|Eynoa 1.67 from HOYA meets the need for lenses that are thin, light, durable, and reliable.|
With so many lens materials available, ECPs should consider seven factors to find the best ones for their patients.
In November 2011, The Vision Council interviewed 70 wholesale optical labs in the U.S. and reported its findings. What stands out in their report is the growth of mid-index, high-index, photochromic, and single vision lenses. The growth in mid- and high-index lens materials suggests that eye- care professionals (ECPs) need some criteria for selecting the one that’s best for their patients.
1. INDEX OF REFRACTION
Index of refraction is a unit of measurement that indicates how much a lens material refracts light rays. The higher the index of refraction, the more the material will bend light rays. This means a higher-index lens will require less curvature and thickness than a lower-index material when the lenses have the same power and diameter. An increased index of refraction will have a difference on the final thickness, weight, and profile of a lens. The reduced thickness translates into a lighter and more comfortable lens for the patient.
There is no official U.S. standard that subdivides ophthalmic lens materials but the optical industry generally follows this breakdown:
• Low index: <1.53
• Mid-index: 1.53 to 1.58
• High-index: 1.59 to 1.66
• Ultra-high or super-high index: >1.66
2. ABBE VALUE
When white light breaks up into its component colors (a rainbow of colored wavelengths), it is called “dispersion.” Patients usually identify it as seeing colored ghost images around objects viewed through the lens or in the periphery of the lens.
A lens’ Abbe value indicates the amount of dispersion a lens material exhibits. Like index of refraction, Abbe value is a unit of measurement so that the higher the Abbe value, the lower the chromatic aberration. Abbe values are a concern with high-index lens materials because these materials have lower values than their lower-index counterparts. A value of 45 or higher is desirable in ophthalmic lenses.
Abbe value cannot be controlled with lens design like some other lens aberrations because it is inherent in the material. The way you control chromatic aberration is to choose a lens material that has a value high enough not to cause problems for the patient.
3. SPECIFIC GRAVITY
The physical term used to describe a lens material’s weight is specific gravity. The higher the specific gravity of a lens material, the heavier the lens will be. When choosing a lens material, balance its thinness with its resulting weight in its final form.
Keep in mind that even if a high-index material’s specific gravity is high, the resulting lens will have a thin lens profile so its total mass will be less than a low-index lens with the same Rx and diameter, resulting in a lighter lens than you might expect from the material’s specific gravity. It’s the final weight of the lens that’s important, not just the specific gravity of the material.
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Impact resistance is an inherent characteristic of a lens material. Materials like polycarbonate, Trivex®, and NXT® have increased impact resistance because they flex upon impact. If you are selecting lenses for safety applications, be sure they are capable of passing the ANSI Z-87.1 impact standards. The tests in this standard help ensure that the lenses you dispense meet the special needs of the safety environment.
With plastic lens materials, durability is determined mostly by their anti-scratch properties because they are easily susceptible to scratching without it. Advancements in anti-scratch coatings have increased their scratch resistance but some inhibit tinting, especially dark colors. This is why you’ll need to do your homework to determine the durability and other properties of the anti-scratch coating used on the lens that you want. Consult your lab or lens company for this information.
6. FRAME CHOICE
The patient’s frame choice can also influence your lens material choice. For example, if the frame is large, you’ll want a material that is as light as possible to counter the frame size. If you’re drilling lenses, you’ll want a material that can take stress and long-term use. Trivex is ideal for rimless because of its impact resistance. It also resists hole enlargement over time.
Polycarbonate is a favored rimless material for its excellent impact resistance but does have issues with crazing and chipping after the holes are drilled. Many ECPs favor 1.67 materials for rimless work because of their superior optical clarity, drilling capability, and considerable lens flexure.
If you can’t process a lens material easily, you won’t use it. No matter how terrific its Abbe or index is or how light it is, etc., you won’t use it because you can’t edge it, surface it, dye it, or do some other processing step with it.
Choosing a lens material is a rewarding part of optical practice. Having a good knowledge of how to choose materials will help you make better recommendations.
Randall L. Smith is the Opticianry Program Director at Baker College in Jackson, MI.
WHERE TO FIND IT:
Carl Zeiss Vision Inc.
800-905-2240 ext. 2012 • zeiss.com/lenses
Essilor of America, Inc.
800-905-2240 ext. 2020 • essilorusa.com
HOYA VISION CARE, North America
800-905-2240 ext. 2024 • hoyavision.com
INDO Lens US Inc.
800-905-2240 ext. 20118 • indo.es
800-905-2240 ext. 2027 • kaenon.com
800-905-2240 ext. 2050 • optima-inc.us
800-905-2240 ext. 20109 • uvco.net
800-905-2240 ext. 2079 • youngeroptics.com