Lenspowermeasurement 121007031943-phpapp02

Lens PowerLens Power
MeasurementMeasurement
Walter Huang, ODWalter Huang, OD
Yuanpei UniversityYuanpei University
Department of OptometryDepartment of Optometry

Methods of Lens PowerMethods of Lens Power
Trial lens hand neutralizationTrial lens hand neutralization
LensometerLensometer

Much greater accuracy may be obtained withMuch greater accuracy may be obtained with
the lensometer than with trial lens handthe lensometer than with trial lens hand
neutralizationneutralization

Trial Lens Hand NeutralizationTrial Lens Hand Neutralization
Two lenses neutralize each other whenTwo lenses neutralize each other when
placed in contact with each other so thatplaced in contact with each other so that
the combined power of the two lenses isthe combined power of the two lenses is
equal to zeroequal to zero
An unknown lens is neutralized by aAn unknown lens is neutralized by a
known trial lens of equal power butknown trial lens of equal power but
opposite in signopposite in sign

This is performed in the absence of aThis is performed in the absence of a
lensometerlensometer
It is used qualitatively as a means forIt is used qualitatively as a means for
estimation in many clinical and dispensingestimation in many clinical and dispensing
situationssituations
It often involves simply identifying if it is aIt often involves simply identifying if it is a
plus, a minus, or a toric lensplus, a minus, or a toric lens
It more accurately estimates low powerIt more accurately estimates low power
plus and minus lenses than toric lensesplus and minus lenses than toric lenses

It is used to measure theIt is used to measure the front vertexfront vertex
powerpower of the lensof the lens

View a large distant cross target throughView a large distant cross target through
the lensthe lens
Hold lens on visual axis, at arm’s distanceHold lens on visual axis, at arm’s distance
Align lens such that the cross target isAlign lens such that the cross target is
continuouscontinuous
Move lens vertically, observe motion ofMove lens vertically, observe motion of
horizontal linehorizontal line
Move lens horizontally, observe motion ofMove lens horizontally, observe motion of
vertical linevertical line

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

For a toric lens, rotational motion is usedFor a toric lens, rotational motion is used
to find the axisto find the axis

If observe “against motion,” use plus cylinderIf observe “against motion,” use plus cylinder
axisaxis

If observe “with motion,” use minus cylinderIf observe “with motion,” use minus cylinder
axisaxis

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

Place known trial lens against front surface ofPlace 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) hasA minus or plus lens (i.e., a spherical lens) has
the same speed and direction of motion in boththe 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., spherocylindricalIn the case of a toric lens (i.e., spherocylindrical
lens), neutralize one limb of the cross target at alens), neutralize one limb of the cross target at a
timetime

ExampleExample

An unknown lens is neutralized in theAn unknown lens is neutralized in the
horizontal meridian with +3.00D and thehorizontal 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 targetLarge distant cross target

Plus lensPlus lens

Minus lensMinus lens

Toric lensToric lens

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

Lensometer PurposeLensometer Purpose
NeutralizingNeutralizing a pair of glassesa pair of glasses

To determine the prescriptionTo determine the prescription
VerifyingVerifying a pair of glassesa pair of glasses

To confirm the accuracy of fabricated glassesTo confirm the accuracy of fabricated glasses
DuplicatingDuplicating a pair of glassesa pair of glasses

To determine the prescription, and the labTo determine the prescription, and the lab
duplicates the exact prescriptionduplicates the exact prescription

It is used to measure theIt is used to measure the back vertexback vertex
powerpower oror front vertex powerfront vertex power of the lensof the lens

To find the back vertex power, place theTo 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 theTo 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 iscylindrical prescription, the lensometer is
used to determine the cylinder axisused to determine the cylinder axis
It is used to locate the optical center of theIt is used to locate the optical center of the
lenslens
The lensometer is used to measure theThe lensometer is used to measure the
amount of prism in the lensamount of prism in the lens

