1.74 high-index lens material can create a super-thin lens profile.
Controlling lens thickness has always been a prime concern of conscientious dispensing opticians. It’s also been a top concern of patients with stronger Rx’s. They both know too well how unsightly a strong Rx can look in a frame when the frame and lens material have not been chosen to minimize lens thickness. Fortunately, today, there are a number of lens materials of varying indices available for opticians to select to help control this issue. One of those materials has the highest index, 1.74.
Before the advent of plastic lenses, opticians didn’t have much to choose from. Glass lenses were offered in three indices: 1.523 Crown Glass, 1.61 known generically as light flint glass, and 1.69 known as dense flint glass. Glass lens material is heavy and the flint materials are even heavier. While they might have been heavy, these materials are what you used since they were the only ones available. Today opticians have an array of materials made largely from some form of plastic that are much lighter and therefore more comfortable.
A THIN ADVANTAGE
1.74 lens material has a specific gravity of 1.47g/cm3 and an Abbe value of 32. This is in contrast to Trivex‘ material’s 1.11g/cm3 specific gravity and 43-45 Abbe value, polycarbonate’s 1.21g/cm3 specific gravity and 32 Abbe value, and CR-39‘‘s 1.32g/cm3 specific gravity and 58 Abbe value. Note that while 1.74’s index of refraction is the highest of all lens materials, its Abbe value is low and its specific gravity is almost that of CR-39, making it the heaviest plastic lens material commonly utilized. This means that while 1.74 material can create the thinnest lens profile, it’s not for every patient. In addition to being priced higher than lower index materials, in lower powered prescriptions, its thinness advantage can be just a few tenths of a millimeter. That kind of difference may not be worth the additional cost. You’ll probably recommend 1.74 to about 3% to 5% of your patients since they are the ones who have prescriptions higher than +/-5.00D and are best suited for it.
When it’s indicated, 1.74 is a handy and useful lens material.
Ed DeGennaro is Director, Professional Content of First Vision Media Group.
Robert Lerose is a freelance writer based in New York.
|WHERE TO FIND IT:
|Carl Zeiss Vision, Inc.
|Essilor of America, Inc.
|HOYA Vision Care, North America
|Nikon Optical USA, Inc.
|Seiko Optical Products of America, Inc.
|Shamir Insight, Inc.
|Signet Armorlite, Inc.
|Transitions Optical, Inc.
|X-Cel Optical Co.