System for Ophthalmic Dispensing 3rd Edition_Brooks, Borish_2006

7/24/2019 System for Ophthalmic Dispensing 3rd Edition_Brooks, Borish_2006

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11830 Westline Industrial DriveSt. Louis, Missouri 63146

SYSTEM FOR OPHTHALM IC DISPENSING, THIRD EDITION ISBN-13: 978-0-7506-7480-5 ISBN-10: 0-7506-7480-6

Copyright 2007 by Butterworth-Heinemann, an imprint of Elsevier Inc.

All rights reserved. No part of this publication may be reproduced or transmitted in any form or byany means, electronic or mechanical, including photocopying, recording, or any information storageand retrieval system, without permission in writing from the publisher.

Some material was previously published.

Permissions may be sought directly from Elseviers Health Sciences Rights Department inPhiladelphia, PA, USA: phone: (+1) 215 239 3804, fax: (+1) 215 239 3805, e-mail:[email protected]. You may also complete your request on-line via the Elsevierhomepage (http://www.elsevier.com), by selecting Customer Support and then ObtainingPermissions.

Notice

Neither the Publisher nor the Authors assume any responsibility for any loss or injury and/ordamage to persons or property arising out of or related to any use of the material contained inthis book. It is the responsibility of the treating practitioner, relying on independent expertise andknowledge of the patient, to determine the best treatment and method of application for thepatient. The Publisher

Previous edition copyrighted 1996

Publishing Direc tor: Linda DuncanSenior Editor:Kathy FalkSenior Developmental Editor: Christie M. Hart

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ISBN-13: 978-0-7506-7480-5ISBN-10: 0-7506-7480-6


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Acknowledgments

For help in preparing the first edition, the authors

would especially like to thank Jacque Kubley forthe original photography and many of the illustra-

tions; Sandra Corns Pickel and Sue Howard for servingas models; and Dr. Linda Dejmek, Kyu-Sun Rhee,Dennis Conway, and Steve Weiss for the artwork andillustrations. For all the help received for the first edition,

we continue to be very grateful.For the second edition, again thanks to Jacque Kubley

for his continued assistance in photography and a numberof the graphics. In the second and now the third edition,thanks to Glenn Herringshaw, who manages IndianaUniversitys optical laboratory, for many helpful ideasand suggestions; and to Glenn and Regina Herringshawfor serving as models for a number of the photographs.

Also thanks to Pam Gondry and Dr. Eric Reinhard forjoining in the modeling team for the third edition. A

specific word of appreciation goes to Robert Woyton ofHilco for reviewing the chapter on repairs and supplyinga number of photographs for both the second and thirdeditions.

Thanks to Ric Cradick of IU Photographic Servicesfor taking the multitude of new color photos for thethird edition. His professional expertise is muchappreciated.

To our students, we owe a debt of gratitude. Theysuffered through preliminary manuscripts, yet wereexceedingly helpful in pointing out omissions, making

valuable suggestions, and asking just the right questions.Finally, special thanks to our many friends within the

profession for offering suggestions and supplying ideasfor improving the text. Without your advice and theinformation you provided, it would have been impossibleto complete the task.

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Frame Types and Parts

The purpose of this chapter is to acquaint thereader with the basic terminology used ineyewear. This knowledge is essential to avoid

misunderstanding the terms used later in the text todescribe in detail the actual dispensing procedures.

BASIC PARTS

The frame is that portion of the spectacles that holds thelenses containing the ophthalmic prescription in theirproper position in front of the eyes.

A frame generally consists of thefront, which in oneform or another contains the lenses, and the temples,

which attach to the front and hook over the ears to helphold the spectacles in place. Frames occasionally do nothave temples and are instead held in place by pressureon the sides of the nose (pince-nez), by attachment toanother frame (clip-ons), or by being held in the hand(lorgnettes).

Frame Fronts

That area of the frame front between the lenses that restson the nose is the bridge. The rim going around thelenses is known as the eyewireor rim. The outer areas ofthe frame front, to the extreme left and right where thetemples attach, are known as the endpieces. A few plasticframes may still have a metal shieldon the front of theendpiece to which rivets are attached to hold the hingein place (Figure 1-1).

The hingeshold the temples to the front, and consistof an odd number of interfitting barrels, the total numberbeing three, five, or seven. Hinges may vary in construc-tion, but for simplicity are usually classified by the totalnumber of barrels they have when assembled, such as athree-barrel hinge.

Some frames have nose pads, which are plastic piecesthat rest on the nose to support the frame. These maybe directly attached to the frame or to connecting metalpieces known asguard armsor pad arms.

Temples

The portion of the temple that is nearest its attachmentto the front is known as the butt portionor butt end. Theplace on the temple where it first bends down to go overthe ear is called the bend. The portion of the temple

between the butt end and the bend is called theshankorshaft, and that portion beyond the bend and behind theear is referred to as the earpiece, bent-down portion, or curl(Figure 1-2).

CONSTRUCTION

Frames

Frames without an eyewire going completely around thelens are called mountings. Lenses are inserted intoframes, but mounted into mountings. Frames them-selves can be classified in a simplified manner by one ofthe following categories of frames or mountings.

PlasticPlastic framesare made of some type of plastic material.Plastic frames were occasionally referred to as shellframes, dating back to the time when eyeglass frames

were made of tortoise shell. This term has fallen intodisuse. Another general term that many still use forcertain plastic frames is zyl, since at one time zylonite(cellulose nitrate) was a commonly used material. Zyloniteis highly flammable and no longer used for spectacleframes. The name zyl continues to be used, but usuallyrefers to the most commonly used plastic material-cellulose acetate. Now, with the emergence of many newmaterials, either the exact name of the plastic materialis used or the frame is simply referred to as plastic(Figure 1-3).

MetalMetal framesare those made of all metal parts, except forthe nose pads and the posterior temple sections, whichare plastic covered. The eyewire runs completely aroundthe lens (Figure 1-4).

Nylon cord framesNylon cord frames,sometimes calledstring mounted framesor nylon supras hold the lenses in place by means of anylon cord that fits around the edge of the lens. Thisgives the glasses the appearance of being rimless. Usuallythe top of the lens is fitted into the upper rim of theframes. The rest of the lens has a small groove cut intoan otherwise flat edge (Figure 1-5).

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C H A P T E R 1


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4 Ophthalmic Dispensing P A R T O N E

frame with plastic eye-wires and metal bridge andtemples.

Half-eyeHalf-eyesare frames made especially for those who needa reading correction but no correction for distance. Theyare constructed to sit lower on the nose than normal,and are only half as high as normal glasses. This allowsthe wearer to look over the top of the glasses. They maybe of plastic, metal, or even nylon cord construction(Figure 1-7). Less common are half-eyes for distant

vision, which allow the wearer to look under the lensesfor reading.

Rimless, Semirimless, and NumontRimless mountingshold the lenses in place by some methodother than eyewires or nylon cords. Often screws areused, but cement, clamps, and plastic posts have beenused. Most rimless mountings have two areas of attach-ment per lens, one nasally and one temporally (Figure1-8). Rimless mountings are sometimes referred to as3-piece mountings.

Semirimless mountingsare similar to the rimless exceptfor a metal reinforcing arm, which follows the upperposterior surface of the lens and joins the centerpieceofthe frame to the endpiece. The centerpiece of a mount-ing consists of bridge, pad arms, and pads (Figure 1-9).

Numont mountings hold the lenses in place only attheir nasal edge. They are seldom seen today. The lensesare attached at the bridge area and the temples areattached to a metal arm that extends along the posterior

Endpiece

Eyewire or rim

Nosepad

Bridge Shield

Figure 1-1. The frame front.

Shaft (shank)

Bend

D.J.C

Earpiece

Butt portion

Dowelhole

Shield

Figure 1-2. Parts of a temple.

Figure 1-3. An example of a plastic f rame.

Figure 1-4. One version of a metal frame.

CombinationCombination framesare commonly frames having a metalchassisand plastic top rims and temples (Figure 1-6). Thechassis includes the eyewire and center or bridge section.

Although this is the most common construction, techni-cally any frame with a combination of metal and plasticcould be included in this category, as in the case of a


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C H A P T E R 1 Frame Types and Parts 5

Figure 1-5. A nylon cord frame or string mount holds the lens in place with a cord thatfits around the edge of the grooved lens.

Figure 1-6. Examples of combination frames.

Figure 1-7. Half-eye frames in use. Half-eyes are made especially for those who need areading correction but no correction for distance vision.

Figure 1-8. An example of a rimless mounting. The central area of the frame is not connectedto the endpieces. The only connecting points are the lenses themselves.


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surface temporally. Thus there is only one point ofattachment per lens (Figure 1-10).

Currently most dispensers refer to any of these threevariations of a rimless mounting as rimless. They donot differentiate between the three.

