highlight pixels corresponding to parallax -induced features 60 during daytime). Consequently, the temporal differencein the image . Accordingly , based on positive determination image obtained with the use of image data from the visiblethat the same pixels are highlighted in the spectral “ differ - light FPA can be quite accurate . The visible FPA can be usedence image 2 - 3 ” used for target species detection , the to compensate for motion in the system as well as many
conclusion is made that the image features corresponding topotential false - alarms in the scene due to motion caused bythese pixels are likely to be induced by parallax and not the 65 people , vehicles, birds, and steam , for example , as long aspresence of target species in the imaged scene. It should be the moving object can be observed in the visible region of
noted that compensation of parallax can also be performed the spectra . This has the added benefit of providing an

US 10 ,254 , 166 B2
17
additional level of false alarm suppression without reducing

18

smaller than the NETD value corresponding to a single BP
filter, thereby providing an experimental proof that the use

the sensitivity of the system since many targets such as gas
clouds cannot be observed in the visible spectral regionof an embodiment of the invention as discussed above
Another method for detection of the gases is to use a provides an unexpected increase in a SNR - type figure ofspectral unmixing approach . A spectral unmixing; approach 5 merit of the IR spectral imaging in comparison with a system
assumes that the spectrum measured at a detector pixel isof related art. In other words , the use of two (or more ) LP

composed of a sum of component spectra , and attempts toestimate the relative weights of these components needed toderive the measurement spectrum . The component spectraor SP filters to extract the imaging data results in a spec

trally - narrowed imaging channel having a higher opera

tional performance as compared with the use of a bandpass

are generally taken from a predetermined spectral library 10 filter centered on the chose wavelength in the same spectral( for example , from data collection that has been empiricallyassembled ), though sometimes one can use the scene toestimate these as well (often called “ endmember determi-

channel.
It is worth noting that the use of optically - filtered FPAS
rather than a more conventional Fourier Transform spec

nation ” ) . For the gas cloud detection , the component spectra trometer (FTS ) in an embodiment of the invention is partly
include the absorption spectra of various gases of interest, 15 explained by a larger number of total pixels available withwhile the “ measurement spectrum ” is not the raw measure a single large format microbolometer FPA array. Morement of spectral intensity values but rather an “ absorption importantly , however, the use of the FTS is well recognized
spectrum ” , which includes the spectrum of background lightto require tightmechanical tolerances, leading to sufficiently

absorbed on transmission through a cloud The spectral

more complex assembly of the imaging system employing

12 optical channels and 12 corresponding microbolometerto regions of the spectrum that the FTS instrument is

unmixing methodology can also benefit from temporal, 20 the FTS as compared to the assembly of the embodiment of
the invention . Additionally , the FTS does not offer a highparallax , and motion compensation techniques .
enough optical throughput for a substantially low number of
Examples of Practical Embodiments and Operation
optical channels (for example, between 4 and 50 opticalchannels ) (in part because many of the sampled wavenum
The embodiment 300 of FIG . 3 was configured to employ 25 ber values in the reconstructed spectrum do not correspond
FPAs as 336 and used to capture a video sequence ( repre sensitive to , and so such sampled data is eventually dis
senting images of a standard laboratory scene ) substantiallycarded and not used for image formation and detection of
immediately after performing calibration measurements
target species) . The FTS is better suited to higher resolution
with the use of a reference source including two shutters , as 30 spectroscopy. The problem with working with highly -rediscussed above ( one at room temperature and one 5° C .
solved spectra , however, is that by sampling the same
above room temperature ). The use of 12 FPAs offers a goodamount of incident light with smaller spectral bins means
chance of simultaneous detection and estimation of thethat image data corresponding each bin is actually noisier.

concentrations of about 8 or 9 gases present at the scene, but Therefore , while improved spectral resolution accorded by
the number of FPAS 336 can vary , depending on the balance 35 the FTS can allow the user to pick locations in the spectrum
between the operational requirements and consideration of that are highly specific to the absorption /emission signaturecost.
Duc to the specifics of operation in the IR range of theof the target species, it also makes such signature weaker
relative to the detection noise .

