机器视觉过滤

FilteringinMachineVision

Therearemanydifferenttypesoffiltersinmachinevisionthatcanbeutilizedtoimproveorchangetheimageoftheobjectunderinspection.Itisimportanttounderstandthedifferenttechnologiesbehindthevarioustypesoffiltersinordertounderstandtheiradvantagesandlimitations.Althoughthereisawidevarietyoffilters,almostallcanbedividedintotwoprimarycategories:coloredglassfiltersandcoatedfilters.

ColoredGlassFilters

Coloredglassfiltersareincrediblycommoninmachinevision,andarecreatedbydopingglassmaterialswithelementsthatselectivelychangetheirabsorptionandtransmissionspectra.Thedopantsvarybasedonwhichwavelengthsareconsideredfortransmission,andthemanufacturingprocessisthennearlyidenticaltostandardopticalglassmanufacturing.Coloredglassfiltersareadvantageousforacoupledifferentreasons:theyareofrelativelylowcostwhencomparedtointerferencefiltersand,moreimportantly,theydonotdemonstrateanyshiftinwavelengthtransmissionwhenusedwithwideanglelensesoratanangle.However,coloredglassfiltersalsotypicallyfeaturewidecut-onwavebands,donothavecurvesthatareassharporaccurateascoatedinterferencefilters,anddonothavetransmissionthroughputlevels(percentages)ashighasinterferencefilters.Figure1showsthetransmissioncurvesforseveralcommoncoloredglassfilters.Notethatthefiltersfeaturewidecut-onwavebandsandhaverelativelyshallowslopesdescribingtheirtransmissionfunctions.

Figure1:TransmissionCurvesforSeveralDifferentColoredGlassFilters

Infrared(IR)cutofffilterscanbeeithercoloredglassfiltersoratypeofcoatedfilterthatisusefulforbothmonochromeandcolorcamerasinmachinevisionapplications.Sincethesiliconsensorsinmostmachinevisioncamerasareresponsivetowavelengthsuptoapproximately1μm,anyIRlightincidentonthesensorthatmayhavebeencausedbyoverheadfluorescentlightsorotherunwantedsourcescancreateinaccuraciesonthesensor.Onacolorcamera,IRlightwillcreateafalsecoloronthesensorthatcandegradeoverallcolorreproduction.Forthisreason,manycolorimagingcamerascomestandardwithIR-cutfilteroverthesensor.Withmonochromecameras,thepresenceofIRlightwilldegradethecontrastoftheoverallimage.

Thereareamultitudeofothertypesofcoloredglassfilters.Forinstance,daylightbluefilterscanbeusedforcolorbalancingwhenpolychromaticlightsourcesandcolorsensorsareused.

CoatedInterferenceFilters

Coatedfilterstypicallyoffersharpercutonandcutofftransitions,highertransmissions,andbetterblockingthencoloredglassfilters.Inadditiontocoloredglassfilters,therearearangeofcoatedfilters,theyrangefromhardcoatedfluorescentfilterstodichroicfilterstopolarizationfilters.Eachcoatedfilterundergoesauniquemanufacturingprocesstoensuretheproperperformance.Wavelength-selectiveopticalfiltersaremanufacturedbydepositingdielectriclayersonaspecificsubstrateofalternatinghighandlowindicesofrefraction.Thesurfacequalityanduniformityofthesubstrateestablishesthebaselineopticalqualityforthefilter,alongwithsettingwavelengthlimitswherethetransmissionofthesubstratematerialfallsoff.Thedielectriclayersproducethedetailedspectralstructureofafilterbycreatingconstructiveanddestructiveinterferenceacrossarangeofwavelengths,aswellasprovidingmuchsharpercut-offandcut-onbandswhencomparedtocoloredglassfilters.

Manytypesofhardcoatedfiltersexist,suchasbandpass,longpass,shortpass,andnotchfilters,eachwithaspecifiedblockingrangeandtransmissionrange.Longpassfiltersaredesignedtoblockshortwavelengthsandpasslongwavelengths.Shortpassfiltersaretheopposite,passingshorterwavelengthsandblockinglonger.Bandpassfilterspassabandofwavelengthswhileblockinglongerandshorterwavelengths.Theinverseofabandpassfilterisanotchfilter,whichblocksabandofwavelengthsandpassesthelongerandshorter.TransmissioncurveshapesforthesefiltertypesareshowninFigure2.

