Eyeglass lenses are perhaps more important than the frames because frames primarily address the style issues while the lens provides proper vision. Selecting frames by your visual assessments is easy, but it is not the same with lenses that need in-depth evaluation. When you shop eyeglasses, you must try to gather as much information about the lenses to judge its suitability and justification for the price. However, it needs some knowledge about lenses to verify the information you gather. This article discusses some basics about lenses for eyeglasses that should help to take a well-informed decision and give the satisfaction of making the right choice.
The lenses referred here are prescription lenses for a variety of eye problems like single vision lenses required for correcting nearsightedness, farsightedness as well as astigmatism, progressive lenses, bifocal lenses and even multifocal lenses used for correcting presbyopia. We begin by discussing the type of materials for the eyeglass lens together with its benefits.
From glass to plastic
The glass used to be the staple material for making eyeglass lenses used for vision correction because it offered exceptional optics, but the flip sides were many. Firstly, glass is heavy that made wearing spectacles quite uncomfortable and secondly, glass is highly fragile and breakable that can cause severe injuries during accidents. There have been instances of eye damage from broken glass, and even there are instances of eye losses.
Lightweight plastic lenses that first made its appearance in California in 1947 provided a suitable substitute for glass for making lenses for eyewear. Columbia resin 39 or CR 39 was the name of the first plastic lens material made from a plastic polymer that became an instant hit because of lightweight, low cost and impact resistant qualities. Most importantly it has excellent optic properties.
Types of plastic lenses
Plastic lenses come under two broad categories – polycarbonate lenses and high-index plastic lenses.
Polycarbonate lenses
While the initial plastic lens was a resin material, subsequent technological improvements offered more options that encouraged lens makers to look for improved plastic material. The search ended in 1970 when the world got its first polycarbonate lens, which despite being plastic was of a much-improved quality that ensured durability due to high impact resistance, clear vision, and lightweight. Initially, polycarbonate lenses were for safety glasses only, but since the eighties, it has become a popular choice for eyeglass lenses too.
Since 2001, newer plastic material for lenses called Trivex has broken into the scene which has the same qualities of polycarbonate but higher Abbe value that determines how wide the lens can disperse different wavelengths of light passing through it.
High-index plastic lenses
High-index plastic lenses are the latest introduction that has been around for two decades. The need for lighter and thinner eyeglasses has led to the discovery of this lens material. These lenses have superior optics because despite being thinners its refractive index is higher than CR 39.
The lens thickness is inversely proportional to refractive index meaning the thinner is the less high is the refractive index.
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