Spectacle Lenses
Glasses, also known as eyeglasses or spectacles, are vision eyewear consisting of glass or hard plastic lenses mounted in a frame that holds them in front of a person’s eyes, typically utilizing a bridge over the nose and hinged arms which rest over the ears.
Breaking Out of the Coke Bottles
Left to Right: plastic, polycarbonate, and high index lenses in the same power. Note the differences in edge thickness
Many of our patients with high prescriptions have a common description for their eyeglasses: “coke bottles.” A lot of them are not aware that there have been significant improvements in ophthalmic plastics, allowing for better cosmetic appearance with the use of thinner lenses.
All ophthalmic lenses have a certain “index of refraction,” which is a number used to indicate the material’s ability to bend light. Generally, the higher the index, the greater the ability to bend light; thus, a material with a high index will often dramatically cut down on lens thickness for higher prescriptions. The material is not as dependent upon increased thickness to satisfy the prescription power needed. Another bonus is that the higher-index materials are more impact-resistant, and lighter in weight.
Regular plastic lenses (CR-39) came along as an alternative to the weight of glass lenses. Gradually the optical industry has developed improved plastics that maintain excellent optical quality. The first was polycarbonate, which today is still a good option for moderately-high prescriptions. Polycarbonate is thinner and lighter than CR-39 plastic. Newer high-index materials, like Trivex, are even thinner than polycarbonate.
HINT: If you have a large prescription and want to improve the appearance of your spectacles, consider a smaller frame in conjunction with a high-index lens material.
