Physicists, mathematicians, chemists and engineers are creating, patenting, and marketing new lens technology at a breathtaking pace. Here is an overview of the theories driving modern ophthalmic lens materials and the most useful lens materials and lens designs for our patients.
Traditional theories of light have described it a ray of light as it passes through an optical system to explain the lower order of aberrations called myopia, hyperopia, and astigmatism. However, up to 17% of all optical aberrations cannot be explained with this theory. A new theory of light called the wavefront theory of light is necessary to explain higher orders of aberrations (HOA) that make up 17% of the aberrations. These HOA include:
Coma- described by patients as a comet when looking at a light source.
Trefoil- Quadrafoil and pentafoil not too problematic as it is peripheral.
Spherical- described as halos around light
One cannot think of light as rays of light when explaining these higher order aberrations or methods to correct them. To explain or correct these HOA we have to think of light, not as a ray of light entering the eye, but as a wave of light entering the eye. Think of light as if it were water. When you throw a rock into the water it will create a wave. If the surface of the water is agitated by wind ripples, this would be analogous to coma, spherical aberrations, and other HOA.
To correct for spherical aberrations the periphery of our spectacle lens must have a different power than the center of the lens. Aspheric lenses correct spherical aberration and increases visual sharpness.
Coma is similar to spherical aberration but it does not occur in a spherical dimension, but rather in a comet shaped dimension. Complex mathematical formulas using the wavefront theory of light are required to control coma on the lens.
Many of the concepts incorporated into correcting HOA in ophthalmic lenses were inspired by technology during LASIK eye surgery. Other ophthalmic lens advancements of better polycarbonate and high index materials have been driven by the CD and DVD industry that requires high quality polycarbonate.
Engineers are continuing to discover, patent, and bring to market sophisticated antireflective coating and photochromic materials which cut glare to enhance visual sharpness and reduce visual stress.
The lens materials and lens designs developed in recent years are truly remarkable. Most of these systems are relatively cost effective.
There is such a wide range of ophthalmic lens materials and lens designs on the market it is hard to know which ones to choose. We must consider lens material properties and how to incorporate these materials into an appropriate design that is cost effective and able to be fabricated in a timely manner. The properties most important to assess are specific gravity, index of refraction (n), impact resistance, scratch resistance, clarity, light transmission, cost, and easy processing.
Specific Gravity = weight. The lower the specific gravity, the lighter the lens.
Index of refraction (n) = how much it bends light. The higher the index of refraction, the thinner the lens.
We generally want lenses that have very low specific gravity and very high index of refraction.
These two are related in that a material like Trivex can be very light, but has an index of refraction that does not bend light very much (mid range n =1.53). So you need a thicker lens to correct 4D of myopia. The results is that a higher index of light lens will be lighter because the lens is thinner and this has less mass so it weighs less (even though the hi index material is heavier than Trivex).
Lenses should be impact resistant for safety reasons. Glass is the least impact resistant and polycarbonate and Trivex are very resistant to breakage.
Scratch resistance is crucial as scratches render a lens unusable from a vision standpoint. This can be controlled with hard coats applied to the surface.
Clarity and light transmission should allow as many useful light rays into the eye and keep out the harmful rays. This is controlled with U.V. filters, A/R coating and photochromic and other tints.
Our goal is to give every patient a lens design to allow them optimum vision at every distance under every light condition. To do this we must use complex design system of SV, multifocal, and variable focus lenses made from different materials with various coatings and tints. The lens system that gives optimum performance while enjoying a summer day on a boat cannot give optimum performance while driving at night. The lens system that will provide optimum computer vision for a presbyope will never provide optimum vision while driving or in the reading plane.