Sensor and pixel sizes - Camera - Vision-Doctor

Pixel sizes of CCD or CMOS sensors ... As a consequence of the miniaturisation of sensors, the pixel sizes grow smaller and smaller. Sensors of consumer cameras ( ... StartServiceIntroductionPracticeSystemsLightingOpticsFiltersCameraInterfaces Camera Theeyeoftheinspectionsystem SensorandpixelsizesofCCDandCMOSsensors TheadvancingtechnologicaldevelopmentofCCDandCMOSsensorsallowsfortheproductionoffinerandfinersemiconductorstructures.Asageneraltrend,sensorandpixelsizesshrinkinordertocutmoreandmoresensorsoutofonewafer.Thisispossiblebecausethesensitivityofthepixelsincreasinglyenhances,too,asmuchasthenoiseperformanceoftheelectronicsisbeingoptimised. Astechnicallimitsarereachedinthisrespect,too,itisworthwhiletocomparecameraswithdifferentsensorandpixelsizeswiththesameresolution,especiallyif… thereislittlelight low-noiseimageswithhighdynamicimageresponsearerequired precisionmeasurementsaresupposedtobeconducted Alargersensorwithlargerpixelsisinalmosteverycasethetechnicallybetterchoice,however,thepriceisalwayshigher. Sensorsizesofstandardcameras Classicmachinevisioncamerashavevaryinglylargesensors,dependingonthecameraandresolutionused.Themajorityofcameraswithsmallersensorsareusedwithso-calledC-mountorpossiblyCS-mountoptics.TheC-mountthreadhasanactualdiameterof1inch,i.e.25.4mmandathreadpitchof1/32inch. Thesensorsusedinstandardcamerasareclearlysmallerandrangefrom4to16mmimagediagonal.Thesesensorsizes,too,areindicatedininches.The1-inchsensorhasadiagonalof16mm. Typicalsensorsizesofindustrialcameras TheinchdataoftheCCDandCMOSsensorsonlyhaveahistoricexplanation:pick-uptubesofTVcameraswereuseduptothemid-1980sandwerelongsuperiortoCCDorCMOSsensorswhichwereinventedinthelate1960s. Theactualimageconverterofthetubecameraswaslocatedinaglassvacuumtube,andthedifferentpick-uptubeswere,amongotherthings,classifiedaccordingtotheirouterdiameteroftheglassbulb.Thediagonalofthelight-sensitivesurfacewithinthetubewasofcoursesmallerandrepresentedapproximatelytwothirdsoftheouterdiameter.EquivalentCCDsensorswhicharesupposedtoreplacethecathode-raytubeshadtocoverexactlythissurface.ACCDthelight-sensitivesurfaceofwhichcorrespondstoa1/2-inchtubewasthereforecalled1/2-inchsensor,evenifthisdoesnotcorrespondtotherealCCDsensorsize. Industrialcamerasusuallyuse1/3"sensorsincaseofresolutionsof640x480pixels,cameraswith1280x1024pixelsmainly1/2".Thequitepopularcameraresolutionof1600x1200pixelsoftenusesasomewhatlargersensorwith1/1.8"withthesamepixelsize. Ingeneralthereisthetrendthatthesensorsbecomesmallerandsmalleronthemasscameramarket.IfastandardVGAsensorhad,insomecases,asizeof2/3"inthelate1980s,itisonly1/3"today.Theminiaturisationisaconsequenceofenhancedproductionprocesseswhichallowforsmallerlight-sensitivesurfaceswitha(hopefully)similarperformance.Itenablesthemanufacturerstoproducealargernumberofsensorsatalowerpricefromonewafer.A1/3"sensor,forexample,hasonlyapproximately40%ofthesurfaceofa1/2"sensorandisthereforecheaper. Important:Ifyouhavethechoicebetweenalargerandasmallersensorforthesamecameraversion,pleasetakethelargervariantifyou… conductprecisionmeasurements,forinstance,orfinestsurfaceinspectionswithaslittlecameranoisefalsifyingtheresultaspossible. planlight-criticalfastapplicationswithashortexposuretime. usecolourcameraswhicharepossiblysupposedtoreplacemonochromecamerasandifonlylittlelightisavailable,astheyrequire3-4xmorelightthanacomparablemonochromesensor. Large-formatsensorsizesofareascancamerasorlinescancameras Incaseofhigh-resolutionareascanorlinescancameras,significantlylargersensorswithasizeofseveralcentimetresareused.Thedimensionsofthesesensorsarenormallynotstandardisedandresultfromtheresolutionandpixelsizesofthesensors.Everythingispermittedandonlylimitedbythebudget. Alinescancamerawith2048pixelswith10μmpixelsizeshasalinelengthof10.48mm,incaseof14μmpixelsizethesensorisalready28.6mmlong.From20mmsensordiagonalon,theC-mountlensconnectioncannolongerbeused. ThesecamerastypicallyuseNikonbayonet(F-mount)orM42toM72aslensconnections.Onlythenhigh-resolutionsensorswithlargepixelscanbeusedinordertobuildlinescancameraswithupto12kpixelsorareascancameraswithupto28millionpixels. PixelsizesofCCDorCMOSsensors