Free-form technology enables lens designers to control aberrations and distortion in single vision lenses.
With all the excitement surrounding progressive addition lenses (PALs) designed and processed using free-form technology, it’s easy to lose sight of the fact that single vision lenses benefit from free-form technology, too. Here are a few examples of how.
Eyeglass lenses have always been plagued by a number of optical aberrations. These aberrations reduce the quality of the lens’ focus, which, in turn, affects the quality of the vision the user obtains from the lens. Aberrations are unavoidable in lenses, because as light interfaces with a lens surface, it does not always pass through undeviated. The minute errors of refraction that occur collectively become the aberrations of the lens. Any lens of good quality attempts to reduce these aberrations to the smallest degree possible.
Some optical aberrations are more troublesome than others. For example, marginal astigmatism is considered the most significant aberration because it is experienced as the eye scans around the lens, away from its optical center. The result of this aberration is the addition of unwanted cylinder and sphere power to the patient’s lens power. The severity of the problem depends on the patient’s prescription, how far away from the optical center the patient looks, and the positioning of the lens to the eye.
Premium lenses designed and processed with free-form technology now enable designers to address coma and spherical aberrations, as well as others. Free-form technology empowers designers to tackle distortion handily, too.
What all this means is that contemporary single vision lenses are far superior to those created by conventional design and surfacing methods. While their designs are not as complicated as PALs, ECPs should employ single vision free-form lenses because they provide the finest optical quality and visual performance a patient can buy.
The advances in single vision free-form lenses are directly proportional to that of free-form progressive lens technology. With computer-aided design and computer numeric control (CNC) manufacturing, a lens designer can choose the optimum base curve, ocular curves, and aspheric curvatures that optimize each prescription. The result is a sharper, wider, and undistorted view across the entire lens. Additional benefits include better contrast and color recognition.
As with premium free-form PALs, frame position is now taken into account with free-form single vision lenses. Specifying where a frame and lenses will sit in front of a patient’s eyes enables the lab to compensate for positional effect and tilt, which affects the power the lens delivers to the patient. In order to obtain this benefit, ECPs are asked to measure the vertex distance, pantoscopic tilt, face form (wrap angle), and vertical optical center height for the patient’s chosen frame. The frame must be pre-adjusted or the measurements will be inaccurate, so be sure not to overlook this important step. The end result of the lab’s compensation is a pair of eyeglasses individually optimized for that patient.
Single vision free-form lenses are also made to a power accuracy of 0.01D, just like their premium free-form progressive siblings. This degree of accuracy is a result of the precision in designing and processing these lenses, and is the reason that they perform so well.
Richard W. McCoy is an opticianry instructor at Hillsborough Community College in Fort Myers, FL.
WHERE TO FIND IT:
Carl Zeiss Vision, Inc. 800-358-8258 • zeiss.com/vision-care
Essilor of America, Inc. 800-542-5668 • essilorusa.com
HOYA Vision Care, North America 877-528-1939 • hoyavision.com
Seiko Optical Products of America, Inc. 800-235-5367 • seikoeyewear.com
Shamir Insight, Inc. 877-514-8330 • shamirlens.com
Signet Armorlite, Inc. 800-759-0075 • signetarmorlite.com