spectrum , the use of the so -called noise -equivalent tempera -A major advantage of the embodiments of the present

ture difference ( or NETD ) is preferred and is analogous to 40 system over instruments of the related art that are configured
the SNR commonly used in visible spectrum instruments .
for target species detection ( for example , gas cloud detec
The array of microbolometer FPAS 336 is characterized toperform at NETD = 72 mK at an f-number of 1.2 . Eachmeasurement was carried out by summing four consecutive

tion ) is that, according to the present invention , the entire

spectrum is resolved in a snapshot mode ( for example,

during one image frameacquisition by the FPA array ). This

frames , and the reduction in the NETD value expected due 45 enables the system of the invention to take advantage of theto such summation would be described by corresponding
factor of V4 = 2 . Under ideal measurement conditions, there

fore , the FPA NETD should be about 36 mK .
The commercially off - the-shelf LP, SP, and/or BP filterscompensation algorithms such as the parallax and motion
compensation algorithmsmentioned above . Indeed , as theimaging data required to implement these algorithms arecollected simultaneously with the target-species related data ,were used as the filters 130 . Using the image data acquired 50 the compensation algorithms are carried out with respect toat each FPA pixel, the processor of the system was used totarget- species related data and not with respect to dataacquired at another time interval , thereby ensuring accuracy
representing a “ mean image ” and a “ standard deviation of data acquisition is much higher ( the present system
image " of the scene.
55 operates at up to video rates ; from about 5 Hz and higher, for
Table 1 summarizes the “mean NETD " values obtained example ), so that the user is enabled to recognize in theimage data across the video sequence , to form a data outputof the data compensation process . In addition , the frame rateby averaging of the NETD values over all pixels of theimages the wisps and swirls typical of gas mixing without

standard deviation image ” , as well as the median NETD

blurring out of these dynamic image features and othervalues obtained in a similar fashion , in degrees Celsius . The

artifacts caused by the change of scene (whether spatial or

top section of Table 1 shows the results for the bandpass 60 spectral) during the lengthy measurements . In contradistincfilters, the middle section of Table 1 shows the results for the
LP and SP filters, and the bottom section of Table 1 presents
data for differences between NETD values of two specified
LP filters . (Note that no filter is treated as equivalent to ation with the imaging systems of the related art that require
image data acquisition over a period of time exceeding asingle -snap -shot time and, therefore , blur the target gas

features in the image and inevitably reduce the otherwise
65 achievable sensitivity of the detection , embodiments of the
The results of Table 1 indicate that the difference between
present invention make detecting the localized concentra
the NETD values of the two chosen LP filters is substantiallytions of gas without it being smeared out and /or averaged
longpass filter here .)US 10 ,254 , 166 B2
19with the areas of thinner gas concentrations. In addition , thehigher frame rate also enables a much faster response rate toa leak of gas (when detecting such leak is the goal): an alarm

can trigger within fractions of a second rather than several

seconds .

TABLE 1
filtermean

medianBP - 8224

0 .691
0 . 938
0 .318

0 .675
0 .923BP - 9480
BP - 9740
BP - 10240
BP - 10700
BP - 10962

BP - 11660
BP - 12227
BP - 13289[none ]

0 . 372
0 .275
0 .4090 .44
0 .5480 . 481 . 309

0 .038LP-8110

0 .063LP - 8500

0 .0760 . 068
0 . 068
0 . 08LP -8110
LP -8305

LP-8500
LP - 9000
LP - 9650

LP-9800
LP-11000

LP -11450

SP -10500[none ]-[LP- 8110 ][ LP -9650 ]-[ LP - 11450 ][LP - 9800 ]-[LP- 11000 ]0 .0730 . 099
0 . 109

0 . 156
0 .2070 .07
0 .07

0 .2080 . 227

estimates are steadily drifting in time, from frame to frame.
5 The curves labelled “ ref” (or “ C ” ) correspond to the detec
tors ' estimates of the temperature of the aperture 338 of theembodiment 300 of FIG . 3A . The results of detection of

NETD (deg C .)BP - 9000

20
the object's temperature, and that difference in object 's
temperature perceived by different detector elements is ashigh as about 2 .5° C . In addition , all of the temperature