Other MountingsBalgrip mountings secure the lens in place with clipsattached to a bar of tensile steel that fits into a nasal anda temporal notch on each side of the lens. The lens canbe easily removed by pulling the clips back from the lens.For this reason, this type of mounting can be used withmore than one pair of lenses for the same frame. Sun-lenses, special purpose lenses, or tinted lenses could thenbe used interchangeably with the patients regular lenses(Figure 1-11). Notches are now more often used in com-bination with drilled holes in rimless mountings to lendstability to the mounting.

Bridge Area

The bridge area of a frame can be constructed of eitherplastic or metal. Because of the variety of nose shapes,

there is also quite an assortment of bridge constructionsin both materials.

Plastic BridgesThe bridge area of a plast ic frame is preformed and sitsdirectly on the bridge of the nose. It is important, then,in picking out a plastic frame that the frame fit the nose

well, since adjustments to this part of the frame areextremely difficult. Bridge adjustments for certain plas-tics, such as nylon, carbon fiber and polyamide, are notpossible.

Thesaddle bridge is shaped like a saddle in a smoothcurve and follows the bridge of the nose (Figure 1-12).

This spreads the weight of the frame evenly over thesides and crest of the nose.

Figure 1-9. A semirimless mounting has a bar behind the topof the lens connecting the endpieces to the bridge area.

Figure 1-10. A Numont mounting has only one nasal pointof attachment per lens.

A

BFigure 1-11. A balgrip mounting. In this form of rimlessmounting, the slotted lenses (A) are held in place with clips(B).


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In the modified saddle, the bridge area looks much thesame as the saddle bridge does when viewed from thefront. The difference is that there are nose pads that arepart of the back of the bridge. These pads help to carrysome of the weight of the frame (Figure 1-13).

The keyhole bridge is shaped like an old-fashionedkeyhole. At the top, the bridge flares out slightly. Thebridge rests on the sides of the nose, but not on the crest(Figure 1-14).

Metal BridgesThe bridge commonly used in metal frames is the padbridge(see Figure 1-8). In the pad bridge, nose pads areattached to the frame by metal pad arms. In this case,the pads alone support the weight of the glasses.

When a metal frame is equipped with a clear plasticsaddle-type bridge, the bridge type is referred to as acomfort bridge.

Metal and rimless frames were, and sometimes stillare, constructed with a metal saddle bridge(Figure 1-15)and enjoyed widespread use for a period of history. Itmay yet appear exactly as before or decoratively in con-

junction with nosepads.With rimless mountings, the crestof the bridge does

not include the pads or straps, but is the center mostarea.

Endpiece Construction

Endpiece construction, like the bridge area construc-tion, can be of either plastic or metal.

Plastic Endpieces ConstructionThere are three general types of endpiece constructionin plastic frames (Figure 1-16). The most common

Figure 1-12. The saddle bridge closely follows the contour ofthe nose, evenly spreading the weight of the frame.

Figure 1-13. The modified saddle bridge has fixed nose padsattached at the back to increase the weight-bearing area of theframe.

Figure 1-14. Besides having an identifying shape, the keyholebridge supports the frame weight upon pads.

Figure 1-15. Metal saddle bridges were originally designed torest directly on the crest of the nose. They may still be used

as originally designed shown in the frame pictured. Often ametal saddle bridge is just for decorative purposes and is usedin conjunction with nosepads.

*Historically the metal saddle bridge was called a W bridge.


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endpiece construction is the butttype, in which the frontis straight and the temple butt is flat, and both meet ata 90-degree angle. The mitreendpiece causes the framefront contact area and temple butt to meet at a 45-degreeangle. In the turn-back type, the frame front bendsaround and meets the temple end to end.

Metal Endpiece ConstructionThe traditional metal endpiece has a construction similarto the turn-back endpiece of the plastic frame (Figure1-17). There are now a wide variety of metal endpiecedesigns.

Endpieces are also noticeable by their absence. Insteadof an endpiece, some frame fronts and temples are madeas one continuous piece(Figure 1-18).

Temple Construction

Temples also vary greatly in their construction. Ingeneral, there are five major categories (Figure 1-19).1. Skull templesbend down behind the ear and follow

the contour of the skull, resting evenly against it.The bent-down portion is narrower at the top of theear and widens toward the end.

A

B

C

Figure 1-16. Endpieces of plastic frames classified as mitre(A), butt (B), and turn-back (C).

Figure 1-17. This traditional metal endpiece has a turn-backdesign.

Figure 1-18. Some metal endpieces are not really endpiecesat all. The endpiece and temple are one continuous piece of

material as in this wrap endpiece design.


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2. Library templesusually begin with an average widthat the butt and increase in width posteriorly. Theyare practically straight and hold the glasses onprimarily by pressure against the side of the head.

They are also known asstraight-back temples.3. Convertible templeswere originally designed so they

could be bent down to take on the form of skulltemples, and converted from the straight-back tothe skull design. Because this temple is versatile andcan be made to fit people with a variety of templelength requirements, it is commonly used. However,it now comes already bent down for a certain templefit. If the bend is in the wrong location, the templemay be easily straightened out and then re-bent tofit the wearer.

4. Riding bow templescurve around the ear, followingthe crotch of the ear where the ear and the headmeet and extend to the level of the earlobe. Theyare sometimes used in childrens and safety frames.

5. Comfort cable templesare shaped the same as ridingbow temples, but are of metal construction with thecurl, or behind the ear portion, constructed from aflexible coiled cable.

Classic Rimless Fronts

The centerpiece of a rimless front consists of the bridge,pad arms, and pads. These parts are the same as for metal

frames. Rimless construction varies considerably. Theclassic rimless point of lens attachment contains astraporstraps. This is the part of the mounting that contactsthe front and back surfaces and the edge of the lens,holding the lens in place. The traditional strap consistsof the shoe and the ear.

Theshoe, also known as theshoulderor collar, contactsthe edge of the lens, bracing it and keeping it fromrocking back and forth in its mounting. On some tradi-tional mountings, there is a small metal springbetweenthe shoe and the lens, which helps keep the lens tight inthe mounting.

The ear, or tongue, is that portion of the strap thatextends from the shoe, contacting the surface of the lens.

There are sometimes two ears per strap, one on each lenssurface, with a screw passing through both ears and thelens to hold the lens in place. The term straps is some-times used to refer only to the ears (Figure 1-20).

The arm is that part of a semirimlessmounting thatextends posteriorly along the top edge of the lens (seeFigure 1-9). The arm is not to be confused with the padarm, which is part of the nose pad assembly. This arm issometimes referred to as a baror brow-bar.

The endpieces of rimless fronts are the same as thoselisted for metal frames. In addition, rimless endpiecesalso have straps to hold the lenses, as well as hinges fortemples.

Coloration

Plastic frames may be partially classified by coloration.Asolidframe is all one color. A vertically gradientframeis darker all the way across the top, including the bridge,and is lighter across the bottom. A horizontally gradientframe is darker at the temporal portions and lightenstoward the central area. Clear bridge frames somewhatresemble the horizontal gradient, but are dark at the top,except for the bridge area. The bridge, along with thelower half of the frame, is clear plastic. The multitudeof color combinations available now makes categoriza-tion beyond this difficult.

FRAME MATERIALS

Plastic Frame Materials

The first classification of a frame is by the material usedin its construction: either plastic or metal. Several typesof both are used to make frames.

The first plastics used for spectacle frames were madefrom bakeliteandgalalith.1These did not perform well incold weather because of their brittleness. Later cellulosenitrate (zylonite)was widely used. Cellulose nitrate acceptsa good polish, but is flammable if brought to a suffi-ciently high temperature. Because of the danger posed,cellulose nitrate has been banned by the FDA and is nolonger used for spectacle frames. However, because thesezylonite frames were the only plastic frames commonlyused for a period of time, plastic frames were known as

A

B

C

D

E

Figure 1-19. Categories of temples are:A,Skull; B,Library;C,Convertible; D,Riding bow (in plastic); E,Comfort cable(in metal).


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the type of sports glasses that are held in place withelastic straps (Figure 1-22) or for shield types of glassesthat may be used either alone or worn over conventionalglasses (Figure 1-23).

KevlarKevlar is a material that is also mixed with nylon. It, too,is a strong, lightweight ophthalmic frame material.Kevlar will remain stable over a large temperature range,but is difficult to adjust. Although it becomes pliable

with heat, it will not shrink or stretch.

RubberSome sports eyewear and sunglass frames may be madefrom a combination of nylon and rubber. As would beexpected, these frames are flexible and will return totheir original shape, but are not adjustable.4

Combinations of Plastic MaterialsThere are numerous possible combinations of plasticmaterials. These include materials sometimes called

memory plastics. Memory plastics are tough and flexible.They can be bent or twisted and still return to theiroriginal shape.

Not all composite plastic materials are memory plas-tics. Other composite plastics combine various materialsto produce frames and frame parts for specific needs andpurposes.

