L&T: RxPertise

Oct
2012

Understanding Digitally Enhanced Lens Performance



By Mark Mattison-Shupnick, ABOM

As an opticianry educator and trainer, I often ask eyecare professionals who attend my classes whether their patients are more satisfied with the vision, performance and value of their digital lenses. Their answer is an overwhelming “yes.” But why?

The answer is actually simple. Ordinarily, any time we ordered eyewear for patients, the standard toric lenses supplied in the frame were shaped and worn differently from the lenses the doctor used to develop the prescription using a phoroptor or refractor. That changed two things. First, it made small changes to the effective power of the lens, i.e., it no longer “saw” as the prescription that the doctor wrote.

Secondly, vision in the periphery changes significantly. This results in a smaller, clear field of view and depending on the amount of cylinder and its axis, a misshaped one as well. You can simulate this if you wear glasses; just tilt the lenses by lifting the temples to an extreme angle (Fig. 1). The result is a new effective power and a change to clear peripheral vision. For low prescriptions the effects are small; for higher prescriptions the effects are larger. However, our patients have been able to use these lenses adequately. That’s because patients learn to adjust to and use the available clear field of view. But in progressives, there’s another effect.

In standard front surface progressives, lenses get steeper from top to bottom. That creates the add power. However, in the periphery and especially below the 180 line, the vertical curve of the lens front changes faster than the horizontal, creating unwanted astigmatism and power error. The result is blurring. Add a cylinder prescription, ground on the back of this traditional progressive and the result is a combination of the front and back cylinders for a new effective prescription. That explains why some patients tell us that the temporal side of one of their lenses is clearer than the other. What can be done?

Design and cut each lens to deliver the prescription as the effective power, centrally and peripherally for the way that the patient will wear their lenses. That’s it—digitally enhancing a lens’ performance. If we can understand the error created for a lens in the way that the patient will wear it, the lens can be designed and its surfaces cut to create the least errors. We may not be able to get rid of all the errors, but it can be significantly better than before and it can in fact deliver the optical lens designer’s intent. Typically, when compared to their previous lenses, patients say they “see” better. Seeing better is the result of a larger, clearer field of view.

TORIC VS. ATORIC SURFACES

Here’s an example. In Fig. 2, start at the right most lens circle. This is a premium front surface progressive design made into a +3.00-1.50 x 135. The patient is wearing this lens at 12 degrees tilt and 12 degrees wrap. (That may be unusual, but we’ll use this extreme for the example.) The map shows the effects.

Unwanted astigmatism created by the lens creates blur. It’s the result of the front and back surface combined and worn with tilt and wrap. The intended design is changed dramatically. However, this is the lens that any of us would have delivered to the patient. In fact, it would be similar to most other lenses that this patient wore before. Vision is probably adequate.

But what if the back surface were made atoric, i.e., not a standard toric surface but aspheric in each of the principal meridians as a way to improve peripheral vision? Each meridian would have the correct asphericity for the power in that meridian. The Atoric map shows an improvement, fewer contours in the periphery (less astigmatism/blur) and a wider near but still not really clear (white) in the full distance, midrange and reading zone.

In the map labeled “Optimized,” errors have been compensated when making the lens but uses an assumption about the way the lens fits, i.e., 9 degrees tilt, 7 degrees wrap and 13.5 mm vertex distance (default fitting values). This makes a real difference in the lens delivered and in fact, this and the atoric design is what most of our patients are commenting on when they say that simple sentence, “I see better.”
Finally, this patient wears his lenses more tilted and wrapped than the default fitting values. If the actual fitting values were used, when ordering the lenses, the lab software would incorporate them when calculating the design surfaces, and the results are shown in the Customized map. This is quite a difference from the standard base curve design. What’s the message here?

A VARIETY OF MESSAGES
First, digitally enhanced lenses help improve a patient’s vision and that’s merchandisable. Develop a marketing plan and advertise on your website, on Facebook and by e-mail to patients about the new digital lenses now being used in your office. Report the reactions of patients with a few of their words and their smiling photos in the new eyewear (with their permission), especially on Facebook.

