了解光学指标.docx
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了解光学指标.docx
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了解光学指标
displacementofthemechanicalaxisfromtheopticalaxis.Themechanicalaxisofalensissimplythegeometricaxisofthelensandisdefinedbyitsoutercylinder.Theopticalaxisofalensisdefinedbytheopticalsurfacesandisthelinethatconnectsthecentersofcurvatureofthesurfaces.Totestforcentration,alensisplacedintoacupuponwhichpressureisapplied.Thepressureappliedtothelensautomaticallysituatesthecenterofcurvatureofthefirstsurfaceinthecenterofthecup,whichisalsoalignedwiththeaxisofrotation(Figure2).Collimatedlightdirectedalongthisaxisofrotationissentthroughthelensandcomestoafocusattherearfocalplane.Asthelensisrotatedbyrotatingthecup,anydecenterinthelenswillcausethefocusingbeamtodivergeandtraceoutacircleofradiusΔattherearfocalplane(Figure1).
Figure2:
TestforCentration
(1)
(2)
whereWisthewedgeangle,oftenreportedasarcminutes,andnistheindexofrefraction.
Parallelism
Parallelismdescribeshowparalleltwosurfacesarewithrespecttoeachother.Itisusefulinspecifyingcomponentssuchaswindowsandpolarizerswhereparallelsurfacesareidealforsystemperformancebecausetheyminimizedistortionthatcanotherwisedegradeimageorlightquality.Typicaltolerancesrangefrom5arcminutesdowntoafewarcseconds.
AngleTolerance
Incomponentssuchasprismsandbeamsplitters,theanglesbetweensurfacesarecriticaltotheperformanceoftheoptic.Thisangletoleranceistypicallymeasuredusinganautocollimatorassembly,whoselightsourcesystememitscollimatedlight.TheautocollimatorisrotatedaboutthesurfaceoftheopticuntiltheresultantFresnelreflectionbackintoitproducesaspotontopofthesurfaceunderinspection.Thisverifiesthatthecollimatedbeamishittingthesurfaceatexactlynormalincidence.Theentireautocollimatorassemblyisthenrotatedaroundtheoptictothenextopticalsurfaceandthesameprocedureisrepeated.Figure3showsatypicalautocollimatorsetupmeasuringangletolerance.Thedifferenceinanglebetweenthetwomeasuredpositionsisusedtocalculatethetolerancebetweenthetwoopticalsurfaces.Angletolerancecanbeheldtotolerancesofafewarcminutesallthewaydowntoafewarcseconds.
Figure3:
AutocollimatorSetupMeasuringAngleTolerance
Bevel
Glasscornerscanbeveryfragile,therefore,itisimportanttoprotectthemwhenhandlingormountingacomponent.Themostcommonwayofprotectingthesecornersistobeveltheedges.Bevelsserveasprotectivechamfersandpreventedgechips.Theyaredefinedbytheirfacewidthandangle(Figure4).
Figure4:
BevelonanOpticalLens
Bevelsaremostcommonlycutat45°andthefacewidthisdeterminedbythediameteroftheoptic.Opticswithdiameterslessthan3.00mm,suchasmicro-lensesormicro-prisms,aretypicallynotbeveledduetothelikelihoodofcreatingedgechipsintheprocess.Itisimportanttonotethatforsmallradiiofcurvature,forexample,lenseswherethediameteris≥0.85xradiusofcurvature,nobevelisneededduetothelargeanglebetweenthesurfaceandedgeofthelens.Forallotherdiameters,themaximumfacewidthsareprovidedinTable1.
ClearAperture
Clearapertureisdefinedasthediameterorsizeofanopticalcomponentthatmustmeetspecifications.Outsideofit,manufacturersdonotguaranteetheopticwilladheretothestatedspecifications.Duetomanufacturingconstraints,itisvirtuallyimpossibletoproduceaclearapertureexactlyequaltothediameter,orthelengthbywidth,ofanoptic.TypicalclearaperturesforlensesareshowinTable2.
Figure5:
GraphicIndicatingClearApertureandDiameterofaFilter
Table1:
BevelTolerances
Diameter
MaximumFaceWidthofBevel
3.00mm–5.00mm
0.1mm
5.01mm–25.4mm
0.25mm
25.41mm–50.00mm
0.3mm
50.01mm–75.00mm
0.4mm
Table2:
ClearApertureTolerances
Diameter
ClearAperture
3.00mm–10.00mm
90%ofDiameter
10.01mm-50.00mm
Diameter–1mm
≥50.01mm
Diameter–1.5mm
SURFACESPECIFICATIONS
SurfaceQuality
Thesurfacequalityofanopticalsurfacedescribesitscosmeticappearanceandincludessuchdefectsasscratchesandpits,ordigs.Inmostcases,thesesurfacedefectsarepurelycosmeticanddonotsignificantlyaffectsystemperformance,though,theycancauseasmalllossinsystemthroughputandasmallincreaseinscatteredlight.Certainsurfaces,however,aremoresensitivetotheseeffectssuchas:
(1)surfacesatimageplanesbecausethesedefectsareinfocusand
(2)surfacesthatseehighpowerlevelsbecausethesedefectscancauseincreasedabsorptionofenergyanddamagetheoptic.Themostcommonspecificationusedforsurfacequalityisthescratch-digspecificationdescribedbyMIL-PRF-13830B.Thescratchdesignationisdeterminedbycomparingthescratchesonasurfacetoasetofstandardscratchesundercontrolledlightingconditions.Thereforethescratchdesignationdoesnotdescribetheactualscratchitself,butrathercomparesittoastandardizedscratchaccordingtotheMIL-Spec.Thedigdesignation,however,doesdirectlyrelatetothedig,orsmallpitinthesurface.Thedigdesignationiscalculatedatthediameterofthediginmicronsdividedby10.Scratch-digspecificationsof80-50aretypicallyconsideredstandardquality,60-40precisionquality,and20-10highprecisionquality.Learnmoreaboutsurfacequalityhere.
