Light Tutorial.docx
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Light Tutorial.docx
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LightTutorial
LightTutorial:
Part1
Thereseemstobeverylittleinformationavailableindetailedformonwhatappearstometobeacrucialsubjectforartistsofallkinds:
lightinalltheguisesinwhichweencounteritonadailybasis.I'vereadcountlessbooksdealingwithdigitalandtraditionalartandthesubjectoflightonlyeverseemstobetouchedonverylightly(pardonthepun).Yettoanyonewishingtocreateanillusionofrealityshouldhaveagoodunderstandingofhowlightbehavesinthephysicalworld.
Theworstculpritsinmyopinionarethe3dtextbookswhichallrepeatthesametiredoldformulasandleavebeginningartistsverylittleinsightonhowtolighttheirscenes.Theresultisthatlightisbadlyunderstoodandpoorlyusedbydigitalartistsonthewhole.Photographersandpaintersnecessarilyhaveabetterunderstandingoflight,butlightisacrucialelementofanyart.Withoutunderstandingitwellitisveryhardtoachieverealism,letaloneatmosphere.
HavingneverfoundadetailedarticleaboutlightontheinternetIdecidedtowritethisone.MostofwhatIsayhereisbasedonmyownobservations,sotheremaybetheoddinaccuracy.Itmaybethatthisstuffissoobviousthatitdoesn'tneedexplaining,howeverthatdoesseemunlikelytomesincecarefulobservationoftheworldaroundisrequiredyoutorealiseexactlyhowlightbehavesindifferentsituations.
ThroughoutthisarticleIwillbeusingadiagramofawhiteballonwhitecardtodemonstratehowlightbehavesindifferenteverydaysituations:
Hereitillustratesasunnyafternoon.Themainsourceoflightisthesun,whilsttheblueskysuppliesasecondsourceoflightwithverydifferentcharacteristics.Somelightisalsobouncingbetweenthewhitebaseandtheballandsuppliesathirdsourceoflight.
Thebrightestlightiscomingfromthesunandiswhitelightemanatingfromasmallsource,whichcausesittocastsharpedgedshadows.Thesecondsource,thebluesky,isaverylargelightsourceandasaresulthasverysoftshadows(whichinanycasearecompletelymaskedbythedirectlightcomingfromthesun).Iwillgointomoredetaillateraboutlightsourcesandshadowsize,butfornowjustrememberthatthesmallerthesourceofthelight,thehardertheshadows.
Thelightcomingfromtheblueskyhasaverystrongcolourcastwhichaffectseverythinginthisscene.Theshadowcastbytheballisbluebecauseitisilluminatedbyblueskylight,sincetheballisshieldingitfromthewhitelightofthesun.Thepartsoftheballwhicharen'tindirectsunlightalsotakeonabluehuebecausetheyarelitbythebluesky.
Finallythelightthatisreflectedbetweenthecardandtheballisalsopredominantlyblue(eventhoughthecardandballarewhite)sinceitisblueskylightthatisbeingreflectedbythewhiteobjects.Thesurfaceswhichareclosertogetherreceivemoreofthisreflectedlightthanareaswhicharefurtherapart,thereforethebottompartoftheballislighterthanthecentrebecauseitisnearerthewhitecard.
Thedarkestareasintheimagearethebaseofthecastshadowandtheborderbetweentheareasinsunlightandshadeontheball:
thiszoneiscalledtheterminator.Thebaseofthecastshadowisverydarkbecauseitreceivesnosunlightandtheballisalsomaskingitfrommostoftheskylightandbouncedlight.Theotherendofthecastshadowislighterbecauseitisreceivingmorelightfromtheskyandalsobouncedlightfromtheball.
Whyistheterminatorthedarkestareaontheball?
Partlybecauseoftheeffectofcontrast,beingsoneartotheverybrightsideoftheballinsunlightmakesitappeartobedarker,butalsoitisreceivinglessofthebouncedlightwhichisbeingreflectedbythewhitecard.Sounliketherestoftheball,whichisreceivingeitherfullsunlightorlightreflectedfromthewhitecard,itsmainsourceofilluminationisthebluesky.Itistheareainbetweenthemainlight(thesun)andthefilllight(thereflectedlightfromthecard).
Whyisthelightfromtheskyblue?
Visiblelightismadeupoftinyparticlescalledphotons,theseparticleshavedifferentwavelengthsdependingontheircolour:
bluelightcomprisesofparticleswithshorterwavelengthswhereasredlightismadeofparticleswithlongerwavelengths.
Whitelightfromthesunismadeupofacontinuousspectrumofcolourswhich,conventionally,isdividedintothecoloursoftherainbow(withprogressivelylongerwavelengths:
violet,indigo,blue,green,yellow,orangeandred).Itisthemixtureofthesecoloursthatproduceswhite.
