Published on 07/18/2005

High Index Lenses are much thinner than conventional lenses. Look here to see how and why.

Degrees of Thickness

As anybody who has had to wear thick lenses can tell you, it can make you a little self-conscious and you would do anything for something a little thinner. Lenses are made from a variety of materials. Many of these materials are designed to be thinner than a conventional lens. This means that they have a higher index of refraction. Index of refraction refers to the lenses ability to bend light. The prescription of a lens is determined by two main factors; the lens?s index of refraction and lens curvature. The higher the index the less curvature that is needed thus the lens can be processed thinner. The lower the index the more curvature is needed for the desired prescription thus making it thicker.

Often you may hear the optician or doctor referring to these lenses by numbers such as such as 1.60, 1.66, and 1.70. These numbers represent the refractive power of the lens. The higher the number the thinner the lens. There are several other benefits to a high index lens. Lenses with a higher index often have a greater tensile strength than other conventional materials. This tensile strength is around 15,000 psi! This makes them suitable for drill mount glasses.

But because a lens is thinner and tougher does not mean that it is without flaws. The higher the index the more reflection that occurs off of the lens surface. The higher the index the more resistance is given to light. This resistance increases the amount of light that is bounces off. For instance a conventional plastic lens with an index of 1.498 will cause 8% of the light to reflect off of the surfaces creating 92% transmission. A lens with the index of 1.70 will cause 13.6% of the light to be reflected creating only 86.4% transmission. The application of an anti-reflective coating can greatly increase light transmission.

Chromatic Aberration

Another aspect of these lenses that wearers often find annoying is a phenomenon called chromatic aberration. Looking through a lens with a high index you may notice that there is a color hue around certain images. The further you move from the center of the lens the more noticeable it becomes. High index lenses have a very high ability to create dispersion. When white light passes through one of these lenses it begins to break up into its component colors. This can be minimized by choosing a frame that is smaller and allows the eye to center well within it. It often takes no longer than a day or so to fully adapt.

Depending on your needs a high index lens may be ideal for you. They are cosmetically appealing and are available in many lens designs including progressive lenses. Proper frame selection and a thin lens can drastically improve appearances.