Lensometry Lensometry What is Lensometry?What is Lensometry?

Lensometry is the procedure used to measure the prescription of a patient’smeasure the prescription of a patient s existing eyeglass lenses or the power of

t t l Alth h lcontact lenses. Although some people refer to this as neutralization, this is technically incorrect. The term neutralization applies to retinoscopy.pp py

Lensometry measures four principal properties of lenses:

• Spherical and cylindrical power p y pin dioptersA i if li d i l• Axis, if cylindrical

• Prism amount and direction if• Prism, amount and direction, if any

• Optical centers

How is Lensometry performed?How is Lensometry performed?

Lensometry is performed with a specialized instrument know as a lensometer.

Manual Manual LensometerLensometer

Automated Automated LensometerLensometerLensometerLensometer

Types of LensometersTypes of Lensometers• Lensometers may be either manual or automatedLensometers may be either manual or automated.

There are several manufacturers for each. • To perform manual lensometry the operator should• To perform manual lensometry, the operator should

have a thorough understanding of lensometry as well as optical principleswell as optical principles.

• Performing automated lensometry requires very littl kill k l d f tilittle skill or knowledge of optics.

• Automated lensometry may be quicker to perform but in most cases the instrument is more expensive to purchase than a manual lensometer.

Performing Manual LensometryPerforming Manual Lensometry

Although most manual lensometers look similar despite a different brand name, the p ,buttons and knobs may be placed differently on certain models and the miresdifferently on certain models and the mires may have a slightly different appearance. The technique is the same for plus andThe technique is the same for plus and minus cylinder.

The following principles may be applied to all manual lensometers:

The first three steps in performing lensometry on lenses of all types are:

1. Focusing the instrument eyepiece2. Positioning the eyeglass lens to be measured2. Positioning the eyeglass lens to be measured

on the specific table (or frame support platform) of the lensometerof the lensometer

3. Measuring the sphere power and, if present cylinder power and axis either plus or minuscylinder power and axis, either plus or minus cylinder form.

EyepieceEyepiece

Focusing the EyepieceFocusing the EyepieceY t f th i i t• You must focus the eyepiece prior to using the lensometer, failure to do so could result in erroneous readings

• With no lens in the lensometer, look ,through the eyepiece of the instrument

• Turn the power wheel until the mires• Turn the power wheel until the mires (the perpendicular crossed lines), viewed through the eyepiece are totallyviewed through the eyepiece, are totally out of focus

Focusing the EyepieceFocusing the Eyepiece

• Turn the eyepiece toward the plus direction• Slowly turn the eyepiece in the oppositeSlowly turn the eyepiece in the opposite

direction until the target is clear• Turn the power wheel to focus the mires• Turn the power wheel to focus the mires• The mires should focus at a reading of zero

(plano)(plano)• If the mires do not focus at plano, repeat

h f h b i ithese steps from the beginning

Positioning the eyeglass lensPositioning the eyeglass lensPower wheel

Platform / table

Gimbal

Power wheel

Axis wheelAxis wheel

Measure the sphere power

Align the lens so that the mires cross in the center of the target (if unable consider prism)

Thin “single” lines that represent the sphereThin, single lines that represent the sphere

Perpindicular, widely spaced thick lines that represent the cylinder

Measuring the sphere powerMeasuring the sphere power

• Turn the power wheel either direction to focus the mires

if all the mires come into focus at the same time…

Record the number on the power wheel, This is a spherical only RxThis is a spherical only Rx

CylinderCylinder

if all the mires do not come into focus at the same time

Si lt l t t th i h l t f d• Simultaneously rotate the axis wheel to focus and straighten the thin sphere lines, record the number on the power wheel this is the sphere poweron the power wheel, this is the sphere power

• Leaving the axis as it is, bring the thick lines into focus by turning the sphere wheelfocus by turning the sphere wheel

Cylinder

• Algebraically add the number shown now on the power wheel with thenow on the power wheel with the number previously recorded as the sphere this is the cylinder amountsphere, this is the cylinder amount, record the axis

• To transpose the cylinder, rotate the axis 90˚and start overa s 90 a d sta t o e

Lensometry technique for multifocal lenses

If the glasses are multifocal the first step is to determine the distance Rx. For a traditional lined segment add you would then reposition the glasses so that the center of the bifocal add is in the lensometer gimbal.

The absolute power of the bifocal segment is always p g ymore plus (or less minus) than the sphere power in the upper (distance) portion of an eyeglass lens. The dd i th t t l diff i di t iadd is the total difference in dioptric power.

