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.)
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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Marking Progressive Lenses (cont.)
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)
Identifying Wanted & Unwanted Prism (cont.)
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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Identifying Wanted & Unwanted Prism (cont.)
A prism looks like a triangle. It has a BASE & an APEX.
Its power is measured in DIOPTERS.
Identifying Wanted & Unwanted Prism (cont.)
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
Identifying Wanted & Unwanted Prism (cont.)
All spectacles lenses can be thought of as simplytwo prisms put together
Identifying Wanted & Unwanted Prism (cont.)
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Identifying Wanted & Unwanted Prism (cont.)
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.
Identifying Wanted & Unwanted Prism (cont.)
Identifying Wanted & Unwanted Prism (cont.)
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.
Identifying Wanted & Unwanted Prism (cont.)
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.
Identifying Wanted & Unwanted Prism (cont.)
What kind ofUNWANTED
prism isbeing
inducedhere?
(Base IN or
Base OUT?)
BASE OUT (each eye)
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Identifying Wanted & Unwanted Prism (cont.)
What kind of
UNWANTEDprism is beinginduced here?
(Base IN or BaseOUT?)
BASE IN (each eye)
Identifying Wanted & Unwanted Prism (cont.)
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