Infrared devices and techniques - Antoni Rogalski

26 Pages·2002·1.63 MB·English

Contributed paper
OPTO-ELECTRONICS REVIEW 10(2), 111–136 (2002)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:4)(cid:6)(cid:7)(cid:8)(cid:7)(cid:6)(cid:9)(cid:10)(cid:11)(cid:6)(cid:12)(cid:8)(cid:5)(cid:2)(cid:7)(cid:8)(cid:13)(cid:6)(cid:11)(cid:14)(cid:2)(cid:10)(cid:15)(cid:16)(cid:6)(cid:12)
A. ROGALSKI*1 and K. CHRZANOWSKI2
1Institute of Applied Physics, 2Institute of Optoelectronics
Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland
Themainobjectiveofthispaperistoproduceanapplications-orientedreviewcoveringinfraredtechniquesanddevices.At
the beginning infrared systems fundamentals are presented with emphasize on thermal emission, scene radiation and con-
trast, cooling technics, and optics. Special attention is put on night vision and thermal imaging concepts. Next section
shortlyconcentratesonselectedinfraredsystemsandisarrangedinordertoincreasecomplexity;fromsmartweaponseek-
ers,imageintensifiersystems,thermalimagingsystems,tospace-basedsystems.Finally,otherimportantinfraredtechniques
anddevicesareshortlydescribedbetweenthemthemostimportantare:non-contactthermometers,radiometers,LIDAR,and
gassensors.
Keywords: thermal emission, contrast, infrared detectors, infrared optics, smart weapon seekers, image intensifier sys-
tems,thermalimagingsystems,space-basedsystems,non-contactthermometers,radiometers,LIDAR,infra-
redgassensors.
(cid:17)(cid:18) (cid:1)(cid:2)(cid:13)(cid:4)(cid:19)(cid:7)(cid:16)(cid:11)(cid:13)(cid:10)(cid:19)(cid:2) field observation for calibration (in this manner, e.g. the
area and content of fields and forests can be determined).
Lookingbackoverthepast1000yearswenoticethatinfra- In some cases even the state of health of a crop be deter-
red (IR) radiation itself was unknown until 200 years ago mined from space. Energy conservation in homes and in-
whenHerschel’sexperimentwiththermometerwasfirstre- dustry has been aided by the use of IR scans to determine
ported [1]. He built a crude monochromator that used the points of maximum heat loss. Demands to use these
a thermometer as a detector so that he could measure the technologies are quickly growing due to their effective ap-
distribution of energy in sunlight. Following the works of plications,e.g.,inglobalmonitoringofenvironmentalpol-
Kirchhoff, Stefan, Boltzmann, Wien, and Rayleigh, Max lutionandclimatechanges,longtimeprognosesofagricul-
Planck culminated the effort with well-known Planck’s turecropyield,chemicalprocessmonitoring,Fouriertrans-
law. formIRspectroscopy,IRastronomy,cardriving,IRimag-
Traditionally, IR technologies are connected with con- inginmedicaldiagnostics,andothers.
trolling functions and night vision problems with earlier Today, only about 10% of the market is commercial.
applications connected simply with detection of IR radia- After a decade the commercial market can grow to over
tion,andlaterbyformingIRimagesformtemperatureand 70% in volume and 40% in value, largely connected with
emissivitydifferences(systemsforrecognitionandsurveil- volume production of uncooled imagers for automobile
lance, tank sight systems, anti-tank missiles, air-air mis- driving [2]. In large volume production for automobile
siles). The years during World War II saw the origins of driversthecostofuncooledimagingsystemswilldecrease
modernIRtechniques.RecentsuccessinapplyingIRtech- tobelow$1000.
nologytoremotesensingproblemshasbeenmadepossible The infrared range covers all electromagnetic radiation
bythesuccessfuldevelopmentofhigh-performanceIRde- longer than the visible, but shorter than millimetre waves.
tectors over five decades. Most of the funding has been Many proposals of division of IR range have been pub-
provided to fulfil military needs, but peaceful applications lished. The division shown below is based on limits of
have increased continuously, especially in the last decade spectralbandsofcommonlyusedIRdetectors.Wavelength
oftwentiethcentury.Theseincludemedical,industry,earth 1 (cid:1)m is a sensitivity limit of popular Si detectors. Simi-
resources, and energy conservation applications. Medical larly, wavelength 3 µm is a long wavelength sensitivity of
applicationsincludethermographyinwhichIRscansofthe PbSandInGaAsdetectors;wavelength6µmisasensitivity
body detect cancers or other trauma, which raise the body limit of InSb, PbSe, PtSi detectors and HgCdTe detectors
surface temperature. Earth resources determinations are optimised for 3–5 µm atmospheric window; and finally
donebyusingIRimagesfromsatellitesinconjunctionwith wavelength15µmisalongwavelengthsensitivitylimitof
HgCdTedetectorsoptimisedfor8–14µmatmosphericwin-
*e-mail:[email protected] dow.
Opto-Electron.Rev.,10,no.2,2002 A.Rogalski 111
Infrareddevicesandtechniques
Table1.Divisionofinfraredradiation. (cid:2) occur at 10.0 (cid:1)m and 12.7 µm, respectively. We need
mp
detectors operating near 10 µm if we expect to “see” room
Region(abbreviation) Wavelengthrange(µm)
temperatureobjectssuchaspeople,treesandtruckwithout
Nearinfrared(NIR) 0.78–1 theaidofreflectedlight.Forhotterobjectssuchasengines,
maximum emission occurs at shorter wavelengths. Thus,
ShortwavelengthIR(SWIR) 1–3
thewaveband2–15µmininfraredorthermalregionofthe
MediumwavelengthIR(MWIR) 3–6 electromagnetic spectrum contains the maximum radiative
emissionforthermalimagingpurposes.
