Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003

FIGURE 14-19 Cutting the nylon cord at an angle makes it easier to thread it through the holes in the frame and allows it to seat smoothly in the groove under the lens. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann [Figure 7-19].)

FIGURE 14-20 The cord is threaded into the lower hole from the inside and back into the upper hole. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann [Figure 7-20].)

threaded into the upper hole, and a length of 1.5 to 2 mm is left (Figure 14-20). For frames that will allow, an alternative method of securing the cord is to melt the end. The nylon balls up and will not slip through the hole (Figure 14-21).

4.Size the lens. The other end of the cord is slipped through the lower hole from the lens side at the nasal point of attachment. It is not threaded

Nylon cord in groove

FIGURE 14-21 With certain types of nylon cord frames an alternative method used to secure the lens in place is to melt the end of the string. This hard melted ball will not pull through the hole.

through the upper hole. With the end of the nylon cord still loose, the lens is slipped into the upper part of the frame. It is not shoved tight into the frame eyewire. The cord is threaded around the lens and the cord is pulled snug (Figure 14-22). The cord is not pulled so tight that it stretches.

5.Remove the lens. The excess end of the cord is held with the thumb so that it does not slide out, and the lens is removed. (Because the lens was not pressed up tight into the eyewire it should be able to be removed without loosening the cord and losing the point of reference for cord length.)

6.Take up the slack in the cord. Because the cord was not pulled tight around the lens, it must be pulled 1.5 to 2 mm farther through the lower hole so that the lens will be tight enough.

7.Thread the excess cord through the remaining hole. The excess cord is threaded through the upper nasal hole while the new position of the cord is maintained in the lower hole.

8.Clip the excess cord. The excess cord is clipped, which leaves 1.5 to 2 mm inside the eyewire. It should be clipped at an angle so that it lays down in the groove smoothly (Figure 14-23; nail clippers work as well as regular cutting nippers.)

FIGURE 14-22 When a nylon cord of unknown length is replaced, the cord is pulled snug around the lens but should not stretch. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, ButterworthHeinemann [Figure 7-21].)

FIGURE 14-23 Once the cord has been sized, extra cord is cut off. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann, [Figure 7-22].)

9.Press the end of the cord into the frame groove. A pair of half-padded nylon jaw pliers is used to push the loose end of the cord down into the groove in the eyewire (Figure 14-24). (Failure to tuck the cord into the groove causes the lens to chip or flake because of the pressure of the cord between the

FIGURE 14-24 Using a half-padded pliers, the free end of the cord at each point of attachment is pressed into the groove. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann [Figure 7-23].)

edge of the lens and the edge of the eyewire.) This is done nasally and temporally.

10.Secure the lens in the upper half of the frame. The lens is inserted into the upper half of the frame beginning in the upper nasal area (Figure 14-25), followed by the upper temporal area (Figure 14-26). The lens should come in behind the nylon cord so that the cord rests on the front surface of the lens.

11.Stretch the cord into the groove around the lens. To secure the lens in the frame, the cord must be stretched to fit into the lens groove. This is done with a plastic strip. Many people use a fabric ribbon. However, a ribbon often frays, which leaves threads wedged between the lens and the cord. These threads are extremely difficult to remove.

Some metal hooks are made specifically for stringing lenses. The main hazard of these hooks is the possibility of causing a small flake of the lens to chip out at the groove area. When this occurs the lens must be replaced.

The plastic strip is slipped between the nylon cord and the lens. The strip then is folded back, and both ends are grasped together. Beginning at the thinnest portion of the lens edge and using the strip to pull the cord around the edge of the lens, the lens is seated into the groove on the way around (Figure 14-27).

12.Check the cord tension. Cord tension can be checked as the plastic strip slides toward the bottom of the

FIGURE 14-25 The lens is inserted in the frame beginning with the upper nasal corner. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann [Figure 7-24].)

