NOV 2016

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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: Technology


Eliminate blinding glare and sunlight reflected off flat surfaces intensified 10 to 100 times that hides objects that you might need to see. The result is reduced glare, with better contrast. Polarized lenses reduce atmospheric scatter (Rayleigh’s Effect). The result is improved vision comfort, reduced haze and improved reaction time.

A must for sunglass clarity is an anti-reflective back lens surface (AR). An AR back reduces or eliminates the magnified back surface reflections (magnified when reflected by a concave surface) that are so easily seen on a dark lens. Those reflections are always there and get in the way because they are so much more visible because of the dark lens. Back surface AR eliminates back lens surface reflections and improves comfort.
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What is the PRP?
So, my first question is, do you know the muffin man? No, sorry. I mean, do you know my little buddy, the prism reference point? When I first asked this question, I was surprised by how many opticians weren’t familiar with this term. Prism Reference Point, or PRP, is the point on a progressive lens that you will use in order to determine if any prism is present within the lens. You can find it on verification masks for all of your progressive lenses, or you can determine by the semi-visible markings on a PAL. You will find that the PRP lives right smack between the watermarks.
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We should consider smarter sunwear. We want to address bright sunlight by reducing or eliminating glare. It’s also helpful to reduce and eliminate scatter and blur so we should also be considering blue management. For ultraviolet radiation, because it is a cause of cataracts, pinguecula, pterygia, sunburn, cancers and AMD, we want to absorb 100 percent of the UV. But for blue light, some are bad, and some are essential. In fact, 25 to 30 percent of sunlight are the blue wavelengths. To properly sort out the blue, we need smarter sunglasses that separate and manage blue better.

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There’s another design consideration for single vision lenses not often thought about. Lens manufacturers make a design choice for the intended distance for which the lens design will be used most. For stock lenses, the decision is typically for far away distance vision. But what if the lens will not be used primarily for far away but a mixture of far and near or for near vision only? That makes a difference. Making that decision improves the way a lens delivers its optics.

Out-of-the-envelope stock single vision lenses are designed for the average wearer for average fitting characteristics and for distance vision. What if you could improve your own technical expertise and separate yourself and your practice by choosing lenses customized for position of wear and then personalized for the way that the patient intends to use them?

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In a Hoya-commissioned study completed by Millard Brown, a company that works with 90 percent of the world’s leading brands to help define brand purpose and engage consumers, they found these answers to the question, “Why do you wear photochromics?”

Half of the respondents cited sunlight sensitivity, glare reduction and UV protection while in one pair of glasses. This shows that protection from the sun and convenience drive a purchase. Did you know that our sensitivity to glare almost doubles every decade of life? This doesn’t mean that a pair of quality-polarized sunglasses isn’t needed, it suggests that this meets the patient’s need when only one pair of glasses may be possible. They might prefer two pairs. It also suggests that this is the perfect solution for clear eyewear since they meet so many more of the ways that patients wear their glasses.

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In the middle of the 19th century, prompted in part by the desire to improve the performance of optical microscopes, the son of a window glass manufacturer, Otto Schott, along with instrument maker Carl Zeiss and mathematician Ernst Abbe, together laid down the basic theory, glass specifications and production techniques required for the production of high quality optical-grade glass and the manufacture of precision optics.

Amongst defining desirable glass qualities, including uniform refractive index and freedom from inclusions and waves, Dr. Abbe created an equation to describe how light is dispersed or spread into its constituent colors as it is refracted, which today is called its Abbe value. Low dispersion Abbe values for single elements of optical grade glass range from 50 to 85, with crown glass having a nominal value of approximately 60.

Over time, the family of crown glasses came to be considered as any glass having a low refractive index and high Abbe value, which includes ophthalmic crown glass, with its 1.530 index and 60 Abbe value. Even today, of all the classic optical aberrations, only the image-inducing blur of poor Abbe value cannot be offset or eliminated by design optimization in eyeglass lenses.

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Compensated Rxs—When Lenses Don’t Measure as Prescribed!
With the popularity of free-form technology, adjusting for power changes that result from frame fit have become commonplace. Compensated powers have become “easy” to implement given this technology. However, providing compensations to prescriptions is not entirely new. In the days of cataract lenses where strong plus powers were needed, vertex compensation to the power of the finished lens was an important factor in providing satisfactory lens performance. For example, if a +10 D lens was prescribed, a lens of a power other than +10 was made so that when positioned at a given vertex distance, it would be seen as a +10. This is the same concept for today’s compensations.

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The current lens methods to attenuate blue wavelengths include yellow or amber lenses, augmented AR, lenses that are a combination of both and lens materials that absorb into the blue. Yellow lenses have never been popular as a fashion item, except for skiers and snowboarders or worn by older adults to enhance contrast. In fact, most lens manufacturers work hard to reduce the yellowness (measured as yellowness index) of their lenses. Augmented AR, using a tuned AR coating can effectively reduce the blue-violet wavelengths. These new AR coatings have a blue to blue-violet reflex color. Recognizing that blue-violet radiation is harmful and blue turquoise is beneficial provides direction to manufacturers to develop coatings that are selective and can differentiate between the two. There are also lenses that are a combination of both, i.e., only slightly yellow or with slightly noticeable color all the way to sun lenses, with AR. Then there are the new lens material substrates that attenuate to about 420 nm. These lenses provide an opportunity for the ECP community to switch over all patients to HEV attenuating products.

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For a fitting height of 18 mm, corridor length roughly calculates as follows: 18 mm minus 4 mm (minimum reading area height) = a 14-mm Corridor Length Value

TIP: Your calculated corridor length value may not always be obtainable in a specific progressive design. Therefore, always keep the minimum useful reading area in mind as you select the next closest corridor available.

In the above example, if the design selected only provides a choice between an 8-mm or 10-mm corridor, the final value chosen will depend upon these additional considerations:

Total Frame Height and Contour: Frame style, size and shape may dictate an adjustment to your target corridor length to ensure the full reading area is not truncated by the contour of the frame’s lower rim. Pupillary distance also places a role here, with narrower PDs more sensitive to being truncated by the upward slope of the frame’s nasal eyewire.

Vertex Distance: Closer vertex distances require shorter corridors to compensate for the reduced drop of the eye’s eclination intercept with the lens surface. And vice versa for long vertex distances, which then may need longer corridors.

Pantoscopic Angle: Greater pantoscopic angles require shorter corridors, and lesser pantoscopic angles require longer corridors for the same conceptual reason stated above in vertex distance.

Rule of thumb: Every 1-mm increase in corridor length will require approximately a 2-degree increase in eye declination.

TIP: The less accommodation reserve, i.e., increasing age, the quicker the wearer will need to get to the target reading add.

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