0 . 315
0 . 369
0 .286
0 .405
0 .437
0 .542
0 . 475
1 . 26
0 .0370 . 063
0 .075
0 .0670 . 067
0 .08
0 .0730 .0980 . 108
0 . 156

0 . 206

0 .07(i.e .methane )(i.e . propylene)(i.e . butane )radiation carried out after each detector pixel has been
subjected to the dynamic calibration procedure described
10
above are expressed with the curved labeled “ obj corr” (or“ B ” ). Now , the difference in estimated temperature of theobject among the detector elements is reduced to about 0 . 5°

C . (thereby improving the original reading at least by a
15 factor of 5 ).
FIG . 8B represents the results of similar measurements

corresponding to a different location in the scene (a location
temperature of the aperture 338 of FIG . 3A ). As shown , the20 correction algorithm discussed above is operable and effective and applicable to objects kept at different temperature.
which is at a temperature about 9° C . above the estimatedindependent .

At least some elements of a device of the invention can be

25 controlled — and at least some steps of a method of theinvention can be effectuated, in operation with a program

mable processor governed by instructions stored in amemory. The memory may be random access memory(RAM ), read -only memory (ROM ), flash memory or any
30 other memory , or combination thereof, suitable for storing

control software or other instructions and data . Those skilledin the art should also readily appreciate that instructions or

To demonstrate the operation and gas detection capability

programs defining the functions of the present invention

frames 1 through 12 representing gas -cloud - detection output

information alterably stored on writable storage media (e . g .

may be delivered to a processor in many forms, including ,of an embodiment of the invention , a prototype was con - 35 but not limited to , information permanently stored on non
structed in accordance with the embodiment 300 of FIG . 3A
writable storage media (e .g . read -only memory devices
and used to detect a hydrocarbon gas cloud of propylene at within a computer, such as ROM , or devices readable by aa distance of approximately 10 feet. FIG . 7 illustrates video computer I/O attachment, such as CD -ROM or DVD disks ),710 (seen as a streak of light) in a sequence from t= 1 to t= 12 . 40 floppy disks, removable flash memory and hard drives) or

The images 1 through 12 are selected frames taken from a
video - data sequence captured at a video -rate of 15 frames /information conveyed to a computer through communica
tion media , including wired or wireless computer networks .

sec . The detected propylene gas is shown as a streak of light

In addition , while the invention may be embodied in soft

710 (highlighted in red ) near the center of each image . The

ware , the functions necessary to implement the invention

nozzle of a gas - contained , while the last image represents

whole using firmware and/ or hardware components , such as

off.
Using the same prototype of the system , the demonstra -other hardware or some combination of hardware , software

first image is taken just prior to the gas emerging from the 45 may optionally or alternatively be embodied in part or in

the system output shortly after the nozzle has been turned

combinatorial logic , Application Specific integrated Circuits(ASICs), Field - Programmable Gate Arrays (FPGAs ) ortion of the dynamic calibration improvement described 50 and/or firmware components .

above by imaging the scene surrounding the system (theWhile examples of embodiments of the system andlaboratory ) with known temperature differences . The result

method of the invention have been discussed in reference toof implementing the dynamic correction procedure is shown

the gas- cloud detection , monitoring, and quantification ( inin FIGS. 8A , 8B , where the curves labeled " obj” ( or “ A ” )cluding but not limited to greenhouse gases such as Carbon
represent temperature estimates of an identified region in the 55 Dioxide , Carbon Monoxide , Nitrogen Oxide as well as

scene. The abscissa in each of the plots of FIGS. 8A , 8Bhydrocarbon gases such as Methane, Ethane, Propane , n -Bu

nate corresponds to temperature (in degrees C .) . Accord

Hydrogen Sulfide, Sulfur Hexafluoride , Ammonia , Benzene ,ingly , it is expected that when all detector elements receivep - and m - Xylene , Vinyl chloride , Toluene, Propylene oxide,

indicates the number of a detector element , while the orditane, iso -Butane, n - Pentane, iso -Pentane , neo -Pentane ,radiant data that, when interpreted as the object' s tempera - 60 Propylene , Methanol, Hydrazine, Ethanol, 1 , 2 - dichloroeth

ture, indicates that the object's temperature perceived by all

detector elements is the same, any given curve would be aane, 1,1 -dichloroethane, Dichlorobenzene, Chlorobenzene ,to name just a few ), embodiments of the invention can besubstantially flat line. Data corresponding to each of thereadily adapted for other chemical detection applications.