FIGURE 14-26 Once the upper nasal corner of the lens is in place, the upper edge of the lens is seated in the rim. This is done through temporal movement across the upper rim while the rim is slipped into the groove. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann [Figure 7-25].)

lens until it is close to the midpoint of the lens cord. The strip should be pulled fairly hard. The strip should pull the cord about 0.5 to 1.0 mm away from the edge of the lens (Figure 14-28). If the tension is incorrect, the lens should be removed

FIGURE 14-27 To thread a nylon cord into the lens groove, the cord is pulled around the lens, beginning temporally and continuing in a nasal direction. The cord must be on the front side of the lens. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann [Figure 7-26].)

and the length of the cord altered. (Using the dental pick may be necessary to free the end of the cord from the frame groove.) Usually the cord will be loose and have to be shortened. If this is the case, the earlier steps are repeated, beginning with step 8.

Once the lens is securely seated, one end of the plastic strip is released and pulled from between the lens and cord.

RETIGHTENING A LOOSE LENS

If a lens in an old nylon cord frame is loose or has fallen out, it is better to replace the cord, rather than to retighten the existing cord. An old cord may have lost some of its elasticity. The new cord has more elasticity and is less likely to break later.

Some have resorted to removing the lens and heating the old cord. The heat will cause the cord to shrink. When the lens is reinserted it will be tighter but only temporarily. Tightening a lens in this manner is not a good practice. The cord will not remain tightened long and the lens may fall out unexpectedly, breaking or scratching the lens.

NYLON CORD FRAMES WITH LINERS

Some nylon cord frames have liners that fit into the top eyewire channel of the frame. These liners are called

FIGURE 14-28 To check for cord tension, the plastic strip is slid toward the bottom central part of the cord and pulled fairly hard. As shown, the cord should stretch some; no more than a 1.0-mm gap should show between the cord and the lens. A gap between 0.5 and 1.0 mm indicates the correct amount of tension. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, ButterworthHeinemann, [Figure 7-27].)

figure-8 liners because, when viewed from the end (in cross-section) the liner looks like the number 8. One part of the 8 is smaller than the other.

If it is necessary to replace the figure-8 liner in the top eyewire, a knife blade, file, or dental pick is used to dig into the liner and slide it out either end. The old length of liner is measured and a new piece of the same length is cut. To aid in insertion of the figure-8 liner back into the top eyewire, the new piece should be cut at an angle.

Using the smallest side of the figure 8 first and beginning either nasally or temporally, the operator slides the liner into the top eyewire channel. The entire piece of liner is fed in and centered in the channel. If the liner seems loose, it is turned around and the larger side is used. Care should be taken not to block any of the four holes used to hold the nylon cord in place.

Some frames use a thin, raised metal ridge in the top rim of the nylon cord frame to lend stability to the mounting instead of a figure-8 liner. Regrooving that section of the lens with a wider groove may be advisable. Then when the frame flexes, the lens is less likely to chip along the groove.

1.Which of the following is not a synonym for a nylon cord frame?

a.Nebulon

b.String mount

c.Nylor

d.Suspension mounting

e.Nylon supra

2.True or False? FDA regulations prohibit the use of glass lenses in nylon cord frames.

3.Which of the following is the minimum edge thickness that is recommended for grooved high-index and polycarbonate lenses?

a.1.5 mm

b.2.4 mm

c.2.8 mm

d.3.2 mm

e.No recommended minimum edge

4.True or False? Edging and grooving lenses on some lens edgers is possible.

5.True or False? A heavy safety bevel on a grooved lens may cause the edge to be weak.

6.If a lens is ordered with polished edges, the edges should be polished at which of the following times?

a.Before the lens is grooved

b.After the lens is grooved

c.Either before or after the lens is grooved—it makes no difference

7.Normally the required groove depth for a lens to be mounted in a nylon cord frame is which of the following?

a.0.4-0.5 mm

b.0.9-1.0 mm

c.1.4-1.5 mm

d.1.9-2.0 mm

C H A P T E R 1 4 N Y L O N C O R D A N D O T H E R G R O O V E M O U N T I N G S

8.Match the lens with the most likely groove placement.

1.In the middle of the edge

2.Closer to the front edge

3.Close to the back edge

a.High-powered minu powered lens

b.Low-powered plus or minus lens

c.Franklin-style lens

9.True or False? If the lens grooving process progresses in the normal manner, a change in the sound of the cutting-wheel means that the lens groove has been cut all the way around the edge.

10.On a lens groover that has two switches—one for lens rotation and the other for cutting wheel rotation—which of the following is turned on first?

a.The cutting wheel rotation switch

b.The lens rotation switch

c.Both must be turned on at the same time.

d.It makes absolutely no difference.

