光学滤波片.docx

光学滤波片.docx

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光学滤波片

OpticalFilters

Opticalfilterscanbeusedtoattenuateorenhanceanimage,transmitorreflectspecificwavelengths,and/orsplitanimageintotwoidenticalimageswithcontrolledbrightnesslevelsrelativetoeachother.Tounderstandtheimportanceofchoosingthecorrectopticalfilterforanyapplication,considerkeyterminology,fabricationtechniques,andthevarioustypesoffiltersavailabletoday.

KeyOpticalFilterTerminology

Beforedelvingintofabricationtechniquesandthetypesofopticalfiltersavailableintheindustrytoday,itisfirstimportanttoreviewkeyterminologyassociatedwiththem.Sinceallfilters,independentofhowtheyaremanufactured,pass,absorb,and/orreflectsomeportionofincidentlight,theysharecommonopticalparameters.

CentralWavelength

CenterWavelength（CWL）isthemidpointbetweenthewavelengthswheretransmittanceis50%ofthespecifiedminimumtransmission,referredtoastheFullWidthatHalfMaximum（FWHM）.Forinterferencefilters,thepeakistypicallynotatthemidpointwavelength.RefertoFigure1foranillustrationofCWLandFWHM.

Bandwidth

Bandwidthisawavelengthrangeusedtodenoteaspecificpartofthespectrumthatpassesincidentenergythroughafilter.BandwidthisalsoreferredtoasFWHM（Figure1）.

BlockingRange

BlockingRangeisawavelengthintervalusedtodenoteaspectralregionofenergythatisattenuatedbythefilter（Figure2）.Thedegreeofitsblockingistypicallyspecifiedintermsofopticaldensity.

Figure1:

IllustrationofCenterWavelengthandFullWidthatHalfMaximum

Figure2:

IllustrationofBlockingRange

OpticalDensity

OpticalDensity（OD）describestheblockingspecificationofafilterandisrelatedtotheamountofenergytransmittedthroughit（Equations1–2）.Ahighopticaldensityvalueindicatesverylowtransmission,andlowopticaldensityindicateshightransmission.Figure3depictsthreedifferentopticaldensities:

OD1.0,OD1.3andOD1.5thatshowthehighertheODvalue,thelowerthetransmission.

（1）

（2）

DichroicFilter

ADichroicFilterisatypeoffilterusedtotransmitorreflectlight,dependingonthewavelength;lightofaspecificwavelengthrangeistransmitted,whilelightofadifferentrangeisreflectedorabsorbed（Figure4）.Dichroicfiltersarecommonlyusedforlongpassandshortpassapplications.

Figure3:

IllustrationofOpticalDensity

Figure4:

IllustrationofaDichroicFilterCoating

Cut-OnWavelength

Cut-OnWavelengthisatermusedtodenotethewavelengthatwhichthetransmissionincreasesto50%throughputinalongpassfilter.Cut-onwavelengthisindicatedbyλcut-oninFigure5.

Cut-OffWavelength

Cut-OffWavelengthisatermusedtodenotethewavelengthatwhichthetransmissiondecreasesto50%throughputinashortpassfilter.Cut-offwavelengthisindicatedbyλcut-offinFigure6.

Figure5:

IllustrationofCut-OnWavelength

Figure6:

IllustrationofCut-OffWavelength

OpticalFilterFabricationTechniques

AbsorptiveandDichroicFilters

Thewiderangeofopticalfilterscanbebrokenintotwomaincategories:

absorptiveanddichroic.Thedifferencebetweenthetwodoesnotlieinwhattheyfilter,buthowtheyfilter.Inanabsorptivefilter,lightisblockedbasedontheabsorptionpropertiesoftheglasssubstrateused.Inotherwords,lightthatisblockeddoesnotreflectoffthefilter;rather,itisabsorbedandcontainedwithinthefilter.Inapplicationswherenoiseinasystemfromunwantedlightisanissue,anabsorptivefilterisideal.Absorptivefiltersalsohavetheaddedbonusofnotbeingveryanglesensitive;lightcanbeincidentuponthefilterfromawiderangeofanglesandthefilterwillmaintainitstransmissionandabsorptionproperties.

Conversely,adichroicfilterworksbyreflectingunwantedwavelengths,whiletransmittingthedesiredportionofthespectrum.Insomeapplications,thisisadesirableeffectbecauselightcanbeseparatedbywavelengthintotwosources.Thisisachievedbyaddingalayer,ormultiplelayers,ofmaterialofvaryingindexesofrefractiontoexploittheinterferencenatureoflightwaves.Ininterferencefilters,lighttravelingfromalowerindexmaterialwillreflectoffahigherindexmaterial;onlylightofacertainangleandwavelengthwillconstructivelyinterferewiththeincomingbeamandpassthroughthematerial,whileallotherlightwilldestructivelyinterfereandreflectoffthematerial（Figure7）.Foradditionalinformationoninterference,viewOptics101:

Level1TheoreticalFoundations.

Unlikeabsorptivefilters,dichroicfiltersareextremelyanglesensitive.Whenusedforanyangle（s）outsideoftheirintendeddesign,dichroicfilterscannotmeetthetransmissionandwavelengthspecificationsoriginallyindicated.Asaruleofthumb,increasingtheangleofincidencethroughadichroicfilterwillshiftittowardsshorterwavelengths（i.e.towardsbluerwavelengths）;anddecreasingtheanglewillshiftittowardslongerwavelengths（i.e.towardsredderwavelengths）.