LongwavelengthIR(LWIR) 6–15
VerylongwavelengthIR(VLWIR) 15–1000
(cid:20)(cid:18) (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:4)(cid:6)(cid:7)(cid:8)(cid:12)(cid:21)(cid:12)(cid:13)(cid:6)(cid:22)(cid:12)(cid:8)(cid:3)(cid:16)(cid:2)(cid:7)(cid:5)(cid:22)(cid:6)(cid:2)(cid:13)(cid:5)(cid:23)(cid:12)
(cid:20)(cid:18)(cid:17)(cid:18) (cid:24)(cid:14)(cid:6)(cid:4)(cid:22)(cid:5)(cid:23)(cid:8)(cid:6)(cid:22)(cid:10)(cid:12)(cid:12)(cid:10)(cid:19)(cid:2)
All objects are composed of continually vibrating atoms,
with higher energy atoms vibrating more frequently. The
vibration of all charged particles, including these atoms,
generates electromagnetic waves. The higher the tempera-
ture of an object, the faster the vibration, and thus the
higher the spectral radiant energy. As a result, all objects
are continually emitting radiation at a rate with a wave-
length distribution that depends upon the temperature of
theobjectanditsspectralemissivity,(cid:1)((cid:2)).
Radiantemissionisusuallytreatedintermsofthecon-
ceptofablackbody.Ablackbodyisanobjectthatabsorbs
all incident radiation and, conversely according to the Fig.1.Planck’slawforspectralemittance(afterRef.3).
Kirchhoff’s law, is a perfect radiator. The energy emitted
byablackbodyisthemaximumtheoreticallypossiblefora
giventemperature.Theradiativepower(ornumberofpho-
(cid:20)(cid:18)(cid:20)(cid:18) (cid:25)(cid:13)(cid:22)(cid:19)(cid:12)(cid:26)(cid:14)(cid:6)(cid:4)(cid:10)(cid:11)(cid:8)(cid:13)(cid:4)(cid:5)(cid:2)(cid:12)(cid:22)(cid:10)(cid:12)(cid:12)(cid:10)(cid:19)(cid:2)
ton emitted) and its wavelength distribution are given by
thePlanckradiationlaw
Most of the above mentioned applications require trans-
missionthroughair,buttheradiationisattenuatedbythe
2(cid:3)hc2 (cid:10) (cid:3) hc (cid:6) (cid:13)(cid:9)1 processes of scattering and absorption. Scattering causes
W((cid:2),T) (cid:2) (cid:2)5 (cid:11)(cid:12)exp(cid:4)(cid:5) (cid:2)kT(cid:7)(cid:8) (cid:9)1(cid:14)(cid:15) W/(cm2µm), (1) achangeinthedirectionofaradiationbeam;itiscaused
by absorption and subsequent reradiation of energy by
(cid:9)1 suspended particles. For larger particles, scattering is in-
2(cid:3)c (cid:10) (cid:3) hc (cid:6) (cid:13)
P((cid:2),T) (cid:2) (cid:2)4 (cid:11)(cid:12)exp(cid:4)(cid:5) (cid:2)kT(cid:7)(cid:8) (cid:9)1(cid:14)(cid:15) photons/(scm2µm), (2) dcoepmepnadreendtwoifthwtahveelwenagvtehl.enHgothweovfetrh,eforardsimatiaolln,ptahreticplreos-,
cess is known as Rayleigh scattering and exhibits a (cid:2)–4
where (cid:2) is the wavelength, T is the temperature, h is the dependence. Therefore, scattering by gas molecules is
Planck’s constant, c is the velocity of light, and k is the negligibly small for wavelengths longer than 2 µm. Also
Boltzmann’sconstant. smoke and light mist particles are usually small with re-
Figure 1 shows a plot of these curves for a number of spect to IR wavelengths, and IR radiation can therefore
blackbody temperatures. As the temperature increases, the penetrate further through smoke and mists than visible
amountofenergyemittedatanywavelengthincreasestoo, radiation. However, rain, fog particles and aerosols are
and the wavelength of peak emission decreases. The latter larger and consequently scatter IR and visible radiation
isgivenbytheWien’sdisplacementlaw to a similar degree.
Figure2isaplotofthetransmissionthrough6000ftof
(cid:2) T=2898µmK formaximumwatts, air as a function of wavelength. Specific absorption bands
mw
of water, carbon dioxide and oxygen molecules are indi-
(cid:2) T=3670µmK formaximumphotons. catedwhichrestrictsatmospherictransmissiontotwowin-
mp
dowsat3–5µmand8–14µm.Ozone,nitrousoxide,carbon
ThelociofthesemaximaareshowninFig.1.Notethat monoxide and methane are less important IR absorbing
for an object at an ambient temperature of 259 K, (cid:2) and constituentsoftheatmosphere.
mw
112 Opto-Electron.Rev,10,no.2,2002 ©2002COSiWSEP,Warsaw
Contributed paper
Fig.2.Transmissionoftheatmospherefora6000fthorizontalpathatsealevelcontaining17mmofprecipitatewater(afterRef.4).
