Glass or Plastic?
Ophthalmic lenses are broadly classified as mineral lenses and organic lenses. Originally, all lenses were made of glass (mineral glass) and it is not easily affected by environmental factors; however, now organic lenses popularly known as plastic are more commonly used in the eyewear industry. Mineral glass is highly scratch resistant and offer excellent optical quality and clarity in comparison to organic lensesbut due to its heavy weight and shattering tendency, they are less used. Organic lenses, in spite of having low optical clarity is used more because of its ability of being mounted on different types of frames, and with different types of surface treatment, these lenses become scratchresistant and lighter.
Criteria for choosing lens materials:-
1. Refractive index-It is the ratio of the speed of light in vacuum to speed of light in a particular medium. It is a measure of much light is bent on passing through a substance. CR 39 has a refractive index of 1.498, polycarbonate lens has 1.59, glass has 1.5 index. A higher refractive index makes the lens thin.
2. Specific gravity-It is the ratio of density of a given material to density of water at a particular pressure at 4 degree Celsius. Specific gravity is the measure of how heavy the substance is. Higher the specific gravity, heavier the lens would be. Glass has specific gravity ranging from
2.54 to 4.00. Organic lenses have a lower specific gravity. Trivex has a specific gravity of 1.11(lowest in all lenses).
3. Abbe value-It is the measure of lens’ materials dispersion of light. A lens with low Abbe value causes a higher dispersion and leads to unwanted chromatic aberration. Higher the Abbe value, better the optical clarity and less distortion. Glass has a Abbe value of 59, CR 39 is 58 while polycarbonate has an Abbe value of 29.
Types of lens materials:-
Glass lens-Glass lenses have superior optics, stable material and high scratch resistant properties. Crown glass has a Abbe value above 50.Some glass is available with high refractive indices of 1.8 and 1.9.However, glass lenses are heavy, thick and dangerous if broken. Hence it cannot be used in all frames.
CR 39 or Columbia resin 39-These are conventional plastic lenses with a refractive index of 1.50 and Abbe value of 58.Lighter than glass lenses and readily tintable - these glasses can be a good choice of lens material. However, these are susceptible to scratches and less suitable for high powered prescriptions.
Polycarbonate-These lenses have a higher refractive index of 1.586 and are thinner. Polycarbonate lenses are highly impact resistant and come with inherent UV protection. These are the best choice of lenses for safety glass, children’s eyewear and rimless frames. However, these lenses have poor optical quality owing to Abbe value of 29 and are susceptible to scratching and stress fractures.
MR-8 or Mitsui Resin-8-The eyewear industry is dominated by MR material. It is characterized by its versatility and offers good UV protection. It is about 8 times more impact resistant than CR-39.These lenses have high refractive index, high Abbe value, low specific gravity ,and good stability. Different types of MR lenses are :-
MR-8 with a refractive index of 1.60 and Abbe value of 41
MR-7 and MR-10 with a refractive index of 1.67 and Abbe value of 31.
MR-174,also known as ultra-thin lens comes with a refractive index of 1.74 and Abbe value of 32.
Trivex-Trivex is the lightest lens material available. It has excellent UV blocking properties and is shatter resistant. With refractive index of 1.53 and Abbe value of 46,Trivex lenses provide excellent optical clarity. These lenses are highly durable. Trivex is also resistant to alcohol and chemical solvents. But being thick and costly these lenses are not widely available in the industry.
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