Generally, the higher the number, the thinner the lens. For instance, if a lens has an index of refraction of 1.67, then light is moving 0.598 the speed in air. The higher numbers indicate that light is moving more quickly through the lens.

Every lens material has an index of refraction. Along with high index, mid index, and ultra high or hyper index lenses are available.

• Normal Index: 1.48-1.54

• Mid Index: 1.54-1.64

• High Index: 1.64-1.74

• Ultra High Index: 1.74 and above.

In the United States, high index lenses are predominantly made from plastic materials, while in Europe, much of the high index market belongs to glass lenses. Glass is a preferred material due to its range of uses, including telescopes, cameras, and medical equipment. It is also available in very high indices such as 1.8 and 1.9. The niche for high index glass is small in America.

This is partially due to the fact that it must be prescribed as being visually necessary, not for cosmetic reasons. The FDA standards in the US are strict, and glass lenses also take longer to fabricate. Lastly, while they may be extremely thin, they are still dense and heavy.

Plastic lenses are more common. They aren't restricted, are not as heavy as glass, and are available in many different lens designs and varieties. High index lenses are also versatile enough for use in modern rimless and 3 piece mount frames.

High index lenses tend to have a lower specific gravity than their traditional glass and plastic counterparts. Translated, these lenses are less dense. Other aspects, such as center thickness, decentration, and frame choice will also impact the finished product. The advantage of this is that a lens can be created using less material, therefore having less bulk and weight.

While high index lenses will provide a more attractive lens all around, there are some factors that should be considered. Abbe value determines the optical integrity of the lens. The higher the abbe value, the more apt the patient is to experience visual discomfort from aberrations in the lens material. Often, lenses with higher refractive indexes also have lower abbe values. It is a delicate balance to find a lens that not only satisfies in terms of aesthetics and weight, but also features acceptable optical clarity.

Many high index materials are also available in an aspheric design. This allows the front curvature of the lens to gradually change toward the periphery. In the case of a minus lens, the curvature will get gradually steeper, while it will flatten in a plus lens. This provides the patient with consistent visual clarity throughout the entire lens, not just the center. Cosmetically, the patient's eyes will not look overly magnified in a plus lens or minified in a minus lens.

The aspheric design itself produces a thinner lens with a more attractive profile. This works well with the properties of high index material to remedy weight and thickness in a two pronged approach.

Minus lenses are thicker at the out edge. A high index lens decreases this, diminishing the 'coke bottle' effect when seen from the side. Internal reflections will also be decreased.

The benefits of high index lenses are more apparent to near sighted patients than to the far sighted. The change in lens material may allow for a thinner center thickness. When matched with an aspheric design, more cosmetic benefit will be noticed in a plus lens.

Lenses with higher indices also reflect more light than standard plastic and glass. Because of this, anti-reflective coating is especially important. The additional reflections will be distracting in day to day activities such as night driving and computer use. The anti-reflective coating will add to the over all aesthetics of the lens by eliminating these reflections in photographs and in person. In fact, most high end high index lenses include anti-reflective coating.

With all the advances that are made in the field of eye glass lenses, the frame selection is also critical. Most patients are best suited with smaller frames. Since there is less physical lens being used, the weight and thickness is already influenced. For near sighted patients, this provides less center thickness, and less edge thickness.

Smaller frames also limit the amount of aberrations the patient will experience. This is helpful to patients with any prescription, but notably for those with stronger amounts of correction. All things being equal, frame choice has the most influence over the thickness of the finished product. When fitting, special attention should be given to changes in base curve, lens material and center thickness. The FDA requirement for center thickness varies and is dependent on the lens material. In higher prescriptions, the 0.5 mm difference between CR-39 and polycarbonate can make a difference both in regard to weight and lens profile.

A binocular pupilary distance should be taken, and care given to the centration of the patient’s eyes in the frame, along with both face wrap and pantoscopic tilt.

Ultraviolet light rays are as damaging to the eyes as they are to the skin. Most high index lenses protect against both UVA and UVB rays.

As patients become more involved in choosing eye wear, the market has increased with options.

Some examples:

• 1.60 Transitions V, Sola Finalite, Hoya Eyas, Trivex

• 1.67 Hoya Eynoa,, Nikon Lite, Seiko 1.67, Pentax Light and Thin, Truclear HD, Varilux Physio 360

• 1.70 Hoya Eyry, Essilor Thin&Lite, Nikon Lite

• 1.74 Optima Hyper Index, Nikon NL5-AS, Varilux Ipseo, Essilor Thin&Lite

• 1.80 Zeiss Tital (glass)

• 1.90 Zeiss Lantal (glass)

Photochromics are available in many lens designs and materials. Tinting, polarization, and mirror coatings should be considered.

Those with stronger correction will benefit from the high and ultra high index lenses. Patients with 4.00 D of correction are good candidates, while those with 7.00D and up would be better suited for higher indices (1.70 and up ). Patients who choose larger frame styles or wrap styles should also consider a higher index lens.