PRESCRIPTION EYEGLASSES IN BELLINGHAM WA: LENS MATERIALS
Prescription Eyeglasses in Bellingham Washington: What types of lens materials are available?
Charron Eye Center offers over a half a dozen types of lens materials for your prescription eyeglasses in Bellingham Wa. In the past, glasses were made with glass (thus the name)! Though glass material is still available, I could count on one hand the number of times we have sold it for our patients. With the advent of lighter weight, more versatile plastic lenses, most all consumers buy a form of plastic glasses lenses. Here is an overview of all the lens types and materials you could get for your eyewear.
The lens material can influence the following factors:
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Optical quality. Each lens has what is known as an ABBE value which refers to how much chromatic abberration that the lens has. If there is a high amount of chromatic abberrations, you may start to notice small waves of colors appearing at object borders with lots of contrast. The lower the abbe value, the more chromatic abberration.
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Thickness
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Weight
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Scratch Resistance
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Ultraviolet Light (UV) Protection
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Impact Resistance (Safety Protection)
GLASS MATERIAL
As stated above, glass was the first type of glasses material. Hardly anyone buys it anymore because it is so heavy and can break easier. It can sometimes be difficult to find optical laboratories to supply it. However there are a few properties of glass lenses which still make it a great choice for your corrective eyewear.
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It doesn't scratch as easy as plastic lens materials
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Crown glass with an index of 1.523 has and Abbe Value of 58 (= to CR-39 plastic) low chromatic abberration which provides good optical quality.
PLASTIC MATERIALS
BASIC PLASTIC (CR-39)
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CR-39 stands for "Columbia Resin" and the 39 refers to the type.
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It is lightweight
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Fairly scratch proof on its own, but will improve with a scratch coating
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Not as impact resistant as polycarbonate or trivex
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High Abbe Value of 58 and which is low in Chromatic Abberrations
POLYCARBONATE
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Polycarbonate is a soft plastic with high impact resistance making it a good choice for safety.
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Scratches easily and therefore should be used with a scratch coating
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Lowest Abbe Value of all the plastic lenses which means it's highest in Chromatic Abberrations
TRIVEX (MID INDEX)
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Another soft plastic with high impact resistance
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Trivex has a the next highest abbe value to CR-39 at 43 which makes it a great option for people looking for impact resistance but also good optics.
HIGH INDEX (1.6 TO 1.74 INDEX OF REFRACTION)
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"High Index" refers to the lens having a higher index of refraction and the higher the index of refraction the thinner the lens.
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Same level of impact resistance as basic plastic (CR-39)
SAFETY LENSES
The American National Standards Institute (ANSI) sets the criteria for lenses to be worn as "safety" eyeglasses. The requirements are based on a few factors listed below:
Thickness:
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Prescription Lenses in Safety Frame: No less than 3 mm thick, but if +3.00 D or higher in any meridian, the requirement drops to 2.5 mm thickness
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Safety glasses with no prescription: No less than 3 mm thick.
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Safety goggles: No less than 3 mm thick
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Cover Plates: No thickness requirement
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Safety Plates (found in helmets/shields): No less than 2 mm thick
Materials:
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As long as the lens passes the impact resistance (drop ball/high velocity/penetration) testing, there is no material requirement, yet almost all safety eyewear is, and should be, made with polycarbonate material.
*Note: The designation of Safety Eyewear can only be made if it meets the requirements for lenses AND frames. If you place lenses which meet the ANSI safety criteria into a regular optical frame, they cannot be labelled or advertised as safety eyewear.
Information source: System For Ophthalmic Dispensing (Second Edition)