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Every direction the eye care professional turns, he or she is bombarded with information about the benefits of high index. But, is it thinner, lighter, better? But what is high index? How is it actually defined? What are the drawbacks to high index and how can they be minimized? To help understand the nature of high index materials, the eye care professional must understand the factors that determine how a lens material is defined. These factors are refractive index, specific gravity, ABBE value and impact resistance.

Refractive Index

The technical definition of high index, within the United Sates, is any lens material that is over 1.530. However, for all practical and marketing purposes, high index is actually any lens with an index above conventional glass or CR-39. Some manufacturers have started to use the term mid–index for lenses in the 1.54 to 1.60 range. However, this term is ill defined and not universally utilized. In plastic lenses, the higher index materials are often derived from polyurethane mixed with sulfur to enable the lens material to bend light to a greater degree than a conventional plastic such as CR-39. In high index glass, metal oxides, such as flint or titanium, are added to crown glass to effect the substrate’s light bending ability.

To help determine what the index of refraction for a lens material is, a lens manufacturer measures the rate of speed that light passes through the substance. This enables the manufacturer to calculate the refractive power of the lens as well as part of the dispersive power of the prisms within certain lens materials. Most manufacturers’ use the yellow wavelength of light (Nd) for this calculation. Yellow is used because human beings see the color of yellow first and foremost. Some manufacturers use the green wavelength of light for this calculation and it can lead to misconceptions about the true index of a lens. The equation for this is:

N= Speed of light in air
Speed of light in material

So, a lens material that has light traveling through it at 110,000 miles per second will have an index of refraction of 1.69. 

Since higher index materials refract light so effectively, they can utilize a thinner prism base to create the same effect as a lower index material. For example, a minus lens with an index of 1.50 would have to be made with a thicker prism base (lens edge) than a minus lens with a 1.69 index. In addition, the lens can be created with a shallower base curve creating a flatter surface. In other words, a -4.00 lens created out of a lens substrate of 1.69 will be approximately 25% thinner than a lens created out of a material with an index of 1.53.

Specific Gravity

Specific gravity, or density, is a measurement of the weight of the lens material. Density is generally measured in grams per cubic centimeter and it is determined by the ratio of the mass of the lens substrate to the mass of an equal volume of water at 4 degrees Celsius. The lower the density the lighter the lens and the more comfortable the glasses are for the patient. Common densities for lens materials are:

In general, when discussing plastic lenses, there is no direct correlation between the index and its density. This is not the case however with high index glass lenses. High index glass have a much higher density than crown glass, especially in mild to moderate powers, therefore the weight reduction in these lenses is insignificant. In fact, some high index glass lenses can be heavier than the thicker crown glass. The use of lead oxide in some high index glass causes the weight increase.

ABBE Value

ABBE value is simply a measure or scale of how much light is dispersed when it enters a specific lens material. Every material has its own ABBE value. Contrary to a popular myth, there is absolutely no correlation between a lens refractive index and its ABBE value. To illustrate this point, look at the original index of crown glass versus Trivex. Both have the identical index of 1.530 yet significantly different ABBE values. Since white light is composed of multiple colors, each color within the spectrum is bent at a different angle than the other during refraction. Therefore, each component of light has its own refractive index. Blue light has a higher refractive index than red light and is therefore bent more than red when it passes through a lens. The result is chromatic aberration or dispersion. This dispersion results in chromatic aberration that can, at times, be observed by the patient. However, most frequently the complaint is that off axis viewing is more blurred than central viewing due to the color images overlapping one another.

The degree to which a lens disperses light is commonly referred to as ABBE value or constringence. Lenses with a high ABBE value have less chromatic aberration than those with a lower ABBE value. Generally, mid – index and higher index materials have lower ABBE values than the conventional lens materials of CR-39 or crown glass. 

Please note that ABBE values are approximate and the same material may have a slight variation from manufacturer to manufacture depending on the manufacturing process that the lens goes through.

