Designing Better Progressives
Progressives must meet the physiological needs of the patient i.e., deliver binocularity, excellent near and distance vision and wide clear comfortable fields of view at all distances. To better accomplish this, new research and manufacturing technologies have found that continued reduction of the blur created by the unwanted astigmatism is critical. The distance prescription affects the placement of the near zone. In addition, the habits and physiology of the myope is different from the emmetrope and the hyperope. Satisfying each type of patient requires new precision in designing lenses that reflect the differences in the way that each patient uses their eyes. In addition, some patients still complain that one eye has somewhat better vision than the other. Localized inconsistencies in the eye system, result in less than clear vision. Therefore, to improve sight, one must be able to measure these effects and provide methods to be used to create a spectacle lens to globally correct them. First, let’s review the effects of prism.
When converging, the distance prescription prism changes the amount the eye needs to turn in to read; myopes converge less than emmetropes, hyperopes more. In addition, the eye of the myope is longer than the hyperope and the shape of the retina at the point of clearest vision is also different. This affects the location of the reading zone and the way the surface power is managed.
When looking through a prism, light is bent towards the base of the prism, the image moves towards the apex. Since all lenses are prisms, when the eye rotates away from the optical axis (optical center) it looks through a prism. When reading, the eye turns in and for a minus Rx, the eye looks through a base in prism, for plus prescriptions, base out. As a result, the myope converges less, the hyperope
Modern progressives’ near zones have variable inset i.e., are designed so that the near location varies based on the distance prescription. In minus prescriptions, near zones are inset less, for plus the near is inset more. This results in better reading vision because the near zone is located where the patient is looking and provides better binocularity (one image from two eye). It’s like looking through a pair of binoculars; vision is only comfortable when both eyes are seeing the same image. Variable inset ensures better reading comfort.
In new Accolade Freedom, eye and frame shape are considered when the prescription is created. The design is changed to improve distance area in large frames and shorten the corridor in smaller frames, all automatically.
Precision fit and an automatic adjustment of the design creates an optimal result for patients, optimizing frame size, PD and fitting height while providing the optician the ability to give patients a more personalized progressive. It provides the optician with more confidence that the design will fit. It also allows the optician the ability to reduce the number of progressives that need to be considered. In a world of so many wonderful and competing designs, this simplifies the task but automatically includes the latest design and manufacturing technologies.