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UnitThirtyFiveCOLOR.SCIENCEANDSHADESELECTIONAnunderstandingofthenatureoflightandhowtheeyeperceivesandthebraininterpretslightascolorisimportantforsuccessfulshadeselection,particularlywhenmetalceramicofall-porcelainrestorationsarebeingmade.Errorsinshadematchingcontinuetobeaproblemwiththeseproceduresandcanbeasourceoffrustrationforthedentistandtechnicianandasourceofdissatisfactionforthepatient.Thischapteroutlinessomeoftheprinciplesoflightandcolorandappliesthem(oadiscussionofshadeselection.LIGHTANDCOLORWithoutlight,color.doesnotexist.Anobjectthatweperceivetobeofacertaincolorabsorbsalllightwavescorrespondingwithothercolorsandreflectsonlythosewavesthatweasthecoloroftheobject.Thusanobjectthatabsorbsblueandgreenlightandreflectsredappearsred.Itsphysicalproperties,thenatureortheincidentlighttowhichtheobjectisexposed,therelationshiptoothercoloredobjects,andthesubjectiveassessmentoftheobserver,influencestheapparentcolorofanobject.Asingletooththereforecanlookdifferenttodifferentpeopleunderdifferinglightconditions.Saleskihaspointedoutthatalthoughlightingstandardsareavailableinindustriesasdiverseasautomobileandtextilemanufacturingthereisnostandardfbrlightingindentistry.DescriptionoflightScientifically,lightisdescribedasvisibleelectromagneticenergywhosewavelengthismeasuredinnanometers(nm)orbillionthsofameter.Theeyeissensitiveonlytothevisiblepartoftheelectromagneticspectrum,anarrowbandwithwavelengthsof380to750nm.Purewhitelightconsistsofrelativelyequalquantitiesofelectromagneticenergyoverthevisiblerange.Whenitispassedthroughaprism,itissplitintoitscomponentcolorsbecausethelongerwavelengthsarebent(refracted)lessthan(heshorterones.QualityoflightThemostcommonlightsourcesindentalofficesareincandescentandfluorescent,neitherofwhichispurewhitelight.Anordinaryincandescentlightbulbemitsrelativelyhigherconcentrationsofyellowlightwavesthanofblueandbluc-grccnwhereasfluorescentceilingfixturesgiverelativelyhighconcentrationsofbluewaves.Dentaloperationsgenerallyhaveincandescentlightbulbscombinedwithbluereflectors,whichgivearatherbluishlight.DescriptionofcolorMunsellcolor_ordcrsystem.ThemostpopularmethodfordescribingcolorjstheMunsellsystem,which,despitehavingcertaindisadvantages,iswidelyusedinthedentalliterature.Initthethreeattributesofcolorarecalledhue,value,andChroma.HueThisisdefinedastheparticularvarietyofacolor,shade,ortint.TheHueofanobjectcanbered,green,yellow,etc.andisdeterminedbythewavelengthofthereflectedand/ortransmittedlightobserved.Theplaceofthatwavelength(orwavelengths)inthevisiblerangeofthespectrumdeterminestheHueofthecolor.Theshorterthewavelength,theclosertheHuewillbetothevioletportionofthespectrum;(helongerthewavelength,thecloseritwinbetotheredportion.IntheMunsellcolor_system,Huesaredividedinto10gradations;yellow,yellow-red,red,red-purple,purple,purple-blue,blue,blue-green,green,andgreen-yellow.Thesearearrangedinawheel.Eachgradationissubdivided;forexample,redcanbewrittenIR,2R-9R,3R-9R.10R,followedby1YR.2YR,3YR-9YR,10YR,followedby1Y,2Y,3Y,etc.;andthesecanbefurthersubdivided.ThusaparticularHuemightbe4.3Yor8.1YR.Mostnaturalteethfanintoarangebetweenyellowandyellow-red.Inastudyof6000teethClarkestablishedaHuerangeof6YRto9.3YRbutfound(hatthedistributionofteethwithinitwasnotatalluniform.ChromaThisisdefinedastheintensityofaHue.ThetermssaturationandChromaareusedinterchangeablyinthedentalliterature;bothmeanthestrengthofagivenHueortheconcentrationofpigment.AsimplewaytovisualizedifferencesinChromaistoimagineabucketofwater.Whenonecupofpaintisadded,asolutionoflowChromaresults.AddingasecondcupofpaintincreasestheChroma,andmon,UntilasolutionisobtainedthatisalmostallpaintandconsequentlyofhighChroma.IntheMunsellcolorsystem,maximumChromadependsontheparticularHuebutcanringfrom10to14.AchromaticshadeshaveaChromanear0.NaturalteetharefoundwithChromarangesfrom0to7.ValueThisisdefinedastherelativelightnessordarknessofacolor_orthebrightnessofanobject.Thebrightnessofanyobjectisadirectconsequenceoftheamountoflightenergy(hatthatobjectsreflectsortransmits.Lingtenergyismeasuredinphotons,anditispossibleforobjectsofdifferentHuestoreflectthesameamountofphotonsandthushavethesamebrightnessorValue.Acommonillustrationwouldbethedifficultyexperiencedintryingtotellagreenfromablueobjectinablackandwhitephotograph.Thetwoobjectswillreflectthesameamountoflightenergyandthusappearidenticalinthepicture.ThisfactledtotheoncepopularmisconceptionthattheValueofacolor_rcsidcdinitsamountofgrayness.IntheMansellmethodofdescribingcolor.Valueisdividedintotengradations,with0beingblackand10beingwhile.NaturalteethrangeinValuefrom4to8.Arestorationthathastoohigh,aValue(istoobright)maybereadilydetectedbyanobserverandisacommonestheticfaultinmetal-ceramicprosthodontics.PerceptionofcolorLightfromanobjectentersthecvcandactsonreceptorsintheretina(rodsandcones).Impulsesfromthesearepassed(otheopticalcenter01thebrain,whereaninterpretationismade.Differentpersonswillmakedifferentinterpretationsofthesamestimulusandthusshadeselectionisdependentonsubjectiveassessment.TheeyeUnderlowlightingconditionsonlytherodsareused(scotopicvision).Thesereceptionsallowandinterpretationofjustthebrightness,notthecolor,ofobjectistobemade.Theyaremostsensitivetoblue-greenobjects.Colorvisionisdependentonthecones,whicharcactiveunderhigherlightingconditions(photopicvision).Thechangefromphotopictoscotopicvisionistermeddarkadaptationandtakesabout40minutestobecomplete.Althoughtheexactmechanismofcolorvisionisnotknown,ithasbeendemonstrated(hattherearethreetypeofcone-sensitivetored,green,andbluelight-whichformanimageinamannersimilartotheadditiveeffectofthepixelsinacolortelevisionpicture.Color_adaptationColor_visiondecreasesrapidlyasanobjectisobserved,theoriginalcolorbecomingapparentlylessandlesssaturateduntililappearsalmostgray.Simultaneously,(hechroma(intensity)ofcomplementarycolorsappearsgreater,aphenomenonthathasledtothesuggestionthatshadeselectioncanbeenhancediftheoperatorwallsarepaintedpaleblue(complementarytoyellow)orapalegray-bluecardisglancedatbetweenlookingatthecolorchoices.DeceptivecolorperceptionSurroundingcolor,particularlycomplementaryones,influencecolor.When,forexample,blueandyellowarcplacedsidebyside,theirchromamayappeartobeincreased.Thecolor_ofteethcanappeardifferentifthepa(5enliswearingbrightlycoloredclothingorlipstick.MetamerismTwocolorsthatappeartobeamatchunderagivenlightingconditionbuthavedifferentspectralreflectancearecalledmetamers,andthephenomenonisknownasmetamerism.Forexample,twoobjectsthatappeartobeanidenticalshadeofyellowmayabsorbandreflectlightdifferently,normallyayellowobjectreflectsyellowlightbutsomemay,infact,absorbyellowlightandreflectorangeandgreen,lbanobsenertheorangeandgreencombinationlooksyellow,althoughwhenthelightischangedthemetamersnolongermatch.Thismeansthatasamplethatappearstomatchunder,forexample,theoperatorlightmaynolongerbesatisfactoryindaylight.Theproblemofmetamerismcanbeavoidedbyselectingashadeandconfirmingitunderdifferentlightingconditionssuchasnaturaldaylightandfluorescentlight.ColorblindnessDefectsincolorvisionaffectabout8%ofthemalepopulation,andlessofthefemalepopulation.Differenttypesexist,fromachromatism(completelackofhuesensitivity)throughdichromatism(sensitivitytoonlytwoprimaryhues,usuallyeitherredorgreennotperceived)toanomaloustrichromatism(sensitivitytoallthreehueswithdeficiencyorabnormalityofoneofthethreeprimarypigmentsinlheretinalcones).Thusalldentistsshouldhavetheircolorperceptiontested,andanythatarefoundtobedeficientinaparticularrangeshouldobtainassistancewhenselectingtoothshades.SHABESELECTIONBecauseshadematchingissub3cctive,consistencyisdifficulttoachieve.Ithasbeendemonstratedthatconsiderablevariationexistsamongdentistsandsomeareunabletoduplicateeventheirownshadeselectionfromonetimetoanother.Fortunately,alifelikerestorationdocsnothavetobeanexactcolorduplicateoftheadjacentofcontralateralteeth.Itshould,however,blendwiththeteethasaresultofthedistributionofceramicmaterialsintherestoration.Shadeselectioncanbeimprovedbyknowledgeoftheprinciplesoflightandcolor,andofthetechniquesinvolvedindentalceramics.CommercialshadeguideThemostconvenientmethodforselectingashadeistouseoneofthemodemcommerciallyavailableporcelainshadeguides.Eachshadetabhasanopaquebackingcolor_andaneckcolor,bodycolor,andincisalcolor.Shadeselectionconsistsofpickingtheshadelab(hatlooks(hemostnaturalandhavingthisreproducedinalaboratorywiththehelpofthematerialsandtechniquesrecommendedby(hemanufacturer.Theprocedureismadeeasieriftheshadeguideisarrangedwithspecimensofthesamehuegroupedtogether.HueselectionInthepopularVitaLuminvacuumshadeguideAl,A2,A3,andA3.5,andA4aresaidtobeofasimilarhue,asarctheB,C,andDshades.Therecommendedtechniqueistochoosethenearesthuefirstandthenselecttheappropriatematchofchromaandvaluefromlhetabsavailable.Itmaybedifficulttodetermineagivenhueaccuratelyifitschromaorintensityislow.Thereforetheregionwiththehighestchroma(i.e.,(hecervicalregionofcanines)shouldbeusedfbrinitialhueselection.ChromaselectionOncethehueselectionhasbeenaccomplished,achoiceismadeofthebestmatchofchroma.Forexample,ifaBhueisdeterminedtomatchbestincolor_varicty,therearcfouravailablegradations(tabs)ofthathue:BI,B2,B3,andB4.Severalcomparisonsareusuallynecessaryindeterminingwhichsamplebestrepresentstheblueanditscorrespondingsaturationlevel.Betweencomparisons,glancingatablueobjectwillresttheoperatorseyeandhelpavoidfatigueoftheretinalcones.ValueselectionFinally,adeterminationismadeforvaluewithasecondcommercialguidewhosesamplesarearrangedinorderofincreasinglightness.Byholdingthesecondshadeguideclosetothepatient,itshouldbepossibletodeterminereadilywhetherthevalueofthetoothisin(herangeof(heshadeguide.Attentionisthenfocusedontherangeoftheshadethatbestrepresentsthevalueofthetoothandhowthatrangerelatestothetabmatchingfbrhueandsaturation.Observationfromadistance,standingslightlyawayfromthechair,andlookingthroughsquintedeyeswillhelpinassessingthevalue.Bysquinting,theobservercanreducetheamountoflightthatreachestheretina.Stimulationoftheconesisreduced,andagreatersensitivitytoachromaticconditionsmayresult.Whilesquinting,theobsenerconcentratesonwhichdisappearfromsightfirst,thetoothortheshadetab;theonethatfadesfirsthasthelowervalue.Whentheproperselectionofvaluehasbeenmade,itwillbetheexceptionratherthantheruleforthistocoincidewiththedeterminationsfbrhueandchroma.Adecisionmustbemadewhetheritisnecessarytochangethepreciouslyselectedshadesample.Iftheindependentvaluedeterminationislowerthanthevalueofthesampleselectedfbrhueandchroma,achangeisusuallynecessarysinceitisnotpossibletoincreasethevalueofandobjectbyaddingsurfacestain(whichalwaysreducesbrightness).Ifthevaluedeterminationishigherthanthehuedetermination,itshouldbedecidedwhetherthisdifferencecanbebridgedthroughinternalorsurfacestainingoftherestoration.Thedeterminationsofhueandchromaaswellasofvaluearcthemcommunicatedtothelaboratory.ShadedistributionchartShadedistributionchartingisapracticalapproachtoaccurateshadeselectionandisrecommendedevenwhenafairlygoodmatchisobtainablefromthecommercialshadesample.Thetoothisdividedintothreeregions:cervical,middle,andincisal.Eachregionismatchedindependently,eithertothecorrespondingareaofacommercialshadesampleortoasinglecolorporcelainchip.Becauseonlyasinglecolorismatched,intermediateshadescanusuallybeestimatedrathereasilyandduplicatedbymixingporcelainpowders.Thejunctionsbetweentheseareasarenormallyfairlydistinctandcanbecommunicatedtothelaboratoryintheformofadiagram.Theshadedistributionandthicknessoftheenamelporcelainareparticularlyimportant.Individualcharacteristicsarereadilymarkedonsuchasketchandwillallowtheceramisttomimicdetailslikehairlinefractures,hypocalcification,andproximaldiscoloration.VOCABULARY1.ceramic陶瓷的，n.陶瓷制占2.porcelain瓷，瓷器a.瓷的3.restoration修复体4.metal-ceramicrestoration金属烤瓷修复体5.porcelainfusedtometalrestoration全瓷熔附金属修复体6.all-porcelainrestoration全瓷修复体7.shade色彩的浓淡8.shadeselection选色9.electromagnetic电磁的1().wavelength波长的11.incandescent白炽的12.fluorescent荧光的13.studio工作室14.colortemperature色温15.reflect反射16.refract折射17.scatter散射18.hue色调19.chroma饱和度，彩度20.saturation饱和度21.value明度，亮度22.photom光子23.perceive看见24.interpret解释25.achromatic非彩色的26.perception感觉，知觉27.retina视网膜28.optical视觉的，光学的29.rod视杆细胞30.cone视锥细胞31.scotopicvision暗视觉32.scotopia暗适应33.thotoma育点34.photopicvision明视觉35.photopia光适应36.coloradaptation色彩适应plementarycolor互补色38.metamer条件等色39.metamerism条件配色40.colorblindncs色盲41.achromatism全色盲42.dichromatism二色性色盲43.anomaloustrichromatism异常三色视觉，色弱44.subjective主观的45.colorimeter比色计，色度计46.stain染色47.prophylaxis预防学面的色全的的的光牙明的的色上不的理学明的学品色的维色透缘部上面化观条复空透明料色粉镜度表医织蓝谱业献离纤选不切颈内表钙美有修真半透染变瓷棱强图牙纺带光商文分fibreoptic48. shadetabopaque49. incisalcenical50. internalstainsurfacestain51. hypocalcificationesthetic52. methodicalprosthodontics53. vacuumtranslucent54. transparentpigment55. discolorationporcelainpowder56. prismintensify57. chartdentistry58. textilebluish59. spectrumcommercial60. literaturesplit
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