11.On a lens groover that has two switches—one for lens rotation and the other for cutting wheel rotation—after the lens has been grooved all the way around, which of the following switches is turned off first?

a.The cutting wheel rotation switch

b.The lens rotation switch

c.Both must be turned off at the same time.

d.It makes absolutely no difference.

12.Reaching into an ultrasonic cleaning unit repeatedly with the unit still on may cause which of the following?

a.Fingernail damage

b.Skin cancer

c.Arthritis

d.Dry skin

e.All of the above

291

13.During grooving of a polycarbonate lens, because polycarbonate is soft, which of the following is correct?

a.The lens should be grooved slightly deeper than normal.

b.The lens should be grooved slightly less deep than normal.

c.The lens should be grooved in two or more passes, with the first pass being half the depth and the second, the full depth.

d.Both a and c are correct.

e.Both b and c are correct.

14.When threading the nylon cord into one side of a nylon cord frame, the operator should start with which of the following?

a.The upper hole and thread it in from the outside of the frame

b.The upper hole and thread it in from the lens side

c.The lower hole and thread it in from the outside of the frame

d.The lower hole and thread it in from the lens side

15.For a nylon cord frame, how much extra cord length should there be on each end of the cord?

a.0.5-1.0 mm

b.1.5-2.0 mm

c.2.5-3.0 mm

d.3.0-3.5 mm

e.4.0-4.5 mm

16.To check for the correct amount of cord tension on a nylon cord frame, using a plastic strip, pull the cord away from the lens edge fairly hard. The strip should pull the cord how far from the lens edge?

a.The cord should not pull away from the lens edge at all.

b.The cord should pull away less than 0.5 mm.

c.The cord should pull away between 0.5 mm and 1.0 mm.

d.The cord should pull away between 1.5 mm and 2.0 mm.

17.True or False? A rapid and acceptable method for retightening a loose lens in a nylon cord frame is to remove the lens, heat the cord in the frame heater to shrink it, and remount the lens.

In many countries it is possible to surface glass lenses as thin as 0.3 mm and still dispense the lenses for regular spectacle lens wear. Such lenses appear thin and are still considered optically excellent. However, they afford little protection for the eyes, and in many situations end up becoming a hazard to the wearer.

FOOD AND DRUG ADMINISTRATION

The U.S. Food and Drug Administration (FDA) began mandating impact resistance for dress ophthalmic lenses in 1971. Since then all eyeglass and sunglass lenses must be impact resistant, except when the optometrist or physician finds that they will not otherwise fulfill the patient’s visual requirements. If the lens cannot be rendered impact resistant, this must be recorded in the patient’s record and the patient also must be notified in writing.

Situations for Dispensing Non–Impact-Resistant Lenses

Some dispensers may assume that a written agreement having the patient assume responsibility makes it possible to dispense non–impact-resistant lenses. This does not ensure freedom from liability. The following are the FDA’s responses to three frequently asked questions regarding dispensing non–impact-resistant lenses2:

Q:Under what circumstances may retailers dispense lenses that are not impact resistant?

A:Lenses that are not impact resistant may be dispensed when a physician or optometrist determines that impact-resistant lenses will not fulfill the visual requirements of a particular patient. The physician or optometrist directs this in writing and gives written notification to the patient.

Q:Can a retailer supply a non–impact-resistant lens if a patient requests it or if the patient/customer agrees to assume all responsibility?

A:No. Non–impact-resistant lenses may be provided only when the physician or optometrist determines that impact-resistant lenses will not fulfill the visual requirements of the patient…. In such cases the physician or optometrist must give notice in writing

2Snesko WN, Stigi JF: Impact resistant lenses: questions and answers, HHS Publication FDA 87-4002, Rockville, Md, Revised September 1987, US Department of Health and Human Services, Public Health Service, Food and Drug Administration, Center for Devices and Radiological Health, pp 11-12.

to the patient, explaining that the patient is receiving a lens that is not impact resistant.

Q: May a physician or optometrist prescribe non–impact-resistant lenses for a patient for purely cosmetic reasons?

