7
Jun/10

9. Durability

by admin under 9. Durability

When choosing a lensmaterial for a patient, durability is an important consideration formany buyers.WhenCR-39™ plastic first became popular for ophthalmic lens use in the early 1970s, lens durability became a topic of discussion. Since glass withstood scratching fairly well, lens buyers wanted to know about the scratch-resistant properties of the new plastic lenses. Unfortunately, it was not that good.The answer to this dilemma was an anti-scratch treatment.This added layer greatly increased the scratch resistance of plastic lenses and has become a standard addition tomost plastic lenses. The way for you to manage lens durability is to choose lenses that have the kind of anti-scratch and tinting properties you want. This means that you’ll have to do your homework to discover which lenses offer the kinds of properties you find appealing. Just ask the lens manufacturer or your lab for this information.

Advancements in hard coatings have significantly brought scratch resistance to impressive levels. These coatings are either organic or inorganic in composition. Inorganic materials produce a hard, quartz-like surface that is very resistant to scratches, but non-tintable. The coating is applied in a vacuum chamber much like a mirror or anti-reflective (AR) treatment.

Organic hardcoatings are usually applied by spraying, dipping, or spinning and cured in an oven. The softer the treatment, the easier the lens will tint, but the more prone it is to scratching. The anti-scratch material “recipe” is what determines its scratch resistance and tintability.

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4
Jun/10

Abbe value

by admin under 8. Abbe value

When white light (e.g., sunlight) is refracted, it breaks up into a rainbow of colored wavelengths, which is referred to as “dispersion.” This optical property of light results in a chromatic aberration. Dispersion reduces the quality of the lens’ focus and can reduce vision. Patients usually identify it as seeing colored ghost images around objects viewed through the lens.
A lens’ Abbe value indicates the amount of dispersion a lens material has. Like index of refraction, Abbe value is a unit of measure that indicates dispersive ability. The lower the Abbe value, the more the material disperses light (and the more troublesome the material can be for the wearer). Abbe values have been a constant concern with high-index lens materials, as higher index materials have lower values than their lower index counterparts.
Abbe value cannot be controlled with lens design like some other lens aberrations because it is inherent in the material. The way you control it is to choose a lens material that has a value that is high enough not to be troublesome for your wearer.

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3
Jun/10

Specific gravity

by admin under 7. Specific gravity

The physical term used to describe a lens material density is specific gravity. Most opticians know it as lens weight. The higher the specific gravity of a lens material, the heavier the lens will be. When choosing a lens material, you need to balance its thinness with its resulting weight. Weights of high-index materials vary considerably, with glass products leading the list as the heaviest. You might try to reduce a lens’ weight by using a higher index material, but often, as the index goes up, so does the material’s specific gravity.
An aspheric lens design is a good way to make lenses thinner, which also makes them lighter. What you need to determine is what the final weight of the lens will be using various index materials. This is the only way to be sure you are providing the lightest lens product. Unfortunately, this is not something you can easily calculate at the dispensing table with a chart, pencil, and paper. A good computer program is the best way to get this information.

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2
Jun/10

CLASSIFYING INDEX OF REFRACTION

by admin under 6. Classifying index of refraction
Since index of refraction is a major factor in thin lens profiles, it’s worth exploring this aspect of material choice. Index of refraction is a unit of measure that indicates how readily a lens material can refract light rays. The higher the index of refraction, the more the material will bend light rays. Therefore, a higher index lens will require less curvature and thickness than a lower index material when the lenses have the same power and diameter.
As the index of refraction increases, a lens of a given prescription and diameter needs less curvature and thickness to produce the same power. For plus lenses, this translates into thinner centers; for minus, it means thinner edges. An increased index of refraction can have a dramatic difference on the final thickness, weight, and profile of a lens. The reduced thickness translates into a lighter and more comfortable product, too. Most ECPs recognize four material index categories—low, mid, high, and ultrahigh. There is no official standard that subdivides these 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- or super-high index: >1.66
 With all of the higher index lens materials available, ECPs as a whole have been moving to higher index lenses. These lenses offer flatter base curves so the resulting lenses shed some thickness. Their thinner, flatter curves look more attractive too. Thinner lenses are lighter in weight, and lighter lenses are more comfortable to wear. Since 92% of prescriptions fall between +/-4.00D, mid-index materials work nicely with this group. Higher index materials are more suitable for higher prescriptions. Other aspects of choosing lenses include lens weight (specific gravity), Abbe value (chromatic aberration), durability (scratch resistance), and impact resistance (safety).
 
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31
May/10

QUESTIONS TO ASK

by admin under 5. QUESTIONS TO ASK

Since questioning is the vehicle used to getting the information you need to make lens material choices, careful questioning is essential. Patients often do not like to fill out forms and answer questions about their health, but it is important for you to ask probing questions if you are going to select the best lens material for them.

Open-ended questions are the best type to ask because they give the patient a chance to provide an in-depth answer instead of a one- or two-word response. Even so, avoid questions like, “What is the most important thing to you when selecting lenses?” It’s just too open-ended and most patients have no idea how to answer or what lens characteristics are important.

A better way to handle this would be to ask something like this: “How would you rank the following lens characteristics in order of importance to your new eyeglass lenses?” Now ask the patient to rank them from one to six with one being the patient’s primary concern.

