In the early days of vision correction, all eyeglass lenses were made of glass.
The main material for glass lenses is optical glass. The refractive index is higher than that of resin lens, so glass lens is thinner than resin lens in the same power. The refractive index of glass lens is 1.523, 1.60, 1.70, 1.80, 1.90. Glass lenses have good transmittance and mechanochemical properties: constant refractive index and stable physical and chemical properties.
Although glass lenses offer exceptional optics, they are heavy and can break easily, potentially causing serious harm to the eye or even loss of an eye. For these reasons, glass lenses are no longer widely used for eyeglasses.
● 1.50 CR-39
In 1947, the Armorlite Lens Company in California introduced the first lightweight plastic eyeglass lenses. The lenses were made of a plastic polymer called CR-39, an abbreviation for “Columbia Resin 39,” because it was the 39th formulation of a thermal-cured plastic developed by PPG Industries in the early 1940s.
Because of its light weight (about half the weight of glass), low cost and excellent optical qualities, CR-39 plastic remains a popular material for eyeglass lenses even today.
● 1.56 NK-55
The most affordable of the higher Index lenses and very tough compared to CR39. As this material is around 15% thinner and 20% lighter than the 1.5 it offers an economical option for those patients who require thinner lenses. The NK-55 has an Abbe value of 42 making it a good choice for prescriptions between -2.50 and +2.50 dioptres.
● High-index plastic lenses
In the past 20 years, in response to the demand for thinner, lighter eyeglasses, a number of lens manufacturers have introduced high-index plastic lenses. These lenses are thinner and lighter than CR-39 plastic lenses because they have a higher index of refraction and may also have a lower specific gravity.
MR™ Series is premium optical lens designed by Japan Mitsui Chemicals with a high refractive index, high Abbe value, low specific gravity and high impact resistance.
MR™ Series is especially suitable for ophthalmic lenses and is known as the first thiourethane bases high index lens material. MR™ series offers a variety of products to provide the best solution for optical lens users.
R.I. 1.60: MR-8
The best balanced high index lens material with the largest share of the R.I. 1.60 lens material market. MR-8 is suited to any strength ophthalmic lens and is a new standard in ophthalmic lens material.
R.I. 1.67: MR-7
Global standard R.I. 1.67 lens material. Great materials for thinner lenses with strong impact resistance. MR-7 has better color tint abilities.
R.I. 1.74: MR-174
Ultra high index lens material for ultra thin lenses. Strong prescription lens wearers are now free from thick and heavy lenses.
|Refractive Index (ne)||1.60||1.67||1.74|
|Abbe Value (ve)||41||31||32|
|Heat Distortion Temperature (℃)||118||85||78|
|Static Load Resistance||Good||Good||Good|
Polycarbonate was developed in the 1970s for aerospace applications, and is currently used for the helmet visors of astronauts and for space shuttle windshields. Eyeglass lenses made of polycarbonate were introduced in the early 1980s in response to a demand for lightweight, impact-resistant lenses.
Since then, polycarbonate lenses have become the standard for safety glasses, sports goggles and children’s eyewear. Because they are less likely to fracture than regular plastic lenses, polycarbonate lenses also are a good choice for rimless eyewear designs where the lenses are attached to the frame components with drill mountings.
Most other plastic lenses are made from a cast molding process, where a liquid plastic material is baked for long periods in lens forms, solidifying the liquid plastic to create a lens. But polycarbonate is a thermoplastic that starts as a solid material in the form of small pellets. In a lens manufacturing process called injection molding, the pellets are heated until they melt. The liquid polycarbonate is then rapidly injected into lens molds, compressed under high pressure and cooled to form a finished lens product in a matter of minutes.
Despite its many advantages, polycarbonate isn’t the only lens material suitable for safety applications and children’s eyewear.
In 2001, PPG Industries (Pittsburgh, Penn.) introduced a rival lens material called Trivex. Like polycarbonate lenses, lenses made of Trivex are thin, lightweight and much more impact-resistant than regular plastic or glass lenses.
Trivex lenses, however, are composed of a urethane-based monomer and are made from a cast molding process similar to how regular plastic lenses are made. This gives Trivex lenses the advantage of crisper optics than injection-molded polycarbonate lenses, according to PPG.
Post time: Apr-08-2022