了解光学指标.docx

了解光学指标.docx

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了解光学指标

UnderstandingOpticalSpecifications

Opticalspecificationsareutilizedthroughoutthedesignandmanufacturingofacomponentorsystemtocharacterizehowwellitmeetscertainperformancerequirements.Theyareusefulfortworeasons:

first,theyspecifytheacceptablelimitsofkeyparametersthatgovernsystemperformance;second,theyspecifytheamountofresources（i.e.timeandcost）thatshouldbespentonmanufacturing.

Anopticalsystemcansufferfromeitherunder-specificationorover-specification,bothofwhichcanresultinunnecessaryexpenditureofresources.Under-specificationoccurswhennotallofthenecessaryparametersareproperlydefined,resultingininadequateperformance.Over-specificationoccurswhenasystemisdefinedtootightlywithoutanyconsiderationforchangesinopticalormechanicalrequirements,resultinginhighercostandincreasedmanufacturingdifficulty.

Inordertounderstandopticalspecifications,itisimportanttofirstreviewwhattheymean.Tosimplifytheever-growingnumber,considerthemostcommonmanufacturing,surface,andmaterialspecificationsforlenses,mirrors,andwindows.Filters,polarizers,prisms,beamsplitters,gratings,andfiberopticsalsosharemanyoftheseopticalspecifications,sounderstandingthemostcommonprovidesagreatbaselineforunderstandingthosefornearlyallopticalproducts.

MANUFACTURINGSPECIFICATIONS

DiameterTolerance

Thediametertoleranceofacircularopticalcomponentprovidestheacceptablerangeofvaluesforthediameter.Thismanufacturingspecificationcanvarybasedontheskillandcapabilitiesoftheparticularopticalshopthatisfabricatingtheoptic.Althoughdiametertolerancedoesnothaveanyeffectontheopticalperformanceoftheopticitself,itisaveryimportantmechanicaltolerancethatmustbeconsiderediftheopticisgoingtobemountedinanytypeofholder.Forinstance,ifthediameterofanopticallensdeviatesfromitsnominalvalueitispossiblethatthemechanicalaxiscanbedisplacedfromtheopticalaxisinamountedassembly,thuscausingdecenter（Figure1）.Typicalmanufacturingtolerancesfordiameterare:

+0.00/-0.10mmfortypicalquality,+0.00/-0.050mmforprecisionquality,and+0.000/-0.010mmforhighquality.

Figure1:

DecenteringofCollimatedLight

CenterThicknessTolerance

Thecenterthicknessofanopticalcomponent,mostnotablyalens,isthematerialthicknessofthecomponentmeasuredatthecenter.Centerthicknessismeasuredacrossthemechanicalaxisofthelens,definedastheaxisexactlybetweenitsouteredges.Variationofthecenterthicknessofalenscanaffecttheopticalperformancebecausecenterthickness,alongwithradiusofcurvature,determinestheopticalpathlengthofrayspassingthroughthelens.Typicalmanufacturingtolerancesforcenterthicknessare:

+/-0.20mmfortypicalquality,+/-0.050mmforprecisionquality,and+/-0.010mmforhighquality.

RadiusofCurvature

Theradiusofcurvatureisdefinedasthedistancebetweenanopticalcomponent'svertexandthecenterofcurvature.Itcanbepositive,zero,ornegativedependingonwhetherthesurfaceisconvex,plano,orconcave,respectfully.Knowingthevalueoftheradiusofcurvatureallowsonetodeterminetheopticalpathlengthofrayspassingthroughthelensormirror,butitalsoplaysalargeroleindeterminingthepowerofthesurface.Manufacturingtolerancesforradiusofcurvaturearetypically+/-0.5,butcanbeaslowas+/-0.1%inprecisionapplicationsor+/-0.01%forextremelyhighqualityneeds.

Centering

Centering,alsoknownbycentrationordecenter,ofalensisspecifiedintermsofbeamdeviationδ（Equation1）.Oncebeamdeviationisknown,wedgeangleWcanbecalculatedbyasimplerelation（Equation2）.Theamountofdecenterinalensisthephysicaldisplacementofthemechanicalaxisfromtheopticalaxis.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,fringesappearwhoseshapedictatesthesurfaceflatnessofth