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OPTOM FASLU MUHAMMED
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Bifocal lenses

Jan 27, 2017

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Page 1: Bifocal lenses

OPTOM FASLU MUHAMMED

Page 2: Bifocal lenses

A presbyopic subject requires a separate correction for distance and near vision, the two prescriptions may be provided as one pair of spectacle in the form of a bifocal lens.

Page 3: Bifocal lenses

A bifocal lens is defined as having two portions of different focal power.

The area of the lens used for distance vision is called Distance portion or DP and the area used for near vision is called the Near Portion or Reading Portion or NP.

Page 4: Bifocal lenses

The first recorded mention of bifocal spectacle lens is a letter written by Benjamin Franklin in 1784.

Page 5: Bifocal lenses

Bifocal lenses can be obtained in four basic constructional types:

1. Fused bifocals 2. Solid bifocal 3. Cemented bifocal 4. Split bifocal

Page 6: Bifocal lenses

The first fused bifocal was the fused kryptok invented by Borsch in the year 1908.

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Page 8: Bifocal lenses

Composite buttons for different bifocal shapes(A) D – Segment flat top bifocal. (B) B – Segment

Page 9: Bifocal lenses

The fused bifocal is produced by fusing a higher index flint to the main crown glass.

Page 10: Bifocal lenses

Solid bifocal can be considered as one piece bifocal, made from single piece of material.

Page 11: Bifocal lenses

A segment is glued onto the surface of a single vision lens.

Page 12: Bifocal lenses

Also known as split bifocal is made of two separate lenses held together by the frame.

Page 13: Bifocal lenses

Round segment bifocal have a segment with a dividing line that is a single circular arc which is least visible compared to the other bifocals.

Page 14: Bifocal lenses

Segments vary from tiny “B segment” to “E-style” in which the whole bottom half of the lens is used for near portion.

Page 15: Bifocal lenses

D segment bifocals are available in solid plastics and fused glass forms

(A) Straight top D bifocal. (B) Curve top bifocal. (C) B segment bifocal. The dot indicates the geometric centre of the segment

Page 16: Bifocal lenses

E- Style Bifocal is one piece bifocal lens with two different curves ground usually on the front surface.

Page 17: Bifocal lenses

IMAGE JUMP

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Page 19: Bifocal lenses
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Any lens that is chosen by careful forethought and positioned for a specialized viewing situation may be classified as an occupational lens.

Some people require intermediate or near viewing while looking upward, including plumbers, pharmacists, librarians, electricians, auto mechanics, and many others in specialized working situations.

Page 21: Bifocal lenses
Page 22: Bifocal lenses

Bifocal segments must be positioned so that the distance and near positions of the lens provide adequate fields of view for distance and near vision respectively.

Page 23: Bifocal lenses

While fitting the bifocal into a spectacle frame three factors must be considered:

1. Segment Inset2. Segment Width and3. Segment Height.

Page 24: Bifocal lenses

Segment inset is specified as the difference between the subjects distance PD and near PD.

In the normal range of PDs, the near PD for a reading distance of 40cms is 4mm less than the distance PD.

Page 25: Bifocal lenses

Segment inset, therefore is usually specified as 2mm for each lens.

There are two reasons for insetting bifocal segment –

1. To ensure that the subject’s line of sight will go through the segment at its optical centre

2.To ensure that the reading fields for the two segments will coincide with one another.

Page 26: Bifocal lenses

Different segment widths are usually available for straight top shaped segments in 22, 25, 28 and 35 mm widths.

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Page 28: Bifocal lenses

1. Choose the final frame and adjust it to fit the subject correctly.

Page 29: Bifocal lenses

2. If the frame is without demonstration lens, attach a vertical strip of transparent adhesive tape to each eye of the frame to enable reference points to be marked

Page 30: Bifocal lenses

Replace the frame on the subjects face and direct the subject to look straight into your eyes.

your eyes are on exactly the same level as those of the subject

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Page 32: Bifocal lenses

Remove the frame and put the frame face down with dots coinciding with a straight line. Draw a straight line from one dot to another

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7. The position of the segment top is usually specified in millimeter from the marked straight line to the lower eye wire of the frame.

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Put back the frame on the face and verify.

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Page 36: Bifocal lenses

Thank u……..