lens power measurement

Lens Power Lens Power MeasurementMeasurement

Walter Huang, ODWalter Huang, OD

Yuanpei UniversityYuanpei University

Department of OptometryDepartment of Optometry

Methods of Lens Power Methods of Lens Power MeasurementMeasurement

Trial lens hand neutralizationTrial lens hand neutralization

LensometerLensometer Much greater accuracy may be obtained with Much greater accuracy may be obtained with

the lensometer than with trial lens hand the lensometer than with trial lens hand neutralizationneutralization

Trial Lens Hand NeutralizationTrial Lens Hand Neutralization

Two lenses neutralize each other when Two lenses neutralize each other when placed in contact with each other so that placed in contact with each other so that the combined power of the two lenses is the combined power of the two lenses is equal to zeroequal to zero

An unknown lens is neutralized by a An unknown lens is neutralized by a known trial lens of equal power but known trial lens of equal power but opposite in signopposite in sign


This is performed in the absence of a This is performed in the absence of a lensometerlensometerIt is used qualitatively as a means for It is used qualitatively as a means for estimation in many clinical and dispensing estimation in many clinical and dispensing situationssituationsIt often involves simply identifying if it is a It often involves simply identifying if it is a plus, a minus, or a toric lensplus, a minus, or a toric lensIt more accurately estimates low power It more accurately estimates low power plus and minus lenses than toric lensesplus and minus lenses than toric lenses


It is used to measure the It is used to measure the front vertex front vertex power power of the lensof the lens


View a large distant cross target through View a large distant cross target through the lensthe lensHold lens on visual axis, at arm’s distanceHold lens on visual axis, at arm’s distanceAlign lens such that the cross target is Align lens such that the cross target is continuouscontinuousMove lens vertically, observe motion of Move lens vertically, observe motion of horizontal linehorizontal lineMove lens horizontally, observe motion of Move lens horizontally, observe motion of vertical linevertical line


For a plus or a minus lens, linear motion is For a plus or a minus lens, linear motion is used to neutralize power used to neutralize power If observe “against motion,” use plus lensIf observe “against motion,” use plus lens If observe “with motion,” use minus lensIf observe “with motion,” use minus lens


For a plus or a minus lens, linear motion is For a plus or a minus lens, linear motion is used to neutralize powerused to neutralize power


For a toric lens, rotational motion is used For a toric lens, rotational motion is used to find the axisto find the axis If observe “against motion,” use plus cylinder If observe “against motion,” use plus cylinder

axisaxis If observe “with motion,” use minus cylinder If observe “with motion,” use minus cylinder

axisaxis


For a toric lens, rotational motion is used to find For a toric lens, rotational motion is used to find the axisthe axis


Place known trial lens against front surface of Place known trial lens against front surface of unknown lensunknown lens

No movement indicates neutralityNo movement indicates neutrality

A minus or plus lens (i.e., a spherical lens) has A minus or plus lens (i.e., a spherical lens) has the same speed and direction of motion in both the same speed and direction of motion in both the vertical and horizontal meridiansthe vertical and horizontal meridians

In the case of a toric lens (i.e., spherocylindrical In the case of a toric lens (i.e., spherocylindrical lens), neutralize one limb of the cross target at a lens), neutralize one limb of the cross target at a timetime


ExampleExample An unknown lens is neutralized in the An unknown lens is neutralized in the

horizontal meridian with +3.00D and the horizontal meridian with +3.00D and the vertical meridian with +2.00Dvertical meridian with +2.00D

Prescription of unknown lens:Prescription of unknown lens:+3.00 -1.00 x 180+3.00 -1.00 x 180

Power cross:Power cross:


Large distant cross target Large distant cross target


Plus lensPlus lens


Minus lensMinus lens


Toric lensToric lens

LensometryLensometry

DefinitionDefinition ““Lenso” = lensLenso” = lens ““metry” = measurement ofmetry” = measurement of

LensometerLensometer

Lensometer PurposeLensometer Purpose

NeutralizingNeutralizing a pair of glasses a pair of glasses To determine the prescriptionTo determine the prescription

VerifyingVerifying a pair of glasses a pair of glasses To confirm the accuracy of fabricated glassesTo confirm the accuracy of fabricated glasses

DuplicatingDuplicating a pair of glasses a pair of glasses To determine the prescription, and the lab To determine the prescription, and the lab

duplicates the exact prescriptionduplicates the exact prescription


It is used to measure the It is used to measure the back vertex back vertex powerpower or or front vertex power front vertex power of the lensof the lens


