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LENSOMETRYHandout SandraFortenberry,O.D. TamiHagemeyer,A.B.O.C. TERMINOLOGY Lensometer–instrumentdesignedtomeasuretheprescriptionofanopticallens. Mires–lines,thickandthinusedasmeasurementimages. PowerDrumorPowerWheel–dialusedtodeterminelenspower. Platform‐stagetheframerestsonwhenlensesarebeingneutralized Instrumentconsistsofanocularforviewingthemires,aflatstageortableforsupportingthe spectacleframe,apowerdial,andanaxiswheel. Sphere–lenswithopticalpowerbeingthesameinallmeridians(conveyedindiopters). Cylinder–lensthathasdifferentrefractive/opticalpowerineachmeridian.Itisusedtocorrect astigmatism. Axis–meridianofcylinderwiththeminimumpowerperpendiculartomaximumpowermeridian. Expressedindegrees. Prism‐atransparent,wedgeshapedmaterialwithtwoflatsurfacesinclinedatagivenanglethatconnect atapointcalledtheapex.Thetwoconnectedsurfacesarerestingonthebaseoftheprism.Prismsare usedtohelptheeyestoworktogetherbybendingorrefractinglight. LensometerFunctions ThefunctionofaLensometeristodeterminethecharacteristicsofalens,including: 1.Power 2.OpticalCenterlocation 3.MajorReferencePointlocation 4.Prismpower/direction 5.Cylinderaxisorientation TheLensometerisalsousedtoplacemarksonalenstoensureproperplacementofthelensduringthe fabricationprocess. LensometryMeasurement: ProcedureforSingleVisionLenses a.Setpowerwheeltozero b.Settheprismcompensatortozero c.Focustheeyepiece 1.Rotatetheeyepiececounterclockwiseuntilthereticleisblurred 2.Turntheeyepiececlockwiseuntilthereticleisjustclear(soasnottooverminusthereticle) d.Checkthepowercalibration 1.Turnthepowerwheelintotheplus,andthenslowlydecreasethepoweruntilthelensometertargetjust sharplyfocuses(DONOToscillatethewheelbackandforthtofindthebestfocus).Thepowerwheelshould readzeroiftheinstrumentisinpropercalibration. 2.Ifthepowerwheeldoesnotreadzero,refocustheeyepieceandcheckthecalibrationagain.Ifthepower wheelstilldoesnotreadzero,theerrormustbecompensatedforonallfuturemeasurementsmadewiththe lensometer. e.Measurethelensbackvertexpowerinminuscylinderform–Alwaysstartwiththerightlens! 1.Startingwiththerightlensofthespectacles,placethelensbacksurfaceagainstthelensstop(temples pointingawayfromyou).Thelensmustbeflatagainstthestop.Centerthelensapproximatelyonthestop. 2.Movethelenstableagainstthebottomsoftheeyewires. 3.Lowerthelensholdergentlyagainstthelensfrontsurface.Ifyouneedtomovethelens,alwaysliftthelens holderslightlyoffthelenstopreventscratchingofthefrontlenssurface. 4.Turnthepowerwheelintotheplus,andthenslowlydecreasethepoweruntilthetargetstartstoclear.If thetargetisnotcenteredonthereticle,movethelensaroundonthelensstopuntilitis. 5.Ifbothsetsoftargetlinesfocusatthesametime,thelensisasphere.Recordthelenspowertothenearest 1/8D(0.12D)Example:OD:‐2.12DS 6.Ifbothsetsoftargetlinescannotbefocusedatthesametime(and/orthelinesappearbroken),thelensisa sphero‐cylinder.Todeterminethelenspower,thesphereandcylinderlinesmustbefocusedseparately. 7.Turnthepowerwheelintotheplus.Slowlydecreasethepowerandrotatetheaxiswheeluntilasetoflines areunbrokenandinsharpfocus.Inordertoreadthelensinminuscylinderform,makethespherelinescome intofocusfirst(i.e.themorepositiveofthetworeadings).Ifthecylinderlinescomeintofocusfirst,rotatethe axiswheel90degrees–thiswillchangetheclearsetoflinestothespherelines.Thepoweratwhichthe spherelinesfocusfirstisthelensspherepower.Recordthismeasurement. 8.Continuefocusingintoortowardtheminusuntilthecylinderlinesareinfocus.Theaxiswheelshouldnot needtobemoved.(Thecylinderlinesmustfocusatamoreminuspowerreadingthanthespherelines.Ifthey donot–changetheaxis90degreesandreturntostep6(1).Notethispowerreading.Thelensminuscylinder poweristhedifferencebetweenthisreadingandthespherepowerreading. 9.Thecylinderaxisisreadfromtheaxiswheel. 10.Recordyourfinalresult.Example–3.00–1.50x175 11.Repeatbackvertexpowermeasurementfortheleftlens. 