You Pay For What You Get
High-index lenses are a practical alternative to ordinary plastic lenses. Although they are somewhat more costly, these innovative eyewear devices can offer those who require stronger prescriptions much more comfortable wear. Optical quality remains consistent - even though the lens is much thinner and more lightweight. Eyecare professionals often recommend this type of lens to appropriate patients.
Newer Frame Designs
Many high-fashion eyeglass frames feature thinner rims than those of conventional eyeglasses. Lens edges may even be totally exposed with many of today's ultra-contemporary rimless designs. In both instances, lens edges are very conspicuous. Thicker edges that are typical of stronger prescriptions can create a less attractive appearance. This is particularly true of nearsighted patients, as their lenses tend to have thicker edges than mid-surface areas are. High-index lenses may be the best option to prevent eyeglasses from dominating observers' view of their entire facial area.
How The Eye Bends Light
The human eye refracts (bends) incoming light in a manner that allows us to accurately visualize the outer world. Sharper visual acuity results when such light is bent to focus on a specific area of the retina. Overall curvature and length of the eye's cornea and lens determine how well the eyeball is able to focus and refract incoming light onto its retina. Refractive deficiencies are essentially eye imperfections that prevent the eye from properly focusing light; thus leading to blurry vision. Farsightedness, astigmatism, and nearsightedness are the primary forms of light refractive errors that patients experience. All such conditions are correctable with the right surgery, eyeglasses, or contact lenses.
Doing More With Less
High-index lenses are capable of greater light refraction than are lenses of a lower index range. The higher an eyeglass index is, the less materials it will require to achieve a given level of prescribed light refraction. Such lenses are categorized by the level of strength with which they are capable of bending light. Relative light refraction indeces range from 1.53 to 1.74 for a high-index lens. By contrast, conventional plastic eyeglass lenses have light refractive indeces that are limited to a range of 1.50 to 1.52. Thus, a lens that features a light refractive index of 1.70 or greater may be at least one-half as thick as traditional lenses without compromising its effectiveness.
Higher-index lenses are typically 20 to 65 percent thinner than those of ordinary plastic construction. Lens power is expressed in units of measurement called "diopters." Nearsighted patients have "minus" diopter lens values. Farsighted patients' lenses fall on the "plus" side. The latter lens type has much thinner edges than centers. Those who require prescriptions that exceed either +2.00 or -2.50 diopters experience notable differences from high-index lens wear. Width reduction is not nearly as dramatic with weaker high-index prescription lenses.
As high-index lenses bend light with much better efficiency, their edges need not be nearly as wide as identical-strength plastic lenses. Thinner edges require the use of less lens materials, thereby reducing overall lens weight. This lighter weight facilitates more comfortable eyeglass wear. High-index glass lenses are also available. They are, however, much heavier than ordinary glass lenses. Thus, they are not nearly as beneficial as their plastic counterparts are.
Farsighted patients with stronger prescriptions require extremely heavy ordinary lenses. Lightweight high-index lenses are therefore extremely beneficial in such cases. These specialized lenses also eliminate the infamous "bug-eyed" look that ordinary lenses often create.
What About Polycarbonate?
Eyecare providers often suggest polycarbonate lenses as an option to high-index alternatives. Although polycarbonate construction features the same aesthetic advantages, it is much less durable and scratches very easily without a scratch resistant coating. Moreover, it is difficult to tint polycarbonate.
High-index lenses do not entail any such tradeoffs. A variety of construction materials may be utilized for different prescriptions. Consult your eyecare provider to determine which option is best for your own vision.