(cid:20)(cid:18)(cid:27)(cid:18) (cid:28)(cid:11)(cid:6)(cid:2)(cid:6)(cid:8)(cid:4)(cid:5)(cid:7)(cid:10)(cid:5)(cid:13)(cid:10)(cid:19)(cid:2)(cid:8)(cid:5)(cid:2)(cid:7)(cid:8)(cid:11)(cid:19)(cid:2)(cid:13)(cid:4)(cid:5)(cid:12)(cid:13)
flectivity. For a 291 K object in a 290 K scene, it is about
0.039 in the 3–5 µm band and 0.017 in the 8–13 µm band.
The total radiation received from any object is the sum of Thus,whileLWIRbandmayhavethehighersensitivityfor
the emitted, reflected and transmitted radiation. Objects ambient temperature objects, the MWIR band has the
that are not blackbodies emit only the fraction (cid:1)((cid:2)) of greatercontrast.
blackbody radiation, and the remaining fraction, 1–(cid:1)((cid:2)), is
either transmitted or, for opaque objects, reflected. When (cid:20)(cid:18)(cid:29)(cid:18) (cid:30)(cid:14)(cid:19)(cid:10)(cid:11)(cid:6)(cid:8)(cid:19)(cid:3)(cid:8)(cid:10)(cid:2)(cid:3)(cid:4)(cid:5)(cid:4)(cid:6)(cid:7)(cid:8)(cid:31)(cid:5)(cid:2)(cid:7)
thesceneiscomposedofobjectsandbackgroundsofsimi-
lar temperatures, reflected radiation tends to reduce the In general, the 8–14 µm band is preferred for high perfor-
available contrast. However, reflections of hotter or colder mance thermal imaging because of it higher sensitivity to
objects have a significant effect on the appearance of a ambient temperature objects and its better transmission
thermal scene. The powers of 290 K blackbody emission through mist and smoke. However, the 3–5 µm band may
and ground-level solar radiation in MWIR and LWIR be more appropriate for hotter object, or if sensitivity is
bandsaregiveninTable2.Wecanseethatwhilereflected lessimportantthancontrast.Alsoadditionaldifferencesoc-
sunlighthaveanegligibleeffecton8–13µmimaging,itis cur;e.g.,theadvantageofMWIRbandissmallerdiameter
importantinthe3–5µmband. oftheopticsrequiredtoobtainacertainresolutionandthat
A thermal image arises from temperature variations or some detectors may operate at higher temperatures (ther-
differences in emissivity within a scene. The thermal con- moelectric cooling) than it is usual in the LWIR band
trastisoneoftheimportantparametersforIRimagingde- wherecryogeniccoolingisrequired(about77K).
vices. It is the ratio of the derivative of spectral photon in- Summarising, MWIR and LWIR spectral bands differ
cidencetothespectralphotonincidence substantially with respect to background flux, scene char-
(cid:4)W (cid:4)T acteristics,temperaturecontrast,andatmospherictransmis-
C (cid:2) . sion under diverse weather conditions. Factors which fa-
W
vour MWIR applications are: higher contrast, superior
The contrast in a thermal image is small when com- clear-weatherperformance(favourableweatherconditions,
pared with visible image contrast due to differences in re- e.g., in most countries of Asia and Africa), higher
Table2.PoweravailableineachMWIRandLWIRimagingbands(afterRef.3).
IRregion Ground-levelsolarradiation Emissionfrom290Kblackbody
(µm) (W/m2) (W/m2)
3–5 24 4.1
8–13 1.5 127
Opto-Electron.Rev.,10,no.2,2002 A.Rogalski 113
Infrareddevicesandtechniques
transmittivity in high humidity, and higher resolution due (A(cid:5)f)1/2
D*(cid:2) (SNR) (3)
to ~3 times smaller optical diffraction. Factors which fa-
(cid:6)
e
vourLWIRapplicationsare:betterperformanceinfogand
dust conditions, winter haze (typical weather conditions, where (cid:6) is the spectral radiant incident power. D* is de-
e
e.g., in West Europe, North USA, Canada), higher immu- finedasthermssignal-to-noiseratio(SNR)ina1Hzband-
nity to atmospheric turbulence, and reduced sensitivity to widthperunitrmsincidentradiationpowerpersquareroot
solar glints and fire flares. The possibility of achieving of detector area. D* is expressed in cmHz1/2W–1, which is
higher signal-to-noise (S/N) ratio due to greater radiance recently called “Jones”. Spectral detectivity curves for a
levelsinLWIRspectralrangeisnotpersuasivebecausethe number of commercially available IR detectors are shown
background photon fluxes are higher to the same extent, inFig.3.Interesthascentredmainlyonthewavelengthsof
and also because of readout limitation possibilities. Theo- the two atmospheric windows 3–5 µm and 8–14 µm,
retically, in staring arrays charge can be integrated for full thoughinrecentyearstherehasbeenincreasinginterestin
frame time, but because of restrictions in the charge-han- longerwavelengthsstimulatedbyspaceapplications.
dlingcapacityofthereadoutcells,itismuchlesscompared ProgressininfraredIRdetectortechnologyisconnected
totheframetime,especiallyforLWIRdetectorsforwhich mainlytosemiconductorIRdetectors,whichareincludedin
background photon flux exceeds the useful signals by or- theclassofphotondetectors.Intheclassofphotondetectors
dersofmagnitude. the radiation is absorbed within the material by interaction
with electrons. The observed electrical output signal results
(cid:20)(cid:18) (cid:18) !(cid:6)(cid:13)(cid:6)(cid:11)(cid:13)(cid:19)(cid:4)(cid:12) fromthechangedelectronicenergydistribution.Thephoton
detectorsshowaselectivewavelengthdependenceofthere-
The figure of merit used for detectors is detectivity. It has sponse per unit incident radiation power. They exhibit both
beenfoundinmanyinstancesthatthisparametervariesin- perfectsignal-to-noiseperformanceandaveryfastresponse.
