Hi Index in Eyeglasses

Most of today’s wearers of eyeglasses are not aware of their eyeglasses materials much more the Hi index in determining the thickness of eyeglasses. Often enough, such technicalities of how the prescription works should be left to the eye experts. But what does Hi index mean?

Hi index in today’s optical industry refers to the material of the lens that is thinner than the standard CR39 — the standard plastic. Within the last 10-15 years, the industry has successfully developed Hi index materials. Technically, the term Hi index is the index of refraction of the material of the lens. Index of refraction is a gauge of the amount of light that the material will bend or refract. Examples of hi index being referred to by opticians are Hi index 1.60 or Hi index 1.67 lenses. The higher the number of the index number, the thinner the lens of the prescribed eyeglasses is.

The standard plastic or CR39 which has an index of 1.49 is a point of reference. Opticians would always recommend to their patients to consider the strength of the eyeglass correction. Higher Hi index lenses mean reduced weight and thickness which would be more comfortable for the user but may not be the best choice.

This aspect should best be discussed with one’s optician for better combination of maximum eye correction and comfort. However, Hi index glasses normally offer natural UV protection as well. And most of them are also scratch resistant due to their special coating.

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