multiple " obj” curves are taken from a stream of video

For example , detection of liquid and solid chemical spills ,frames separated from one another by about 0 . 5 seconds ( for 65 biologicalweapons, tracking targets based on their chemicala total of 50 frames ). The recorded “ obj” curves shown in

FIG . 8A indicate that the detector elements disagree about

composition , identification of satellites and space debris,

opthalmological imaging, microscopy and cellular imaging,US 10 , 254, 166 B2
21
endoscopy , mold detection , fire and flame detection , andpesticide detection are within the scope of the invention .

22
5 . The system of claim 1, wherein the system is config

ured to identify target species in the scene .

6 . The system of claim 1 , further comprising two spectral
What is claimed is :, each filter positioned in a respective one of said two
1. An infrared (IR ) imaging system , the imaging system 5 filters
opticalchannels.
comprising:
7 . The system of claim 6 , wherein at least one of the twoan optical system including an optical focal plane array spectral
filters is a long pass (LP ) filter.
(FPA ) unit, the optical system including components

8 . The system of claim 6 , wherein at least one of the two
associated with two optical channels , said two optical spectral
is a short pass (SP ) filter.
channels being spatially and spectrally different from 10 9 . The filters
system
6 , further comprising at least two
one another, each of the two optical channels posi- 10 reimaging lenses ,ofeachclaimreimaging
lens positioned to transmit
tioned to transfer IR radiation incident on the opticalaportionoftheIR
radiationtransmitted
through a respective
EPAaportionofsystem towards the optical FPA unit, the optical FPA one of said two spectral filters towards the
optical FPA unit.
unit comprising two groups of detector pixels disposed
10.
The
systemofclaim
1,whereinthethermalreferenceat a distance from two corresponding focusing lenseshe; 15 is a field reference comprising an optical aperturethata thermal reference, wherein radiation emitted from the
15 circumscribes
15
said
two
opticalchannels.
thermal reference directed towards the optical FPA unit
The system of claim 1, wherein the thermal reference
is received by at least one of the two groups of detector has11a. uniform
temperature across its surface .
pixels simultaneously with radiation emitted from a
12.
The
systemof claim 1 , wherein the thermal reference
scene viewed by the imaging system ; andsystem 2020 is configured to obscure a peripheral portion of the incident
a data -processing system , said data -processing system
IR radiation .
configured to :. The system of claim 1 , wherein a parameter of one ofacquire a plurality of image frames with the two groups the13two
groups of detector pixels is adjusted based on theof detector pixels having regions in the plurality ofimage frames that correspond to radiation emitted measurement obtained by the other of the two groups of

25 detector pixels .
from the thermal reference; and14 . The system of claim 1 , wherein the measurements
obtained
the two groups of detector pixels using thebased on a measurements obtained by the two groups radiation byreceived from the thermal reference are temperaof detector pixels using the radiation received fromture estimates of the thermal reference .
the thermal reference .
2 . The system of claim 1 , wherein the optical FPA unitit 3030 15 . The system of claim 14 , wherein a parameter of oneof the two groups of detector pixels is adjusted based on the
dynamically calibrate the two groups of detector pixels

does not require a cooler to cool during normal operation .
temperature estimate of the thermal reference obtained by
3 . The system of claim 1 , wherein the IR radiation
comprises wavelengths between about 1 micron and about the other of the two groups of detector pixels.
16 . The system of claim 1, wherein one of the two groups
20 microns.
ocessing 35 of detector pixels comprises a first focal plane array and the

4 . The system of claim 1, wherein the data -processing 35 0 of the two groups of detector pixels comprises a
system is configured to map an overall image datacube otherrepresenting a spatial distribution of concentrations c of a second focal plane array.
****target species.