Metal Frame Materials

In the past, gold-containing alloys were the more pre-dominant metals used for spectacle frames. (See Chapter2, Gold Classifications for Metal Frames with SubstantialGold Content.)Today few frames contain any gold.

Great progress has been made in metal frames becauseof the electrolytic treatment techniques, which allow forcorrosion resistance and finished beauty. Any nostalgiaover the disappearance of gold alloy frames from themarketplace should be dispelled by the beauty and ser-

viceability of the product that has taken its place.It is also common for frames to be made with more

than one material. The temples may be from one mate-rial for flexibility, the frame front from another, and theconnecting pieces something different still.

Nickel-Based MaterialsNickel is a material that is often used for eyeglass frames.It is strong and malleable. The main disadvantage is thenumber of people who may have an allergic reaction tonickel. It is reported that 10% of the population may beallergic to nickel.5Fortunately high-quality ophthalmicframes are coated with a protective material that bothprevents corrosion and keeps the metal from coming indirect contact with the skin while the coating remainson the frame.

Pure Nickel. Nickel resists corrosion. Because ofmalleability, pure nickel frames are easily adjusted.Nickels characteristic of accepting color well makesthese frames versatile.

Figure 1-22. Polycarbonate sports frames can be ordered from the manufacturer with planolenses already in place. They can also be ordered without lenses for prescription use.

Figure 1-23. Safety frames with plano lenses can be moldedas one unit. In the sample shown, both the frames and lenses

are molded together from polycarbonate material.


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Nickel Silvers. Nickel silvers contain more than 50%copper, 25% nickel, and the rest zinc. But nickel silvercontains no silver. Copper gives the material its pliabil-ity, zinc adds strength, and nickel gives the alloy a whitishappearance. When the nickel content of nickel silverexceeds 12%, the copper color no longer shows through.1

Another name for nickel silver is German silver.Monel Metal. Monel is whitish in color, is pliable for

good adjustment, resists corrosion, and accepts a highpolish. It is made from nickel, copper, iron, and tracesof other elements. The largest component of the material(63% to 70%) is nickel. The second largest componentis copper. Iron constitutes only 2.5%, and there aretraces of silicium, carbon, and sulfur.1 Monel is usedquite often as a frame material.

AluminumAluminum is both strong and extremely lightweight. Itcan be finished in a wide variety of colors and does notcorrode. Aluminum does not solder or weld well, so mustbe made such that its parts are assembled with screws orrivets.6It holds the adjustment well, but has no flexibility.If it bends, it stays that way.

Stainless SteelIn the nineteenth century, some frames were made fromregular (nonstainless) steel material. Stainless steel wasdeveloped in the early 20th century. It is made mainlyfrom iron and chrome and is highly resistant to corro-sion. Stainless steel is strong. When made very thin, ithas an element of springiness and flexibility that makesit well suited for temples. Yet that very springiness meansthat adjustments are difficult and often do not hold.7Stainless steel is one of the more nonallergenicmaterials.

TitaniumTitanium is a versatile and abundant material that hasbecome increasingly common for use in ophthalmicframes. The advantages include the following:Titanium is extremely l ight in weight. When

compared with conventional metal frame materials,titanium is 48% lighter.8

Titanium is very strong, which allows t itaniumframes to be designed exceedingly thin. Thinnessalso contributes to still more weight reduction.

Titanium is very corrosion resistant. This makestitanium an excellent choice for people in hotclimates or those working in conditions where they

would be perspiring a great deal.Titanium is hypoallergenic. It should be noted that

titanium is often used in combination with othermetals. If the wearer is allergic to another of themetals in the alloy, then, unless the frame isappropriately coated, allergic reactions could stilloccur. But when titanium is not mixed with othermetals, it is the metal of choice for those with skin

allergies related to frame wear. This makes titaniuma very attractive frame material for those with skinallergies.

When used in combination with other metals,titanium allows frames to be made so that they are

very flexible. It should be noted that some framesuse titanium in combination with nickel to increaseflexibility. Without an appropriate coating on theframe, this would increase the likelihood of anallergic response for some.The disadvantagesof titanium are fewer. These include

the following:Titanium is hard to solder or weld. Because the manufacturing process is more

demanding, titanium is more expensive thanconventional materials.Titanium Marking Guidelines and Classifications.

The Vision Council of America (VCA) has establishedvoluntary marking guidelines for frames containing tita-nium. The reason for these guidelines was to end someof the confusion that arises when frames are labeledtitanium but are actually only part titaniumor do notcontain titanium at all.9 Because these are voluntaryguidelines, this means that there may still be some con-fusion in marking. However, if frames are markedaccording to VCA standards, then the buyer shouldknow what that particular frame contains. To be certi-fied, the titanium content of the frame must be tested byan independent accredited laboratory. Here are theguidelines:10

Certified 100% TitaniumAll major components ofthe frame are at least 90% titanium by weight and,to assure there will be no problems with wearerallergy, the frame must not contain any nickel(Figure 1-24,A).

Certified Beta TitaniumAll major components ofthe frame are at least 70% titanium by weight, andthere must be no nickel content (Figure 1-24, B).

TItaniumVision Council

of America

A

Certified 100%

B

TItaniumVision Council

of America

Certified beta

Figure 1-24. The Vision Counci l of America marking guide-lines for titanium uses a symbol that would normally appearon the demonstration lens of the display frame. A,Certified100% Titanium means 90% titanium and there is no nickelcontained in the frame. B, Certified Beta Titanium means70% minimum titanium with no nickel content.


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Not included in the Vision Council of American clas-sification is what has been called combination titaniumaname applied to frames with titanium for the major partsof the frame and trim pieces made from other metals.8

The name nickel titaniumorshape memory alloy (SMA)isapplied to a titanium alloy made with 40% to 50% tita-nium and the rest nickel.11 Sometimes simply calledmemory metal,12 this material is extremely flexible andreturns to its original shape after being twisted or flexed.(It should be noted that there will be other types of metalframe materials that will also function like a memorymetal.)

BronzeBronze is a metal alloy traditionally made from copperand tin. It is suited for spectacle frames because it is cor-rosion-resistant, lightweight, and takes color well.

MagnesiumMagnesium is even lighter in weight than titanium.Frames made from magnesium are extremely lightweightand exceptionally durable. The exterior of the frame isnormally sealed because of the corrosiveness of raw mag-nesium. Magnesium is also used as part of an alloy incombination with other metals.

Other Materials and AlloysThere are other materials that are also suitable forframes, including cobalt, palladium, ruthenium, andberyllium.

As would be expected, there are many different pos-sible combinations of the previously listed metals thatmay be combined to optimize certain characteristics.Some have trade names applied especially for a particu-lar combination used by a given frame manufacturer.One, for example, called FX9 is a combination of copper,manganese, tin, and aluminum engineered to yield ahypoallergenic, lightweight, and malleable material.13

Another, referred to as Genium, combines 12% carbon,17.5% to 20% manganese, 1% silicone, 17.5% to 20%chrome, and 58.9% to 63.9% steel. These materials arecombined to create a hypoallergenic frame that is thin,strong, lightweight, flexible, and durable.14As framedesigns change, metal alloy combinations will vary tomeet these new design demands.

ALLERGIC REACTIONS TOFRAME MATERIALS

As previously noted, most frame manufacturers will usea coating on their plastic frames to protect the framesand also to reduce any possibility of allergic reactions.However, sometimes this is not enough.

To reduce the possibility of a reaction for people whohave a history of skin reactions to wearing frames, useframe materials that are known to be hypoallergenic.Here are some that are reported to be hypoallergenic:

Optyl material Polyamide/CopolyamideTitanium Stainless steel

If a person is already having a reaction to their frame,here are some things that may be done to the frame toreduce allergic reactions: Have a clear coat finish applied to a frame.

Companies that specialize in frame repairs may offerthis service. (Incidentally, some dispensers have triedto just coat the inside of the temples with clear nailpolish to solve the problem. Unfortunately, this doesnot work for very long.)

Use ultrathin, clear heat-shrink tubing over thetemples. Optical shrink tubing is available fromoptical suppliers of spare pairs, pliers, andaccessories.If a person has an allergic reaction to nosepads, there

are replacement pads available that will eliminate theproblem. These pads are: Gold-plated metal nosepadsTitanium nosepads Crystal nosepads

(See also the section in Chapter 10 on HypoallergenicNosepad Materials.)

For allergic reactions to metal cable temples, use atemple cover to cover the temple. Temple covers comein plastic, vinyl, and silicone materials. There is alsoheat shrink tubing sold for this purpose, which report-edly takes care of eliminating allergic reactions. (Formore on this see the section in Chapter 10 on AddingCovers to Cable Temple Earpieces.)

An additional note on allergies: There is a liquid lensliner sometimes used in the groove of a frame to makea loose lens more secure. This material contains latexand should not be used on frames whose wearers havelatex allergies.

