In the year 1784, Benjamin Franklin invented bifocal lenses by placing one lens on top of the lower portion of another. The top portion of his invention was for viewing images off in a distance and the lower half was for viewing fine print and detail up close.
Modern simple bifocal lenses still have the basic two parts: the upper portion normally used for viewing images at a distance and the lower portion used for close up or near vision for things like reading or viewing fine detail on objects such as a penny.
Bifocal lenses are needed for people who have reached an age of around 40 to 45 and now need a little help reading and seeing close up. This is because they now have a condition called presbyopia; the need for assistance to see clearly close up due to the muscles in the eye have reached an age where they are unable to work as quickly as before. Over time the muscles attached to the crystalline lens do not work as well bringing the lens into focus when viewing images at different distances. The lens doesn’t constrict as precise as it did before therefore needs the use of a special lens; a bifocal lens for near vision such a reading. Bifocals prevent the need to switch glasses to accommodate distance vision and then to near vision. This was the basis of Ben Franklin’s invention; he did not want to keep switching his glasses.
In the beginning when bifocals were first invented and for some time after, bifocal eyeglasses had a line running completely across the lens. It makes sense this would be the style since in Ben Franklin’s design was a rough one, but served its purpose well for the time. As technology improved the bifocal lens improved as well. Lenses became thinner, though not much for quite some time. The placement of the segment, reading portion of the bifocal lens, became smaller and indifferent shapes. This meant choices for the consumer:
- Executive Bifocal, a.k.a. Franklin, or Ribbon Style (bifocal running across the entire bottom of the lens)
- Flat top or straight top (half moon look)
- Round Segment (a.k.a. CT, a.k.a. round seg).
- Ribbon Segment (narrow rectangular strip running completely across the lens)
Not so long ago technology began to make huge strides with bifocal lenses and meeting the needs of consumers. The no-line bifocal was invented and this too has made great strides as innovative technology has continues to be driven by companies wanting to be the best and have the latest and greatest product. There are many manufacturers today competing to come out with the next generation of no-line bifocals.
No-line bifocal lenses have a number of variations, but the all basically work the same. The top has the distance Rx and gradually changes as it moves down the lens into the near Rx. There is a center corridor holding the intermediate Rx; this accommodates vision at about an arms length distance.