Figure2:TransmissionCurveExamplesofLongpassandShortpass(a)andBandpassandNotchFilters(b)

Filtersdesignedfordeepblocking(highOpticalDensity)andsteepslopes(sharptransitionfromblockingtotransmission)areusedinapplicationswherepreciselightcontroliscritical.Mostmachinevisionapplicationsdonotrequirethislevelofprecision;typically,anyfilterwithanOpticalDensity(OD)of4orgreaterismoreprecisethanrequiredandaddsunnecessarycost.

Becausehardcoatedfiltersutilizeopticalinterferencetoachievesuchprecisetransmissionandrejectionbands,theyintroducesomedifficultieswhenusedinmachinevisionapplications.AllinterferencefiltersaredesignedforaspecificAngleofIncidence(AOI),generally0°unlessspecificallydefinedotherwise.Whenusedinmachinevision,thesefiltersaregenerallyplacedinfrontofthelens;doingsuchcausesthefiltertoacceptlightcomingfromanglesdictatedbytheangularfieldofviewofthelens.Especiallyinthecaseofshortfocallength(largeangularfieldofview)lenses,thelightthatistransmittedthroughthefilterwilloftendisplayanunwantedeffectknownasblueshift.Forexample,a4.5mmfocallengthlens(wideangle)willhaveamuchlargerblueshiftthana50mmfocallengthlens(narro王le).AstheAOIonaninterferencefilterincreases,theopticalpathlengththroughthefilterlayersincreases,whichcausesthecut-onandcut-offwavelengthstodecrease(Figure3).Therefore,atdifferentfieldpointsintheimage,thefilterwillbehavedifferentlybytransmittingdifferentwavelengthranges:thefartheroutinthefield,themorepronouncedtheblueshift.Inmostcases,interferencefilterscanstillprovidebetterfilteringcontroloveracoloredglassfilter,butbeawareofthepotentialpitfallswhenusinganinterferencefilterwithawide-anglelens.

Figure3a:Interferencefiltersfunctionbasedonthedistancethatlightincidentuponthefiltertravels.Atthecorrectangleofincidence,thelightwavesincidentonthefilterdestructivelyinterfere,disallowingthemfrommakingitthroughthefilter.Atadifferentangle,thedestructiveinterferenceisnotaseffective,effectivelychangingthetypeoffilter.

Figure3b:AnExampleofBlueShift,shownwithaBandpassFilterusedata15°AngleofIncidence.Notenotonlytheshifttowardsalowercenterwavelength,buttheshallowingoftheslopeaswell.Thedashedcurveisideal,whenthefilterisusedata0°angleofincidence.

ApplicationswithMachineVisionFiltering

Whendesigningamachinevisionsystem,itisimportanttoenhancethecontrastoftheinspectedobject’sfeaturesofinterest.Foranintroductiontocontrast,seeourapplicationnote.Filteringprovidesasimplewaytoenhancethecontrastoftheimagewhileblockingoutunwantedillumination.Therearemanydifferentwaysfilterscanenhancecontrast,andthefiltertypeisdependentontheapplication.Somecommonfiltersusedinmachinevisionarecoloredglass,interference,NeutralDensity(ND),andpolarization.

Coloredglassbandpassfiltersaresomeofthesimplestfiltersavailablefordrasticallyimprovingimagequality.Thesefiltersworkincrediblywellatnarrowingthewavebandthatisvisiblebythevisionsystem,andareoftenlessexpensivethancomparableinterferencefilters.Coloredglassfiltersworkbestwhenusedtoblockoutcolorsontheoppositesideofthecolorwheel(Figure4).

