基于TCS230颜色传感器的色彩识别器的设计 外文翻译.docx

基于TCS230颜色传感器的色彩识别器的设计 外文翻译.docx

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基于TCS230颜色传感器的色彩识别器的设计外文翻译

SensingcolorwiththeTAOSTCS230

TheTAOSTCS230isasmall,highlyintegratedcolorsensingdevicepackagedinaclearplastic8-pinSOIC.Itreports,asanalogfrequency,theamountofshortwave（blue）,mediumwave（green）,longwave（red）,andwideband（white）opticalpowerincidentontothedevice.Itcanbeusedinavarietyofcolorsensingapplications.Detailsofthedevicecanbefoundinitsdatasheet.ThiswhitepaperdetailstheconceptsandcalculationsinvolvedincolorsensingusingtheTCS230.

WewillusetheColorCheckerchartasanopticalstimulustoworkthroughanumericalexampleofcolorsensing.Thechart,depictedinFigure1,ismanufacturedanddistributedbyGretagMacbeth.Thechartmeasuresapproximately13inchesby9inches（330mmby230mm）;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