verselywiththesquarerootofboththedetector’ssensitive But to achieve this, the photon detectors require cryogenic
area, A, and the electrical bandwidth, (cid:5)f. In order to sim- cooling. Cooling requirements are the main obstacle to the
plify the comparison of different detectors, the following morewidespreaduseofIRsystemsbasedonsemiconductor
definitionhasbeenintroduced[5] photodetectorsmakingthembulky,heavy,expensiveandin-
Fig.3.ComparisonoftheD*ofvariouscommerciallyavailableinfrareddetectorswhenoperatedattheindicatedtemperature.Chopping
frequencyis1000Hzforalldetectorsexceptthethermopile(10Hz),thermocouple(10Hz),thermistorbolometer(10Hz),Golaycell
(10 Hz) and pyroelectric detector (10 Hz). Each detector is assumed to view a hemispherical surround at a temperature of 300 K.
Theoreticalcurvesforthebackground-limitedD*foridealphotovoltaicandphotoconductivedetectorsandthermaldetectorsarealso
shown(afterRef5).
114 Opto-Electron.Rev,10,no.2,2002 ©2002COSiWSEP,Warsaw
Contributed paper
convenient to use. Depending on the nature of interaction, compressed air and a Joule-Thompson minicooler [6]. The
the class of photon detectors is further sub-divided into dif- operation of Joule-Thompson cooler is based on the fact
ferent types. The most important are: intrinsic detectors that as the high-pressure gas expands on leaving a throttle
(HgCdTe, InGaAs, InSb, PbS, PbSe), extrinsic detectors valve,itcoolsandliquefies.Thegasusedmustbepurified
(Si:As,Si:Ga),photoemissive(metalsilicideSchottkybarri- to remove water vapour and carbon dioxide which could
ers) detectors, and quantum well detectors (GaAs/AlGaAs freeze and block the throttle valve. Specially designed
QWIPs). Joule-Thompson coolers using argon are suitable for ul-
The second class of infrared detectors is composed of tra-fastcool-down.
thermaldetectors.Inathermaldetector,theincidentradia-
tionisabsorbedtochangethetemperatureofmaterial,and
the resultant change in some physical properties is used to
generate an electrical output. The detector element is sus-
pendedonlags,whichareconnectedtotheheatsink.Ther-
mal effects are generally wavelength independent; the sig-
nal depends upon the radiant power (or its rate of change)
but not upon its spectral content. In pyroelectric detectors,
a change in the internal spontaneous polarisation is mea-
sured, whereas in the case of bolometers a change in the
electrical resistance is measured. In contrast to photon de-
tectors,thermaldetectorstypicallyoperateatroomtemper-
ature. They are usually characterised by modest sensitivity
and slow response but they are cheap and easy to use.
Bolometers, pyroelectric detectors, and thermopiles have
found the greatest utility in infrared technology. Typical
valuesofdetectivitiesofthermaldetectorsat10Hzchange
intherangebetween108to109cmHz1/2W–1.
Uptilltheninetiesofthe20thcentury,thermaldetectors
have been considerably less exploited in commercial and
militarysystemsincomparisonwithphotondetectors.The
reason for this disparity is that thermal detectors are popu-
larlybelievedtoberatherslowandinsensitiveincompari-
sonwithphotondetectors.Asaresult,theworldwideeffort
to develop thermal detectors was extremely small relative
to that of photon detector. In the last decade, however, it
has been shown that extremely good imagery can be ob-
tained from large thermal detector arrays operating
uncooledatTVframerates.Thespeedofthermaldetectors
is quite adequate for non-scanned imagers with
two-dimensional (2D) detectors. The moderate sensitivity
ofthermaldetectorscanbecompensatedbyalargenumber
ofelementsin2Delectronicallyscannedarrays.Withlarge
arraysofthermaldetectorsthebestvaluesofNEDT,below
0.1K,couldbereachedbecauseeffectivenoisebandwidths
lessthan100Hzcanbeachieved.
(cid:20)(cid:18)"(cid:18) (cid:30)(cid:19)(cid:19)(cid:23)(cid:10)(cid:2)#
Thesignaloutputofaphotondetectorissosmallthatator-
dinarytemperaturesitisswampedbythethermalnoisedue
to random generation and recombination of carriers in the
semiconductor.Inordertoreducethethermalgenerationof
carriers and minimise noise, photon detectors must be
cooled and must therefore be encapsulated. The method of
cooling varies according to the operating temperature and
the system’s logistical requirements. Most 8–14-µm detec-
Fig. 4. Three ways of cooling IR detectors: (a) four-stage
tors operate at about 77 K and can be cooled by liquid ni- thermoelectriccooler(Peltiereffect),(b)Joule-Thompsoncooler,
trogen. In the field, however, it is more convenient to use and(c)Stirling-cycleengine.
Opto-Electron.Rev.,10,no.2,2002 A.Rogalski 115
Infrareddevicesandtechniques
The use of cooling engines, in particular those em- ison with those for visible range, particularly for wave-
ploying the Stirling cycle, has increased recently due to lengthsover2.5µm.
theirefficiency,reliabilityandcostreduction.Stirlingen- There are two types of IR optical elements: reflective
gine requires several minutes cool-down time; the work- elements and refractive elements. As the names suggest,
ing fluid is helium. Both Joule-Thompson and en- the role of reflective elements is to reflect incident radia-
gine-cooleddetectorsarehousedinprecision-boredewars tion and the role of refractive elements is to refract and
into which the cooling device is inserted (see Fig. 4). transmitincidentradiation.
