Rigid gas permeable lens identification using refractometry

Clinical Article

Rigid Gas Permeable Lens Identification Using Refractometry Neil

R. Hodur,

OD,

Janice

Jurkus,

OD,

A pkntitude of rigid gas permeabk lenses have become availto the contact kns practitioner. The materials vary in monomer compounds that produce specific optical polymers, refractive indices, specific gravities, and oxygen pewneabilities us well as unique wetting characteristics. An in-office method of material identification and differentiation is desirabk. In many instances, the clinical contact kns practitioner needs to know the type of contact lens mated that a patient is wearing. This paper investigates the use of the Atago refractometer to test contact lens materid identification. The refractive index of 20 unidentified rigid contact lens materials was measured by four investigators using the Atago IV3000 refractometer. The contact lenses were fabricated and coded so that the material was unknown to the investigators before the study begun. The study showed that the measured values for the refractive ind.ex were consistent for individual lens materials by each investigator. Specific lens identification was possible as well as a simple differentiation between a lens with and without fluorine. A refractive index v&e less than 1.460 indicated a fluropolymer, whereas a value of 1.460 or greater identified a silicone-acrylate polymer. A value of 1.49 indicated poly(methyl methacrylate). a&

Keywords: Rigid gas permeable contact lenses; refractive refractometry

index;

Introduction The contact lens practitioner has many different rigid gas permeable contact lens materials to choose from in clinical practice. These materials vary in chemical formulation that

Address reprint requests to Dr. Hodur at the Illinois Eye Institute, 3241 South Michigan Avenue, Chicago, IL 60616, USA. Accepted

for publication

January 1992.

0 1992 Butterworth-Heinemann

MBA,

and Gary

Gunderson,

OD,

MS

produces a characteristic refractive index, specific gravity, surface wettability, optical quality, and machinability. An in-office method of material identification and differentiation is desirable. In many instances, the clinical contact lens practitioner needs to know the type of contact lens material that a patient is wearing. Certain lens materials may provide a better optical and/or physiological contact lens response in a given patient and exact duplication of the lens will be beneficial. Hoffman and Allen described a method of lens identification based on the specific gravity of the lens materials. ’ The Opti-Mis’” system uses a float/sink comparison in different liquids to identify the lens material. Although accurate, this system can be time-consuming and somewhat messy. At the present time, there are 16 specific gravity solutions that can be used with the Opti-Mis system. Fatt and Chaston reported the use of a refractometer to measure the effect of temperature on the refractive index of hydrogel lenses.’ An Abbe refractometer was used to determine refractive index changes as compared with temperature changes. Brennen reported the use of a refractometer to measure the water content of hydrogel lenses.3 An Atago N2 was used to determine the water content of 18 different hydrogel lenses. Differences in water content will be reflected in a difference in the refractive index of hydrogel materials. According to Brennen, measurement of the water content of soft contact lenses is clinically simple with the hand-heJd refractometer. This could lead to the identification of a hydrogel material. The refractive index of a substance, such as a gas permeable contact lens, can be determined by dividing the speed of light in a substance into the relative speed of light in air. The speed of light is not constant in all medias. Oblique light when entering the interface between two

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ClinicalArrick medias will bend due to the difference in speed between the two medias, i.e., Snell’s Law.4 The relative speed of light, in air, is found by experimental means and is considered to have a refractive index of 1.00, which is very close to the speed of light in a vacuum: 186,000 miles per second. The speed of light in a vacuum, the absolute speed of light, considers a vacuum to have a refractive index of exactly 1.00. The relative refractive index indicates the speed of light through a specified medium as compared to the speed of light in air.5 The refractive index is a physical constant to a material. The index of refraction is defined by the equation

index of rigid contact lens materials in an easy and reliable way. Also, the temperature will be room temperature and the light will be available room light. A glass prism is used as the face plate of the refractometer and the critical angle of light through the glass prism is known in air. When an unknown material is placed against the prism, the critical angle of light changes. This change in angle is noted on an internal reticule scale that reads the change in the reflected light angle directly in the scale. The sensitivity of the instrument is +O.OOl.

n = sin i/sin p,

Twenty unidentified contact lenses made of 12 different lens materials were obtained in coded form from a local fabrication laboratory. Each lens was measured with the Atago hand refractometer (Figure 1). This experiment was repeated four times by four separate observers in four locations. Each measurement was made at room temperature with available white light in each location. A PMMA contact lens (refractive index 1.49) was used for calibration. The procedure used was as follows:

where i is the angle that incident light (in air) makes with a perpendicular to the interface and p is the angle that the refracted light (unknown medium) makes with the perpendicular. A refractometer is an instrument that measures the speed of light by critical angle of an unknown material and a known glass plate. Monochromatic light will give more precise measurements for index of refraction as compared to white light. Most indices of refraction are reported for the D-line or sodium wavelength, which is 589.3 nanometers. The standard temperature of measurement is 20°C. If the temperature at the time of measurement is not 20°C a temperature correction should be made. The magnitude of correction is 0.00045 per degree from 20”C6 The index of refraction decreases with increasing temperature so the correction must be added to the observed index when the measuring temperature is above 20°C. If the temperature of measurement is below 20°C the correction is subtracted from the measured index. The Abbe refractometer is a standard instrument that uses these principles.’ The Atago refractometer uses the Abbe refractometer principles. The Atago refractometer (Figure 1) is a relatively inexpensive and easy to use instrument.