A: No. If medical problems are related to cosmetic considerations, however, the physician or optometrist may invoke special exemption provisions of the regulation based on professional judgment. For example, if the patient’s prescription cannot be filled by impact-resistant lenses because the physician or optometrist knows from previous experience that the weight of the heavy lenses may cause headaches, undue pressure on the bridge of the nose or ears, pressure sores, etc., the physician or optometrist may find that the visual requirements of the patient cannot be met by use of impact-resistant lenses.

For lenses to qualify for impact resistance, they must meet certain qualifications.

MINIMUM THICKNESS FOR DRESS OPHTHALMIC LENSES

Formerly the American National Standards Institute (ANSI) Z80.1 prescription lens standards had a minimum thickness recommendation of 2.0 mm. Now no thickness recommendation exists. FDA impactresistance requirements are—and have always been— performance-based; the lens must be capable of withstanding a predetermined amount of impact. If that requirement can be met with lenses that are thinner than 2.0 mm, the lens is acceptable. Today many lenses can meet current impact-resistance requirements and still be below 2.0 mm, including some types of glass lenses.

IMPACT-RESISTANCE TEST

REQUIREMENTS

The standard “referee test” for determination of impactresistance suitability for dress ophthalmic lenses is the drop-ball test. This test has specific administration guidelines. However, the FDA states that this does not inhibit the lens manufacturer from using equal or superior test methods to test for impact resistance.

THE DROP-BALL TEST

To be judged acceptable, first a lens is placed front- side-up on a neoprene gasket. It must be capable of

294

FIGURE 15-1 A drop-ball tester drops a steel ball on the front surface of a lens from 50 inches.

withstanding the impact of a 5/8-inch steel ball weighing 0.56 ounce, dropped from a height of 50 inches (Figure 15-1). (The exact specifications for the drop-ball test are given in Appendix C.)

The area of contact that the dropped steel ball makes with the surface of the lens influences the outcome of the test. Because with use these steel balls deform over time they should be replaced periodically. Similarly, the neoprene gasket compresses and loses some elasticity over time. If a worn gasket is not replaced, the percentage of glass lenses that pass the drop-ball test will decrease by 25%.3

Timing of Drop-Ball Test

Glass lenses must be tested after the lens has been edged and hardened and before it is placed in the frame. Plastic lenses may be tested in the “uncut-finished” stage before they have been edged.

C H A P T E R 1 5 L E N S I M PA C T R E S I S TA N C E A N D T E S T I N G

Drop-Ball Testing of Glass Lenses

With few exceptions, all glass lenses must be hardened and individually subjected to the drop-ball test. Only lenses that could be damaged by the test are exempt. These lenses must still be hardened but do not need to be tested. Glass lenses that are exempt from testing are the following:

•Raised multifocal lenses (lenses that have a ledge area on the lens, such as an Executive lens)

•Prism segment multifocals

•Slab-off lenses

•Lenticular cataract lenses

•Iseikonic (size) lenses

•Depressed-segment one-piece multifocals

•Biconcave, myodisc, and minus lenticular lenses

•Custom laminate lenses (such as polarizing lenses)

•Cement assembly lenses

INDIVIDUAL VERSUS BATCH TESTING

Batch testing is the practice of selectively testing a statistically significant number of lenses in a manufactured group. This prevents having to test individually lenses that could sustain damage by the test. The practice of batch testing is permitted for plastic lenses and nonprescription lenses such as mass-produced sunglass lenses. Glass, plano-powered sunglass lenses that are produced individually in a finishing laboratory must still be individually drop-ball tested.

Performing Batch Testing

Most—but not all—lens manufacturers normally batchtest for finished lens product impact resistance. This allows finished plastic lenses edged in a finishing laboratory to avoid individual testing or batch testing. Currently, nearly every lens manufacturer states that laboratories must perform testing to ensure impact resistance for lenses that have been surfaced from semifinished product.

If the lens is altered after having been received, as when it is sent out for antireflection (AR) coating, its impact resistance is altered. A great many types of coatings could be applied to the lens. Each of these coatings affects the impact resistance of the lens differently.

The envelope or box that the lens comes in should state whether the lens has been batch tested. If the package has nothing on it about impact testing, even if it is polycarbonate, it must be tested for FDA compliance.4

Typically the AR-coating laboratory batch tests lenses being coated. Laboratory personnel use lenses of the

3Torgersen D: Impact resistance questions and answers, OLA Tech Topic (Optical Laboratories Association), February 1998, pp 3-4.