Mountedinthevacuumspaceattheendoftheinnerwall Mirrors used extensively inside IR systems (especially
of the dewar, and surrounded by a cooled radiation shield in scanners) are most often met as reflective elements that
compatiblewiththeconvergenceangleoftheopticalsys- serve manifold functions in IR systems. Elsewhere they
tem, the detector looks out through an IR window. In need a protective coating to prevent them from tarnishing.
some dewars, the electrical leads to detector elements are Spherical or aspherical mirrors are employed as imaging
embedded in the inner wall of the dewar to protect them elements.Flatmirrorsarewidelyusedtofoldopticalpath,
from damage due to vibration. andasreflectiveprismareoftenusedinscanningsystems.
Many detectors in the 3–5-µm waveband are thermo- Four materials are most often used for mirrors fabrica-
electrically cooled. In this case, detectors are usually tion: optical crown glass, low-expansion borosilicate glass
mounted in a hermetic encapsulation with a base designed (LEBG),syntheticfusedsilica,andZerodur.Opticalcrown
tomakegoodcontactwithaheatsink. glass is typically applied in non-imaging systems. It has a
relatively high thermal expansion coefficient and is em-
(cid:20)(cid:18)$(cid:18) (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:4)(cid:6)(cid:7)(cid:8)(cid:19)(cid:26)(cid:13)(cid:10)(cid:11)(cid:12) ployed when thermal stability is not a critical factor.
LEBG, known by the Corning brand name Pyrex, is well
The optical block in IR system creates an image of ob- suited for high quality front-surface mirrors designed for
served objects in plane of the detector (detectors). In the low optical deformation under thermal shock. Synthetic
case of scanning imager, the optical scanning system cre- fusedsilicahasaverylowthermalexpansioncoefficient.
ates image with the number of pixels much greater that Metallic coatings are typically used as reflective coat-
numberofelementsofthedetector.Inaddition,theoptical ings of IR mirrors. There is four types of most often used
elements like windows, domes and filters can be used to metallic coatings: bare aluminium, protected aluminium,
protect system from environment or to modify detector silver, and gold. They offer high reflectivity, over about
spectralresponse. 95%, in 3–15-µm spectral range. Bare aluminium has a
There is no essential difference in design rules of opti- very high reflectance value but oxidises over time. Pro-
calobjectivesforvisibleandIRranges.DesignerofIRop- tected aluminium is bare aluminium coating with a dielec-
tics is only more limited because there is significantly tricovercoatthatarreststheoxidationprocess.Silveroffers
fewermaterialssuitableforIRopticalelements,incompar- better reflectance in near IR than aluminium and high
Fig.5.Transmissionrangeofinfraredmaterials(afterRef.7).
116 Opto-Electron.Rev,10,no.2,2002 ©2002COSiWSEP,Warsaw
Contributed paper
reflectance across a broad spectrum. Gold is widely used con optics must have antireflection coatings. Silicon offers
material and offers consistently very high reflectance twotransmissionranges:1–7µmand25–300µm.Onlythe
(about99%)inthe0.8–50-µmrange.However,goldissoft first one is used in typical IR systems. The material is sig-
(it cannot be touched to remove dust) and is most often nificantlycheaperthangermanium.ItisusedmostlyforIR
usedinlaboratory. systemsoperatingin3–5-µmband.
The most popular materials used in manufacturing re- Singlecrystalmaterialhasgenerallyhighertransmission
fractive optics of IR systems are: germanium (Ge), silicon than polycrystalline one. Optical-grade germanium used for
(Si), fused silica (SiO ), glass BK-7, zinc selenide (ZnSe) thehighestopticaltransmissionisn–typedopedtoreceivea
2
andzincsulfide(ZnS).TheIR-transmittingmaterialspoten- conductivity of 5–14 (cid:17)cm. Silicon is used in its intrinsic
tiallyavailableforuseaswindowsandlensesaregatheredin state.Atelevatedtemperaturessemiconductingmaterialsbe-
Table3andtheirIRtransmissionisshowninFig.5. comeopaque.Asaresult,germaniumisoflittleusedabove
Germanium is a silvery metallic-appearing solid of 100(cid:18)C.Inthe8–14µmregion,semi-insulatingGaAsmaybe
veryhighrefractiveindex((cid:16)4),thatenablesdesigningof usedatthetemperaturesupto200(cid:18)C.
high-resolution optical systems using minimal number of Ordinary glass does not transmit radiation beyond
germanium lenses. Its useful transmission range is from 2.5 µm in IR region. Fused silica is characterised by very
2µmtoabout15µm.Itisquitebrittleanddifficulttocut low thermal expansion coefficient that makes optical sys-
but accept a very good polish. Additionally, due to its tems particularly useful in changing environmental condi-
very high refractive index, antireflection coatings are es- tions. It offers transmission range from about 0.3 µm to
sential for any germanium transmitting optical system. 3 µm. Because of low reflection losses due to low refrac-
Germanium has a low dispersion and is unlikely to need tiveindex((cid:16)1.45),antireflectioncoatingsarenotneeded.It
colourcorrectingexceptinthehighest-resolutionsystems. ismoreexpensivethanBK-7,butstillsignificantlycheaper
In spite of high material price and cost of antireflection thanGe,ZnSandZnSe,andisapopularmaterialforlenses
coatings, germanium lenses are particularly useful for of IR systems with bands located below 3 µm. BK-7 glass
8–12-µm band. Significant disadvantage of germanium is characteristics are similar to fused silica; the difference is
seriousdependenceofitsrefractiveindexontemperature, onlyabitshortertransmissionbandupto2.5µm.
so germanium telescopes and lenses may need to be ZnSe is expensive material comparable to germanium;
athermalised. hasatransmissionrangefrom2to22µm,andarefractivein-
Physical and chemical properties of silicon are very dexabout2.4.Itispartiallytranslucentinvisibleandreddish
similar to properties of germanium. It has high refractive in colour. Due to relatively high refractive index, antireflec-
index((cid:16)3.45),isbrittle,doesnotcleave,takesanexcellent tion coatings are necessary. The chemical resistance of the
polish, and has large dn/dT. Similarly to germanium, sili- materialisexcellentandsuperiortogermaniumandsilicon.