REFERENCES

1. Ophthalmic optics files: 8. Spectacle Frames, Paris,undated, Essilor International.

2. Todays frame material for tomorrow, Munich, Germany,undated, Optyl Holding GmbH & Co.

3. August EC: Professional selling skills and frame materials,Eye Quest Magazine, 2:40, 42, 1992.

4. Bruneni JL: Perspective on lenses 1995, Merrifield, Va,Optical Laboratories Association.

5. Parker L: Titanium tacticspart 2: translating titaniuminto sales, Eyewear, 1999.

6. Barnett D: Whats in a frame? Eyecare Business, Septem-ber, p.76, 1988.

7. DiSanto M: Rimless eyewear: making the right choice,20/20, New York, NY, 2004, Jobson Publishing.

8. Szczerbiak M: The ABCs of titanium frames, Visioncare-products.com, vol 2, no 1, January/February 2002.

9. OMA debuts titanium guidel ine, Eyecare Business,August , p. 22, 1999.


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10. Vision Council of America, Titanium marking guidelines,http://www.visionsite.org/s_vision/sec.asp?TRACKID =&CID=266&DID=397,February 2006.

11. Hohnstine, Nicola, Spina: Make it a lite...a titanium lite,20/20 Online, 30:11, 2003.

12. What are the different frame materials? Essilor website:Http://www.essilorha.com/frames.htm, excerpted from

OLA Perspective on Lenses, Optical Laboratory Asso-ciation, 1997.

13. OKeefe J: Make mine metal, Visioncareproducts.com, vol4, April 2004.

14. OKeefe J: Frame materials go beyond zyl and monel,Visioncareproducts.com, vol 3, May 2003.

Proficiency Test

(Answers can be found in the back of the book.)

Match the name of the frame with its description:

1. ____ Commonly has a metalchassis and plastic top rims.

2. ____ Has two holes per lensand a metal reinforcing armthat follows the upperposterior surface of the lens.

3. ____ Holds the lenses in placeonly at their nasal edge.

a. Balgripb. Half-eyesc. Combinationd. Semirimlesse. Numont

Match the name of the frame with its description:

4. ____ Made especially for thoseneeding a reading correction

but no distance correction

5. ____ Secures the lenses in placewith clips attached to a bar oftensile steel that fits into a sloton each side of the lenses

6. ____ Secures the lenses in placeby means of a small string thatgoes around the lenses

a. balgripb. half-eyes

c. semirimlessd. nylon cord

Match the following terms:

7. ____ Optyl

8. ____ lorgnettes 9.____ aluminum

10. ____ half-eyes

11.____ shell

12. ____ convertible

13.____ nylon cord

14.____ Numont

15. ____ earpiece

a. hand-heldb. zyl

c. readingd. curle. only nasalf. anodizedg. straight backh. has memoryi. string mount

16. Which type of temple curves around the ear

following the crotch of the ear where ear and headmeet, extending to the level of the earlobe? Thistype of temple is usually plastic, and is often usedin childrens and safety frames.a. libraryb. skullc. riding bowd. convertible

17. This follows the bridge of the nose smoothly,spreading the weight of the frame and using nosepads attached to the back of the bridge.a. keyhole bridgeb. modified saddle bridgec. saddle bridged. pad bridgee. none of the above


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farthest part of the groove on the right side of the box(Figure 2-3). Do not filt the box.

In measuring a lens, the measurement begins at theapex, or point, of the bevel on the left side of the boxenclosing the lens and extends to the apex of the bevelon the right side of the box. Remember, the A dimensionis the width of the enclosing box. It is not the width ofthe lens at the middle of the shape.

Effective Diameter

The effective diameter of a lens is found by doubling thedistance from the geometric center of the lens to theapex of the lens bevel farthest from it (see Figure 2-2).

This measurement helps determine the smallest lensblank from which the lens can be cut. (See Chapter 5:Determining Lens Blank Size.)

Frame Difference

The difference between the horizontal and the verticalmeasurements is known as the frame difference and ismeasured in millimeters. The larger the difference, themore rectangular the enclosing box appears (Figure 2-4).Frame difference is sometimes referred to as lensdifference.

Distance Between Lenses (DBL) or Bridge Size

The boxing system also makes it possible to define thedistance between lenses (DBL). The DBL is the distancebetween the two boxes when both lenses are boxed offin the frame. This is usually synonymous with bridgesize, although it is important to note that manufacturersnot adhering to the boxing system may mark a bridgesize that does not correspond to the distance betweenlenses.

Bridge size or DBL is measured on the frame as thedistance from the inside nasal eyewire grooves across thebridge area at the narrowest point (Figure 2-5). Thisdistance is measured in millimeters. Naturally, twoframes having the same DBL will not necessarily fit thesame person in the same manner because of variationsin lens shapes.

Geometric Center Distance (GCD)

The distance between the two geometric centers of thelenses is known as the geometric center distance (GCD).

Datum center

Datum Line

Mid-datum

depth

Figure 2-1. In the datum system, the middatum depth maynot always be equal to the distance between the horizontaltangents. The datum eye size is the width of the lens at thelevel of the datum line. The datum system eye size and theboxing system eye size are not the same. Some measurethe eye size according to the datum system, thinking they areusing the boxing system. The two eye size measures are notthe same.

Geometricalcenter(GC)

Datum line

Effectivediameter

(ED)

(ED)

A

C

B

Seg drop

Seg height

Geometrical center distance(GCD or frame PD)

DBLor

Bridgesize

Lens size oreyesize

Figure 2-2. In the boxing system, the A dimension is the horizontal boxing width. If theframe is properly marked, the eye size will be equal to the A dimension of the frame. The Bdimension is the vertical boxing length. The C dimension is the width of the lens along thehorizontal midline. This dimension is seldom used today. The C dimension should not beconfused with the C-size of a lens. The C-size of a lens is the distance around the lens(i.e., its circumference). The dispenser uses the C-size to ensure that a lens ordered by itself(without the frame) will be exactly sized for that frame.


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C H A P T E R 2 Frame Measurements and Markings 19

Figure 2-3. To measure the horizontal dimension of a frame,the measurement begins at the inside of the groove on one sideand extends across the lens opening to the farthest part of thegroove on the other. We cannot see the inside of the groovewhen looking from the front. This means we can estimatewhere it will be and hold the ruler so that the zero point is at

the position of the left-hand side of the groove. Then we needto read the ruler at the position where the groove will be onthe right. If the opening itself is measured, then about mmper side needs to be added to the measure to allow for thedepth of the groove. This may vary somewhat, dependingupon the depth of the groove.

Frame differenceof 20

48

28

Frame differenceof 10

48

38

Figure 2-4. The difference between the horizontal and verti-

cal measurements of a frame is known as the framedifference.

It can be measured more easily as the distance from thefar left side of one lens opening to the far left side of theother (i.e., from the left side of one box to the left sideof the other box.) Or the geometric center distance canbe calculated by simply adding the eye size to the DBL.

The result is the same.

A

BFigure 2-5. A, The DBL or bridge size is measured on theframe as the distance from the inside nasal eyewire groovesacross the bridge area at its narrowest point. When measuringthe bridge size, we cannot see the inside of the groove andmust estimate its location. B,If the measurement is made fromlens opening to lens opening, then approximately mm pergroove must besubtracted from the measure depending uponthe depth of the groove.

The GCD is also known by three other names:1. Distance between centers(DBC)2. Framecenter distance3. Frame PD

The term frame PD is commonly used in dis-pensing, but has no relationship to the wearers


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interpupillary distance or distance between pupilcenters.*

Seg Height

When specifying bifocal or trifocal segment height, thereference points are given in millimeters as either (l) thedistance below or above the horizontal midline (called

seg dropor seg raise), or (2) the distance from the lowerline of the boxing system rectangle enclosing the lens

shape (calledseg height). In the actual measuring process,the level of the lower line of the box corresponds to thelowest point in the eyewire groove. This level may be dif-

ferent from the depth of the point on the lens edge founddirectly below the pupilas can be seen by looking carefullyat Figure 2-2.

TEMPLE LENGTH

Most temples are currently marked with the total, oroverall, temple length. Temple lengths are expressed inmillimeters. Temple length may be measured in one ofthe following ways.

Overall Temple Length

The overall temple length is the distance from the centerof the center barrel screw hole to the posterior end ofthe temple, measured along the center of the temple(Figure 2-6, A). Many times the center of the barrel

screw hole will match the position of the butt end of thetemple. But this is not always the case. Also, when mea-suring the overall temple length, it is necessary tomeasure around the bend and not in a straight line,unless of course the temple is straight. The easiest wayto do this is shown in Figure 2-7, Athrough D.

Comfort cable temples are measured in terms ofoverall length. The actual measurement is done bygrasping the tip and extending the temple along theruler (Figure 2-8).