4Young J: Technical Information for Labs, Lab Talk, 25(11):25, 1997.

same material and minimum thickness as those sent to them for coating. Finishing laboratory personnel are responsible to communicate with the company that applies the coating to determine that testing requirements have been fulfilled.

DEFINING ‘MANUFACTURER’

A large number of participants are involved in the process of making a pair of glasses. One company makes the lenses, another may surface the lenses, a third may edge the lenses, and someone else could coat the lenses. Who then is the manufacturer of the finished eyeglasses? Although in a lawsuit, each participating party is likely to be named, final responsibility lies heavily with the unit that performed the final process on that lens. The following is the FDA’s response to the question:

Q: In terms of the regulation, who is the manufacturer?

A: The manufacturer is the person who puts the lens in the form ready for its intended use or who alters the physical or chemical characteristics of the lens by such acts as grinding, heat treating, beveling, or cutting. For the purpose of this regulation the term “manufacturer” includes a company that imports eyeglasses for resale.5

In this chain of manufacturing events, the question of record-keeping may arise. The following is how the FDA poses and answers this question:

Q: What are the record keeping requirements on partially finished lenses furnished by one manufacturer for completion by another?

A: Records must be kept to show how lenses were rendered impact-resistant, when and how they were tested for impact resistance, and by whom in the processing chain these actions were accomplished.6

This means that if the retailer is the manufacturer, then the record-keeping requirements of the manufacturer apply. Retailers also have a 3-year requirement of keeping the names and addresses of persons buying prescription eyewear.

5Snesko WN, Stigi JF: Impact resistant lenses: questions and answers, HHS Publication FDA 87-4002, p 7, Rockville, Md, Revised September 1987, US Department of Health and Human Services, Public Health Service, Food and Drug Administration, Center for Devices and Radiological Health.

6Snesko WN, Stigi JF: Impact resistant lenses: questions and answers, HHS Publication FDA 87-4002, p 10, Rockville, Md, Revised September 1987, US Department of Health and Human Services, Public Health Service, Food and Drug Administration, Center for Devices and Radiological Health.

EFFECT OF LENS PROCESSING ON IMPACT RESISTANCE

A number of processes can be performed on a lens that affect the way the lens is able to resist the impact of an object. One of the most obvious is lens thickness: the thinner the lens, the less the impact resistance will be. Other factors may reduce impact resistance. Some of the more significant ones are listed below.

Effect of Lens Coatings on Impact Resistance

When a plastic lens is either scratch-resistant–coated or antireflection-(AR-)coated, the impact resistance of the lens normally decreases. This seems opposite to what would be expected.

Both scratch-resistance and AR coatings are harder than the plastic lens material to which they adhere. When a lens breaks, the break starts at the weakest point. If a plastic lens is hit by an object, the lens may flex but may not break. However, if the coating is harder than the lens, as the lens flexes, the harder (more brittle) coating cracks before the uncoated lens would. When the coating is bonded strongly to the lens, the energy that is concentrated at the first crack is released. The released energy travels through the lens and may cause it to break.

Corzine and colleagues7 used a static load form of testing8 and compared uncoated CR-39 lenses to the following:

•Scratch-resistance–coated lenses

•Five-layer AR-coated lenses

•Lenses that had been prepped for AR coating but were not yet AR-coated

The mean fracture load required to break the lenses in each category were as follows:

Uncoated CR-39: 587 Scratch-resistance coated CR-39: 505 AR-coated CR-39: 465

AR-prepped, but not AR-coated CR-39: 609

As can be seen from the results, the weakening of the lens is as a result of the coating itself, not by the process the lens is subjected to in preparation for coating.

This tendency of the coating to reduce impact resistance can be countered to some extent by using cushion coatings or primers applied to the surface of the lens before the application of a hard coating.9

7Corzine JC, Greer RB, Bruess RD, et al: The effects of coatings on the fracture resistance of ophthalmic lenses, Optometry and Vision Science 73:8, 1996.

8Static load testing is where an increasing amount of pressure is applied to the lens until the lens finally breaks.

9Torgersen D: Impact resistance questions and answers, OLA Tech Topic (Optical Laboratories Association), February 1998, p 5.

