基于TCS230颜色传感器的色彩识别器的设计 外文翻译Word文件下载.docx
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itcontains24coloredpatchesarrangedina6by4array.Figures2through5overleafshowthespectralreflectanceofthepatchesineachofthefourrowsofthechart–thatis,thefractionofincidentlightthatisreflected(withrespecttoanidealdiffusereflector),asafunctionofwavelengthfrom350nmto750nm.
Figure1TheColorCheckercontains18coloredpatchesanda6-stepgrayseries.
Figure2ColorCheckerspectra,toprow.
Figure3ColorCheckerspectra,secondrow.
Figure4ColorCheckerspectra,thirdrow.
Figure5ColorCheckerspectra,bottomrow(neutralseries)
Figure6Conesensitivitiesofconephotoreceptorsareshown.Theshortwave-sensitivephotoreceptorsaremuchlesssensitivethantheothertwotypes.Theresponsesofthemediumwaveandlongwavephotoreceptorshaveagreatdealofoverlap.Visionisnotsensitivetotheprecisewavelengthofthestimulus:
Whatattersisopticalpowerintegratedundereachresponsecurve.
Introductiontocolorvision
Photoreceptorcellscalledconesintheretinaareresponsibleforhumancolorvision.Therearethreetypesofconecells,sensitivetolongwave,mediumwave,andshortwaveradiationwithintheelectro-magneticspectrumbetweenabout400nmand700nm.Becausetheconesensitivitiesareveryroughlyinthepartsofthespectrumthatappearred,green,andblue,colorscientistsdenotethecelltypesasρ,γ,and,theGreeklettersforr,g,andb.(TodenotethesensorsR,G,andBwouldwronglysuggestaclosercorrespondence.)EstimatesofthespectralresponseoftheconetypesaregraphedinFigure6above.
Lightinthephysicalworldcanbecharacterizedbyspectralpowerdistributions(SPDs).Coloredobjectscanbecharacterizedbyspectralreflectancecurves,suchasthoseoftheColorChecker.However,visionisinsensitivetotheexactwavelengthofastimulus:
Accordingtothemoderntheoryofcolorscience,allthatmattersistheintegralofopticalpowerunderneatheachresponsecurve.Thatthereareexactlythreetypesofconecellsleadstothepropertyoftrichromaticity:
Threecomponentsarenecessaryandsufficienttocharacterizecolor.Somepeoplemightusethephrase“colorassensedbytheeye,”butIcon-siderthatqualifiertoberedundantatbest,andmisleadingatworst:
Colorisdefinedbyvision,sothereisnoneedtousethequalifyingphrase“assensedbytheeye,”ortousetheadjectivevisiblewhenreferringtocolor.
OverviewofCIEColorimetry
ThespectralresponsesoftheconecellsthatIgraphedinFigure6wereunavailabletoresearchersinthe1920s.Researchersatthetimeusedpsychophysicalexperiments,suchasthefamouscolormatchingexperiment,toteaseoutthedata.TheCIEistheinternationalbodyresponsibleforcolorstandards.In1931,thatorganizationadoptedthecolormatchingfunctionsdenotedx(λ),y(λ),andz(λ),graphedinFigure7.
Figure7CIE1931,2°
color-matchingfunctions.Acamerawith3sensorsmusthavethesespectralresponsecurves,orlinearcombinationsofthem,inordertocaptureallcolors.However,practicalconsiderationsmakethisdifficult.Theseanalysisfunctionsarenotcomparabletospectralpowerdistributions!
WeightingaphysicalSPDundereachofthesethreecurves(thatis,formingthewavelength-by-wavelengthproduct),andsummingtheresults,formsatripleofthreenumbers,denotedX,Y,andZ.Incontinuousmathematics,threeintegralsneedtobecomputed;
indiscretemath,amatrixproductissufficient.TheX,Y,andZtristim-ulusvaluescharacterizecolor.Theyarelinear-lightquantities,propor-tionaltoopticalpower,thatincorporatethewavelengthsensitivityofhumanvision.TheYvalueisluminance,whichisordinarilyexpressedinunitsofcandelapermetersquared(cd·
m-2).Ifyouaremeasuringreflectance,thereflectedtristimulusvaluesdependuponthespectralcharacteristicsoftheilluminant,andtheiramplitudesscalewiththepoweroftheillumination.Relativeluminanceistheratioofreflectedluminancetotheluminanceoftheillumination;
itisalsoknownastheluminancefactor.
Figure8SPDsofvariousilluminantsaregraphedhere.IlluminantA,showninorange,isrepresentativeoftungstenlightsources;
itisdeficientinshortwavepower,andmaycauseerrorsinsensingbluecolors.ThebluelinegraphstheSPDofaNichiawhiteLED.Thereisapeakintheblueportionofthespectrum:
Uncorrected,thesensorwouldreportexcessivebluevalues.TheotherfourlinesrepresentCIEstandardilluminantsC,D50,D55,andD65.