Asaconsequenceoftheminiaturisationofsensors,thepixelsizesgrowsmallerandsmaller.Sensorsofconsumercameras(8to12megapixelsfor200euros)havepixelsizesofmostly1.7μmtoday,thelight-activesurfaceperpixelisthereforeonlyapproximately3μm2.Thisresultsinanextremelystrongsensornoiseincaseofnon-optimallightingconditions.Forqualitycontrolusingcameras,thisisabsolutelyinacceptable. Machinevisioncameras(C-mount)withresolutionsfromVGAto2megapixelsnormallyhavepixelsof4.6to6.5μmwitha10-15timeslargerlight-activesurfacesandthusclearlybettersignalresults.Ifyouneedimagesasnoise-freeaspossibleandprecisemeasuringresults,lookforpreferablylargesensorpixels,evenifthesecamerasaremoreexpensive! Typicalpixelsizesofindustrialcameras Pixelswithanedgelengthof14or10μmarepreferentiallyusedinlinescancameras.Duetothehighlinefrequencyof18Hz,forinstance,themaximumexposuretimeis1000/18000=55μsforonecapturedimageline.Thelight-activesurfaceofthepixelcanneverbelargeenoughinthiscase. Fullwellcapacityofapixel Thisspecificationdescribeshowmanyelectronsapixelelementcanholdbeforeitiscompletelysaturated.Apixelof5.5μmstructuresizecanaccumulateapproximately20,000electrons,a7.4μmpixel40,000electrons. Thelargerthefullwellcapacity,yetthebetterthemaximumsignal-noiseratio.Consumercameraswithpixelsizesof1.7μmrequireonlyabout1,000photonsforthepixelsaturation.Incaseofdigitalisationwith8,10,oreven12bits,othernoiseeffects(photonnoise,digitalisationnoise,darknoise)canalreadyassumesignificantscales,interferewiththesignalandthusinfluencetheimageinanextremelynegativeway. Thelargerthefullwellcapacity,yetthebetterthemaximumsignal-noiseratio.Consumercameraswithpixelsizesof1.7μmrequireonlyabout1,000photonsforthepixelsaturation.Incaseofdigitalisationwith8,10,oreven12bits,othernoiseeffects(photonnoise,digitalisationnoise,darknoise)canalreadyassumesignificantscales,interferewiththesignalandthusinfluencetheimageinanextremelynegativeway. Importantformachinevision Thesmallerthepixels,themorelightisrequiredtocaptureanimage.Incaseofshortinspectiontimesthefactthatthelightisnotsufficientcaneasilybeaproblem. Incaseoflittlelight,smallpixelsdeliverclearlynoisierimagesthanlargepixels,thedynamicimageresponseisreduced.Noiseisdisturbingfortheapplication.UsebrightilluminationorLEDflashcontrollersinordertohavemorelightavailable! Manymegapixelsdonotnecessarilyhelp.Smallpixelstructuresrequirehigh-qualityopticimaging,thatishigh-resolutionlenses.Otherwise,blurredimageswithmanypixelsarecreated,butnorealstructuredetails. Smallcamerapixelsfurthermorerequireanextremelyprecisemechanicalignmentofthesensor,asthedepthoffieldsignificantlydecreases.Thetiltingofa5μmsensorinthehousingmustonlybehalfaslarge(+/115μmataperture=2.8)asincaseofpixelswithastructuresizeof10μm.Thereforelookforsuppliersproducinghighquality,otherwisethebestsensorinthecameraisinvain. MenuCamerasSelctionofperfectcameraCameratechnologybasicsResolutionofsensorsSensorandpixelsizesCCDsensorsInterlinetransferCCDFullframetransferCCDFrametransferCCDCMOSsensorsTriggerfunctionsElectricreadoutfunctionsProcessingfunctionsCameraelectronicsCamerainterfacesAreascancamerasSingle-chipmonochromecamerasSingle-chipcolourcamerasBayercolourinterpolationThree-chipcolourcamerasDual-chipcamerasExposuretimeareascancameraLinescancamerasLinescancamerabasicsLinescanfrequency&resolutionUseofencodersColourlinescancamerasCalibrationlinescancameraIlluminationlinescancameraSmartcameras Needtobuytherightcamera? Vision-Doctor.co.ukisaprivate,independent,non-commercialwebsiteprojectwithoutcustomersupport. Forthebestadvice,trainingandsalesofmachinevisioncameraspleaseclickhere. (externallinkpartnerwebsiteinEurope) Partner&Network StartServiceIntroductionPracticeSystemsLightingOpticsFiltersCameraInterfaces