8.2 History of Multifocal Lenses

Who created the first bifocal?

Benjamin franklin

What is the Franklin bifocal?

two lenses put together

What are the advantages to a Franklin bifocal?

centered optical system (seg crosses OC), therefore no image jump

What are the disadvantages of the Franklin bifocal?

dividing line is annoying

falls apart easily

What was the first commercially successful one piece bifocal lens?

solid up curve bifocal

What is a solid upcurve bifocal?

upper back surface is flattened to reduce plus power for distance portion

What is the advantage for a solid up curve bifocal?

invisible

What are the disadvantages of a solid upcurve bifocal ?

decentered optical system therefore significant image jump

What was the first cemented bifocal?

thin glass wafer with same index of refraction of main lens added to back surface

What are the disadvantaged to the first cemented bifocal?

collected dirt

adherence changed with temperature

cemented darkened with use

What is a cemented kryptok bifocal?

segment with a higher index is added to front of lens and cemented in place

What is the first bifocal with segment of higher index?

cemented kryptok bifocal

What are the disadvantages of the cemented kryptok bifocal?

requires grinding + polishing 6 surfaces, expensive

cement darkens

cover glass is fragile, lenses break apart

What is a fused kryptok bifocal?

segment with a higher index is fused to main lens with heat

Where must cyl power go on a fused kryptok bifocal?

on back (minus cyl form)

What are the advantages of a fused kryptok bifocal?

Doenst collect dirt, fall out, to chip

dosent discolor

relatively invisible

What is an ultex bifocal?

bifocal power ground on back surface with longer radius of curvature than major lens portion

Where must the cyl power go on an ultex bifocal?

cyl must be ground on front surface (plus cyl form)

Who made the first flattop bifocal?

Univis Lens Company

Who made the first executive bifocal?

american optical company

A fused bifocal is made out of what material?

glass

A one-piece bifocal is made out of what material?

plastic

glass if its executive

What is the modern fused bifocal?

heat fused glass lens that using a higher index material to make the add segment

Where must the cyl go for a modern fused bifocal?

cyl on back (minus cyl lens form)

All segmented bifocals are what?

one-piece bifocals

What cyl form are segmented bifocals (one-piece bifocals)?

can be either + or - cyl, - cyl in the US

What are the designs of a modern one-piece bifocal?

round bifocal

flat-top bifocal

executive bifocal

PAL