296

Newer methods of coating probably will be engineered with impact resistance in mind.

In consideration of the effect a coating may have, a point worth remembering is that highly impactresistant lenses such as polycarbonate still have plenty of impact resistance, regardless of the coating. The main concern is for plastic lenses, especially some highindex plastic lenses. These lenses might be surfaced very thin and then have a coating applied without the benefit of cushion or primer coatings. As the number of finished stock lenses that have been AR-coated at the factory increases, there may be a resulting increase in impact-resistance quality. This result is because cushion or primer coatings can be engineered for the specific plastic substrate/lens coating combination being delivered in a factory-finished, already-coated, uncut lens blank.

Lenses sent out for coating have the chemical characteristics of the lens altered by the coating process. It then becomes the responsibility of the coating laboratory to comply with testing requirements for impact resistance. The edging laboratory personnel should be certain that the coating laboratory personnel fulfill these responsibilities. Otherwise the edging laboratory becomes responsible.10

Effect of Re-edging on Impact Resistance

Re-edging a plastic lens to another shape after it has been edged once does not significantly affect impact resistance. However, edging or re-edging a glass lens that already has been hardened will affect impact resistance. So may a hardened glass lens be re-edged to a new shape and then worn? The following is the FDA’s response to the question:

Q: May a glass lens, after it has been chemically or thermally treated for impact resistance, be processed further in any way?

A: Lenses that are treated for impact resistance by induced surface compression may be re-edged or modified for power. However, the beneficial effects of surface compression may be reduced substantially. Such lenses must be retreated and tested before they are dispensed to the patient.11

10Torgensen D: The effect of coatings on impact resistance, Clearvisions (Optical Laboratories Association), 3(7):8, 2000.

11Snesko WN, Stigi JF: Impact resistant lenses: questions and answers, HHS Publication FDA 87-4002, p 9, Rockville, Md, Revised September 1987, US Department of Health and Human Services, Public Health Service, Food and Drug Administration, Center for Devices and Radiological Health.

C H A P T E R 1 5 L E N S I M PA C T R E S I S TA N C E A N D T E S T I N G

Effect of Drilling and Grooving on Impact Resistance

Drilled glass lenses that are heat treated are not safe to wear. They may pass the drop-ball test in their unmounted state, but the compounded stress brought about by the mounting causes the mounted lenses to fail too easily.

Drilled lenses that are tempered chemically will pass the drop-ball test and are not as affected by drill mounting as are heat-treated lenses. Nevertheless, glass lenses are seldom used in a drill mounting even when chemically tempered.

In fact, glass lenses are seldom used with grooved lenses either. In 1993, Optical Laboratories Association Technical Director George Chase addressed the glass lens grooving/drilling issue in an OLA Tech Topics paper. He indicated that even though drilled and grooved glass lenses normally would pass the drop-ball test, the unprotected, exposed lens edges were likely to chip or microcrack with normal use and reduce impact strength. If drilled or grooved glass lenses are to be made, the OLA encourages optical laboratories to first obtain a waiver from the person ordering the lenses.12

Effect of Surface Scratches on Impact Resistance

A scratched lens surface reduces impact resistance. The scratch introduces a weak spot on the lens and creates a sort of “fault line.” The scratch provides an easy area for stress to build during impact, which makes breakage more likely. To better imagine how this works, the reader may think about how panes of glass are “scored” with a diamond so that they may be broken along the scored line.

Contrary to intuition, scratches on the back surface of a lens reduce lens impact resistance more than front surface scratches. Glass or CR-39 lenses with front surface scratches were reduced in impact resistance by 20%, whereas CR-39 lenses with back surface scratches were reduced in impact resistance by 80%.13

‘Duty To Inform’

Eyeglasses come in a large variety of lens materials. Each of these materials varies in impact resistance. Lenses that have a lower impact resistance may be acceptable for a person with a sedentary lifestyle but would not be appropriate for children who run and play. Laboratory personnel have no way of knowing the lifestyle of the person whose name appears at the top of

12Chase G (as quoted by Torgerson D): Impact resistance questions and answers (May 26, 1993), OLA Tech Topic (Optical Laboratories Association), February 1998, p 4.

13Torgersen D: Impact resistance questions and answers, OLA Tech Topic (Optical Laboratories Association), February 1998, p 4.