Statement of Problem This research will determine if the Atago N3000 handheld refractometer can be used to identify the refractive

Procedure

The refractometer prism was cleaned with nonpreserved saline and wiped dry before each measurement. The eye piece was adjusted so that the reticule was easily seen. A clean dry PMMA lens was placed convex side down on the face of the refractometer and a drop of nonpreserved saline was placed on the concave side of the lens. The saline neutralized the air space between the lens and the prism cover. Care was taken to hold the cover plate firmly against the contact lens to produce a full optical contact and alignment with the prism. The refractometer was pointed toward an indirect light source, such as a light-colored wall, to illuminate the instrument and scale. A boundary line appears in the field of the eyepiece dividing the field into at least three parts. The top part is dark, and below, a single or series of white lines appeared. Below the white line(s), the bottom part of the field was dark. The uppermost white line indicated the marking for the refractive index, which was read on the internal scale. A reading of the refractive index was taken from the internal scale. The scale was calibrated by a screw ring to an index of 1.49. The prism face of the refractometer was wiped dry and an unknown lens was put through the measurement procedure, and the refractive index was read from the scale. The refractometer was recalibrated after every 10 lenses that were measured.

Results Figure 1. Atago N3000 hand refractometer.

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The individual measurements are found in Table I.

and average of each lens

RGP lens ID wing refractometry: edge of which material a contact helpful for several reasons:

Table 1. Average Measurements Trial 1

Trial 2

Trial 3

Trial 4

Average

Std. Dev.

::

1.430 1.474

1.430 1.471

1.434 1.477

1.435 1.478

1.432 1.475

0.002309 0.003162

;: 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

1.466 1.468 1.468 1.456 1.430 1.457 1.430 1.462 1.471 1.464 1.456 1.452 1.447 1.464 1.452 1.473 1.464 1.430

1.467 1.442 1.467 1.456 1.430 1.459 1.430 1.462 1.470 1.464 1.456 1.451 1.447 1.462 1.451 1.477 1.465 1.430

1.449 1.468 1.469 1.460 1.435 1.460 1.433 1.462 1.471 1.466 1.454 1.453 1.447 1.465 1.446 1.475 1.463 1.435

1.449 1.467 1.470 1.460 1.436 1.462 1.435 1.465 1.473 1.476 1.456 1.456 1.449 1.467 1.454 1.479 1.467 1.437

1.447 1.468 1.469 1.458 1.433 1.460 1.432 1.463 1.471 1.468 1.456 1.453 1.448 1.465 1.451 1.476 1.465 1.433

0.000577 0.0033 16 0.000957 0.002309 0.003201 0.002081 0.002449 0.001500 0.001258 0.005744 0.001000 0.002160 0.001000 0.002081 0.003403 0.002581 0.001707 0.003559

The average measurements were arranged in order of lowest refractive index to highest refractive index and then the actual lens material was decoded. Tabk 2 shows the averaged measured index for the material.

Discussion A need exists for the contact lens practitioner to easily identify specific rigid contact lens materials. The knowl-

Table 2. Average Measured Index

Material 2. 9. 20. 7. 15. 4. 17. 14. 13. 6. 8. 10. 16. 19. 12. 3. 5. 11. 1. 18.

RxD RxD Equalens Equalens Fluoroperm 92 Fluoroperm 92 Fluorex 700 Fluorex 700 Fluoroperm 30 Fluoroperm 30 Paraperm EW SGP II SGP II Boston IV Boston IV SGP I Boston II Optacryl 60 Ocusil Ocusil

Average Measured Refractive Index 1.432 1.432 1.433 1.433 1.448 1.447 1.451 1.453 1.456 1.458 1.460 1.463 1.465 1.465 1.468 1.468 1.469 1.471 1.475 1.476

Hodur et al.

lens is made of can be

1. The contact lens practitioner is not always aware of the rigid contact lens material that a new patient is currently wearing. 2. Many patients have multiple pairs of contact lenses. Identifying the lens material a patient is currently wearing is helpful for evaluation. 3. Knowing the contact lens material provides the assurance that a duplicated lens will be in the same material as the original. 4. Patients who complain of dry eyes, lens coating, or other possible material-related problems may benefit by changing the lens material from that which they are currently using. 5. Some materials may be more stable than others in specific lens designs. Knowing the existing material may allow the practitioner to monitor base-curve warpage, surface buildup, and scratch resistance. 6. Patients may need to have care products of their lenses reviewed for handling and care. Knowledge of the contact lens material may indicate certain handling and/or care procedures. 7. Knowledge of the material will allow the practitioner to determine if a dispensed lens has been approved for continuous wear. 8. The ability to check a contact lens material allows the practitioner to know if a lens that the laboratory fabricated was made of the material specified. Material identification has been done in the past by a specific gravity, sink-float method (Opti-Mis). We found the refractive index method of lens identification offers several advantages over the specific gravity method of identification. The advantages are 1. Using the Atago refractometer 2. Using the Atago refractometer 3. Using the Atago refractometer

is simple. is inexpensive. is clean and fast.

