The index of refraction is an expression of how much lens material bends light.
Understanding the differences in the types of Hi-Index, or high density, lenses is not difficult once you know some basic details about the lenses. Hi-Index lenses are thinner and lighter than standard optical lenses and available in different materials and densities. The key to understanding the different choices in high index lenses involves a number, called the Index of Refraction.
Index of Refraction
The index of refraction (IR) is the difference in the speed of light as it passes through air and into lens material, and is expressed as a number. Regular plastic lenses have an IR of 1.50. High index lenses bend light more efficiently because the lens material is compressed. Thinner lenses have a higher IR number.
Plastic and Glass Lenses
Plastic is the most common material used in spectacle lenses. Standard plastic lenses have an IR of 1.49 or 1.50. Glass lenses are not as popular as plastic because glass is about twice as heavy. Standard glass lenses have an IR of 1.52. A lens with an IR higher than 1.52 is considered to be a Hi-index lens. The most common range for hi-index lenses is between 1.54 and 1.74.
Polycarbonate
Polycarbonate lenses are lighter than plastic and recommended for children, safety and sports because the material is impact-resistant. Polycarbonate is a hi-index material with an IR of 1.586. The lenses are lighter but, in certain types of prescriptions, the optics are not as crisp or clear as with plastic or glass lenses.
Advancements in Technology in Hi-Index
A lightweight lens, called Trivex, and manufactured by PPG is now available with clearer vision than polycarbonate. The lenses are still lightweight and shatter-resistant but because of the way they are manufactured, the lenses have crisper optics. Trivex has an IR of 1.53. The lenses are more scratch-resistant and tintable than polycarbonate but are slightly thicker.
Choosing a Hi-Index Lens
Your eye care practitioner or optician will help you select the best high density lens for your specific needs. Lens selection usually depends on the strength of your prescription and the size and type of your frame.
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