Table3.Principalcharacteristicsofsomeinfraredmaterials(afterRef.7).
Material Waveband dn/dT Density
n ,n Othercharacteristics
(µm) 4µm 10µm (10–6K–1) (g/cm3)
Ge 3–5,8–12 4.025,4.004 424(4µm) 5.33 Brittle,semiconductor,canbediamond-turned,
404(10µm) visiblyopaque,hard
Si 3–5 3.425 159(5µm) 2.33 Brittle,semiconductor,diamond-turnedwith
difficulty,visiblyopaque,hard
GaAs 3–5,8–12 3.304,3.274 150 5.32 Brittle,semiconductor,visiblyopaque,hard
ZnS 3–5,8–12 2.252,2.200 43(4µm) 4.09 Yellowish,moderatehardnessandstrength,can
41(10µm) bediamond-turned,scattersshortwavelengths
ZnSe 3–5,8–12 2.433,2.406 63(4µm) 5.26 Yellow-orange,relativelysoftandweak,canbe
60(10µm) diamond-turned,verylowinternalabsorption
andscatter
CaF 3–5 1.410 –8.1(3.39µm) 3.18 Visiblyclear,canbediamond-turned,mildly
2
hygroscopic
Sapphire 3–5 1.677(n ) 6(o) 3.99 Veryhard,difficulttopolishduetocrystal
o
1.667(n ) 12(e) boundaries
e
AMTIR-1 3–5,8–12 2.513,2.497 72(10µm) 4.41 AmorphousIRglass,canbeslumpedto
near-netshape
BF7(Glass) 0.35–2.3 3.4 2.51 Typicalopticalglass
Opto-Electron.Rev.,10,no.2,2002 A.Rogalski 117
Infrareddevicesandtechniques
(cid:20)(cid:18)((cid:18) (cid:24)(cid:14)(cid:6)(cid:4)(cid:22)(cid:5)(cid:23)(cid:8)(cid:10)(cid:22)(cid:5)#(cid:10)(cid:2)#(cid:8)(cid:12)(cid:21)(cid:12)(cid:13)(cid:6)(cid:22)(cid:8)(cid:11)(cid:19)(cid:2)(cid:11)(cid:6)(cid:26)(cid:13)(cid:12)
ZnS has excellent transmission in the range from 2 µm
to12µm.Itisahighqualitycrystalthatshowsonlyalight
yellow colour. Because of relatively high refractive index Thermal imaging is a technique for converting a scene’s
of2.25,antireflectioncoatingsareneededtominimiseflux thermal radiation pattern (invisible to the human eye)
reflection. The hardness and fracture strength are very into a visible image. Its usefulness is due to the follow-
good. ZnS is brittle, can operate at elevated temperatures ing aspects [3]:
and also can be used to colour correct high-performance (cid:127) it is totally passive technique and allows day and night
germaniumoptics. operation,
ThealkalihalideshaveexcellentIRtransmission,how- (cid:127) it is ideal for detection of hot or cold spots, or areas of
ever,theyareeithersoftorbrittleandmanyofthemareat- differentemissivities,withinascene,
tacked by moisture, making them generally unsuitable for (cid:127) thermal radiation can penetrate smoke and mist more
industrialapplications. readilythanvisibleradiation,
For more detailed discussion of the IR materials see (cid:127) itisareal-time,remotesensingtechnique.
Harris[8]andSmith[9]. The thermal image is a pictorial representation of tem-
perature difference. Displayed on a scanned raster, the im-
(cid:20)(cid:18)%(cid:18) &(cid:10)#(cid:14)(cid:13)’(cid:9)(cid:10)(cid:12)(cid:10)(cid:19)(cid:2)(cid:8)(cid:12)(cid:21)(cid:12)(cid:13)(cid:6)(cid:22)(cid:8)(cid:11)(cid:19)(cid:2)(cid:11)(cid:6)(cid:26)(cid:13)(cid:12) age resembles a television picture of the scene and can be
computerprocessedtocolour-codetemperatureranges.Or-
iginallydeveloped(inthesixtieslastcentury)toextendthe
Night-vision systems can be divided into two categories:
thosedependinguponthereceptionandprocessingofradi- scopeofnightvisionsystems,thermalimagersatfirstpro-
ation reflected by an object and those which operate with vided an alternative to image intensifiers. As the technol-
ogyhasmatured,itsrangeofapplicationhasexpandedand
radiation internally generated by an object. The latter sys-
nowextendsintothefieldsthathavelittleornothingtodo
temsaredescribedinsection2.9.
withnightvision(e.g.stressanalysis,medicaldiagnostics).
