Most eyeglass wearers with a moderate to strong eyeglass revision have probably heard of Hi index lenses at one time or another. In the optical industry today Hi index can refer to any lens material that is thinner than the acceptable plastic, generally known as Cr39. In the last 10 to 15 years many new Hi index materials have been developed, production it more confusing for the midpoint consumer to know what is best for them.
[b]Lens[/b]The term Hi index is a reference to the index of refraction of a particular lens material. The index of refraction is a measure of how much a lens material will bend or refract light. When referring to Hi index lenses, the Eye physician or Optician may refer to the index of the lens. For example, Hi index 1.60 or Hi index 1.67 lenses. In general, the higher the index of a lens, the thinner it will be for a given eyeglass prescription. As a point of reference, the index of a plastic Cr39 lens is 1.49.
As a professional eyeglass dispenser, my suggestion for Hi index lenses is primarily determined by the force of the eyeglass correction. For eyeglass prescriptions that are less than +/- 2.00, the benefits of hi index lenses for reducing weight and thickness are negligible. Often times the added cost associated with Hi index lenses are not justified for lower corrections. If the eyeglass prescribe is greater than +/- 3.00 I would normally suggest a 1.60 Hi index lens. This lens material is 25 to 30% thinner that Cr39 plastic and will supervene in a lens that is noticeably thinner and more enchanting for the wearer. For prescriptions that are greater than +/- 5.00 a 1.67 Hi index lens would be my lens of choice. A 1.67 Hi index lenses is 40% thinner than Cr39 plastic and works verily well in this prescribe range.
The most recent generation of Hi index lenses are 1.71 and 1.74 index lenses. I will often times refer to these as ultra or hyper index simply as a means of differentiating them from the other Hi index materials. These lenses I will generally suggest to habitancy with corrections greater than +/- 8.00. The 1.74 Hi index lenses are nearly half the thickness of the Cr39 lenses. If you are one the few habitancy with eyeglass lenses in this range I can tell you that the cosmetic benefits of the 1.74 lenses are astonishing.
In addition to being thinner, Hi index lenses also have the advantage of being natural Uv blockers. Because of this it is not needful to add a Uv safety or coating to these lenses. Hi index lenses come acceptable with a scratch protective coating. Some Hi index lenses also contain an anti reflective coating, which is very useful because Hi index lenses will have more glare due to the increased density of the material.
In conclusion, these recommendations are guidelines and there are exceptions. For example, when it comes to childrens eyewear the lens material of choice is polycarbonate or trivex. These materials have a much greater impact resistance and there for are much safer. For full rimless frames, trivex or 1.67 hi index lenses are recommended. Other lenses materials are not good choices for drilling and tend to break very verily in a full rimless frame. To perform the best lens profile with a high index lens it is prominent to select the smallest frame that is still cosmetically appealing. choosing a frame that your eyes are centered in will work the best. A frame that is too big can add thickness to a lens, especially with higher corrections.
understanding Hi Index Eyeglass Lenses
0 comments:
Post a Comment