Inmanyapplications,tristimulussignals(includingluminance)scalewiththeillumination,andareotherwiseuninterestinginthemselves.Whatismoreinterestingistheratiosamongthem,whichcharacterizecolordisregardingluminance.TheCIEhasstandardizedtheprojectivetransformationofEquation1,inthemargin,totransform[X,Y,Z]valuesintoapairof[x,y]chromaticitycoordinatesthatrepresentcolordisregardingluminance.Thesecoordinatesaresuitableforplottingintwodimensionsonachromaticitydiagram.
Eq1Chromaticitycoordinates
Illumination
Anonemissiveobjectmustbeilluminatedinordertobevisible.TheSPDreflectedfromanilluminatedobjectisthewavelength-by-wave-lengthproductoftheilluminant’sSPDandthespectralreflectanceoftheobject.Beforelightreachestheeye,theinteractionamonglightsourcesandmaterialstakesplaceinthespectraldomain,notinthedomainoftrichromaticity.Toaccuratelymodeltheseinteractionsrequiresspectralcomputations.WhenapplyingtheTCS230,attentionmustbepaidtothespectralcontentoftheilluminationandtopoten-tialinteractionbetweentheilluminationandthesamplestobesensed.Generally,thelessspikythespectra,thebetter.Figure8graphsseveralilluminants.
Yourapplicationmayinvolvesensingcolor,inwhichcasetheprecedingdescriptionapplies.However,someapplicationsoftheTCS230involvenotsomuchestimatingcolorasseenbytheeyebutrathersensingphysicalparametersassociatedwithopticalpowerinthevisiblerange.Insuchapplications,toapproximatethevisualresponsemaynotbethebestapproach:
Itmaybemoreeffectivetotakeamoredirectapproachtoestimatingtheparametersoftheunderlyingphysicalprocess.
TheColorChecker
Equippedwithknowledgeofhowspectraarerelatedtocolors,theplottingofchromaticitycoordinates,andthedependenceofcolorsuponillumination,wecanreturntotheColorChecker.GretagMac-bethdoesn’tpublishorguaranteethespectralcompositionofthepatchesoftheColorChecker.However,nominalCIE[X,Y,Z]valuesarepublished.ThepatchesinthebottomrowoftheColorCheckercontainneutralcolors;
thenumericnotationsinthelegendsofFigure5reflectonetenthofthelightness(L*)valuesofthosepatches.
Thespectragraphedonpages2and3representthephysicalwave-length-by-wavelengthreflectanceofthepatches.Thesespectralreflec-tanceshavebeenmeasuredbycolormeasurementinstrumentcalledaspectrophotometer.Ifyouhadaccesstoalightsourcehavingperfectlyevendistributionofpoweracrossthevisiblespectrum,thenthereflectancecurvesgraphedherecouldsimplybescaledtorepre-sentthereflectanceinyourapplication.Practicallightsourcesdonothaveperfectlyevenspectraldistributions,socompensationisneces-sary:
Youmustcomputethewavelength-by-wavelengthproductoftheilluminant’sSPDwiththespectralreflectanceofthechart.
WewillfirstcalculatetheCIE[X,Y,Z]valuesfromthechart.(ThesevaluesshouldagreewiththefiguresprovidedbyGretag.)Thenwewillcalculatethe[R,G,B]valuesthatwillbedetectedbyaTCS230.
TocalculateCIE[X,Y,Z],wetakethe31×
3matrixrepresentingthecolormatchingfunctions(CMFs)oftheCIEStandardObserver,andperformamatrixproductwith31spectralresponsevaluesascorrectedforillumination.Thisproducesthe[X,Y,Z]tristimulusvalues.Whenchromaticitycoordinates[x,y]arecomputedfrom[X,Y,Z]throughtheprojectivetransforminEquation1,thenplotted,thechromaticitydiagraminFigure9results.Thehorseshoe-shapedfigure,closedatthebottom,containsallcolors:
Everynon-negativespectraldistributionproducesan[x,y]pairthatplotswithinthisregion.Thelightly-shadedtriangleshowstheregioncontainingallcolorsthatcanbeproducedbyanadditiveRGBsystemusingsRGB(Rec.709)primarycolors.Thisregiontypifiesvideoanddesktopcomputing(sRGB).ThepointsplottedinFigure9arethecolorsoftheColorChecker.Whiteandgrayvaluesareclusterednearthecenterofthechart.
Figure9CoordinatesofColorCheckerpatchesaregraphedontheCIE[x,y]chromaticitydiagram.Thehorseshoeenclosesallcolors;
thetriangleenclosesthecolorsthatcanberepresentedinvideo(Rec.709)andindesktopcomputing(sRGB).
TheTCS230
Figure10showstheresponsesofthefourchannelsoftheTCS230.Theblackcurveshowstheresponseofthe