According to corrective functions, glass lenses can be classified into four categories: single vision glass lens, bifocal glass lens, trifocal glass lens and multifocal glass lens. These four types can meet the needs of people with different vision problems.
From top to bottom, single vision glass lenses have the same focal power. They are responsible for myopia, hyperopia or a combination of them. Therefore, single vision glass lenses can be subdivided into three types based on eyesight problems: Concave lens for nearsighted correction, convex lens for farsightedness and aging eye and cylindrical lens for astigmatism. CR-39 is the widely used single vision lens.
Bifocal glass lens consists of two parts: the upper part for distance vision and the lower part for near vision. Bifocal glass lens has a line going across the entire length of the lens. People over 45 may suffer from both these vision problems. With bifocal glass lenses, there is no need for separate single vision glasses. Bifocal glass lens was invented by Benjamin Franklin, one of the founding fathers of the USA.
Just as their name reflects, trifocal glass lenses have three sections: the top section for distance vision, the center for intermediate vision and the bottom for near vision. This kind of glass lens is more useful since it adds a third section for seeing objects within an arm’s reach.
Bifocal and trifocal are included in multifocal glass lenses. But there is another special multifocal glass lens: progressive addition glass lens. It is also called no-line bifocal. These kind of glass lenses eliminate the visible lines in bifocal and trifocal lens so that provide a more appealing appearance.
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