cognex视觉培训教程

Lighting&OpticsSession11Objective•UnderstandsomeofthedetailsofLightingandOptics•Optics–•ImageQuality•Typesoflens•Lighting–•Usage•Techniques•Filters•MorethoroughWebinarsandInteractiveTutorialsareavailableonline•LocateFeatures•Seethepart•KeepinFocus•Abilitytoseedetails•RemovePerspectiveDistortion•Avoidlensdistortionwhenpossible•MaximizeContrast•Detailsaremoredistinct11-4FiveParametersofImageQuality•Resolution•Abilitytoreproduceobjectdetail•Contrast•Keepingorenhancinglight/darktransition11-5DepthofField•Rangeoverwhichimageremainsinfocus.•Criticalingaugingapplications.•Directlyrelatedtoaperture–thesmallerthebetter.•f/#=focalratio,f-number,f-stop-largerthenumber,themorelightrequired11-6FiveParametersofImageQuality•Distortion•Opticalaberrationbylens(centertocorner)•PerspectiveError•Sizeofitembasedondistance11-7TypesofLensesTelephoto•Focallengthhigherthandiagonalofsensor.e.g.25mm•Shallowdepthoffield;enlargesdistantobjects.WideAngle•Focallengthlessthandiagonalofsensor;for1/3”CCD:e.g.6mm•Widedepthoffield,closerfocusingdistance.Zoom•Lenswithvariablefocallengths.35-70mmMacro•Lensoptimizedforhighmagnificationapplications•Sizeofimage=sizeofobject(1:1)Telecentric•Noperspectivedistortion.•Cannotbelargerthanthefrontapertureoflens11-8LensAdvisor…?•Ifyou’refocusedonapartandneedlargerFOV•Gotoasmallerfocallengthlens•25mm-16mm•Ifyou’refocusedonapartandneedtomountcameracloser(WD)•Gotoasmallerfocallengthlens•25mm-16mmThesmallerthefocallengthnumber,theclosertheWDandlargerFOV12mm16mm25mm11-10WhatdoIdo…?(cont.)Spacers–Allowlargerthannormalmagnification•Thehighertheamountofspacers,thegreaterthemagnificationaffect•Youmustchangeworkingdistanceandfocustogetasharpimage•Spacerscanreducethequalityoftheimagesocaremustbetakenwhendecidingbetweenusingaspaceroranotherlens16mm/nospacer16mm/2mmspacer(infinity)16mm/2mmspacer(closer)11-11LightingLightingConsiderationsIsthesurfacefinishspecular(smoothandglossy)ordiffuse(roughanddull)?Doesthesurfaceexhibitdirectionalreflectance(reflectslightinaspecificdirection)?Doestheappearanceofthepartchangeunderdifferentcolorsoflight(darkenorlighten)?Isthepart’ssurfaceflator3-dimensional?Curved?Irregular?Etched?Embossed?Raised?Isthesurfaceofthepartstableorwillitchangeovertime(tarnish,oxidize,fade)?Howmightambient(allaround)lightaffectthepart?11-13•Light:electro-magneticRadiation.DetectablebyhumaneyeLight800nm400nm1mmThermalRadiation50µm2500nmMid-IRClose-IR315nm280nm1nmUV-AVisibleLightUV-BUV-C100nmX-UVNFRadioWaves3000km10km30cm300µmMicroWavesOpticalRadiationX-RayGamma1nm1mm10pm640nm600nm570nm490nm430nm555nmV(l)505nmV‘(l)11-14LightandCamera•Imagesarecreatedwhenlightreflectsintothecamera•Itisbrightwhenreflectionisdirect(orfromstraylightenteringthelens)•Itisdarkwhenlightraysmissthecamera•Thisallowsforedgesortexturetobecomeprominent11-15LightingOptions•Placementofpartwithrespecttolightandcamera•Angleanddirectioncangreatlychangeappearanceofpart11-16LightingOptionsSamePart-DifferentLightPosition11-17BrightField•Raysareperpendiculartosurface•Shapeandcontourareenhanced•Diffusesurfacesaredark•Flat,polishedsurfacesarebright•Usedtoemphasizeheightchanges11-18DarkField•Raysareatanangletothesurface•Shapeandcontourareenhanced•Diffusesurfacesarebright•Flat,polishedsurfacesaredark•Usetoemphasizeheightchanges11-19GoingfromBrightFieldtoDarkFieldDistancefromObjectNearFar11-20DiffuseandCollimatedLightCollimatedLightRaysScatteredLightRaysFrostedGlassorPlasticCollimatedLightRaysScatteredLightRaysCondenserLens11-21Use:ConstantorStrobedStrobedNotStrobed•Alwayson•Consistentlight•Easytosetup•Heatsup•Onlyonatacquisition•Stopsmovement•Requireslightthatrespondsquickly•Needstobecontrolled11-22LightingTechniquesTechniquesDirect(orFront)StructuredBackLightDiffuseOff-AxisDiffuseOn-Axis11-24BackLight•Createsoptimalcontrast•Verylittlelightlost11-25BackLight•Pros•Maximumcontrast•Simplifiesimagebycreatingsilhouetteofpart•Cons•Surfacedetaillost•DifficulttousewithobjectsinafixtureHints:•Keeplightclean•Usecollimatedfilmforhighaccuracy•Applications•Gaugingandmeasuringdimensions(especiallyholes)11-26BackLightApplicationsRingLightBackLight11-27BackLightApplicationsRingLightBackLight11-28DirectFrontLight11-29DirectFrontLight•Pros•Easytosetup•Maximumcontrast•Cons•3Dpartswillcastshadows•CausesspecularreflectiononshinypartsHints:•Use2ormorespotlightstominimizeshadows•Shadowscanbeusedtoimprovecontrast•Applications•Maximumcontrastforlowcontrastimages•Usedasastrobetofreezemovement11-30DirectFrontLightApplicationFrontLightAccentuateShadows11-31StructuredLightUseslaserorfiberopticlinelight11-32StructuredLight•Pros•Inexpensiveformeasuringheight/depth•Showssurfaceprofileonlowcontrast•Cons•Lasersareexpensiveandmustbehandledwithcare•ZdirectionisnothighlyaccurateHint:Usefiberopticasopposedtolaserwhenpossible•Applications•Gaugingcontinuousfeatures•Verylowcontrastpart(linedeformation)11-33StructuredLightApplicationsLineLightChangeinDepth11-34DiffuseOn-Ax