Minimizing Problems
Taking Measurements and ensuring fit

Although there are several factors that help to ensure patient satisfaction when fitting a high index lens, the main factor is taking accurate measurements. It is important that the eye care professional take monocular PDs and determine the optical center placement both vertically and horizontally. The frame:

· should fit well.
· have a minimal decentration. 
· have a pantoscopic tilt is between 10 and 15 degrees in most instances.
· exhibit an effective diameter (ED) within 2mm of the frames A dimension. 

Determining thickness

Once these measurements are taken, it is best to determine the lens thickness to ascertain if it meets the needs of the patient. This can be done by utilizing the sag approximation formula and adding the result to the predetermined center thickness or by applying optical calculators such as those found on OptiCampus.com. For reference, the sag approximation formula is:

Sag = ((d/2)2 X D) / 2000(n-1)
where d is diameter in mm
D is power
n is index

Utilize Anti-Reflective Coating

Do not sell a high index lens without anti-reflective (AR) coating. This may seem extreme, but an uncoated CR-39 lens reflects 8% of light, whereas an uncoated high index lens will reflect up to 50% more than CR-39. A rule of thumb is the higher the index, the greater the amount of light reflected. This can lead to increased difficulty with night driving as well as eye fatigue due to decreased light transmittance. With an AR coated lens however, the light transmittance can increase to 99.5%. As a result, reflections and chromatic aberrations are reduced, the patient has a clearer view of his or her surroundings, and the glasses have a better cosmetic appearance. By applying AR, the rate of non-adapt to the visual differences in high index material over lower index materials will decrease. Fortunately, AR coatings have really improved over the last few years and are an essential enhancement to high index with the new scratch, dirt, and oil resistant formulas available.

Know your patient

Does the patient have a high prescription? Work with the public? Want a high fashion or minimalist look? Is he or she a child or very active? All of these questions help determine how well a high index lens will fit with a patient’s lifestyle.

The thinness of high index makes it an ideal lens material for individuals with a higher prescription. A general guideline is to recommend high index materials for any patient with a prescription of +/-3.00 D or more – depending on the frame parameters. With minimal decentration and a reasonable size, high index may not be required at this prescription level. Anything less than this prescription and the weight and thinness benefits are usually minimal.

High index, especially the index of 1.67 and 1.70, is an excellent choice for individuals who want the minimalist look of drilled rimless eyeglasses. Due to the thinness of the material’s edge, the lens does not look out of place or heavy when placed in a drilled rimless. In addition, high index materials do not crack as easily when drilled and the hole maintains its size and shape more readily than lower index materials such as CR-39.

It is also essential for the newer, larger frame designs that are making a comeback. With the increasing popularity of larger frames, edge thickness problems due to decentration are going to become more prevalent. High index can eliminate some of this but remember that it is very important to consider the lens prescription when fitting a larger frame.

High index lenses are not ideal for everyone. One such instance is children or individuals who have severely decreased vision in one eye. In these cases, the safety and impact resistance of polycarbonate is necessary to protect the patient’s vision. Also, the flatter base curves of high index lenses prevent the lenses from fitting into the wrap designs that are popular in some frame designs and are therefore not recommended. Another frequently forgotten aspect of polycarbonate is the thinness of the center. Frequently a polycarbonate lens with a 1.0 mm center will be equal to or thinner than some high index lens materials with more substantial center thickness – and safer as well.

Conclusion

High index is the most advanced lens material available today. When utilized properly it provides a thin profile, lighter weight and advanced optics to the patient. By taking all factors into consideration, the eye care professional can utilize high index lenses to provide a superior product to the patient as long as the ECP remembers to utilize good optical principles along with the lens selection. We cannot expect the lens to overcome poor frame decisions, so be sure to cover the basics prior to recommending the high index lens material. Putting solid optical practice together with a high index lens material can provide the patient with the ultimate eye care experience.

With contributions from: Brian A. Thomas, P.h.D, ABOM

Carrie Wilson
BS, LDO, ABOM, NCLE-AC