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2 bifocal.ppt
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- 2. • Benjamin Franklinwas invented the bifocal lens in which a pair of lenses for nearsightedness and a pair for farsightedness are cut in halves and inserted in the tops and bottoms of the eyewires.
- 3. SOLID AND FUSEDBIFOCALS • Bifocal were originally made in glass in one of two ways. • Solid : the lens is made of one piece of crown glass. Where the prescription changes for reading segment ,the curve of the lens changes and you can feel a bump on the lens. • Fused : the distance portion of the lens is made of ordinary crown glass and the segment is fused in to the main lens, hence no bump.
- 4. D -SEGMENT (FLATTOP) • Available in segment sizes 25mm, 28mm, 35mm, 40mmand 45mm. • The segment size is measured across the widest part of segment. • This type of bifocal is suitable for first time wearers, especially in negative prescriptions. • The reason for this is that there is relatively little image “jump” when customers look down into the reading area.
- 5. CONT.…. • Jump causeby looking through a prism in the lens. It cause object viewed through the segment to appear to jump up. • The D segment is the most recent type of bifocal design and keeps this effect to a minimum. • This is because the optical center of distance portion of the lens and that of the segment are close to one another. • The size of segment will depend on the customer’s occupation and or lifestyle.
- 6. CONT… • The D28is the most common first time bifocal lens as it offers an adequate field to view at near vision and clear all round distance vision. • If the customers job or hobby changes you can advise a large segment without having to change you can advise a larger segment without having to change the type of bifocal.
- 7. EXECUTIVE BIFOCAL • Theexecutive bifocal is worn largely by executive the reading area extends across the full width of the lens. • This type of bifocal provides the largest reading area available in a bifocal combined with a large distance area. • It also has a very little jump. • This is largely a vocational lens, as it appearance is sometimes off- putting for the wearer .
- 8. CONT.… • The dividingline on an executive bifocal is very prominent. • Both for the wearer and to anyone looking at them. • It can be distracting for the wearer moving around as well as being cosmetically un attractive. • The blink size available in limited specially in glass( maximun blank 65Ø), where the thickness and weight is a major consideration. Often, then, an executive bifocal wearer may want to have a different type of lens for social wear. • But more effective for children .
- 9. ROUND SEGMENT • Roundsegment bifocals are available in 24mm,28mm,30mm and 38mm. • The segment size on existing spectales is measured either across the widest part of the segmentor by placing the lens on the dispensing ruler.
- 10. • Round segemntsare usuallly dispensed when the customer is already wearing them (and is happpy with them ).it is possible to change the size of the segment without changing the type of bifocal. • Bifocal lenses of this type are cosmetically appealing,particularly fused bifocal where the reading segment is almost invisible .hoever ,image jump is incresded in higher power which can cause visuqal problems.
- 11. C -SEGMENT (CURVEDTOP) • Available in segemnt size 28mm. • This type of bifocal has all the attributes of a D –segment bifocal with additional benefits of being more cosmetically appealing .due to the curve at the top of the segment it is not visible as a D-segment.the only drawbake to this lens is its limited size availability. • This lens ,while being very popular in continental europe is not widely dispersed in the U.K.