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