Download LENSOMETRY Handout - Optometry`s Meeting

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