Length to Bend (LTB)

An older method of measuring temple length is in termsof the length to bend (LTB). This is measured from thecenter of the barrel to the middle of the bend (Figure2-6, B). The distance from the middle of the temple bendto the end of the temple is known as the length of drop(see Figure 2-6, B).

Front to Bend (FTB)

If the endpieces wrap around in a swept-back manner,there is a distance between the plane of the frame frontand the actual beginning of the temple. In this case,the temple length could be specified as frame to bend(FTB) (Figure 2-6, C), which would be slightly longerthan LTB. This measurement method is seldom used.

FRAME MARKINGS

Most frames are marked according to size with threemeasurements: eye size, DBL, and temple length. Metalframes that are manufactured from rolled gold are alsomarked as to the amount of gold found in the frame.Rolled gold frames were used regularly a good while ago.

Any new rolled gold frames are very expensive.

Eye Size and DBL

When a frame marking such as 5020 is seen, it meansthat the eye size is 50 mm and the distance betweenlenses is 20 mm. The box between the numbers meansthat the eye size is measured according to the boxingmethod; it also serves to separate the two numbers andprevent confusion. The eye size and DBL are sometimessimply marked 50-20 or 50/20.

Location of Markings

On a plastic frame the marking may be found in any ofseveral places. It may be printed on the inside of thenosepad, or it may be found on the upper outer sectionof the eyewire. Some frames had the size printed on theback side of the endpiece, and the temple must be foldedclosed to find it. Sometimes the eye size is printed onone endpiece and the DBL on the other. As it should be,temple length is printed on the inner side of the temples.Some manufacturers put all three measurements on thetemple. This is done because most frames are sold as acomplete unit rather than a frame front with a matching

*The term Frame PD may have originated when frame size was

determined by selecting the correctly fitting bridge size, then choos-

ing an eye size so that the wearers pupils would be at the geometric

centers of the frames lens openings.

Overall temple length

Lengthofd

rop

A

Length to bend

Front to bend

B

C

Figure 2-6. A-C, Various methods used in specifying templelengths.


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C H A P T E R 2 Frame Measurements and Markings 21

A B

C DFigure 2-7. Measuring overall temple length.A, Here is a temple marked with a temple lengthof 140. We will be measuring this temple and comparing our results with what is marked.B, Begin the measurement by placing the zero on the ruler at the center of the hinge barrel,

as seen in th is measuring view. C, Looking at the temple from the side it is evident that thezero point is not at the butt end of the temple. Often times the position of the center of thebarrel and the butt end of the temple are at the same location. It is obvious from the photothat in this case they are not and the beginning point for measuring does not start at the endof the temple. D,Turn the ruler around the temple bend and note where the end of the templefalls on the ruler scale. This is the overall temple length.

Figure 2-8. The overall temple length for a cable temple is obtained by stretching the cabletemple along the ruler.


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of metal is used in combination with gold to make thegold alloy.

The karat system is used to determine the amount ofgold present. The number marked on the article is theamount of gold by weight in comparison to a total of 24units: an article marked 12k is an alloy made up of one-half gold and one-half another metal.

Fine GoldFine goldis the name used for an article that has no metalin it other than gold. The gold found in it is chemicallypure. Although this is the purest form, it is not alwaysthe most practical, as is the case in spectacle frames.Frames of fine gold would be too malleable and wouldbend and dent too easily to be practical. Using the karatsystem, fine gold is 24 karats fine, which means that by

weight, 24 parts out of 24 are gold.

Solid GoldSolid goldarticles are actually an alloy of gold and anothermetal, a mixture of gold and a base metal. Thus the termis misleading, as it does not mean all gold. The solid goldarticle is made entirely of the gold alloy. It maintains itsluster regardless of how far down it is worn throughuse.

The symbol qis used to denote a 10k solid gold bridge;the symbol to denote a 12k solid gold bridge.

Gold FilledGold-filled articles are made of a metal other than goldand then covered with a gold alloy. The term does notindicate that the article is filled with gold, but ratherthe opposite: an outer wrapper of gold alloy is filled

with a baser metal. To be classified as gold filled, aminimum of one twentieth of the articles total weightmust be gold.

Articles in this classification are marked with a frac-tion, a karat rating, and the abbreviation for gold filled.

The fraction shows what part of the total weight of thearticle is represented by the gold alloy covering. Thekarat rating shows, as always, the amount of gold by

weight in the gold alloy in comparison to a total of 24units. The GF classifies the article as gold filled. Forexample: 1/1010% of the total weight of the article is alloy. 12k12 parts out of 24 parts of the covering alloy

by weight are gold. GFThe article is classified as gold filled.

Thus the article would bear the marking of 1/10 12kGF.

A gold-filled article retains its luster until the goldcovering eventually wears through.

If a frame is made from parts having different per-cents of gold, the frame must be marked according tothe part containing the least amount of gold. If, forexample, the temples are 1/8 12k GF and the front is1/10 12k GF, the frame must be marked 1/10 12k GF.

set of temples. Unfortunately this leads to confusionwhen temples are exchanged.

On metal frames and frames with metal chassis, theeye size and DBL are usually on the inside of the bridge,although occasionally they are printed on the undersideof a top reinforcing bar, or again, on the temples.

Frame Manufacturer Name, Color, and Country

of OriginFrames should also be marked as to country of origin,manufacturer, and frame name. Many frame manufac-turers use a number rather than a name. This can beconfusing if the frame color is also specified by numberand both numbers are stamped on the frame. Consultinga frame reference catalog or database will help.

Safety Frame Markings

Frames that are suitable for use as safety glasses musthave Z87 or Z87-2 and the name or logo of themanufacturer stamped on the frame front and on bothtemples. This is as specified by the American NationalStandards Institute (ANSI) in their standard called

American National Standard Practice for Occupational andEducational Eye and Face Protection. The standard is num-bered as Z87.1. If a pair of glasses has safety lenses, butis not in a frame marked Z87 or Z87-2, the glassesare not safety glasses. (For more on safety eyewear, seeChapter 23.)

Gold Classifications for Metal Frames WithSubstantial Gold Content

Metal frames may not have any gold or any significantamount of gold in the frame. This does not imply any-thing about the quality of the frame. (See Chapter 1 formore on frame materials.)When a frame has a substan-tial gold content, numbers other than those indicatingthe size of the frame are printed on the frame to indicatethe nature of the gold content. Gold or part-gold articlescan be classified as fine gold, solid gold, gold filled, orhaving gold plating or gold flashing (Table 2-1).

The color of a frame with gold content has nothingto do with its quality. The color depends on what type

TABLE 2-1

Gold Classifications

Name Meaning

Fine gold 100% pure goldSolid gold Gold plus base metal evenly mixed

throughoutGold filled Base metal inside a solid gold coatingGold plating A base metal thinly plated with goldGold flashing A base metal with gold thinly and quickly

applied in a manner similar to that ofgold plating


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Measuring the InterpupillaryDistance

This chapter provides the methodology for mea-suring the interpupillary distance (PD). Failureto accurately determine the interpupillary dis-

tance results in a misplacement of the optical center ofthe lenses. This induces unwanted prismatic effects,requiring the wearer to turn his eyes inward, or evenoutward, to keep from experiencing double vision. Overtime, this effort causes visual discomfort and can resultin a decreased ability of the eyes to work together inbinocular vision.

DEFINITION

The anatomic PDis the distance from the center of onepupil to the center of the other pupil, measured in mil-limeters. Before ordering prescription glasses or evenbefore doing a visual examination, the distance betweenthe pupils must be determined. It can be measured in a

variety of ways.

DISTANCE PD

Binocular PDThe most common method used to measure the PD alsorequires the least amount of equipment. The techniqueuses a simple millimeter ruler, commonly referred to asaPD rule.

TechniqueWhen the PD is to be measured, the dispenser shouldbe positioned at a distance of 40 cm (16 inches) directlyin front of the subject, with his or her eyes at the same

vertical level as those of the subject. The PD rule ispositioned across the subjects nose with the measuringedge tilted back so that it rests on the most recessed partof the nose. The dispenser holds the PD rule betweenthumb and forefinger and steadies the hand by placingthe remaining three fingers against the subjects head.

The dispenser closes the right eye and sights with theleft (Figure 3-1). The subject is instructed to look atthe dispensers open eye while the dispenser lines up thezero mark of the rule with the center of the subjectspupil.

When the zero mark is lined up correctly, the dis-penser closes the left eye and opens the right. The subject

is instructed to look at the dispensers open eye. ThePD for the distance prescription is read off as thatmark falling in the center of the subjects left pupil(Figure 3-2).

The dispenser now closes the right eye and opens theleft. The subject is again instructed to look at the dis-pensers open eye. This step is primarily a recheck tomake sure the zero mark is still properly aligned. (Thistechnique is summarized in Box 3-1.)

