MARCH 2014

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Your monthly guide to staff training outside the box

Eyes / Lenses / Fitting Lenses / Free-Form / Frames / Sunwear / Patient Solutions / In-office / Standards

LENSES—Material Performance

Breakage or Star Cracks

Notch sensitivity is a major factor in rimless lens choice and the less the better. Trivex material has virtually no notch sensitivity so even if a drill hole has a small micro-crack, it won't turn into a runner, star crack or lens breakage. Also, while being extremely impact resistant, it isn't as easily stressed by compression. That results in clearer delivery of the prescription. Tied to that is its higher Abbe number (45 to 47) so there is less chromatic effects in the lens periphery especially in higher lens powers. Its lower index is somewhat moderated because of its high impact resistance so centers can be made as thin as high-index materials so for the core range of prescriptions (+3 to -3), there are negligible differences. In higher prescriptions, the differences can be determined so you can make a considered decision. 

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Eyewear returned
for star cracks

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Radiation Protection

UV/HEV Absorbing Materials: Start with the right lens material since it is the platform to which you'll add the design and then the lens treatments. However, UV and HEV blocking is the result of the total lens and how it's constructed. The materials that are inherently 100 percent UV absorptive are Trivex material, new Tribrid, polycarbonate, 1.60, 1.67, 1.70 and 1.74 high index. Standard hard resin plastic (about 90 percent UV-absorptive) requires additional UV treatment. Crown glass is only about 40 percent UV-absorptive. No clear lens materials by themselves are blue light-absorbing (at least not yet).

UV/HEV Attenuating Treatments: Lens treatments like photochromics and polarization or combinations of the two (Transitions Drivewear, NikeMax Transitions, others) are 100 percent UV-absorptive. For blue light, lenses with color treatments provide selective absorption using either polarizing films or tints. Some sun lenses are specifically designed to filter or selectively absorb more of the HEV than others (Vision-Ease Coppertone gray and brown, Oakley VR28 Black Iridium Polarized, others). Therefore, knowing the transmission characteristics of lenses before and after coating allows you to deliver the best outdoor lenses that block UV and HEV. Ask for your lab's recommendations.

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Higher index lenses require less curvature to produce lenses of the same power. As a result, the edges of minus lenses and the centers of plus lenses are thinner. Less lens volume also makes lenses lighter. Lower density creates an even lighter lens. For example, Table 1 describes how polycarbonate makes a thinner and lighter lens.

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Radiation, Technically Speaking

A green photon at 550 nanometer wavelength is eye-safe whereas a UV photon at 275 nanometers is not, but what about photons with in-between energies? Can they interfere with the normal chemistry of the eye? The 400 to 500 nanometer wavelength range (violet to blue) is known as high-energy visible (HEV) light, and some experts have eye health concerns regarding this part of the visible spectrum. Exposure to high intensity light in this wavelength range has caused eye damage in laboratory animals, suggesting the possibility that long-term exposure to lower levels of HEV could be harmful to human eyes. If so, the explanation of the causes will depend on the imagined "You Must Have This Amount of Energy to Break this Bond" sign on the biological molecules involved.

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Children's Eyewear: Safe, Clear and Comfortable

Children will also be reluctant or refuse to wear their glasses if they're experiencing distortion and/or discomfort. Lenses should be safe, clear and comfortable to wear.

Safe–Safety encompasses protection from impact, scratches and ultraviolet radiation. While all lenses are considered as "medical devices" and must meet standards for impact resistance as required by the U.S. Food and Drug Administration (FDA 21CFR801.410, commonly referred to as "the drop-ball test"), children require lenses that significantly exceed the FDA standard.

Clear–Corrected vision delivered with the utmost clarity is always the goal. With children, this is even more critical because during their first 12 years, visual learning accounts for 80 percent of the learning process. Visual clarity of a lens begins with the material's fundamental ability to effectively align the spectrum of light waves that pass through it.

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Making Eyewear Is Like Building a House

Lens material, like a strong foundation, contributes to the protective performance, clarity, looks and comfort of the final pair of glasses.

For example, impact resistance, notch sensitivity, index and frame type determine minimum center and edge thickness. Index, specific gravity, lens size and decentration determine weight and comfort. Absorptive, reflective and impact characteristics contribute to the protective performance of lenses.

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