Thehumanvisualperceptionsystemisoptimisedtoop-
In most present-day thermal imagers, an optically focused
erate in daytime illumination conditions. The visual spec-
image is scanned (mechanically or electronically) across
trumextendsfromabout420nmto700nmandtheregion
detectors [many elements or two-dimensional (2-D) array]
of greatest sensitivity is near the peak wavelength of sun-
the output of which is converted into a visual image. The
light at around 550 nm. However, at night fewer visible
optics,modeofscanning,andsignalprocessingelectronics
lightphotonsareavailableandonlylarge,highcontrastob-
arecloselyinterrelated.Thenumberofpicturepointsinthe
jectsarevisible.Itappearsthatthephotonrateintheregion
scene is governed by the nature of the detector (its perfor-
from 800 to 900 nm is five to seven times greater than in
mance)orthesizeofthedetectorarray.Theeffectivenum-
visibleregionaround500nm.Moreover,thereflectivityof
ber of picture points or resolution elements in the scene
various materials (e.g. green vegetation, because of its
may be increased by an optomechanical scanning device
chlorophyllcontent)ishigherbetween800nmand900nm
whichimagesdifferentpartsofthesceneontothedetector
than at 500 nm. It means that at night more light is avail-
able in the NIR than in visual region and that against cer- sequentiallyintime.
The performance of a thermal imager is usually speci-
tainbackgroundsmorecontrastisavailable.
fied in terms of temperature resolution. It can be shown,
A considerable improvement in night vision capability
can be achieved with night viewing equipment which con- that the temperature sensitivity of an imager, so called
noise equivalent temperature difference (NETD), can be
sists of an objective lens, image intensifier and eyepiece
givenby[10]
(see Fig. 6). Improved visibility is obtained by gathering
more light from the scene with an objective lens than the 4f2((cid:5)f)1/2
unaided eye; by use of a photocathode that has higher NETD (cid:2) # , (4)
photosensitivityandbroaderspectralresponsethantheeye; A1/2topM*
andbyamplificationofphotoeventsforvisualsensation.
wheref isthef-numberofthedetectoroptics(f =f/D,fis
# #
thefocallengthandDisthediameterofthelens),t isthe
op
transmission of the optics, and M* is the figure of merit
which includes not only the detector performance D* but
also the spectral dependence of the emitted radiation,
((cid:4)S/(cid:4)T) , and the atmospheric transmission t . It is given
(cid:2) at
bythefollowingequation
(cid:19)
(cid:3)(cid:4)S(cid:6)
M* (cid:2) (cid:20)(cid:5) (cid:8) t D*d(cid:2). (5)
(cid:4)(cid:4)T(cid:7) at(cid:2) (cid:2)
0 (cid:2)
NETDisthedifferenceoftemperatureoftheobjectre-
quired to produce an electric signal equal to the rms noise
Fig.6.Diagramofanimageintensifier.
118 Opto-Electron.Rev,10,no.2,2002 ©2002COSiWSEP,Warsaw
Contributed paper
at the input of the display [5]. Temperature resolution de-
pendsonefficiencyoftheopticalsystem,responsivityand
noiseofthedetector,andSNRofthesignalprocessingcir-
cuitry.
Forhighsensitivity,theNETDmustbelow.Thesensi-
tivityincreasesinverselyasthesquarerootoftheelectrical
bandwidth. For a given size of IR scene, the electronic
bandwidthisinverselyproportionaltothenumberofparal-
lel detector elements, and so the thermal sensitivity in-
creasesasthesquarerootofthetotalnumberofdetectorel-
ements, irrespective of parallel or serial content of the ar-
ray.
(cid:27)(cid:18) (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:4)(cid:6)(cid:7)(cid:8)(cid:12)(cid:21)(cid:12)(cid:13)(cid:6)(cid:22)(cid:12)
This section shortly concentrates on selected IR systems
andisarrangedinordertoincreasecomplexity;fromsmart
weapon seekers to space-based systems. A comprehensive
compendium devoted to infrared systems was copublished
in 1993 by Infrared Information Analysis Centre (IRIA)
and the International Society for Optical Engineering
Fig.7.Opticaldiagramofatypicalpassivenon-imagingseeker.
(SPIE) as The Infrared and Electro-Optical Systems Hand-
book (executive editors: Joseph S. Accetta and David L.
Shumaker).
PassiveimagingseekershaveaTVcameraorathermal
camera in their optoelectronic head. Location of a target is
(cid:27)(cid:18)(cid:17)(cid:18) (cid:28)(cid:22)(cid:5)(cid:4)(cid:13)(cid:8))(cid:6)(cid:5)(cid:26)(cid:19)(cid:2)(cid:8)(cid:12)(cid:6)(cid:6)*(cid:6)(cid:4)(cid:12)
determinedfromanalysisoftheimagegeneratedbyacam-
era. Some of the air-to-ground missiles attack and destroy
The seeker is the primary homing instrument for smart
ground targets, particularly large non-movable targets like
weapons that include missiles, bombs, artillery projectiles,
bridges,bunkers,buildings,etc.However,significanttech-
and standoff cruise missiles. They can be categorised into
nical limitations exist. First, the seekers using TV cameras
there groups: passive non-imaging seekers, passive imag-
can operate only in the day light conditions. Second, it is
ingseekers,andactivelaserguidedseekers.
verydifficulttodesignathermalcameraforthehigh-speed
Passive non-imaging seekers use circular optical plate
missiles.Suchacameramustbeofsmall-size,veryfastop-
with adjacent transparent and non-transparent parts called
erating, reliable, ready to withstand harsh environmental
reticlethatisfixedattheimageplaneoftheimagingoptics
requirements,andoflowmanufacturingcost.Thereforethe
of the missile head (Fig. 7). A single IR detector, of the
imaging missiles using thermal cameras in their optical
sizeabitlargerthanthereticle,isplacedjustbehindit.Lo-
headarestillatadevelopmentstage.
cation of the point image of the target on the reticle plate
Active laser guided seekers can be divided into two
changes,evenwhenthetargetdoesnotchangeitsposition,
subclasses: seekers homing on the irradiated target and
due to rotation of the reticle or rotation of the imaging op-
seekers irradiated with a laser beam (see Fig. 9). Seekers
tics. Therefore radiation emitted by the target generates
homing on the irradiated target cooperate with a laser illu-
electrical pulses at the detector output. Pulse duration and
minator and use the laser radiation reflected by the target.
phase of these pulses give information about angular posi-
tionofthetarget(Fig.8).