Figure4:ColorWheelDemonstratingthatWarmColors首ldbeusedtoFilteroutCoolColorsontheOppositeSideoftheWheel

ColorFilters

ConsidertheexampleshowninFigure5,wheregelcapsulesarebeinginspected.Asshown,tworedcapsulesareontheoutersidesofapairofgreencapsulesandunderawhitelightbacklight.Thisisasortingapplicationwherethepillsneedtobeseparatedbycolortoreachtheirrespectivelocations.Imagingthecapsuleswithamonochromecamera(Figure6)providesacontrastbetweenthegreenandredcapsulesofonly8.7%,whichisbelowtheminimumadvisablecontrastof20%.

Figure5:FourLiquidCapsulesunderInspectionwiththesameVisionSystem,shownhereinColor

Figure6:CapsulesbeingviewedwithaMonochromaticCamera,yieldingaContrastof8.7%

Inthisparticularexample,minorfluctuationsinambientlight,suchasindividualswalkingpastthesystem,candecreasethealreadylowcontrastvalueof8.7%enoughsothatthesystemisnolongercapableofoperatingproperly.Severalsolutionstothisproblemexist:abulkyandcostlylightbaff领systemcanbebuilttocompletelyenclosetheinspectionsystem,theentirelightingschemeofthesystemcanbereworked,orafiltercanbeaddedtoenhancethecontrastbetweenthegreenandredpills.Inthisinstance,thesimplestandmostcosteffectivesolutionistoutilizeagreencoloredglassfilterinordertoimprovethecontrastbetweenthetwodifferentcoloredcapsules.AsshowninFigure7,thecontrastimprovesfrom8.7%to86.5%:anincreaseofnearlyafactorof10.

Figure7:CapsulesbeingviewedwithaMonochromaticCameraandGreenColoredGlassFilteryieldingaContrastof86.5%

NeutralDensityFilters

Neutraldensityfiltersareusedincertainapplicationswhereitisadvantageoustohaveadditionalcontroloverthebrightnessofanimagewithoutchangingtheexposuretimeoradjustingthef/#.Althoughtherearetwoprimarytypesofneutraldensityfilters(absorbingandreflecting),theiroverallresponsibilityisthesame:uniformlylowerthelightthatistransmittedthroughthelensandontothesensor.Forapplicationslikeweldingwheretheimagercanbeoverloadedregardlessoftheexposuretime,neutraldensityfilterscanprovidethenecessarydropinthroughputwithoutneedingtochangethef/#(whichcanimpacttheresolutionofthesystem).Specialtyneutraldensityfilters,likeapodizingfilters,existtohelpwithhotspotsinthecenterofanimagecausedbyaharshreflectionfromanobject,buttheopticaldensitydecreaseswithradialdistanceawayfromthecenterofthefilter.

PolarizingFilters

Polarizationfiltersareanothercommontypeoffilterusedinmachinevisionapplicationsastheyallowbetterimagingofspecularobjects.Inordertoproperlyusepolarizingfilters,itisimportantthatboththelightsourceandthelenshavepolarizationfiltersonthem.Thesefiltersarecalledthepolarizerandtheanalyzer,respectively.Figure8showsanexampleofhowpolarizationfilterscanmakeadifferencewhenviewingspecularobjects.IntheFigure8a,aCCDimagerisbeinginspectedwithbrightfieldilluminationandFigure8bshowsthesameilluminationsetupwithapolarizeronthelightsourceandananalyzeronthelens.

Figure8:ImagestakenwithnoFilter(a)showingHighGlareandwithPolarizationFilters(b)whichReduceGlare

AsshowninFigure8b,augmentingthesystemwithpolarizersprovidessuperiorperformanceastheharshreflectionsareabsorbedbythefilteronthelens.Toensurethemaximumextinctionofunwantedglare,thepolarizeronthelightsourcemustbealignedwithitspolarizationaxis90°fromthepolarizationaxisofthepolarizeronthelens,otherwise,thelenswillstilltransmitsomeoftheharshlyreflectedlightintothesystem,causingglare.

Itiscriticaltounderstandthatfiltersexisttomanipulatethecontrastofanimageinordertohelpincreasetheaccuracyoftheimagingsystem.Whetheritissimplecolorfilteringorpolarizationfiltering,eachfilterexiststosolveauniqueproblem;itisimportanttounderstandwhatfilters首ldbeusedforspecificapplications.

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