To find the back vertex power, place the To find the back vertex power, place the concave side of lens against lens stopconcave side of lens against lens stop


To find the front vertex power, place the To find the front vertex power, place the convex side of lens against lens stopconvex side of lens against lens stop


In the case that the lens is a sphero-In the case that the lens is a sphero-cylindrical prescription, the lensometer is cylindrical prescription, the lensometer is used to determine the cylinder axis used to determine the cylinder axis

It is used to locate the optical center of the It is used to locate the optical center of the lenslens

The lensometer is used to measure the The lensometer is used to measure the amount of prism in the lensamount of prism in the lens

Lensometer SystemsLensometer Systems

Observation system (Keplerian telescope Observation system (Keplerian telescope with two plus lenses, an inverted target with two plus lenses, an inverted target image)image) Objective lensObjective lens Eyepiece (ocular lens)Eyepiece (ocular lens) Reticle (concentric circles and cross hairs Reticle (concentric circles and cross hairs

focused by eyepiece)focused by eyepiece)

Lensometer SystemsLensometer Systems

Focusing system (Badal lens system)Focusing system (Badal lens system) Light sourceLight source Target (cross hairs)Target (cross hairs) Standard lens (+20.00D)Standard lens (+20.00D) Lens stopLens stop Power wheelPower wheel

Lensometer SchematicLensometer Schematic

Observation SystemObservation System

The Keplerian telescope consists of an The Keplerian telescope consists of an objective lens, an eyepiece, and a reticleobjective lens, an eyepiece, and a reticleThe two plus lenses are positioned so that The two plus lenses are positioned so that their two focal points coincide with each their two focal points coincide with each otherotherThe unknown lens whose power is to be The unknown lens whose power is to be measured or neutralized is positioned at measured or neutralized is positioned at the lens stop (the location of the the lens stop (the location of the secondary focal plane of the standard secondary focal plane of the standard lens)lens)

Lensometer OperationLensometer Operation

With the instrument set at zero, an With the instrument set at zero, an illuminated target (light source) is positioned illuminated target (light source) is positioned at the focal length of a plus lens (standard at the focal length of a plus lens (standard lens) usually a +20.00D lenslens) usually a +20.00D lens

Diverging rays of light from the illuminated Diverging rays of light from the illuminated target are bent by the standard lens and target are bent by the standard lens and parallel light emerges from focusing system parallel light emerges from focusing system into the observation system, which is viewed into the observation system, which is viewed through the telescope by the observerthrough the telescope by the observer


When the lens of unknown power is When the lens of unknown power is introduced, the image of the illuminated introduced, the image of the illuminated target is thrown out of focustarget is thrown out of focus


The target is movableThe target is movable

By moving the target closer to or farther By moving the target closer to or farther from the standard lens, the refractive from the standard lens, the refractive power of the unknown lens can be power of the unknown lens can be neutralizedneutralized Closer to standard lens for plus lens Closer to standard lens for plus lens

neutralizationneutralization Farther from standard lens for minus lens Farther from standard lens for minus lens

neutralizationneutralization


The physical distance forward or backward The physical distance forward or backward that the target moves indicates the power that the target moves indicates the power of unknown lens for the meridian being of unknown lens for the meridian being measuredmeasured

eyepiece

Lens holder Lens stop Axis wheel

Power wheel

Lens stage

Risley prism

Lensometer AnatomyLensometer Anatomy


reticule

target

123

Sphere line

Cylinder lines

Lensometer PreparationLensometer Preparation

Focus the eyepiece of the lensometer for Focus the eyepiece of the lensometer for the examiner’s eyethe examiner’s eye With the power wheel set on zero, turn the With the power wheel set on zero, turn the

eyepiece as far counter-clockwise as possibleeyepiece as far counter-clockwise as possible Then slowly turn it clockwise until the reticule Then slowly turn it clockwise until the reticule

first comes into sharp focusfirst comes into sharp focus

Lens Measurement PreparationLens Measurement Preparation

Insert the spectaclesInsert the spectacles If testing a pair of glasses, always check the If testing a pair of glasses, always check the

right lens firstright lens first Place the pair of glasses in the lensometer with Place the pair of glasses in the lensometer with

the ocular surface away from youthe ocular surface away from you The lens is held in place by the lens holder and The lens is held in place by the lens holder and

is held level on the lens tableis held level on the lens table Center the lens by moving it so that the image Center the lens by moving it so that the image

of the lensometer target is aligned in the center of the lensometer target is aligned in the center of the eyepiece reticleof the eyepiece reticle

