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.

 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.

 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 Inset
2. Segment Width and
3. 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.

Bifocal lenses

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Bifocal lenses