Howeverwhathappenstolightwhenittravelsthroughtheatmosphereoftheearthisthattheshorterwavelengthsoflightbecomescattered.Ouratmosphereismadefromvariousgasesandtheatomsandmoleculesthattheseareformedfromaresuspendedwithinit.Photonstravellingthroughtheatmospherephysicallycollidewiththeseatomicparticlesandacollisionwilldeflectthephotonsandmakethembounceinanotherdirection.Shorterwavelengthsaremorelikelytobedeflectedthanlongerones,sothatthephotonswhicharescatteredinalldirectionsbythesecollisionsarepredominantlyblue.
Longerwavelengthsoflight,suchasred,cantravelfurtherthroughtheatmospherewithoutbeingscattered.Thisiswhysunsetsarered:
asthesunlighttravelsthroughathickerlayerofairtoreachuswhenitislowerintheskyalotofthebluelightislostfromscattering,andtheremaininglightispredominantlyred.
Theeffectofbouncingbluephotonsinalldirectionsisthattheatmosphereisactuallyglowingwithbluelight,aneffectwhichisclearlyvisiblefromspace.Thisbluelightisstrongenoughtoilluminateareasthatarenotindirectsunlight,whichiswhyyoucanstillseewhenyouareinopenshade.
Whenlighthitsasurfaceiteitherbouncesorisabsorbedbyit,dependingonwhatcolourthatsurfaceis.Awhiteobjectwillreflectallwavelengthsequally,whereasablackobjectwillabsorbthemall.Whenwhitelighthitsaredsurfacetheblueandgreenwavelengthsareabsorbedandtheredlightisreflected(I'mjustusingtheprimarycolours,ratherthanthefullspectrumhere,forthesakeofsimplicity).
Soifwhitelighthitsaredsurfacethephotonsthissurfacereflectswillbered.Whenthesephotonshitthenextsurfaceintheirpaththeywillthereforebeilluminatingitwithredlight.Thisphenomenoniscalledradiance,andthecoloursofadjacentobjectswillhaveanaffectoneachotherbecauseofthis.
Radianceisusuallyasubtleeffect,andittakesagreatdealoflightforittobecomeapparent.Insoftordimlightitmaynotbevisibleatall,howeverinbrightlightitcanaddalotofcolourtotheobjectsitaffects.Iflightisreflectingbetweenobjectsofthesamecolouritcancreateaverysaturatedeffectasthebouncedlightreinforcestheexistingcolouroftheunderlyingsurface,makingthecolourglowvividly-youcansometimesseethisphenomenoninbrightdaylight.
Howwechoosetorepresentasceneissubjectiveandopentointerpretation.Mostsituationswillhaveabalancebetweenlightandshadethatproducesanaverageormediumgreybecausethat'swhatweperceiveasnormal.Howevertherearesomesituationswherethenaturalorderofthingstendstowardseitherextremeoflightordark,suchasfogorsnowontheonehand,ornighttimeontheother.Alternativelyanartistmightchoosetoemphasiseoneoftheseextremesforvisualimpact,ortoconveyaspecificfeeling.
Highkeyimageshaveapredominanceofwhiteorverylighttonesandtendtolooklightandairy.Highkeylightingisoften(butnotalways)soft,anddetailisgenerallylow.Innaturehighkeylightingisfoundinfogandsnow,whereevenshadowsarelightduetotheamountofreflectedlightbouncingaround.
Lowkeyimageshavebytheirverynatureverylittlelightinthem.Contrastisusuallyhighandthelightinghard.Lowkeylightingcancreateaverymoodyatmosphereandisoftenusedtothiseffect.Themostobvioussettingforlowkeylightingisnighttime,butitcanalsobefoundinothersituationssuchasstormsandininteriors.
Mostlightsourcesthatweencounterineverydaysituationshaveacolourcast,howeverourbrainisverygoodatfilteringthisout.Aslongasthereisavaguemixtureofthethreeprimariesinthelightourbraininterpretsitaswhite.Evenunderlightingwithverystrongcolourwehavetheabilitytofiltertheinformationoureyesreceivesandmakesenseofthecolourssothatweperceivetheminarelativeratherthanabsolutemanner.
Themostobviouswayofdemonstratingthisistouseadigitalcamerawiththewhitebalancesettodaylight:
thisisaneutralsettingwhichwillreflectthecoloursthatareactuallythere.IntheexamplebelowIhaveshottheimagewithawindowactingasthelightsource.Thelightiscomingindirectlyfromanovercastskyandisrelativelyneutral.
InthenextphotographIhaveclosedtheblindsandusedastandardhousehold60wattlightbulbasmylightsource:
Thestrengthofthecolourcastinthisimagemaywellhavesurprisedyou,sincewedon'ttendtoperceivetungstenlightingasbeingsuchabrightyellow/orange.Ourbrainconvertsthecolourstomakethemresemblethefirstimage,butinthiscaseit'sthecamerawhichispaintingthetruepicture.
Oneveryeasywayofconfirmingthisfactistolookatwindowsfromtheoutside:
nexttimeyouareoutdoorsduringtheeveninglookatthecolourthatcomesfromhousesandyouwillseethattheirinteriorsareabrightorange.Whenwearen'tdirectlyunderthelightsourceweareabletoseeitstruecolour.
Somethingverysimilarhappenswhenwearesta
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