Lenses for 3D Time-of-Flight (ToF) Image Sensors

1 1 SBAA217 July 2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLensesf or 3D Timeof flight (ToF) ImageSensorsApplicationReport SBAA217 July 2016 Lensesfor 3D Time-of-Flight (ToF) ImageSensorsSubhashCh andraSadhuABSTRACTIn a camera,the lens ensuresthat light comingfroma givenpointin a scenereachesa ideallens is in perfectfocus,light comingfroma givenpointin the imagewill reacha is an importantopticalcomponentwhichenablescam eras,including3D Time-of-Flight (ToF) cameras,to ToF camerashavecertaindistinctcharacteristic swhichhavespecialrequirementsto be metwith whileselectingor designingthe to decidethe specificationsfor3D ToF also describeshow to chooseor designlens of of 3D ToF camerasis giveninTime-of-FlightCamera An Introduction(SLOA190).

2 This documentdescribesthe specificswhichset the 3D 3D ToF cameraworkson cannotget any depthsignalfromthe itis importantthat the light collectionabilityof the lens is as efficientas light collectionabilityof the lens dependson the lens apertureand the transmissionefficiencyin the wavelengthsof rangeof wavelengthsthe userwouldbe interestedin is the transmissionspectrumof the LEDorLaserbeingusedfor the effectivelight collectionabilityof the lens is not the samefor eachand reducesas you movefartherand fartherfromthe centerof the lens applicationsrequiringgoodperformanceat the edgesandcornersof the scene,relativeilluminationis an orderto optimizethe camera,the light powerincidenton eachpixelhas to be the pixelprovidesa increasestheSignal-to-NoiseRatio(SNR)of thesystemresultingin an the largestabsoluteaperturedo notnecessarilygive fl + 2fl(B2#2) July 2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLensesf or 3D Timeof flight (ToF) ImageSensorsThe mathematicalrelationsneedto be the constantlight intensityon the targetis I,thenthe per pixellight power,P is givenas.

3 Where I = Lightintensityon the target(W/m2) = Pixelsize (m) A = Lensaperture(m) = Lenstransmissionefficiency f = Lensfocallength(m)(1)Amongtheseparameter s,I is purelydependenton the illuminationand the objectdistanceand isdependenton the the cameradesign,the lens designcan affectin termsof the F numberandthe July 2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLensesf or 3D Timeof flight (ToF) ImageSensors2 LensAn examplelens specificationwhichworkswell for a 3D ToF systemis shownin LensSpecificationsSpecificationValueSens orFormatHorizontal80pixelsVertical60pixe lsSensorPixelsizeHorizontal30umVertical3 0umLightWavelength850 nmEffectFocalLength(EFL) >85%@ 850 nm>80%@ 835 nm to 870 nm>70%@ 800 nm to 900 nmLensTotalLength< (deg)75 Vertical(deg) Diagonal(deg)90 Resolution(MTF)On Axis> 70%contrastat 17 lp/mmFull Field> 50%contrastat 17 lp/mmTV distortion(Trad.)

4 < 10%RelativeIllumination(Ref.)70%ChiefRay Angle< 10%MaximumImageCircle(MIC) mmBarrelThreadSizeM12 eachof thesetopicsis freelyavailableon the documentdealsonlywith the specificationsimportantto the Number:The F numberis the ratioof the focallength(f) to the aperture(A). This is a commonway of specifyingtheapertureof the number= Focallength/ Aperture(2)The pixellight collectionpowerscalesas a secondpowerof the F is importantto havea verylow F numberto get the low F numberhas otherside effectsto accountfor. Low F numberlenseshavea very narrowdepth-of-fieldowingto the the rangeof distancesoverwhichthe cameraappearsto be in lens depth-of-fieldmustbe carefullydesignedconsideringthe applicationforthe limitsof lens depth-of-fieldshouldmatchor exceedthe minimumand maximumdistancerequirementsof the final camerasthat the lens wouldbe usedin.

5 Depth-of-fieldincreasesas the 's 3D ToF thelargerpixelsize,the OPT8241will havea very widedepth-of-fieldcomparedto a typicalRGBcameraforthe July 2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLensesf or 3D Timeof flight (ToF) theAnti-ReflectiveCoatings(ARC)usedon the visiblelight haveARCwhichare wide-bandin ToF camerasneedtoworkfor only a narrowrangeof makesthe ARCmore monochromatic (narrowband)in a generalrule,it is possibleto makeARCswith bettertransmissionefficienciesfor a smallerrangeof if a lens has 85%transmissionefficiencyfor a visiblerangeof wavelengths,itshouldbe possibleto changethe coatingson the lens elementsto havetransmissionefficiencybetterthan85%.

6 July 2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLensesf or 3D Timeof flight (ToF) ImageSensors3 LensHolderLensholderis the componentwhichholdsthe lens to the has to be designedor selectedconsideringthe mechanicaldimensionsof the lens holdershouldmatchthe threaddiameterand the threadpitchof the examplemetricthreadspecificationis M12 This meansthat the threadis of 12 mm diameterand 1 full rotationofthe lens in the holderwouldmoveit verticallyby holdershouldhaveclearancefor the lensBackFocalLength(BFL).Backfocallength of the lens isthe physicaldistancebetweenthe bottomof the lens barrelto the top of the sensorin air.

7 The concernisthat the holdermightnot haveenoughclearanceto allowthe lens to screwin all the way to is also possiblein somerare casesthat the holderdoesnot haveenoughthreadlengthtoallowthe BFL of the havea refractiveindexgreaterthan1. So the equivalentpathlengthin air has to be consideredas BFL for the actualphysicaldistancebetweenthe lens and thesensorpackageglasstop. Examplecalculationsfollow:Sensorpackageg lassthickness,d: mmSensorpackageglassrefractiveindex, : (specifiedin datasheet) coverglass= d = physicaldistancefromsensor= BFL ( 1) dPhysicaldistancefromcoverglass= BFL sensorneednot haveits optically-activepixelareain the centerof the instance,theOPT8241has its July 2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedLensesf or 3D Timeof flight (ToF)

8 ImageSensorsThe lens opticalaxis has to passthroughthe pixelarea s mightinvolveusinglens holderswhichhavean offsetin the lens is also possibleto use muchlargerlens holderswhichstillmanageto encompassthe othercomponentspresentaroundthe sensorpackageas decouplingcapacitorsand othersuchcomponentswhichhaveto be placedvery closeto the sensorpackagewill haveto beaccommodatedunderthe lens holderhas to be chosenor designedfor the PCBfootprintareatakenup by the componentsas well as the profileheightof the fit underthe lens respectto centeringis the centeringtoleranceof the lens makinguse of screwsmighthelp centerthe lens up to a toleranceof 100 m.

9 In theOPT8241,13 pixelsare 200 m. It gets progressivelyharderto correctfor the lens distortionsand themountingtolerancesas the centeringgoesoff. Use lens holderswith isdoneby LensHolderThesealignmenttabs get insertedinto high precisionholesmadein the ,thesecan bemadeto fit tightlyinsidethe alignmenttabs give muchbettercenteringas comparedto still be usefulas an alternativeto glue to affix the holderon to the documentdescribedthe following: Detailson the opticselectionfor 3D ToF cameras Differencein the lens requirementsfor 3D ToF camerasand typicalcameras Basicconcernswhiledesigningor choosinglens mountsfor a givenlensVeryfew lensesavailablein the free marketfit the requirementsof 3D ToF is usefulto identifysourcesof compatiblelensesand shareit with the lens sourcewith the communitywouldhelp its subsidiaries(TI)

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