When difficulty is experienced in determining theexact center of the pupil, the edge of the pupil may beused as a measuring point if both pupils are the samesize. Measurement is read from the left side of one pupilto the left side of the other. Measuring from the insideedge of one pupil to the inside edge of the other wouldgive an artificially low reading; from the outside edge ofone pupil to the outside edge of the other, an artificiallyhigh reading.

When a person has dark irises or unequally sizedpupils, it may be difficult to use either the center or theedge of the pupil. In this case, the dispenser may use thelimbus edgethe sharp demarcation between whitesclera and dark iris (Figure 3-3). (Because the pupil isdisplaced 0.3 mm nasal ward from the center of thelimbal ring,1 a limbal measure will be approximately0.5 mm greater than the measure found using pupilcenters.) The same rule must be applied when using thelimbus edge as when using the pupil edge: the same sidesof the limbus (both left or both right) must be used, oran extremely large error is induced.

Common Difficulties and Their SolutionsDispenser Cannot Close One Eye. Occasionally the

person doing the measuring is unable to close one eyeindependent of the other. This can be remedied byoccluding (covering) the eye with the free hand. Thepractice of holding the lid down with one finger gives anunprofessional appearance, especially when wearingglasses. Occluding the eye with the hand held flat appearsto be a natural part of the test and does not reveal apersons inability to close only one eye.

Dispenser Visually Impaired in One Eye. If the dis-penser is blind in one eye, or has visual acuity too poorto allow the ruler to be read accurately, then the tech-nique is modified. The dispenser places the good eye

25

C H A P T E R 3


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directly in front of the subjects right eye and at thenormal distance. The zero mark is lined up as usual. Thedispenser then moves sideways until the good eye ispositioned in front of the subjects left eye and the mea-surement is read. Unfortunately this method can easilylead to parallax errors. The most desirable solution forsomeone with this difficulty is to use another type ofinstrument that only requires the use of one eye.

Subject Is Strabismic. The strabismic subject, whoseeyes are in a tropic position (i.e., with one eye pointingin a different direction from the other) presents a specialproblem, since the PD rule method of measurement maythen give an artificially high or low reading. To deter-mine a true reading, simply cover the subjects eye not

being observed. This ensures that the subject is fixatingwith the eye under observation and ensures that it is notturned unless eccentric fixation is present. Even if eccen-tric fixation is present, the PD measurement is stillcorrect, since the subject never uses this eye in any otherposition relative to the dominant eye.

In some instances where one eye turns out constantly,the prescribing doctor may determine that the wearer isbetter served if the lenses are centered in front of thepupils, even for the eye that is turned. This will requirethat a separate measure be taken for each eye. Onemeasurement will then be considerably larger than theother.

Subject Is an Uncooperative Child. If the subject isyoung or uncooperative, making normal PD measure-ments impossible, the dispenser may have to take acanthus-to-canthus measurement. (The canthus is thecorner of the eye where the upper and lower lids meet.)

This is done by measuring from the outer canthus of oneeye to the inner canthus of the other eye. Unfortunately,

Figure 3-1. Position of the dispenser for beginning the PDmeasurement using just a PD ruler.

Figure 3-2. The dispenser uses his or her left eye to establish the zero point of the PD rulein the center of the pupil of the subjects right eye as shown here. The subject is looking atthe dispensers left eye. Next the subject looks at the dispensers right eye. The dispenser useshis or her right to read the pupillary distance at the center of the subjects left eye. (This isnot what is seen in this photo.)

Steps in Measuring the Binocular Distance PD

1. Dispenser posit ions at 40 cm (16 in).

2. Dispenser closes right eye, subject fixates on

dispensers left eye.

3. Dispenser lines up zero point on subjects right eye

at the pupil center, left pupillary border, or left limbus.

4. Dispenser closes left eye, opens right eye; subject

fixates right eye.

5. Dispenser reads off scale directly in line with left

pupil center, left pupillary border, or left limbus.

6. Dispenser closes right eye, opens left; subject

fixates left eye.7. Dispenser checks to make sure zero point is still

correct.

BOX 3-1


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C H A P T E R 3 Measuring the Interpupillary Distance 27

this measurement is not entirely exact, since the innercanthi of the eyes encroach farther across the sclera with

younger children.

Common Causes of ErrorsThere are several common causes of errors inherent in

using a PD rule.21. There will be an error in measurement if the

measurers PD differs significantly from thesubjects because the lines of sight are not parallel.For example, if the measurers PD is 16 mm largerthan the subjects, the reading will be 1 mm toohigh because of this parallactic error.

2. The above error will be increased if the PD rule isnot tilted on the subjects nose so that the scale is inthe most recessed area. The most recessed areacorresponds to the approximate position where thespectacles will be worn.

3. Just as error will be increased when the measurers

PD is significantly different from the subjects, theparallactic error will also be increased even more ifthe dispenser is too close to the subject. Too close iscloser than the normal 40 cm (16 inch) distance.

4. A significant error will be induced if the subject isstrabismic (one eye turns in or out) or if the subjectdoes not fixate binocularly* during the PDmeasurement.

5. An error can result if the subjects head moves.6. An error can result if the person measuring moves

his or her head.7. An error will result if the person measuring does

not close or occlude one eye at a time to ensure

sighting from directly in front of the subjects eyeunder observation.

8. The subject may not look directly at the measurerspupil during the test, as he or she should, which willresult in an error.

Monocular PD

Since faces are not always symmetrical, it is often neces-sary to specify the PD for each eye independently. Themain goal in taking the PD is to eventually place theoptical centers of the lenses directly in front of the sub-

jects eyes to prevent any undesired prismatic effect.If one eye is set closer to a persons nose than is the

other and the optical centers of the lenses are placedsymmetrically in the frames, the wearers lines of sight

will not pass through the optical centers of the lenses.The error is not too serious if the lenses are of the samepower and are not strong. If, however, one lens is verydifferent from the other, the centers must be placedaccurately to prevent unwanted binocular prismaticeffects (Figure 3-4). Monocular PDs are also important

when using aspheric lenses or high index lenses, includ-ing polycarbonate lenses. High index lenses have morechromatic aberration than crown glass or regular (CR-39) plastic lenses. The negative effect of chromatic aber-ration on vision is increased if the eye is not lookingthrough the optical center of the lens. (For more infor-mation on high index lenses, see Chapter 23. For moreinformation on aspheric lenses, see Chapter 18.)

Procedure for Measuring Monocular PDsUsing a Ruler

The monocular PD is best taken using a pupillometer.When a pupillometer is not available, monocular PDsare taken by measuring from the center of the nose tothe center of the pupils. The procedure consists of thefollowing three steps:1. Measure the binocular PD as described earlier in

the chapter. Use the center of the pupil as thereference point.

2. Before moving the ruler, note the scale reading onthe ruler at the center of the nose. This is the rightmonocular PD.

3. Subtract this reading from the binocular reading toobtain the left monocular PD.For example, the binocular PD is 66. The scale reading

at the center of the nose is 32. The monocular PD forthe right eye is then 32. To calculate the monocular PD

Figure 3-3. When the subject has dark irises, the outside edge of the l imbus may be used asthe zero reference point and the inside limbal edge of the other eye as the measuring point.

*What does not fixating binocularly mean? It means that one eye

may have a tendency to turn in or out when the subject is not con-

centrating. In simple terms, they will only be using one eye to see

instead of both eyes. When this does happen, one eye usually turns

outward somewhat and the measurement is then too large.


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for the left eye, subtract 32 from 66, to get a reading of34. The procedure is the same as in taking a binocularPD measurement, except that the two readings are inde-pendent of one another and, for purposes of measuring,the center of the pupil is always used. (There are othermethods that are considerably more dependable thanthis method in their ability to yield consistently accurateresults.)

Procedure for Measuring Monocular PDsUsing the FrameOne error inherent in using a ruler alone appears whena person has an asymmetrical nose. An asymmetricalnose often occurs when a nose has been broken. In thiscase, the frame positions itself somewhat to the left orright. For the lenses to be accurately placed, this factormust be taken into account. It is possible to use an over-head transparency marking pen and the glazed* lenses

in the sample frame. If the sample frame does not haveglazed lenses, clear tape may be placed over the lensopening of the empty frame.

The procedure for measuring monocular PDs beginsby adjusting the frame. The frame should occupy theexact position it will have with the lenses in place. Thedispenser should be at the same level as the wearer andapproximately 40 cm away. The dispenser closes theright eye. The wearer is instructed to look at the dis-pensers open left eye. Since there is no ruler used, thedispenser uses an overhead transparency marking penand marks a cross on the right glazed lens. If there is nolens in the frame, the clear tape placed over the lensopening is marked instead, directly over the center ofthe wearers right pupil (Figure 3-5).

Next the dispenser closes the left eye and opens theright eye. The subject is instructed to look at the dis-pensers open eye. The dispenser then marks a cross onthe lens or tape directly over the left pupil center.

