A stunning 75 percent of ophthalmic lenses sold globally are standard single vision lenses (spherical and aspheric). Roughly 80 percent are in stock finished single vision (FSV), and 20 percent in surfaced semi-finished single vision lenses. While the standard single vision lens category has served us well all these years, there was ample room for improvement especially in the lens peripheral optics.
The poor peripheral optical performance in standard spherical single vision lenses is made worse when the wrong base curve is used to meet the demand for flatter and thinner lenses. Best-form or corrected curve theory tells us that spherical lenses have an ideal base curve for the Rx power that will minimize oblique aberrations, which means that each lens power prescribed should be matched with an ideal (best-form) base curve. But in the real world, this is impractical. Inventory requirements to match every Rx power with its ideal base curve is unfeasible. So instead, the range of prescriptions used for a base curve falls outside the range for a best-form base curve. The result increased lens aberrations affecting visual clarity and limiting the clear field of view through the lens periphery. In addition, the base curve of a standard spherical SV lens affects magnification, lens bulge, thickness, weight and peripheral lens optical performance. Even when best-form is used, the lens can be thicker, heavier and more bulging, and these attributes clearly do not appeal to the demand for thinner, flatter and more cosmetically appealing lenses.
To make matters worse, upwards of 70 percent of all ophthalmic lenses have cylinder correction. Sphero-cylinder lenses have a spherical power meridian and a cylinder power meridian. This forces the lens designer of standard single vision lenses to compromise since standard spherical lenses have a single base curve and a toric (two curves) back surface. The lens designer had to choose whether to use the best-form base curve to minimize power aberrations in the spherical meridian or the cylinder meridian, or for a spherical equivalent (average of the sphere and the cylinder).
Lens casters initially responded to this problem by using hi-index aspheric front curves where the curvature changed from the center to the edge. This combination was cosmetically better and produced better optics since the atoric front surface used a different amount of apshericity for the spherical versus the cylinder meridians. While aspheric and atoric and double aspheric single vision lenses improved peripheral optics, there was still a lot of room for improvement. In our March CE, “A Revolution in Single Vision Lenses,” you can learn about ZEISS ClearView and how 700 free parameters are optimized across one entire quadrant of the lens, which is then mirrored two times to cover the whole lens surface. Freeform design optimization is a giant leap in upgrading single vision stock and semi-finished lenses. ClearView is an exciting new product that achieves the cosmetic objective of a very flat and thin SV lens while also achieving the optical performance upgrade that produces excellent lens optics from the lens center to the periphery.
• Deborah Kotob
Pro to Pro Director