Spectacle Lenses – Design

Spectacle Lenses
The Basics

Single vision spectacle lens designs, where the lenses are formed to minimise unavoidable aberrations, have remained very similar over the last 100 years or so- this is not because there is no investment or research, but more a testament to the knowledge of our predecessors!

These “best form” lenses are still used today and are designed using spherical curves which do not change across the lens surface. Recently however, there have been some marked advances in spectacle lenses, for example aspheric lenses and Varifocals. Both these lenses could only be designed using complex mathematics, and could only be manufactured with the advent of computer controlled equipment- aspheric lenses were invented in theory years before their manufacture became possible!

There are basically two important properties of a spectacle lens- its refractive index (the material’s ability to bend light) and the curvatures of its surfaces. To maintain the power of the lens, if one goes up the other can come down, which means that Higher Refractive Index Lenses can be made with flatter curves, which results in thinner lenses. High refractive index lenses make a marked difference in Minus Lenses especially.

It is worthwhile to mention that when selecting Hi-Index lenses, the Abbe number is important- this gives us a value of the material’s tendency to cause chromatic aberration, which the wearer may see as coloured fringes at the edge of objects. A high Abbe number is preferable.

An aspheric lens has a curvature similar to a traditional best form lens at its centre, but the curvature slowly decreases towards the edge of the lens. Not only does this improve the optical performance of the lens- the wearer will see better than before when looking towards the edges of the lenses, but it also results in lenses which are flatter and thinner than with traditional curves. Aspheric lenses make a marked difference, especially in Plus powers. Aspheric lenses cause less magnification than standard lenses, giving the wearer more natural vision and minimising the apparent size of their eyes to others.

Anti Reflection coatings can dramatically improve the appearance of any lens, allowing others to see the wearers eye and reducing annoying reflections when at the computer or night driving. These are especially important with Hi-Index lenses (many of which come coated as standard) because they would otherwise cause more reflections than normal index lenses.

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