12.Note:Whenusinganunfamiliarlensometer,thefollowingruleallowsyoutoconfirmthetargetsphereand cylinderlines:Whenthecylinderlinesareinfocusatthemoreminuspowerposition,theline’sangular positionwillmatchthereadingontheaxiswheel. ProcedureforMultifocalLenses a.Note:Mostbifocalandtrifocalsareonfrontsurfacedesign;therefore,toaccuratelymeasuretheaddition power,frontvertexpowersshouldbemeasured.Thisentailsturningthelensaroundsothefrontsurfaceis againstthelensstop.Eventhoughthisisthemore“correct”methodtomeasureaddpowers,clinicallyitisnot usedoften.Whenthedistanceandnearpowersarebothsmall,mostpeopledonotbothertoturntheglasses around.However,aspowersincrease,significantdifferencesbetweenthefrontandbackvertexpowersdo occur,makingfrontvertexmeasurementsverynecessary.Inthislaboratory,wewillmeasurefrontvertexadd powerswhenthedistancespherepowerisgreaterthan+/‐5.00DS.Ifthebifocal/trifocalisofbacksurface design,backvertexaddpowersshouldbemeasured. 1.Fusedsegmentsarefrontsurfacebifocals 2.One‐piecedesignsareusuallyfrontsurface,butfeelthefrontandbacksurfaces. The‘ledge’ofthebifocalwillbefeltonthefrontwithfrontsurfacebifocalsandonthebackwithbacksurface bifocals. b.Measurethebackvertexpowerofthedistancecarrierasyouwouldasinglevisionlensandrecordyour results(abovethebifocalsegment). c.Focusthemostverticalsetoftargetlines(maybeeitherthesphereorcylinderlines).Notethepoweron thepowerwheel. d.Slidethelensup,sothattheadditionsegmentisoverthelensstop. e.Addpluspoweronthepowerwheel,pastthepointoffocus,andthenslowlydecreasetheplusuntilthe samesetoftargetlinesareagaininclearfocus.Notethispowerreadingonthepowerwheel. f.Theadditionpoweristhedifferencebetweenthesetwomeasurements(stepscande).Itisalwaysa sphericaladditiontothedistancepower.Therefore,thereisnoneedtomeasurethespherepower,cylinder power,andaxislocationofanadditionsegment. g.Recordthetotallenspower.Example:OD:+2.00‐1.00x080Add:+1.25Repeatforleftlens i.Note:Usually,BUTNOTALWAYS,theaddpowersareequalinbothlensesofaspectacleprescription. j.Ifthelensisatrifocal,youmustcompletestepsc‐fforeachsegment.Thepowerofeachsegmentisalways thedifferencebetweentheadditionpowerandthedistancepower. 1.Thelowestportionofatrifocallensisusedfornearvision.Itspoweristhelensaddpower(theaddpower thatisorderedfromtheopticallaboratory). 2.The“middle”segmentorthe“intermediate”segmentisusuallyusedforvisionatdistancesbeyond“arm’s length”butnotatdistance.(i.e.computers,grocerystoreshelves,etc.)thepoweroftheintermediatepoweris measuredexactlythesameasabifocalsegment.Itspowerisspecifiedasanintermediatepower(amountof+ poweraddedtothedistancepower)oraspercentageofthenear(bifocal/reading)portionofthesegment. Prism: Horizontal Step1:Placetheglassesonthepatient • Withafelt‐tippedpen,markthecenterofthepatient’spupils • Verticalpositioningisnegligiblebutshouldbeclose Step2:Centerthemarkfortherightlensonthelensometer • Removeanyverticaldisplacement Step3:Anyremainingdeflectioninthexaxisistheamountofhorizontalprismforthislens • Noticetheringsthatarenumbered,eachringrepresentsadegreeofprism,thedirectionofthe prismisthedirectionthemiresarewhenfocused(canbeupordownforvertical) • Recordtheeye,amountanddirection(i.e.2.5∆BOOD) Step4:Repeatfortheleftlens Step5:Totalhorizontalprismisthesumofeacheye’sprism. • Basesinthesamedirectionareadditive(BO/BOorBI/BI) • Basesintheoppositedirectionaresubtractive(BO/BIorBI/BO) Vertical Step 1: Measure lens powers • Determine which lens has the highest power in the vertical meridian • The higher powered lens will be the reference lens Step 2: Center the target lines on the center of the reticle • Move the lens table so that the eyewire is now resting on the lens table Step 3: Without moving the lens table, slide the other lens onto the lens stop. • Position the second lens so that there is no horizontal prism • Rotating the reticle so the “crossline” is vertical makes this procedure easier. • The remaining vertical displacement is the vertical prism present in the glasses. Step 4: Record the amount, base direction and eye when specifying vertical prism • This will give you the total prism in the glasses, often prism is split between both eye Transposition Sometimesprescriptionscomeinpluscylinder;however,optometryworksinneutralizinglensesin minuscylinderform.Toconvertthelensprescriptiontoaminuscylinderformiscalledtransposition. Transpositioncanalsobeusedtotransposeminuscylinderintopluscylinder. Step1: Algebraicallysubtractthecylinderfromthespherepower;thiswillbethespherepowerofthelens. Step2: Changethepowersignofthecylinder(thepowerwillremainthesame) Step3: Add90degreestotheaxis