CSE486, Penn StateRobert CollinsLecture 26:Color and Lightnot in textbook (sad but true)CSE486, Penn StateRobert CollinsPhysics of Light and Color• Light is electromagnetic radiation– Different colors correspond to different wavelengths λ– Intensity of each wavelength specified by amplitude• Visible light: 400-700nm. rangeROYGBIV(ROYGBIV)CSE486, Penn StateRobert CollinsWhat is Color?• Objects don’t have a “color”• Color is a perception; what we “see”• It is a function of– light source power at different wavelengths– proportion of light at each wavelength reflected off object surface– sensor response to different wavelengthsCSE486, Penn StateRobert CollinsSketch: Light TransportSource emits photonsPhotons travel in astraight lineThey hit an object. Some areabsorbed, some bounce offin a new direction.And then some reachan eye/camera andare measured.CSE486, Penn StateRobert CollinsLight TransportSource emits photonsPhotons travel in astraight lineThey hit an object. Some areabsorbed, some bounce offin a new direction.And then some reachan eye/camera andare measured.IlluminationCSE486, Penn StateRobert CollinsColor of Light SourceSpectral Power Distribution:UVIRVisibleWavelength λAmplitudeRelative amount of lightenergy at each wavelengthCSE486, Penn StateRobert CollinsSome Light Source SPDsCSE486, Penn StateRobert CollinsCSE486, Penn StateRobert CollinsLight TransportSource emits photonsPhotons travel in astraight lineThey hit an object. Some areabsorbed, some bounce offin a new direction.And then some reachan eye/camera andare measured. SurfaceReflectionCSE486, Penn StateRobert Collins(Ir)radianceIncominglightsurfaceOutgoinglightIRRADIANCERADIANCECSE486, Penn StateRobert CollinsSpecular SurfacesLight rays purely reflect via Snell’slaw (angle of reflection = angle ofincidence)Properties:Outgoing light has same SPD(“color”) as incoming light.If you stand in the right place yousee a little picture of the lightsource reflected off the surface.CSE486, Penn StateRobert CollinsLambertian SurfacesPurely “matte” surface.Properties:Apparent brightness is proportionalto cosine of angle betweenobserver’s line of sight and thesurface normal (Lambert’s Law)Outgoing light has SPD thatdepends on spectral albedo ofsurface (what wavelengths getabsorbed vs transmitted).CSE486, Penn StateRobert CollinsMore General SurfacesHave both a specular and diffuse reflections.embedded colorante.g. paintCSE486, Penn StateRobert CollinsSpectral albedo for several different leavesSpectral AlbedoRatio of outgoing to incoming radiation at different wavelengths.(proportion of light reflected)CSE486, Penn StateRobert CollinsSpectral Radianceto aSpectralIrradianceSpectralRadianceSpectral AlbedoCSE486, Penn StateRobert CollinsLight TransportSource emits photonsThey hit an object. Some areabsorbed, some bounce offin a new direction.And then some reachan eye/camera andare measured.Sensor ResponseCSE486, Penn StateRobert CollinsHuman Vision• Human eyes have 2 types of sensors:– CONES• Sensitive to colored light, but not very sensitive todim light– RODS• (very) Sensitive to achromatic lightCSE486, Penn StateRobert CollinsHuman Eye: Rods and ConesS cones (blue)M cones (green)L cones (red)rods (overall intensity)near foveanear peripheryCSE486, Penn StateRobert CollinsPutting it all Together = Color3 conesCOLOR!CSE486, Penn StateRobert CollinsSimple Example600 700Wavelength (nm)01 albedoSpectral albedoof apple (red) Relative powerRelative Spectral PowerDistribution of White Light01Wavelength (nm)400 700.* Relative powerSpectral Radiance01Wavelength (nm)600 700continuedCSE486, Penn StateRobert Collins Relative powerSpectral Radiance01Wavelength (nm)600 700Simple Example (continued)relative numericresponse (area)=100= 0= 0Relative responseGreen Cones01Wavelength (nm)600 700400 500Relative responseRed Cones01Wavelength (nm)600 700400 500Relative responseBlue Cones01Wavelength (nm)600 700400 500.*.*.* response response responseGreen Cones01Wavelength (nm)600 700400 500Red Cones01Wavelength (nm)600 700400 500Blue Cones01Wavelength (nm)600 700400 500(no response)(no response)looks “red”CSE486, Penn StateRobert CollinsSimple Example600 700Wavelength (nm)01 albedoSpectral albedoof apple (red) Relative powerRelative Spectral PowerDistribution of Blue Light01Wavelength (nm)400 700.* Relative powerSpectral Radiance01Wavelength (nm)400700continued500(no radiance)CSE486, Penn StateRobert Collins Relative powerSpectral Radiance01Wavelength (nm)600 700Simple Example (continued)relative numericresponse (area)= 0= 0= 0Relative responseGreen Cones01Wavelength (nm)600 700400 500Relative responseRed Cones01Wavelength (nm)600 700400 500Relative responseBlue Cones01Wavelength (nm)600 700400 500.*.*.* response response responseGreen Cones01Wavelength (nm)600 700400 500Red Cones01Wavelength (nm)600 700400 500Blue Cones01Wavelength (nm)600 700400 500(no response)(no response)looks “black”(no radiance)(no response)CSE486, Penn StateRobert CollinsThe Abyss Clip“One-way ticket” clip from DVDCSE486, Penn StateRobert CollinsWhat is Going On in This Clip?Under yellowish green light,both the blue/white wire and theblack/yellow wire look identical.Now for the spectral explanationof why this happens…CSE486, Penn StateRobert CollinsBlack/Yellow under Yellow Light0.*.*yellowblackblackyellowyellowyellowIrradianceSpectral AlbedoRadianceIrradianceSpectral AlbedoRadianceCSE486, Penn StateRobert CollinsBlue/White under Yellow Light0.*.*yellowblueblackyellowwhiteyellowIrradianceSpectral AlbedoRadianceIrradianceSpectral AlbedoRadianceCSE486, Penn StateRobert CollinsLesson LearnedSurfaces materials that look different under whitelight can appear identical under colored light.CSE486, Penn StateRobert CollinsMetamersDefinition: two different spectral reflectances that appearindistinguishable to a given observer under givenillumination conditions.Illumination metamerism: two color distributions look thesame under a given illuminationObserver metamerism: two color distributions look thesame to a given observer.CSE486, Penn StateRobert CollinsSample MetamersFrom http://www.iitp.ru/projects/posters/meta/Metameric curves for human eye under daylightTest pattern viewed under daylightOvercoming metamerism by viewingunder different illuminationOvercoming metamerism by having a different observerViewed underincandescent lightingViewed by camerawith more