Page 62 - Demo

but for those whose prescriptions are near the edge of a base curve range, the areas of clear vision were limited. The limitations were more acute for patients with stronger prescriptions.
Freeform lenses, also known as digital lenses, still use lens pucks with a limited number of base curves. The difference is that through real-time customization, compensation can be added to the design that “bridges the gap” between the ideal base curve for the prescription and the actual curve on the lens puck. The effect is as if every patient were receiving a lens made with the ideal base curve for the prescription. This will result in larger elds of clear vision for many wearers and consistently excellent performance across the full prescription range (Fig. 4).
The problem is even more signi cant for patients with signi cant amounts of astigmatism. As mentioned before, optimizing for both sphere and cylinder powers requires an atoric lens; these lenses are dif cult to manufacture and have limited availability. With freeform manufacturing, however, lenses can be made atoric as part of the surfacing process, greatly improving the visual experience for patients who have astigmatism.
Another advantage of freeform is the ability to account for the position in which the lenses are worn by the patient. This wearing position is a result of the interaction between the frame chosen by the patient and the patient’s facial features. The distance between the front of the eye and the back of the lens (vertex distance), the vertical tilt of the frame (pantoscopic angle) and the horizontal curve of the frame (face-form wrap) can create a difference between the powers prescribed by the doctor and the powers experienced by the wearer.
The freeform customization process allows the wearing position to be considered in the design of the lens, resulting in lens powers that are compensated for the position of wear. As a result the powers of the lens are not the exact powers prescribed, but allow the patient to experience the effect of the prescribed powers when wearing the lenses. This means sharper vision for the patient than can be provided with uncompensated lenses. This type of compensation has two implications for the dispensing of eyewear. First, the dispenser must measure the vertex distance, pantoscopic angle and frame wrap with the patient wearing the chosen frames. This can be done with manual tools, but is ideally performed with a digital centration device,
such as the ABS Smart Mirror or Zeiss’ i.Terminal 2. Lenses delivered from the lab will include a sheet showing the prescribed powers along with the compensated powers. This requires the dispenser to trust that the powers were compensated correctly, but ultimately it is the only way for the patient to receive the precise visual experience that the doctor intended.
BEYOND SINGLE VISION: AUGMENTATION FOR YOUNGER WEARERS
The development of the original single vision lenses was in part an acknowledgement of one of the dif culties of civilized living: reading is challenging to the eyes, especially older eyes. In contemporary life, the challenge is even greater, thanks to the almost universal use of digital devices among Americans of all ages. These challenges don’t just affect those who have reached the traditional age of presbyopia; they affect younger wearers as well. In fact, the greatest number of complaints about digital eyestrain come from wearers under 40.
One problem is that digital screens don’t have the clarity or contrast of printed pages, and pixels that comprise a digital image (either of text or graphics) can be confusing to the eye. In addition, on average we check our smartphones about 52 times per day, and the constant refocusing required can tire the eye’s ciliary muscles, causing fatigue.
Another issue relates to how we hold a smartphone or other handheld device. On average, we hold the phone about 25 percent closer to the eye than we tend to hold printed matter. We think of presbyopia as beginning when we can no longer hold reading material far enough away to see it clearly. But the decline in near visual acuity actually begins when we are quite young, and the nearest point on which we can focus continually moves farther away. Since we hold smartphones closer than books, it only makes sense that we lose the ability to focus comfortably on them earlier in life. That means that people below the traditional age of presbyopia who are heavy smartphone users can experience presbyopia- like symptoms that a conventional single vision lens can’t relieve.
The solution is a lens that provides distance vision with an area of low add, typically 0.50 to 1.00 diopters, below the 180 line. This digital “bump,” positioned for the typical viewing angle of a handheld device, alleviates some of the eye’s focusing effort, relieving symptoms of digital eyestrain (Fig. 5). These lenses
EYEZONE Issue 91-92 May / August 2020 62