Because of the movement involved in marking pupilcenters and the ease with which unintentional head

NOSE

32 32

64

6434302 mm 2 mm

Figure 3-4. Here the PD has been measured binocularly as shown on the top measurements.However, the wearer has very dif ferent monocular PDs. Even though the distance PD is 64,the monocular PDs are not 32 and 32. Instead they are 30 and 34. When the lenses are madeas if the wearer had 32/32 monocular PDs, in this case there will be unintended Base Outprism caused by the misplaced lenses.

Figure 3-5. To measure monocular PDs using a marking pen and a frame with lenses in theframe, the same procedure is followed as would be used with a ruler. It is essential that the

wearer be looking at the dispensers eye that is direct ly in f ront of the eye being measured.In other words, to mark the location of the wearers right pupil center, the wearer looks atthe dispensers open, left eye. (The dispensers right eye is closed.) To mark the location ofthe wearers left pupil center, the wearer looks at the dispensers open right eye. (The dis-pensers left eye is closed.)

*Glazed lenses are also called coquilles, dummy lenses, or

demo lenses.


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movement can occur, it is important that these markingsbe carefully rechecked.

When the dispenser is confident that pupil centers areaccurately marked, the frames are removed and the dis-tances from the center of the bridge to the center of eachcross are measured and recorded. (These steps are sum-marized in Box 3-2.)

PD Measuring Instruments

The interpupillary distance is most easily measured byusing an instrument especially designed for this purpose.Readings taken using this instrument are not nearly assubject to parallax errors as those taken using a PD rule.Such a device also solves the problems caused when theperson doing the measuring is monocular or is amblyo-pic in one eye.

Most instruments have an occlusion system, whichallows for individual monocular measurements, witheach eye fixating alternately in cases of strabismus.

A well-designed PD measuring instrument shouldrest against the bridge of the subjects nose exactly as aframe would. This most accurately approximates the waythe glasses will position themselves. It should also posi-tion the measuring plane at the approximate spectacleplane.

The subject will see a ring of white or colored lightaround a dark, central dot within the instrument. Thedispenser will see the subjects eye and a scale appearingon it, from which a direct measure is read. Alternately, insome instruments, a split image of the pupil may be seen.

Instruments Using Corneal ReflexesAlthough some instruments use a method of taking thePD where the reference point is the geometric center ofthe pupil itself, the popular alternate corneal-reflexmethod is used in instruments such as the Essilor pupil-lometer (Figure 3-6) or the Topcon PD-5, PD Meter.

The instruments are supported by means of pads posi-tioned so as to cause the instrument to rest on the nose

where the average frame would rest. This is superior toa forehead support system used alone.

The dispenser asks the subject to hold his or her endof the pupillometer so that the pads rest on the nose(Figure 3-7). The forehead support should be against theforehead. The dispenser uses one eye to look into theinstrument. (A real advantage for dispensers with good

vision in only one eye.)An internal light produces an image by reflection on

each cornea, and the hairline within the device is moveduntil coincident with this corneal reflection (Figure 3-8).

The measurement is assumed to correspond with thesubjects line of sight, but is an objective measurementof the position of the corneal reflection rather than theposition of the line of sight. In addition to a distance PD,near PD may be measured for near points from 30 or35 cms to infinity.

The line of sight is defined as a l ine passing from thecenter of the pupil to the object of regard. This is theline that desirably passes through the optical center of

Steps in Measuring Monocular PDs Using

the Sample Frame

1. The selected frame is adjusted in exactly the same

manner as it will be when worn.

2. Dispenser positions at 40 cm from the wearer and at

the same level.

3. Dispenser opens left eye, closes right eye, and

instructs wearer to look at dispensers open (left)

eye.

4. Dispenser marks location of wearers right pupil

center on glazed lens.

5. Dispenser opens right eye, closes left eye, and

instructs wearer to look at dispensers open (right)

eye.

6. Dispenser marks location of wearers left pupil

center on glazed lens.

7. Dispenser rechecks the locations of the marked

crosses by repeating steps 3 and 5 and notes the

positions of the marked crosses.

8. If one or both crosses are wrong, the frames are

removed and the cross(es) erased using a dampcloth.

9. When crosses are accurate, monocular PDs are

measured from frame center to cross center.

BOX 3-2

Figure 3-6. The digital version of Essi lors pupillometer dis-plays monocular PDs for the right and left eyes, as well as thebinocular PD. It can be set to measure distance or near PDs.


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the lenses and is the basis upon which the measurementof interpupillary distance rests.

Corneal reflections are observed along a line whichintersects perpendicularly the center of curvature of theanterior surface of the cornea. (Technically this line isreferred to as the pupillary axis.) This line intersects theline of sight at the entrance pupil of the eye. It varies inits orientation by an angle,* which for the average eyeis approximately 1.6 degrees.1 This places the cornealreflection somewhat toward the nose. Thus a PD deter-mined on the basis of corneal reflections will vary slightlyfrom that determined by the centers of the pupils.

It is possible to use a corneal-reflection-style instru-

ment to measure a PD based on pupil center distances.To do this, the hairline within the device is moved tothe center of the pupil rather than the center of thecorneal reflection. The corneal reflection method isdefinitely the method of choice when measuring a PDfor someone with pupils dilated from a recent eyeexamination.

Using Corneal Reflections to Measure the PD withouta PupillometerIt is possible to use corneal reflections to measure inter-pupillary distance with even a PD ruler, or by using theframe with glazed lenses. Procedures need only beslightly modified. The dispenser should be positioned atthe near working distance. The dispenser holds a penlight directly below his or her eye and shines it into theeye of the subject. The subject looks either at the penlight or the dispensers eye. The reflection of the penlight on the cornea is used as the reference point insteadof the geometric center of the pupil. The sequence ofmeasurements is followed exactly as outlined in Boxes3-1 and 3-2, except that the dispenser must position thepen light directly below his or her open eye through-out the sequence.

Photographic Instruments for Measuring PDThere are instruments available for taking a wearersinterpupillary distance that make use of a photograph ofthe wearers eyes with the frame in place. The framesare adjusted as they are to be worn. The wearer fixatesa light in the instrument, and the photo is taken. PD andsegment height measurements are determined using thepicture. Up to this point, no photography-based PDmeasuring system has successfully penetrated the U.S.ophthalmic market.

NEAR PD

The near PD is required for single vision reading glassesor for multifocals.

Forsingle vision reading glasses, the lenses are set so thattheir optical centers will be in the lines of sight of theeyes when the eyes are converged for reading.

For multifocals, the distance portion is ground to cor-respond to the distance PD, while the bifocal or trifocalportion is decentered inward to be properly situated fornear vision. The near PD can be either measured orcalculated.

Measuring Near PD With a PD Rule

To measure the near PD with the PD rule, the dispenseris positioned at the subjects working distance; that is, atthe distance for which the reading portion isprescribed.

Closing his or her poorer eye, the dispenser aligns hisor her better eye directly before the subjects nose andinstructs the subject to look into that open eye.

The PD rule is lined up with the zero point corre-sponding to the center of the subjects right pupil. Itshould also be held in the same place that the subjectsnew frames will rest because this will also affect thereading.

The dispenser then notes the mark corresponding tothe center of the subjects left pupil. This is the near PD(Figure 3-9). The subject is not required to shift gaze,

Figure 3-7. To use a pupillometer, the subject (on the right)holds the pupillometer so that the pads rest on the nose in themanner of normal eyeglasses. The dispenser views the sub-

jects eyes through the instrument.

Figure 3-8. The corneal reflection as seen through Essilorspupillometer. The hair line is adjusted to the center of thecorneal reflex. (Courtesy of Essilor, Inc.)

*This angle is angle lambda, but is often commonly designated as

angle kappa.


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and the dispenser is not required to change eyes duringthe procedure. (See Box 3-3 for a summary of thistechnique.)

It should be added that it is also possible to use theedge of the pupil or the limbus for reference points intaking the near PD, as long as only the right or only theleft edges are used, and not both outer or both inneredges.

In practice, many who use a PD rule to measure thebinocular distance PD, measure the near PD at the sametime. This is done as follows:

(The first three steps are how binocular distance PDmeasures begin.)1. Dispenser is positioned at 40 cm.2. The dispenser closes his or her right eye and the

subject, using both eyes, fixates on dispensers lefteye.

3. Dispenser lines up zero point of ruler on center ofsubjects right pupil. (This next step allows for thenear PD measurement.)

3A. The dispenser looks over at the subjects left eyeand reads the scale on the ruler at the location ofthe left pupil center. This is a measure of the near

PD for the distance from the subject to thedispenser.

The dispenser now continues the steps for finding thebinocular distance PD as listed in Box 3-1.

Taking Near PD Using a Pupillometer

Usually a PD measuring instrument will allow both dis-tance and near PD to be measured. This is done throughthe use of a movable internal lens that changes the imagedistance and convergence for the subject. The near read-ings are carried out in the same manner as the distancereadings.