Single Vision Lens Single Vision Lens MeasurementMeasurement

To measure single vision lenses, either To measure single vision lenses, either back vertex powers or front vertex powers back vertex powers or front vertex powers must be found must be found


Determine which part of the target is used Determine which part of the target is used for determining the spherical component for determining the spherical component and which part of the target is used for and which part of the target is used for determining the cylindrical componentdetermining the cylindrical componentRotate the power wheel until the lines (or Rotate the power wheel until the lines (or the spots) are in clear focusthe spots) are in clear focusIf the power is spherical, all the lines (or If the power is spherical, all the lines (or spots) will be clearspots) will be clearNote the power on the power wheelNote the power on the power wheel

Types of TargetTypes of Target


If the spherical and cylindrical lines do not If the spherical and cylindrical lines do not come into focus at the same time, the lens come into focus at the same time, the lens has a cylindrical componenthas a cylindrical component

Rotate the power wheel until the spherical Rotate the power wheel until the spherical lines focus with the lines focus with the less minusless minus (or more (or more plus) powerplus) power

Orient the target rotation dial (axis wheel) Orient the target rotation dial (axis wheel) so that the spherical lines are perfectly so that the spherical lines are perfectly straightstraight


Read the power and record as the Read the power and record as the sphericalspherical component of the prescriptioncomponent of the prescription

Focus the cylindrical lines by rotating the power Focus the cylindrical lines by rotating the power wheel to wheel to more minusmore minus (or less plus) power (90 (or less plus) power (90 degrees away)degrees away)

The difference in power between the two The difference in power between the two principal meridians is the amount of principal meridians is the amount of minus minus cylinder power cylinder power in the lensin the lens

Read the axis of the cylinder from the axis Read the axis of the cylinder from the axis wheelwheel


ExampleExample +1.00 -2.00 x 120+1.00 -2.00 x 120 Power wheel sphere settingPower wheel sphere setting


ExampleExample +1.00 -2.00 x 120+1.00 -2.00 x 120 Power wheel cylinder settingPower wheel cylinder setting


Mark the optical center (OC)Mark the optical center (OC) Make sure that the lens is centered and the Make sure that the lens is centered and the

spectacle is sitting on the lens table evenlyspectacle is sitting on the lens table evenly Use the OC marker on the lensometer to spot Use the OC marker on the lensometer to spot

the lensthe lens Three dots will be markedThree dots will be marked

The center dot marks the OCThe center dot marks the OCThe other two dots indicate the 0 to 180 horizontal The other two dots indicate the 0 to 180 horizontal lineline

Position and read the second lensPosition and read the second lens


When both lenses have been measured When both lenses have been measured and marked, measure the distance and marked, measure the distance between optical centers of the lenses between optical centers of the lenses (DBOC or geometric center distance)(DBOC or geometric center distance)

Multifocal Lens MeasurementMultifocal Lens Measurement

To measure bifocal and trifocal lenses, To measure bifocal and trifocal lenses, front vertex powers must be foundfront vertex powers must be found This is especially true for lenses with high This is especially true for lenses with high

distance and near powersdistance and near powers Again, front vertex power is measured by Again, front vertex power is measured by

turning spectacles around with back surface turning spectacles around with back surface of the lens toward the operator (i.e., the of the lens toward the operator (i.e., the convex side of the lens against the lens stop)convex side of the lens against the lens stop)

Multifocal Lens MeasurementMultifocal Lens Measurement

Measure the distance portion of multifocal Measure the distance portion of multifocal lenses, in the same way as with single vision lenses, in the same way as with single vision lenseslensesTurn the glasses around backward so that the Turn the glasses around backward so that the temples face the operatortemples face the operatorFind the Find the distance front vertex powerdistance front vertex powerFind the Find the near front vertex powernear front vertex powerRecord the addition power (Add), which is the Record the addition power (Add), which is the difference between the distance and near difference between the distance and near prescriptionsprescriptions

PrismPrism

Changes direction of light going through a Changes direction of light going through a lenslens

Prism PurposePrism Purpose

To treat a binocular vision problemTo treat a binocular vision problem

To shift the visual fieldTo shift the visual field

To improve the lens appearance through To improve the lens appearance through prism thinningprism thinning

PrismPrism

It is often divided equally between the two It is often divided equally between the two lenses of spectacles for balance and lenses of spectacles for balance and aesthetic reasonsaesthetic reasons

Types of PrismTypes of Prism

Horizontal prismHorizontal prism Base in (BI)Base in (BI) Base out (BO) Base out (BO)

Vertical prismVertical prism Base up (BU)Base up (BU) Base down (BD)Base down (BD)

