What is the Best Lens Material for Glasses?
Although the term glasses implies that the lenses inside are made of glass, it is very uncommon today to find lenses made from glass of any form. Instead, a variety of plastics are utilized for prescription lenses due to being more light-weight, less fragile, and easier to manipulate.
Deciding on which plastic material to have your lenses made from is a big decision that has many factors that go into it. Your optometrist or optician should be able to assist you in deciding which material is the best fit for your eyewear needs.
Common Lens Materials in Glasses
Some of the most common lens materials are CR-39 plastic, Polycarbonate plastic, and 1.67 High Index plastic. Each of these materials has advantages and is the most desirable in certain circumstances.
Factors in Choosing Lens Materials for Your Glasses
Two important factors help determine the best material to use for your lenses: index of refraction and specific gravity.
The index of refraction determines how thick a lens needs to be to provide the needed prescription; the higher the index of a material, the thinner the lenses can be made.
The specific gravity determines the weight of the lenses made from the material. The higher the value of specific gravity, the heavier the lenses made from the material are.
Additionally, impact resistance, ability to be tinted, and cost of the material play a role in determining the best material for you.
When CR-39 Plastic is the Best Lens Material for Glasses
If you have a relatively low plus or low minus prescription and do not need additional safety protection from your lenses, CR-39 plastic may be the best choice of material.
Among the oldest plastics to be used in ophthalmic manufacturing, the CR-39 plastic has a very good lens material profile with an index of refraction of 1.52, a specific gravity of 1.32, and can be tinted and manipulated into frames.
The impact resistance of CR-39 is moderate, and it is generally avoided when resistance is a concern. CR-39 is a very affordable material and is the most common material used in many optical labs.
When Polycarbonate is the Best Material for Glasses
If impact resistance is a concern or you have a moderately high plus or high minus prescription, polycarbonate plastic may be the best choice of material.
Polycarbonate plastic has an index of refraction of 1.586, a specific gravity of 1.20, and is very impact resistant. This allows lenses to be thinner, lighter, and much more impact resistant than CR-39.
Polycarbonate is considered the best choice of material for children and for safety glasses.
When You Should Choose 1.67 High Index Plastic
If you have a very high plus or very high minus prescription, 1.67 high index plastic may be the best choice for your lens material.
In general, a high index material will cost slightly more than CR-39 or polycarbonate and is used when the thickness of the lenses would be too great with either CR-39 or polycarbonate.
The 1.67 high index plastic has an index of refraction of 1.67, a specific gravity of 1.37, and is moderately impact resistant.
Always Consult with your Optometrist or Optician
Knowing what material options you have for your lenses is very beneficial as you consider what material will best suit your needs. Your optometrist or optician will likely make specific recommendations for your spectacles and the material of the lenses.
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