USING THE NEAR PD FOR BIFOCAL INSET

For the near reading area of a pair of glasses to be usedmost comfortably, it must be positioned accurately in thelens. Horizontal placement of the near segment viewingarea is determined by the near PD. (Vertical placementdepends on frame depth and the individuals visual needand will be covered extensively in Chapter 5.)

The horizontal position of bifocal segments is speci-fied as the distance from the farpoint PD that the seg-ments are set in toward the bridge. The total inset is thedifference between the distance PD and the near PD.

Because of the possibility of unequal monocular PDs,segment inset is usually specified individually for eacheye. Ordinarily segment inset is the difference betweenthe distance PD and the near PD, divided by 2:

Segment Inset

distance PD near PD=

( ) ( )2

For example, if the distance PD is 68 and the near PDis 64, then the segment (seg) inset for each eye is2 mm.

Where inequality of the monocular PDs exists, thisrule may result in errors, since both eyes may not berequired to converge equal angular amounts for nearfixation. The actual amount of error is usually so slight,however, that it is usually ignored. The exceptions wouldbe cases of very marked differences in monocular PD or

very strong lenses.If there is a large difference in monocular PDs, inset-

ting the bifocal segments accordingly may result in arather unusual-looking pair of glasses (Figure 3-10).

This effect can be made less noticeable by using a bifocalwith a wider segment.

Calculating the Near PD

There are several other factors to be considered whencalculating the near interpupillary distance, most notablythose that cause differences in segment inset.

CalculationThe most logical way to calculate the interpupillary dis-tance is to draw a triangle with the center of rotation of

0

Subject

Dispenser

61Spectacle plane

Figure 3-9. Using a PD rule, near PD may be taken with thedispenser positioned as shown. The distance between dis-penser and subject is equal to the subjects working distance.

Steps in Measuring the Near PD

1. Dispenser places his or her dominant eye in front of

subjects nose at the subjects near working

distance. This is the distance for which the near

prescription is intendednormally 40 cm (16 in).

2. Dispenser closes the nondominant eye.

3. Subject fixates dispensers open eye.

4. Dispenser places zero point of PD rule at center of

subjects right pupil.

5. Dispenser reads scale marking at center of subjects

left pupil.

BOX 3-3


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the eyes being two points of the triangle and the nearpoint of fixation being the third. A similar triangle isthen constructed by drawing a line corresponding to thespectacle plane.

By similar triangles, the monocular near PD can becalculated from the monocular distance PD (Figure3-11).

When using a prewritten prescription, the workingdistance will normally never exceed the reciprocal of thepower of the near addition. For example, a +2.00 diopternear addition will indicate a working distance no furtherthan 50 cm.

1

2 000 50 50

+= =

.. meters cms

Unless the professional situation or physical build ofthe wearer indicates otherwise, the customary near

working distance can be assumed to be 40 cm. If,however, the power of the near addition (add power) is

greater than +2.50 diopters, then the working distance

a b

Figure 3-10. If there is a large difference in monocular PDs,insetting the bifocal segments from these points may result ina rather unusual-looking pair of glasses. Using a wider segmentsize or changing to a progressive addition lens is a betterchoice.

40cm

Spectacle plane

Vertex distance

13.5mms

Center of rotation

a

b

Figure 3-11. a = monocular distance PD; b =calculated monocular near PD. The distancefrom the front surface of the cornea to the centerof rotation of the eye is normally considered tobe 13.5 mm. (The diagram is, for clar ity, notdrawn to scale.)


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will be the reciprocal of the add power. For example,a +3.00 diopter add indicates a working distance of331/3cm.

1

3 000 33

1

333

1

3+= =

..

Dmeters cms

Gerstman3 has simplified the calculation of the nearinset with a rule which he calls the three-quarter rule.

The three-quarter rule states that for every diopter ofdioptric demand, the optical center of each reading lens,or the geometric center of each bifocal addition, shouldbe inset 0.75 (three-quarters) mm. Dioptric demand isthe inverse of the reading distance in meters and is inde-pendent of the actual bifocal addition power.

Example 3-1

For a reading distance of 40 cm, and an add power of +1.00D, what is the inset per lens?

Solution

To find the answer, we first need to know the dioptric demand.

The dioptric demand is the inverse of the working distance,

not the inverse of the +1.00 add power. Therefore sincethe working distance is 40 cm or 0.40 m, the dioptr ic

demand is

1

0 402 50

..= D

Having found the dioptric demand, we can find the inset

per lens by multiplying by three-quarters, as the rule name

implies. Therefore the inset per lens is

2 503

41 9. . = mm

The three-quarter rule tends to give the appropriate inset

at all reading distances for the typical adult. Gerstmandefines the typical adult as one whose interpupillary dis-

tance is between 62 and 68 mm. For those whose PDs do

not fall within this range, it becomes necessary to refer to

an inset table (Table 3-1). This table is a quick reference for

determining the segment inset when reading distance

(working distance) and distance PD are known.

The Influence of Distance Lens Power onSegment Inset

The power of the distance prescription has an effect onbifocal inset. When a person looks at a near object, theeyes turn inward and are no longer looking throughthe optical centers of the lenses. Negative power, orminus lenses, keep the eyes from converging as much asthey normally would because of the Base In prismaticeffect at this point on the lens. Positive power, or pluslenses, cause the eyes to converge slightly more thanthey normally would because of their Base Out prismaticeffect.

For positive lenses then both the measured or theGerstman-calculated near PD would need to be reduced(i.e., the segment inset of the bifocal increased). Forminus lenses, the near PD would need to be increased(i.e., the inset of the segment reduced).

The position of the near reading area becomes moreimportant when the reading area is small. This meansthat for progressive addition lenses, the position of theintermediate and near readings areas is very important.Progressive addition lens designers are now taking dis-tance power into consideration when determining howmuch inset the near viewing area should have.

Segment Inset Formula. There have been severalfactors listed as having an effect on segment inset. These

were:

TABLE 3-1

Segment Inset as Determined by the Wearers Distance PD and Near Working Distance

Near Working Distance (cm)

100.0 50.0 40.0 33.3 25.0 20.0 16.7 14.3 12.5 11.1 10.0

50.0 0.7 1.4 1.7 2.0 2.6 3.1 3.7 4.2 4.7 5.2 5.655.0 0.8 1.5 1.8 2.2 2.8 3.4 4.0 4.6 5.1 5.6 6.1

60.0 0.8 1.6 2.0 2.4 3.1 3.8 4.4 5.0 5.6 6.1 6.765.0 0.9 1.8 2.2 2.6 3.3 4.1 4.8 5.4 6.1 6.7 7.270.0 1.0 1.9 2.3 2.8 3.6 4.4 5.1 5.8 6.5 7.2 7.875.0 1.0 2.0 2.5 3.0 3.8 4.7 5.5 6.3 7.0 7.7 8.3

1.00 2.00 2.50 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Dioptric Demand*

*The dioptric demand is the reciprocal of the working distance expressed in meters.

ForPD

(mm)


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The distance the lenses are from the eyesThe distance PDThe near working distanceThe power of the distance lens

Taking all these factors into consideration, Eller-brock4derived the following formula for segment inset.

i

P

s f

= + 1

1 1

where P is one half the distance PD, wis the distance ofthe lens from the working nearpoint, s is the distancefrom the lens to the center of rotation of the eye, and fis the focal length of the lens in the 180-degree merid-ian. All measurements are expressed in millimeters.

Example 3-2

What would the segment inset be for a person with a 70 mm

distance PD who is wearing a prescription of +6.50D?Assume they are wearing a +2.50 add, but are working at anear working distance of 20 cm. The spectacle lenses are

25 mm from the center of rotation of the eye to the back of

the lens.

Solution

We are using Ellerbrocks formula. In Ellerbrocks formula P

is half the distance PD, so

P mm= =70

235 .

The value of w is the distance from the lens to the near

working point in millimeters. This distance is given as 20 cm,

which is the same as 200 mm.The focal length of the lens is the reciprocal of the power

of the lens. This is

1

6 500 1538

153 5.

.

.

=

=

Meters

mm

Since the lens is a sphere, the power in the 180-degree

meridian is the same as the power in any other meridian.

The distance from the lens to the center of rotation of the

eye is given as 25 mm, so s =25 mm.Inserting all of this into Ellerbrocks formula results in

iP

s f

mm

=+

=+

=

11 1

35

1 2001

25

1

153 5

4 5

i

.

.

So the inset per lens for this wearing situation is 4.5 mm

per eye.

Summary of FactorsFortunately the variations in segment inset caused by allthese factors are not radically different from that foundusing the measured near PD. This assumes, of course,that the near PD is measured at the appropriate workingdistance.

Table 3-2 summarizes the effect of distance lenspower on segment inset for the normal working distance(40 cm or 16 in).5

Recommendations For Finding The Near PD

System for Ophthalmic Dispensing 3rd Edition_Brooks, Borish_2006

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