The refractive index method of contact lens identification may be a convenient and economical method for the contact lens practitioner to identify lens materials. One problem associated with the refractive index method of identifying contact lens materials is the cracking of a contact lens. When making optical contact with the prism cover, added pressure could damage a lens when too much pressure is applied. Practice with the instrument will alleviate this problem. An initial list of measured refractive indices (Table 3) using the Atago refractometer can be used as a guide to identifying 12 common contact lens materials. As other contact lens materials become available, the list can be updated easily. The data indicate that the difference between a silicone-

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Clinical Article Conclusions Table 3. Refractive

Index Guide Using the ATAGO N3000

Hand Refractometer Index

Material Fluorosilicone acrylate

1.432 1.433 1.447 1.452 1.457

&Zlens Fluoropenn 92 Fluorex 700 Fluoropenn 30 Silicone acrylates Paraperm EW SGP II Boston IV SGP I Boston II Optacryl 60 Ocusil

1.460 1.464 1.465 1.468 1.469 1.471 1.476

acrylate and a fluorosilicone acrylate lens is easily differentiated. A refractive index of less than 1.460 indicates a fluorosilicone acrylate and an index of 1.460 or greater indicates a silicone acrylate material. The individual lens materials within the two groups of fluorosilicone acrylate and silicone acrylate materials can then be compared to the published contact lens data or the experimental method performed in this research, with the Atago refractometer.

The Atago N3000 hand-held refractometer is a viable instrument for use in identifying the material of a fabricated contact lens. The refractive index method is a convenient, accurate method for the practitioner to use when identification of contact lens materials is needed.

Acknowledgments The authors would like to thank Mr. Michael Johnson, from Art Optical, Contact Lens Inc., for his assistance in preparing and coding the fabricated contact lenses used in this research and to Art Optical, Contact Lens Inc., for fabricating the contact lenses.

References 1. Hoffman WC, Allen MD: Identifying rigid lens materials. Contact Lens Forum 1985;April:35-39. 2. Fatt 1, Chaston J: The effect of temperature on refractive index, water content and central thickness of hydrogel contact lenses. ICLC 1980;7(6):250-255. 3. Brennen N: A simple instrument for measuring the water content of hydrogel lenses. ICLC 1983;10(6):357-362. 4. Fincham WHA, Freeman MH: Optics, 9th ed. ButterworthHeinemann, Oxford, UK, 1980, p. 19. 5. Fincham WHA, Freeman MH: Optics, 9th ed. ButterworthHeinemann, Oxford, UK, 1980, p. 16. 6. Atago N3000 Hand Refractometer Instruction Manual. NSG Precision Cells, Inc., Farmingdale, NY. 7. Fincham WHA, Freeman MH: Optics, 9th ed. ButtenvorthHeinemann, Oxford, UK, 1980, p. 53.

Clinical Implications

Verification of lenses, either ordered from the laboratory or as presented by a new patient, is one of the essential steps in problem solving and quality assurance in contact lens practice. As the number of rigid lens materials and manufacturers continues to grow, the ability to identify a lens polymer adds a significant dimension to the practitioner’s knowledge of a lens. The author has accurately listed the numerous situations in which it would be particularly helpful to be able to quickly and accurately identify a rigid lens material. The tables provided in the paper should prove a useful reference for use at every contact lens verification station. The previously available method of polymer identification by specific gravity, as the author points out, is both time- and space-consuming as well as messy. Practitioners who already use a refractometer for hydrogel identification will be happy to have this new information and new use for this device. For others, there is now added justification for acquiring a useful, yet inexpensive instrument.

Peter D. Bergenske, OD, FAA0 83 13 Greenway Boulevard Middleton, WI 53562

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RGP lens ID using refractometry:

Dr. Neil R. Hodur is an associate professor of College of Optometry. He is a 1975 graduate Optometry. Dr. Hodur’s current responsibility in geometrical optics and physical optics at extensively on the topic of contact lenses States.

Hodur et al.

Optometry at the Illinois of the Illinois College of is teaching the courses I.C.O. He has lectured throughout the United

Janice M. Jurkus, OD, MBA, is an associate professor at the Illinois College of Optometry. As chief of their Specialty Contact Lens Service, she is responsible for the didactic, laboratory, and clinical aspects of the contact lens curriculum. She has lectured and written extensively in the area of contact lenses. Dr. Jurkus is an officer of the Association of Optometric Contact Lens Educators and a member of the AOA and Academy.

Dr. Gary Gunderson is a 1979 graduate of the Illinois College of Optometry. He received his Master of Science degree from Northern Illinois University in cell physiology. Dr. Gunderson is currently an assistant professor at the Illinois College of Optometry. His major areas of interest include ocular physiology and related contact lens research.

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