Figure7:

DepositionofMultipleLayersofAlternatingHighandLowIndexMaterialsontoaGlassSubstrate

ExploringDichroicBandpassFilters

Bandpassfiltersareusedinawiderangeofindustriesandcanbeeitherdichroicorcolorsubstrate.Dichroicbandpassfiltersaremanufacturedbytwodifferenttechniques:

traditionalandhardsputtered,orhardcoated.Bothtechniquesachievetheiruniquetransmissionandreflectionpropertiesbyadepositionofnumerouslayersofalternatinghighandlowindexoffractionmaterialsontoglasssubstrates.Infact,dependingupontheapplication,therecanbemorethan100layersofmaterialdepositedperfaceofagivensubstrate.

Thedifferencebetweentraditional-coatedfiltersandhard-sputteredfiltersisthenumberofsubstratelayers.Intraditional-coatedbandpassfilters,layersofvaryingindexmaterialsaredepositedontomultiplesubstrateswhicharethensandwichedtogether.Forexample,imaginetheillustrationinFigure7repeateduptoandevenmorethan100times.Thistechniqueleadstoathickfilterwithreducedtransmission.Thisreductionintransmissioniscausedbyincidentlighttravelingthroughandbeingabsorbedand/orreflectedbynumeroussubstratelayers.Conversely,inhard-sputteredbandpassfilters,materialsofvaryingindicesaredepositedontoonlyasinglesubstrate（Figure8）.Thistechniqueleadstothinfilterswithhightransmission.Foradditionalinformationonfabricationtechniques,viewAnIntroductiontoOpticalCoatings.ViewtheBenefitsofHardCoatingstohelpyouselectthecorrectfilterforyourapplication.

Figure8:

TraditionalFilter（Left）andHard-SputteredFilter（Right）

TypesofOpticalFilters

Toaidinunderstandingthesimilaritiesanddifferencesbetweenthelargevarietyofopticalfiltersavailabletoday,considertenofthemostpopulartypes.Thefollowingselectionguidecontainsabriefdescription,aswellassampleproductimagesandperformancecurvesforeasycomparison.

OpticalFilterSelectionGuide

SampleImage

OpticalFilterType–SampleCurve

BandpassFilters

Bandpassfiltershaveextremelynarrowband（<2nmto10nm）orbroadband（50nmand80nm）transmittanceacrossthesubstrate.Theyareparticularlyanglesensitive,socareshouldbetakenwhenmountingandplacingthemwithinanopticalsetup.Hardsputteredfiltersshouldbechosentomaximizethetransmissionofselectedwavelengths.

LongpassFilters

Longpassfilterstransmitallwavelengthslongerthanthespecifiedcutonwavelength.Longpassfiltersincludecoldmirrors,coloredglassfilters,andThermosetADC（opticalcastplastic）filters.

ShortpassFilters

Shortpassfilterstransmitallwavelengthsshorterthanthespecifiedcutoffwavelength.ShortpassfiltersincludeIRcutofffilters,hotmirrors,andheatabsorbingglass.

HeatAbsorbingGlasses

Heatabsorbingglasseswilltransmitvisiblelightandabsorbinfraredradiation.Theabsorbedenergyisthendissipatedasheatintotheairaroundtheglass.Forcedaircoolingistypicallyrecommendedtoremovetheexcessheat.Heatabsorbingglasscanalsobeusedasshortpassfilters.

ColdMirrors

Coldmirrorsarespecifictypesofdichroicfiltersdesignedtohavehighreflectivityinthevisiblespectrumwhilemaintaininghightransmissionintheinfrared.Coldmirrorsaredesignedforuseinanyapplicationwhereheatbuild-upcancausedamageoradverseeffects.

HotMirrors

Hotmirrorsarespecifictypesofdichroicfiltersdesignedtohavehighreflectivityintheinfraredspectrumandhightransmissioninthevisible.Hotmirrorsareusedprimarilyinprojectionandilluminationsystems.

NotchFilters

Notchfiltersaredesignedtoblockapre-selectedbandwidthwhiletransmittingallotherwavelengthswithinthedesignrangeofthefilter.Notchfiltersareusedtoremoveasinglelaserwavelength,ornarrowband,fromanopticalsystem.

ColorSubstrateFilters

Colorsubstratefiltersaremanufacturedfromsubstrateswithinherentlydifferentabsorptionandtransmissionpropertiesacrossaspecificspectralregion.Colorsubstratefiltersareoftenusedaslongpassandbandpassfilters.Theboundarybetweentransmissionandblockingislesssharpcomparedtosomecoatingbasedfilters.

DichroicFilters

Dichroicfiltersarecoatedwiththin-filmstoachieveadesiredtransmissionandreflectionpercentageacrossagivenspectrum.Theyareoftenusedascolorfilters（bothadditiveandsubtractive）.Dichroicfiltersareslightlyanglesensitivebutaremuchmoreforgivingthaninterferencefilters.

NeutralDensity（ND）Filters

Neutraldensity（ND）filtersaredesignedtoreducetransmissione