Oblique prismOblique prism A combination of horizontal and vertical A combination of horizontal and vertical

prismsprisms

Types of Prism and LensTypes of Prism and Lens

Methods of Achieving Prism in Methods of Achieving Prism in LensLens

GrindingGrinding Usually applied when a large amount of prism is Usually applied when a large amount of prism is

requiredrequired Lens is custom-madeLens is custom-made Optic center is often not on lensOptic center is often not on lens

DecentrationDecentration Usually applied when a small amount of prism is Usually applied when a small amount of prism is

requiredrequired With spherical prescription, it is easier to deal with With spherical prescription, it is easier to deal with

decentering than with spherocylindrical prescriptiondecentering than with spherocylindrical prescription

Prism MeasurementPrism Measurement

The purpose is to verify if the prescription The purpose is to verify if the prescription contains the desired prismatic effectcontains the desired prismatic effect

Verification of prism in prescription is done Verification of prism in prescription is done similarly to measurement of lens power similarly to measurement of lens power using the lensometerusing the lensometer

The only difference is in the means by The only difference is in the means by which the target is positioned in the which the target is positioned in the lensometer lensometer

Prism SpecificationPrism Specification

Prism VerificationPrism Verification

To verify the prescribed prism when the To verify the prescribed prism when the amount of prism is knownamount of prism is known The center of the illuminated target is The center of the illuminated target is

positioned at the location on the circular mires positioned at the location on the circular mires corresponding to the prism requiredcorresponding to the prism required


Example 1:Example 1: If the right lens calls for 2 prism diopter BU, If the right lens calls for 2 prism diopter BU,

then the illuminated target would be then the illuminated target would be positioned at the “2” ring above the center of positioned at the “2” ring above the center of the miresthe mires


Example 1:Example 1:


Example 2:Example 2: If both lenses contain 1.5 prism diopter BO, If both lenses contain 1.5 prism diopter BO,

then the target would be placed at the 1.5 ring then the target would be placed at the 1.5 ring to the left of the center of the mires for the OD to the left of the center of the mires for the OD lens and to the right of the center of the mires lens and to the right of the center of the mires for the OS lensfor the OS lens

Mark the major reference point (MRP) with the Mark the major reference point (MRP) with the OC marker and measure the distance OC marker and measure the distance between the MRP of the lenses (i.e, the between the MRP of the lenses (i.e, the patient’s distance PD)patient’s distance PD)


Example 2:Example 2:

Prism MeasurementPrism Measurement

In an unknown lensIn an unknown lens Patient may come in with a prescription that Patient may come in with a prescription that

you are not sure if it contains prism in the you are not sure if it contains prism in the lenseslenses

After power of the lenses are neutralized and After power of the lenses are neutralized and the optic centers are marked, measure the the optic centers are marked, measure the distance between the optic centers (DBOC)distance between the optic centers (DBOC)

If DBOC does not equal to the patient’s If DBOC does not equal to the patient’s distance PD, then there is prism in the lensdistance PD, then there is prism in the lens

Prentice’s RulePrentice’s Rule

P = d * FP = d * F P = prism power (in prism diopters)P = prism power (in prism diopters) d = decentration (in cm)d = decentration (in cm) F = refracting power of the lens (in diopters)F = refracting power of the lens (in diopters)


Example:Example: Rx = -4.00DS OURx = -4.00DS OU Patient’s distance PD = 62mmPatient’s distance PD = 62mm DBOC = 72mmDBOC = 72mm P = d * F P = d * F = (72 – 62mm)/2 x 1cm/10mm * (-4.00D) = (72 – 62mm)/2 x 1cm/10mm * (-4.00D) = 0.5cm * (-4.00D) = 0.5cm * (-4.00D) = 2 prism diopters BI in each lens= 2 prism diopters BI in each lens


Example:Example:

RecordingRecording

Record the prescription for each lens Record the prescription for each lens separatelyseparately

Record the amount of induced prism in Record the amount of induced prism in each lens, if applicableeach lens, if applicable

RecordingRecording

Example 1Example 1 OD -3.00DSOD -3.00DS Add +2.00Add +2.00 OS -1.50 -0.75 x 180OS -1.50 -0.75 x 180 Add +2.00Add +2.00

RecordingRecording

Example 2Example 2 OD +2.50 -0.75 x 080OD +2.50 -0.75 x 080 2 prism diopter BI2 prism diopter BI OS +1.00 -0.25 x 110OS +1.00 -0.25 x 110 2 prism diopter BI2 prism diopter BI

lens power measurement

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