Old Dog, New Tricks

While a great deal of work was placed in improving lens materials, processing, and coatings, the emphasis placed on the nature of optical design as it applied to spectacles was relatively unexplored. Aspheric lenses were nothing new. They had, however, not managed to find their way out of scientific or specialty ophthalmic applications. From the 1960's through the 80's aspheric lenses of various types were primarily marketed for visual correction of aphakia. Over time, the need to address the comfort and cosmetic concerns of the average high-power prescription patient became more urgent. The general trend in the market was, and still is, lighter and thinner. Aspheric lens design offered the means to address these issues while improving visual performance.

A Warped Perspective

To understand what aspheric lens design is all about, you must first take a look at some of the limitations of traditional spherical designs. Spherical lenses have a uniform curve which runs across the entire surface of the lens. Various types of chromatic and astigmatic aberrations occur in spherical lenses. If all patients had relatively good vision and required only minor refractive correction, and if we moved only our heads, instead of out eyes, spherical design would be more than adequate. The fact is, however, that many patients require significant amounts of refraction to achieve good visual acuity. In spherical design, these high diopter requirements result in much thicker edges or centers, which are cosmetically unappealing, uncomfortable due to weight, and can place limitations on frame selection.

An additional problem encountered is the optical distortions or aberrations caused by spherical lens design. Objects viewed through these lenses can appear out of scale to reality and in the lens periphery, objects and surroundings can take on a bowed or spherical contour. The more the patients eye pans away from the optical center of the lens or off-axis, the more distorted the image becomes, thus limiting best visual acuity to small functional zone surrounding the optical center. Depending on the degree of correction, off-axis performance can shift dramatically from the on-axis acuity at the optical center. These astigmatic aberrations are compounded by color fringes or chromatic aberrations, caused by differential refraction of light waves at the periphery.

Hey, Nice Curves!

Aspheric design lenses combine multiple curves in their front surface which allow for improved off-center optical performance without large increases in thickness. Aside from the improved optical characteristics, this greatly enhances the cosmetics of the spectacles in a variety of ways. First, and most obvious, is that the lens doesn't have that "coke bottle" appearance. Second, is that the lens can be housed in a much wider array of frames. Many patients have been frustrated after being told their first, or second, or even third fashion frame selection could not accommodate their high prescription requirements. Let's also not forget that the lenses are far more comfortable to wear due to the diminished thickness and weight. Finally, and perhaps most importantly, is that the complex curves of the aspheric lens do not cause the dramatic "bug eye" or magnification effect inherent in high power spheric designs.

Pretty yet Practical

Given all the cosmetic advantages imparted by aspheric lenses, one might expect to sacrifice something in terms of optical performance. The fact is aspheric lenses perform better than their spherical counterparts in a number of key areas. The main advantages to aspheric design to keep in mind when informing prospective patients are:

1. Minimized center thickness in plus power lenses
2. Minimized edge thickness in minus power lenses
3. Minimized off-axis aberration creating a larger overall visual field
4. Better appearance due to the flatter lens profile
5. More frame choices for higher powers
6. Reduced magnification and minification in plus and minus powers respectively

Complex Curves Mean Complex Processing, Right?

Since the complex curves of the aspheric lens are in the front surface, prescriptions for these lenses can be processed in much the same way conventional spherical design lenses. Aspheric lenses are available in all materials and can be provided with every premium feature available to spherical lenses.

Article provided courtesy of Optical Services International