As seen from the illustration maps, these lenses can deliver benefits in a good, better and best form that fits any patient’s budget. It also makes it easy to implement the use of digitally enhanced lenses in your practice. There’s no lack of manufacturers and brands.

The best of these new highly sophisticated designs also require a new set of measure ments and you should be able to say, “This office is specially trained” in taking them. Call your lab reps to try the variety of desk, floor and iPad-based measuring devices that quickly deliver vertex, tilt and wrap. You want every patient saying, “No one ever did this before.”

WHERE TO START?
For high prescriptions (more than 3D in sphere power), order optimized designs. It’s virtually a guaranteed success. Order two to four atoric jobs for single vision and flat tops for patients with a 1.25D cylinder (they’re your target) so you can get some fast experience. This will also enable patients to see the benefit. Then consider changing the patient’s entire standard design platform to atoric designs, which would represent “good” in a good-better-best scenario.

Talk to your lab about the availability of atoric jobs and place the order. When you deliver the job, watch what the patient says about how they see. These lenses will deliver a definite improvement at a small increase in price for the wearer. Visit opticianshandbook.com for more information.

WHICH LAB?
Your lab is now a very important choice since they have become the manufacturer of the lens design. Be sure to order from a lab that has experience processing free-form lens designs. Many labs today not only offer brand name lenses, but also offer their own house brand digital lenses, which usually offer good optical performance for a lower price.

By understanding the basics of digital lens performance, providing your lab with the proper patient measurements and selecting an experienced lab, you should be able to deliver the latest lens technology to your patients. It’s a proven recipe for patient satisfaction.

The Challenge of Digital Design Starts with the Exam

Lens designers make certain assumptions when they create any lens design. These assumptions must be clearly understood for the best digital outcomes. Of course, the ophthalmic prescription begins in the exam room. Doctors need to set up their testing appropriately if they want the very best that digital lens technology has to offer.

The lens designers assume that the testing is done with the lines of sight at 90 degrees to the plane of the lenses. To achieve this in the vertical, you can simply use an inclinometer (also used to measure the pantoscopic tilt of frames). Some inclinometers can be pressed against the refractor lens apertures to take a reading, others are used from the side with a sighting line. Also the refractor should be oriented so that it is neither turned to the right or left with regard to the chart. The easiest way to do this is to put the refractor’s near point rod down and be sure it is pointing at the chart. The chart must also be at the same level as the patient’s lines of sight.

Another assumption is that the lines of sight are passing through the optical centers of the test lenses. To assure this is the case, use pinholes in both refractor cells and a dissociating vertical prism at the start of the subjective refraction portion of your testing. If the patient does not see two charts, ask him to move a bit until he does. Then ask him to hold that position. After your subjective refraction, replace the pinholes and dissociating prism, and watch closely to be sure he doesn’t have to move in order to see diplopically.

If you use a short (e.g., 10-foot) refracting room and compensate your prescriptions, having the chart too high or low by a few inches will be a greater error than if the same discrepancy happened at a longer distance. The easy way to check whether the chart is at the right height is to place a vertical (held flat against the wall) mirror at the height of the chart and ask if the patient can see his own eye. If he can, his lines of sight are level. This will work with mirrored refracting rooms also, but getting the plano mirror exactly vertical is more challenging.

Manufacturers begin their digital calculations with the prescription you send them. Using this data can be a problem for strong Rxs (e.g., power > +5.00). You may need to compensate the power if the frame vertex and refracting distance are not the same. For free-form and individualized lenses, these calculations should be rounded to 0.05 diopter rather than to 0.12 or 0.25. Your lab should be able to do this upon request if you supply both the refracting vertex and the frame vertex. If you use a lab with knowledgeable consultants, you should be able to easily deal with these and other digital challenges. Digital and particularly individualized lenses are a huge step forward when properly used.

—Palmer R. Cook, OD

 

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