Lensometer SystemsLensometer Systems
Observation system (Keplerian telescopeObservation system (Keplerian telescope
with two plus lenses, an inverted targetwith two plus lenses, an inverted target
image)image)

Objective lensObjective lens

Eyepiece (ocular lens)Eyepiece (ocular lens)

Reticle (concentric circles and cross hairsReticle (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 anThe Keplerian telescope consists of an
objective lens, an eyepiece, and a reticleobjective lens, an eyepiece, and a reticle
The two plus lenses are positioned so thatThe two plus lenses are positioned so that
their two focal points coincide with eachtheir two focal points coincide with each
otherother
The unknown lens whose power is to beThe unknown lens whose power is to be
measured or neutralized is positioned atmeasured or neutralized is positioned at
the lens stop (the location of thethe lens stop (the location of the
secondary focal plane of the standardsecondary focal plane of the standard
lens)lens)

Lensometer OperationLensometer Operation
With the instrument set at zero, anWith the instrument set at zero, an
illuminated target (light source) isilluminated target (light source) is
positioned at the focal length of a plus lenspositioned at the focal length of a plus lens
(standard lens) usually a +20.00D lens(standard lens) usually a +20.00D lens
Diverging rays of light from the illuminatedDiverging rays of light from the illuminated
target are bent by the standard lens andtarget are bent by the standard lens and
parallel light emerges from focusingparallel light emerges from focusing
system into the observation system, whichsystem into the observation system, which
is viewed through the telescope by theis viewed through the telescope by the
observerobserver

When the lens of unknown power isWhen the lens of unknown power is
introduced, the image of the illuminatedintroduced, 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 fartherBy moving the target closer to or farther
from the standard lens, the refractivefrom the standard lens, the refractive
power of the unknown lens can bepower of the unknown lens can be
neutralizedneutralized

Closer to standard lens for plus lensCloser to standard lens for plus lens

Farther from standard lens for minus lensFarther from standard lens for minus lens

The physical distance forward orThe physical distance forward or
backward that the target moves indicatesbackward that the target moves indicates
the power of unknown lens for thethe power of unknown lens for the
meridian being measuredmeridian being measured

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 forFocus the eyepiece of the lensometer for
the examiner’s eyethe examiner’s eye

With the power wheel set on zero, turn theWith 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 reticuleThen 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 theIf testing a pair of glasses, always check the
right lens firstright lens first

Place the pair of glasses in the lensometerPlace the pair of glasses in the lensometer
with the ocular surface away from youwith the ocular surface away from you

The lens is held in place by the lens holderThe lens is held in place by the lens holder
and is held level on the lens tableand is held level on the lens table

Center the lens by moving it so that the imageCenter the lens by moving it so that the image
of the lensometer target is aligned in theof the lensometer target is aligned in the
center of the eyepiece reticlecenter of the eyepiece reticle

Single Vision LensSingle Vision Lens
To measure single vision lenses, eitherTo measure single vision lenses, either
back vertex powers or front vertex powersback vertex powers or front vertex powers
must be foundmust be found

Determine which part of the target is usedDetermine which part of the target is used
for determining the spherical componentfor determining the spherical component
and which part of the target is used forand which part of the target is used for
determining the cylindrical componentdetermining the cylindrical component
Rotate the power wheel until the lines (orRotate the power wheel until the lines (or
the spots) are in clear focusthe spots) are in clear focus
If the power is spherical, all the lines (orIf the power is spherical, all the lines (or
spots) will be clearspots) will be clear
Note the power on the power wheelNote the power on the power wheel

Types of TargetTypes of Target

If the spherical and cylindrical lines do notIf the spherical and cylindrical lines do not
come into focus at the same time, the lenscome into focus at the same time, the lens
has a cylindrical componenthas a cylindrical component
Rotate the power wheel until the sphericalRotate the power wheel until the spherical
lines focus with thelines 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 perfectlyso that the spherical lines are perfectly
straightstraight

