928_progressive_IPD

8/7/2019 928_progressive_IPD

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ADVANCED LENSOMETRYWORKSHOP

Instructors: M. Patrick Coleman &

MSgt Brent SonnierMSgt. Donald Murphy

Things you will (hopefully) learn:

How to neutralize a bifocal (multifocal) lens

How to measure segment heights

How to mark a progressive add lens (PAL)

How to identify wanted and unwanted prism

Lets begin with

How to neutralize a bifocal lens (or maybewe should just go with the term multifocal?)

Neutralizing a Multifocal

FIRST:

Neutralize the Rx of the top portion oflens, as taught in Basic Lensometry

This will usually be the distanceRx (but ifthe specs are for office/CPU use, the topportion may represent the intermediate Rx)

NEXT:

Center the bifocal (or lowest) segment inthe lensometer

Neutralizing a Multifocal Neutralizing a Multifocal (cont.)

NOW:

Turn the power wheel until the spherelines are in focus;Note:Note:The reading on the lensometer powerThe reading on the lensometer power

wheel iswheel is NOTNOT your ADD power!your ADD power!

MATH TIME: Calculate the Distance Traveled from the

sphere power reading in the top of the lens tothe sphere power reading in the bottom ofthe lens. THAT IS YOUR ADD POWER!


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Neutralizing a Multifocal (cont.)

Example:

SPHERE power reading in the top of lens was-1.50

SPHERE power reading in bottom of lens was

+0.50

To get from -1.50 to +0.50, you had to travel2.00 diopters in the PLUSPLUSdirection. So your

ADD power is +2.00

Neutralizing a Multifocal (cont.)

What about TRIFOCALS!?

Dont freak out!

IGNORE THE MIDDLE SEGMENT.

Dont read it on the lensometer.

It will almost always be 50% of the ADD power

found in the lowest segment of the specs

If this confused youjust remember:

IGNORE THE MIDDLE SEGMENT

Since we are looking at

Multifocals, what about seg

heights? Lets look at

measuring

SEGMENTHEIGHTSnow

Measuring Segment Heights Measure from lowest part of the lens to thelowest part of the lens to the

top of the segment.top of the segment. Do this in the center ofthe segment (not off to the left or right)

If the millimeter ruler shows 17, then the

segment height is 17 for rimless designs, or18 for rimmed frames (see picture.)

Measuring Segment Heights (cont.)

OhMyGodBECKY!Can it really be that easy?

Uhyes, it can!

So thats all greatbut what aboutPALs?

Lets look at how to mark up a PAL to measureit correctly


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Marking Progressive Lenses

Determinemanufacturer of the

PAL (& there are aLOT of them!)

You have to find thesymbolsymbol on the lens,then look in thethebookbook to find out who

made that lens.

PAL IDENTIFIER BOOK

Marking Progressive Lenses (cont.)

Find template marks on lens. They will be nasal& temporal on the lens, about half way downfrom the top.

These marks are USUALLY just small circles.(Sometimes you can only find one, as the otherside can get ground off when fitting the lens into

the frame.)

Dot those template marks with an erasablemarker


Marking Progressive Lenses (cont.) Place glasses on manufacturers template &

mark the distance Rx area & the optical centerarea (again, use an erasable marker)


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Now neutralize the distance Rx like normal

(thru the semi-circle area toward top) To get the ADD power, just look at the

marking on the front of the lens to seewhat number the manufacturer put on it.(Reallythats it.)

You will see stuff like 22 which is a +2.25ADD, or 17 which is a +1.75 ADD or 27which is a +2.75 ADD

Multifocals and PALS are not so hardto figure out. But what about PRISMPRISM?

Identifying Wanted & Unwanted Prism

First off, WHAT IS PRISMWHAT IS PRISM?

1. A 1970s Canadian Rock Band.

2. A polyhedron w/two congruent & parallel faces (the

bases) whose lateral faces are parallelograms.

3. A fictional super-villain character in the MarvelComics universe; he is a member of the Marauders.

He is a mutant & a clone & his first name is Robbie4. A transparent piece of glass that refracts or

disperses light.

ALL OF THE ABOVE! (but #4 is the one we like)

Identifying Wanted & Unwanted Prism (cont.)

WANTEDWANTED PRISM:

Prism that is prescribed by the doctor.

Prism we want the labs to put in the lenses.

When you measure the specs, you want theprescribed amount of prism to be in them &you want that prism to be going in thedirection prescribed:

i.e. Base UP (BU), Base DOWN (BD), Base IN

(BI) or Base OUT (BO)


UNWANTEDUNWANTED PRISM is:

Prism that is found in the spectacles that

that was not prescribedand

exceeds the allowable amountspecified by ANSI standards.

ANSI STANDARDS allow UP TO:

0.33 Diopters UP or DOWN prism

0.66 Diopters IN or OUT prism


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A prism looks like a triangle. It has a BASE & an APEX.

Its power is measured in DIOPTERS.


Prisms can cause:

DEVIATIONDEVIATION

Prisms can cause:

DDIISSPPEERRSSIONION

Identifying Wanted & Unwanted Prism (cont.) We are interested only in the DEVIATION aspect of

prisms today.----------------------------------------------------------------------------

Light gets deviated toward the BASE.

Identifying Wanted & Unwanted Prism

(cont.)

ImagesImagesappear to getdeviated toward theAPEXAPEX


All spectacles lenses can be thought of as simplytwo prisms put together



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Light passing thru the OPTICAL CENTER

of a lens is NOT deviated.

To avoid UNWANTED PRISM, theoptician needs to measure the patientsinterpupillary distance (IPD or just PD)accurately ANDthe lab needs to makethe specs so that the optical centersoptical centers(OCs) match the patients PD.(OCs) match the patients PD.



In a perfect world, all patients eyes would lineup perfectly (heterophoria) and no spectacleswould ever need prism in them.

Continuing in our perfect world: Every opticianwould measure every patients PD perfectly.

Still in our perfect world: Every lab would makethe optical centers of every pair of specs line up

with the patients PD so the patient was alwayslooking through the optical centers of the lenses.

Finally, the perfect worldwould have allglasses fit and adjusted to the patient perfectly.


SNAP OUT OF IT!

Perfect World doesnt exist

Patients eyes are not always perfectly aligned(i.e., patient has heterotropia), so patients needprism to see clearly and without diplopia.

Some opticians arent perfect and mismeasurepatient PDs.

Some labs fail to make specs correctly, to thespecifications given them.

Some opticians do a poor job of adjusting specsto sit properly on the patient.


What kind ofUNWANTED

prism isbeing

inducedhere?

(Base IN or

Base OUT?)

BASE OUT (each eye)


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What kind of

UNWANTEDprism is beinginduced here?

(Base IN or BaseOUT?)

BASE IN (each eye)


What kind ofUNWANTED prismis being inducedhere?

(Base IN or BaseOUT?)

BOTH!BOTH!

Base IN-OS Base OUT-OD

In summary, you learned how to

Neutralize a bifocal (multifocal) lens

Measure segment heights

Mark a progressive add lens (PAL)

Identify wanted and unwanted prism

THANK YOU! If you like what was presented today, please

say so on your evaluation cards!

If you DID NOT like what you saw today,please tell us personally so we can hear whatwe could do better in the future.

Sincerely,

M. Patrick Coleman and MSgt Brent Sonnier

928_progressive_IPD

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