• Thin lenses

• Light weight

• Durability (scratch resistance)

• Impact resistance

• Glare protection

• UV protection

This is an interesting way of obtaining this information because it lets the patient tell you what he thinks are the most essential aspects of his eyewear and why. Often, ECPs get the information they need but do not always understand which lens parameters are the most important. This questioning technique solves that situation. The answers that patients provide will also guide you to the lens material that will address their concerns.

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28
May/10

Subjectively speaking

by admin under 4. Subjectively speaking

Once you have reviewed the prescription and considered its power in regard to material
choice, the rest of your information will come from subjective questioning. The
importance of this kind of analysis cannot be understated. Careful questioning is the
key to discovering a patient’s lifestyle. Once you understand the demands he feels
are important for his vision and eyewear, you can match the features and benefits of
the proper lens material to him.
The trick to making this work the best for you is to construct your questions carefully so
that they unearth the appropriate information. The questioning needs to not only discover
the patient’s eye use information as well as his wants and needs, but also to determine the
degree to which each issue he raises bears on the lens material choice.
For example, if a patient indicates that she would really like to have fashionable
lenses with a flash mirror treatment, ultra-thin lenses, and a high degree of impact
resistance, which one of these is the most important in the lens material choice?
Which is the second most important? Which is the third? Sure, you could offer this
patient a 1.74 plastic lens material—the highest plastic material on the market, but
is this the right thing to do? Perhaps, but only if she (and you) feel that thinnest is
the most important aspect of her lenses while the other factors are addressed in
some way. Perhaps the impact resistance should take priority since it will help protect
the wearer’s eyes better than a less impact-resistant material. Decisions like this
are all a part of lens material selection.

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27
May/10

By the numbers

by admin under 3. By the numbers

Before you ask the patient any questions, gather some objective preliminary information on your own by looking at the patient’s Rx. What power is used in each eye? In many cases, it will fall between +/-3.00D. In a study conducted by Younger Optics this year, it was discovered that 85% of all prescriptions fall into this category. According to the study, 92% of all prescriptions fall between +/-4.00D while 95% fall between +/-5.00D. At +6.00D to -6.00D, you’ll cover 97% of prescriptions. The Rx’s that fall above +/-6.00D constitute only 3% of all Rx’s dispensed. What is this objective Rx data telling you? While it may be surprising to learn that the vast majority of prescriptions fall between +/-3.00D, it is an important statistic because it means that most patients do not require high-index lenses. With all the emphasis being placed on high-index lens materials these days, some eyecare professionals (ECPs) have defaulted to one or two of them for all patients. Sure, any lens will be thinner if made using a higher index material than a lower index one, but a lens has many more features than just index of refraction. Think of a lens as a package of features. In other words, when you recommend a lens, it’s a package deal—the patient receives all the features it has, not just one or two. The trick to recommending the most appropriate material for the patient is to determine the patient’s needs and match them to the best set of lens features you can find. As you analyze the data presented above, you begin to realize that you can begin
forming an idea of which lens material might be suitable for the patient based on her Rx. This is surely not the final decision; it is simply the first piece of the puzzle. To help you formulate a way of making the decision of which one is the most appropriate, consider creating a table of all the lens materials that you use and place them vertically into one column from the lowest to the highest index. Next to each, put the range of Rx’s you feel might be most suitable for it. If you do this, you will probably distribute the Rx’s across the material offerings, not place them all into the row of one lens material. That’s the logical thing to do…but are you actually doing this in practice? Probably not. Practicing opticianry means using your judgment. Making a lens material choice is one of the most important decisions you will make for any eyewear patient. Its selection should be done with careful consideration of many factors. If one lens material is really the best choice for all patients, then a machine can do lens selection since your personal analysis and interpretation is not needed. Contact lens practitioners do this kind of objective investigation as they do their workup of a patient. It’s an important part of deciding which lens material is suitable for a given patient.

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27
May/10

Getting Started

by admin under 2. Getting started

In order to decide which lens material to use for a patient, you should establish criteria for selecting it. These criteria should be both objective (based on reviewing measurements and data, and observing physical conditions) and subjective (based on answers that patients give to questions—their opinion of what they need or want).

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27
May/10

Intro

by admin under 1 Intro

If you were an optician in the 1960s, you remember that choosing a lens material was a pretty simple task. You almost always recommended a lens made of Crown glass. Occasionally, you would use a high-density flint glass that had a high index of refraction. Today, there are a wide variety of plastic lens materials on the market along with a few glass choices, all with specific features, benefits, and applications. The question is: which one is the right one for any given patient and how do you decide which one to use?

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27
May/10

Course description

by admin under HOW TO CHOOSE THE RIGHT LENS MATERIAL FOR A PATIENT GUIDE

Choosing a lens material for a patient is arguably one of the most important recommendations an eyecare professional (ECP) makes during the eyewear selection process. Today’s ECP can choose from plastic lens materials in 11 indices, ranging from 1.50 to 1.74, and in five indices from 1.52 to 1.90 in glass. Which one is the right one for your patient and how do you decide? This course will explore aspects of lenses you should consider and recommend how to make the right decision in order to provide the appropriate lens material for any patient.

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