The grandfather of passive IR seekers is the Side-
winderseekerdevelopedinthe1950’s;employedvacuum
tubes and lead salt single-element detector. During next
decades it has been found, that despite their simplicity,
the passive non-imaging seekers are very effective for
guidingthemissileswhenthetargetisonauniformback-
ground. Therefore at present, majority of currently used
short-range smart missiles use this type of seekers. How-
ever, effectiveness of passive non-imaging seekers de-
creases significantly for targets on non-uniform back-
ground like typical ground military targets or in presence
ofcountermeasures.Thereforethetrendoffuturesystems Fig.8.Exemplaryreticleandthesignalgeneratedatdetectoroutput
is toward passive imaging seekers. byafewtargetsofdifferentlocation.
Opto-Electron.Rev.,10,no.2,2002 A.Rogalski 119
Infrareddevicesandtechniques
Fig.9.Principleofworkofactivelaserguidedseekers:(a)seekers
homingontheirradiatedtarget,and(b)seekersirradiatedwitha
laserbeam.
These seekers enable very accurate location of small tar- Fig.10.Representativeimaging(staring)seekerarchitecture(after
gets in a highly non-uniform background and are particu- Ref.6).
larlywellsuitedforair-to-groundmissilesorbombs.How-
ever, it is an active method and employing warning sys-
tems or other countermeasures can significantly reduce its and imaged, the missile chooses an aimpoint and conducts
effectiveness. final manoeuvres to get to the target or selects a point and
Seekers irradiated with a laser beam are kept on their timetofuseandexplode.
flighttothetargetwithinthebeamemittedbythelaserillu-
minatorthatirradiatesthetarget.Laserradiation,thatgives (cid:27)(cid:18)(cid:20)(cid:18) (cid:1)(cid:22)(cid:5)#(cid:6)(cid:8)(cid:10)(cid:2)(cid:13)(cid:6)(cid:2)(cid:12)(cid:10)(cid:3)(cid:10)(cid:6)(cid:4)(cid:8)(cid:12)(cid:21)(cid:12)(cid:13)(cid:6)(cid:22)(cid:12)
informationontargetlocation,comesdirectlyfromtheillu-
minator to the sensors at the back of the missile, not after The image intensifiers are classed by generation (Gen) num-
the reflection by the target as in the previous method. bers.Gen0referstothetechnologyofWorldWarII,employ-
Thereforelowpowerilluminatorscanbeusedhereandthe ing fragile, vacuum-enveloped photon detectors with poor
effectivenessofthewarningsystemsisreduced. sensitivity and little gain. Gen1 represents the technology of
The new generation of standoff weapons relies on theearlyVietnamEra,the1960s.Inthisera,thefirstpassive
real-time target recognition, discrimination, tracking, navi- systems, able to amplify ambient starlight, were introduced.
gation,andnightvision.Itispredictedthatthesmartweap- Throughsensitive,thesedeviceswerelargeandheavy.Gen1
ons will tend to replace the radar emphasis as stealth plat- devicesusedtri-alkaliphotocathodestoachievegainofabout
formsareincreasinglyusedforlow-intensityconflicts.Itis 1000.Bytheearly70’s,themicrochannel-plate(MCP)ampli-
more difficult to perform IR missile warning than radar fierwasdevelopedcomprisingmorethantwomillionmicro-
guidedmissilewarning. scopic conducting channels of hollow glass, each of about
A representative architecture of a staring seeker is 10 (cid:1)m in diameter, fused into a disc-shaped array. Coupling
shown in Fig. 10. To keep seeker volume, weight, and theMCPwithmulti-alkaliphotocathodes,capableofemitting
power requirements low, only the minimum hardware more electrons per incident photon, produced GenII. GenII
needed to sense the scene is included. We can notice, that devicesboastedamplificationsof20000andoperationallives
the seeker’s output is going to a missile-based processor, of 2500 hrs. Interim improvements in bias voltage and con-
behind the FPA in the seeker, to perform tracking and struction methods produced GenII+ version. Substantial im-
aimpoint selection. A concept of seeker’s operation in- provementsingainandbandwidthinthe1980’sheraldedthe
cludes a standby turn-on, followed by a commit, which advent of GenIII. Gallium arsenide photocathodes and inter-
cools the FPA. At the beginning, the seeker is locked onto nalchangesintheMCPdesignresultedingainsrangingfrom
its target by an external sensor or a human. Next, the mis- 30000to50000andoperatinglivesof10000hrs.
sile is launched and flies out locked onto its target, match- Many candidate technologies could form the basis of a
ing any target movement. Finally, when the target is close GenIV, ranging from enhanced current designs to com-
120 Opto-Electron.Rev,10,no.2,2002 ©2002COSiWSEP,Warsaw