Bifocal lenses

OPTOM FASLU MUHAMMED

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.

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.

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

Bifocal lenses can be obtained in four basic constructional types:

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

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

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

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

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

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

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

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.

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

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

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

IMAGE JUMP

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.

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.

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

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

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.

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.

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

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

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

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

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

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.

Put back the frame on the face and verify.

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