Bifocal reading glasses have undergone a long history since its invention which happened more than two hundred years ago. A basic point of these eyeglasses is that they can provide both presbyopia and nearsightedness/farsightedness correction. This can also be considered as the most significant improvement compared with regular reading glasses. In fact, bifocal specs have been continuing its progression after making such a significant breakthrough. For certain, the basic design principle will not change as its name suggests. A pair of bifocal eyeglasses will provide those basically two forms of visual aids for ever. However, a lot of new, specific designs have been brought into existence during the past years since its origin. Currently, all lens designs of bifocals can be grouped into two mainly categories: Franklin bifocals and bifocals with reading segments in a small size.
The first category is also the original bifocal design. This initial form was developed by Benjamin Franklin by cutting two lenses in half and combining them together. So, a bifocal lens of Franklin’s style has a half portion for reading and the other half for distance vision. Using these bifocal specs, wearers can naturally look down to read materials and look up through to view objects in the distance. All things seem good and feasible. One point of this simple design is that these two focal points have been fixed into each half part. In some cases, it is inflexible for certain users to perform customized visual tasks. The current situation in the market is that bifocal reading glasses of this design can seldom be seen. There exist more flexible and helpful designs.
The second category of bifocal lens designs takes reading segment as the fundamental concept. Bifocal specs within this category all take use of reading segments molded into a primary lens. These different designs differ in the shape and position of the reading segment. For instance, available shapes include half-moon or D shape, round, narrow rectangle and so on. While most reading segments are situated in the lower part of the lens, there is a special design called double-D that molds two reading segments into the upper and lower sections respectively.
C: Bifocal specs have developed several different designs after more than two hundred years’ progression. There are mainly two categories, respectively the Franklin bifocals and bifocals with small reading segments.
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