Measuring trifocal powerMeasuring trifocal power

To measure the trifocal segment directly, To measure the trifocal segment directly, follow the same procedure as for the bifocalfollow the same procedure as for the bifocalfollow the same procedure as for the bifocal follow the same procedure as for the bifocal segment, reading the distance segment first, segment, reading the distance segment first, the intermediate segment second and thethe intermediate segment second and thethe intermediate segment second, and the the intermediate segment second, and the near segment last. It is standard for the near segment last. It is standard for the trifocal power to betrifocal power to be 5050% of the bifocal power;% of the bifocal power;trifocal power to be trifocal power to be 5050% of the bifocal power; % of the bifocal power; therefore the usual protocol would be not to therefore the usual protocol would be not to measure the power of the trifocalmeasure the power of the trifocalmeasure the power of the trifocal.measure the power of the trifocal.

Progressive add multifocal lensesProgressive add multifocal lenses•Progressive add lenses are different from•Progressive add lenses are different from traditional bifocal or trifocal eyeglasses in that there are no visible segments dividing the distance andare no visible segments dividing the distance and reading portions of the lenses.

• In order to create this “no-line” appearance the manufacturing processes often produce unwanted cylinder power, distortion, or blurred transition zones between the distance and near segments. This can make lensometry a little tricky.

Progressive add multifocal lensesProgressive add multifocal lenses• When performing lensometry on progressive-p g y p g

add eyeglasses try to select the area with the least distortion in both distance and the reading

ti f th l b f t ki diportions of the lenses before taking a reading.

• Because of the nature of the progressive add, p g ,the strongest portion of the add is close to the bottom of the lens, so try to read the add as close to the bottom of the lens as possible.

• Other than these issues lensometry is performed Other than these issues lensometry is performed the same way as with conventional multifocals.

Placement of optical centers• Optimal vision correction is achieved when

pp

looking through the optical center of the eyeglass lens. The lensometer may be

d t if th iti f th ti lused to verify the position of the optical center of a lens.

• Position the frame in the lensometer as if f i b i l t kperforming basic lensometry, make sure

the frame is sitting flat on the platform and is lined up properly Focus the mires andis lined up properly. Focus the mires and center in the target.

Placement of optical centersp

the picture on the left is an example of properly aligned mires and

target, at this point you would dot the lens

Placement of optical centersPlacement of optical centers

• If equipped, use the dotting device on the lensometer to mark the lens. Thisthe lensometer to mark the lens. This mark will be the optical center. If th i d tti d i th• If there is no dotting device, then use a nonpermanent marker to mark the approximate center of the lens.

Placement of optical centersPlacement of optical centers

• Once the optical center of both lenses has been found, a millimeter ruler ishas been found, a millimeter ruler is used to measure the distance between the marks on the lensesthe marks on the lenses.

• This distance should match the patient’s interpupillary distance unless there is prism in the lenses.p s t e e ses

Lensometry technique for prisms

L t t l

Lensometry technique for prisms

• Lensometry measures not only the power of a prism but also thethe power of a prism but also the orientation of the prism base.

• The prism orientation may be base in (toward the nose) basebase in (toward the nose), base out (toward the temple), base up,out (toward the temple), base up, or base down.

Lensometry technique for prisms

If i i d i th l

Lensometry technique for prisms

• If prism is ground in the lens you will not be able to align the mireswill not be able to align the mires with the target.

• As mentioned in the previous chapter regarding optical centerschapter, regarding optical centers, if the pupil measurement is “off”if the pupil measurement is off prism will be induced.

Measuring prism power and orientationMeasuring prism power and orientation

• OK, you can’t line the mires up with the target…so you have prism, right? Now g y p , ghow do you find out how and what type?...

the picture on the left is a representation of mires that cannot

be aligned with the target due to prism (not drawn to scale)

To measure the amount of prismTo measure the amount of prism

• Count the number of circles from the central cross of the target to the center gof the mires (each circle represents 1 prism diopter)prism diopter)

• Record the direction of the base faccording to the displacement of the

mires (example: if mires are displaced ( p pdown, the prism is base down)

Some lensometers are made to Some lensometers are made to compensate for prism If yourcompensate for prism If yourcompensate for prism. If your compensate for prism. If your lensometer has a feature of this type, lensometer has a feature of this type, please refer to the instruction manual please refer to the instruction manual for more information Also somefor more information Also somefor more information. Also some for more information. Also some lensometers have auxiliary prisms in lensometers have auxiliary prisms in the case that there is more prism than the case that there is more prism than there are circles on the target.there are circles on the target.there are circles on the target.there are circles on the target.

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Th k Y !THE END

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