Read the power and record as theRead the power and record as the
sphericalspherical component of the prescriptioncomponent of the prescription
Focus the cylindrical lines by rotating theFocus the cylindrical lines by rotating the
power wheel topower wheel to more minusmore minus (or less plus)(or less plus)
power (90 degrees away)power (90 degrees away)
The difference in power between the twoThe difference in power between the two
principal meridians is the amount ofprincipal meridians is the amount of
minus cylinder powerminus cylinder power in the lensin the lens
Read the axis of the cylinder from the axisRead 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 theMake 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 spotUse 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 OC
The other two dots indicate the 0 to 180 horizontalThe 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 measuredWhen both lenses have been measured
and marked, measure the distanceand marked, measure the distance
between optical centers of the lensesbetween 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 highThis is especially true for lenses with high
distance and near powersdistance and near powers

Again, front vertex power is measured byAgain, front vertex power is measured by
turning spectacles around with back surfaceturning spectacles around with back surface
of the lens toward the operator (i.e., theof 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 multifocalMeasure the distance portion of multifocal
lenses, in the same way as with single visionlenses, in the same way as with single vision
lenseslenses
Turn the glasses around backward so that theTurn the glasses around backward so that the
temples face the operatortemples face the operator
Find theFind the distance front vertex powerdistance front vertex power
Find theFind the near front vertex powernear front vertex power
Record the addition power (Add), which is theRecord the addition power (Add), which is the
difference between the distance and neardifference between the distance and near
prescriptionsprescriptions

PrismPrism
Changes direction of light going through aChanges 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 throughTo improve the lens appearance through
prism thinningprism thinning

PrismPrism
It is often divided equally between the twoIt is often divided equally between the two
lenses of spectacles for balance andlenses 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 verticalA combination of horizontal and vertical
prismsprisms

Types of Prism and LensTypes of Prism and Lens

Methods of Achieving Prism inMethods of Achieving Prism in
LensLens
GrindingGrinding

Usually applied when a large amount of prism isUsually 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 isUsually applied when a small amount of prism is
requiredrequired

With spherical prescription, it is easier to deal withWith 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 prescriptionThe purpose is to verify if the prescription
contains the desired prismatic effectcontains the desired prismatic effect
Verification of prism in prescription is doneVerification of prism in prescription is done
similarly to measurement of lens powersimilarly to measurement of lens power
using the lensometerusing the lensometer
The only difference is in the means byThe only difference is in the means by
which the target is positioned in thewhich the target is positioned in the
lensometerlensometer

Prism SpecificationPrism Specification

Prism VerificationPrism Verification
To verify the prescribed prism when theTo verify the prescribed prism when the
amount of prism is knownamount of prism is known

The center of the illuminated target isThe center of the illuminated target is
positioned at the location on the circular mirespositioned 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 bethen the illuminated target would be
positioned at the “2” ring above the center ofpositioned at the “2” ring above the center of
the miresthe mires


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 ringthen the target would be placed at the 1.5 ring
to the left of the center of the mires for the ODto the left of the center of the mires for the OD
lens and to the right of the center of the mireslens and to the right of the center of the mires
for the OS lensfor the OS lens

Mark the major reference point (MRP) with theMark the major reference point (MRP) with the
OC marker and measure the distanceOC marker and measure the distance
between the MRP of the lenses (i.e, thebetween the MRP of the lenses (i.e, the
patient’s distance PD)patient’s distance PD)

Prism MeasurementPrism Measurement
In an unknown lensIn an unknown lens

Patient may come in with a prescription thatPatient may come in with a prescription that
you are not sure if it contains prism in theyou are not sure if it contains prism in the
lenseslenses

After power of the lenses are neutralized andAfter power of the lenses are neutralized and
the optic centers are marked, measure thethe 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’sIf 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 * FP = d * F

= (72 – 62mm)/2 x 1cm/10mm * (-= (72 – 62mm)/2 x 1cm/10mm * (-
4.00D)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 lensRecord the prescription for each lens
separatelyseparately
Record the amount of induced prism inRecord 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

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