SurfaceFlatness
Surfaceflatnessisatypeofsurfaceaccuracyspecificationthatmeasuresthedeviationofaflatsurfacesuchasthatofamirror,window,prism,orplano-lens.Thisdeviationcanbemeasuredusinganopticalflat,whichisahighquality,highlypreciseflatreferencesurfaceusedtocomparetheflatnessofatestpiece.Whentheflatsurfaceofthetestopticisplacedagainsttheopticalflat,fringesappearwhoseshapedictatesthesurfaceflatnessoftheopticunderinspection.Ifthefringesareevenlyspaced,straight,andparallel,thentheopticalsurfaceundertestisatleastasflatasthereferenceopticalflat.Ifthefringesarecurved,thenumberoffringesbetweentwoimaginarylines,onetangenttothecenterofafringeandonethroughtheendsofthatsamefringe,indicatetheflatnesserror.Thedeviationsinflatnessareoftenmeasuredinvaluesofwaves(λ),whicharemultiplesofthewavelengthofthetestingsource.Onefringecorrespondsto½ofawave.1λflatnessisconsideredtypicalgrade,¼λflatnessisconsideredtobeprecisiongrade,and1/20λisconsideredhighprecisiongrade.
Power
Power,atypeofsurfaceaccuracyspecification,appliestocurvedopticalsurfaces,orsurfaceswithpower.Itistestedinafashionsimilartoflatness,inthatacurvedsurfaceiscomparedagainstareferencesurfacewithahighlycalibratedradiusofcurvature.Usingthesameprincipleofinterferencecausedbytheairgapsbetweenthetwosurfaces,theinterferencespatternoffringesisusedtodescribethedeviationofthetestsurfacefromthereferencesurface.Adeviationfromthereferencepiecewillcreateaseriesofrings,knownasNewton'sRings.Themoreringsthatarepresent,thelargerthedeviation.Thenumberofdarkorlightrings,notthesumofbothlightanddark,correspondstotwicethenumberofwavesoferror.
Irregularity
Irregularity,atypeofsurfaceaccuracyspecification,describeshowtheshapeofasurfacedeviatesfromtheshapeofareferencesurface.Itisobtainedfromthesamemeasurementaspower.Regularityreferstothesphericityofthecircularfringesthatareformedfromthecomparisonofthetestsurfacetothereferencesurface.Whenthepowerofasurfaceismorethan5fringes,itisdifficulttodetectsmallirregularitiesoflessthan1fringe.Thereforeitiscommonpracticetospecifysurfaceswitharatioofpowertoirregularityofapproximately5:
1.Formoredetailedinformationonopticalflatsandinterpretingfringepatternstotestsurfaceflatness,powerandirregularity,viewOpticalFlats.
SurfaceFinish
Surfacefinish,alsoknownassurfaceroughness,measuressmallscaleirregularitiesonasurface.Theyareusuallyanunfortunateby-productofthepolishingprocess.Roughsurfacestendtowearfasterthansmoothsurfacesandmaynotbesuitableforsomeapplications,especiallythosewithlasersorintenseheat,duetopossiblenucleationsitesthatcanappearinsmallcracksorimperfections.Manufacturingtolerancesforsurfacefinishrangefrom50ÅRMSfortypicalquality,20ÅRMSforprecisionquality,and5ÅRMSforhighquality.
MATERIALSPECIFICATIONS
IndexofRefraction
Theindexofrefractionofamediumistheratioofthespeedoflightinvacuumtothespeedoflightinthemedium.Typicalindicesofrefractionforglassrangefrom1.4-4.0;visibleglasseshavelowerrangesthanthoseoptimizedfortheinfrared.Forexample,N-BK7(apopularvisibleglass)hasanindexof1.517,whereas,germanium(apopularIRglass)hasanindexof4.003.Formoreinformationoninfraredmaterials,viewTheCorrectMaterialforInfrared(IR)Applications.Theindexofrefractionofanopticalglassisanimportantpropertybecausethepowerofanopticalsurfaceisderivedfromboththeradiusofcurvatureofthesurfaceandthedifferenceintheindexofrefractionofthemediaoneithersideofthesurface.Inhomogeneity,specifiedbytheglassmanufacturer,describesthevariationofindexofrefractioninaglass.Itisspecifiedaccordingtodifferentclasses,whereclassandinhomogeneityareinverselyrelated–asclassincreases,inhomogeneitydecreases(Table3).
Table3:
InhomogeneitySpecifications
InhomogeneityClass
MaximumPermissibleVariationofIndexofRefraction
0
+/-
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