Ophthalmic Dispensing: Multifocal Lenses.pptx

MULTIFOCAL LENSES
Optom. Sahibzada Hakim Anjum Nadeem
M.Phil Optometry K.E.M.U, Post Professional Doctor of Optometry
B.Sc. Vision Sciences, Dispensing Optician (USA), JCAHPO Certified
Focal Person Dispensing Opticianry Training Course COAVS, MHL
Incharge Dispensing Opticianry Training Lab, COAVS, Mayo Hospital LHR

PRESBYOPIA
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.

Bifocals
 Bifocals provide two separate
prescriptions, usually for distance and
near vision clarity.
 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.
 Bifocal lenses share the common
design of correction for distance and
near vision in a single lens.

 Benjamin Franklin invented the first
bifocal lens in 1785

Bifocals
 Benefits: No need to keep two
spectacles, each for distance & near
 Disadvantages:
 Dividing line seen, so cosmetically looks
poor.
 Intermediate targets are blurred.
 Ring scotoma around the segment

Constructional Types
Bifocals are usually constructed in four
main ways:
 Fused Bifocals
 Solid/One piece Bifocals
 Cemented Bifocals
 Split Bifocals

Fused Bifocals
 Fused multifocal are available only in
glass.
 Segment is made of glass having higher
refractive index than the distance
carrier lens.
 The surfaces of fused lens have no
change of curvature.

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

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

Solid/One piece Bifocal
 One piece bifocals are made from one
lens material.
 Change in power in segment portion is
due to the change in the curvature of
the lens.
 If either ledge or change in curvature is
felt, the lens is likely a one piece design.
 One piece multifocals can be made
from any lens material.

One piece bifocals are usually either;
 Full segment Franklin style
 Large round segment lenses

Cemented Bifocals
 Cemented lenses are custom made
lenses
 Have small segment glued onto the
distance lens.
 Cemented lenses are usually in small
round segment

Split Bifocals
 Earliest design
 A distance lens whose flat bottom abuts
the flat top of separate near lens.

 Split bifocals can therefore be made exactly
as required for centration, prism,
prescription, tint etc. The segment top
position is determined in the usual way.
 The disadvantages with this lens are that
the dividing line between the two halves
can be quite noticeable (depending on the
prescription), the ridge can collect dirt and
the lenses can separate even though they
are bonded at the joint.
 Split bifocals are better glazed into a metal
frame as the tension produced reduces the
risk of the lenses coming apart. Split
bifocals are often referred to as Franklin

 Choices in multifocal lenses today
include segment style (shape and size),
position, material, and number of
segments.
 These specifications also indirectly
specify other items, such as refractive
index and fused or one-piece designs.
 Factors that should be considered when
deciding between these choices are the
patient's occupational and recreational
needs, expense, cosmesis, and optical
performance, including aberration.

Types of Bifocals
There are few major types of bifocal
segment style/shape including ;
 Round segment
 Flat top segment
 Curve top
 Panoptik
 Franklin or Executive style

Round Segment
 Round segments vary in size from a
small lens of 22mm upto the largest
40mm.
 The most common size is 22 mm.
 Round segment can be rotated but not
look tilted
 An advantage of round seg bifocals is
that the line separating the distance
and near zones of the lens tends to be
less noticeable than the upper line of a

Flat Top Segments/ D-
Shaped
 The most commonly used bifocal design is the flat-
top bifocal.
 It comes in four common sizes, 22, 25, 28, and 35,
referring to the horizontal width in millimeters,
respectively, of the flat top of the segment.
 The flat-top 28 is the most commonly prescribed.
 Smaller sizes are generally used for smaller frames.
 The flat-top 35 can be used when a larger near-
viewing area is desired.
 Flat tops are also known as D-Segs.

Flat-Top Bifocal (D-seg; Straight-Top)

 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 center

Curve Top and Panoptik
 Curve Top segments look similar to flat
tops, except that the upper line is
arched, rather than flat.
 There is a distinct point on either
corner.
 The top of the panoptik segments are
curved as well but the corners are
rounded.

Ribbon segment
 A ribbon-segment bifocal is a
modification of the flat-top with the top
and portion cut off, giving the patient a
distance viewing area below the
segment (for patients who may want to
see stairs or play golf).
 There are two types
 A B and an R segment

Ribbon segment
 The B is 9mm deep and is good for
someone who want distance vision
below the segment area.
 The R has 14mm depth.
 It is seldom used as a regular bifocal
lens.
 Ribbon segments are only available in
glass.

Double-segment bifocals
 Double-segment bifocals have two equal but
separate reading areas within a single lens.
 They are useful for people with special
occupational needs such as painters,
carpenters, electricians, or others who
need to perform near work above and
below primary gaze.
 They are available as Double-D (double flat-
top), double-ribbon, or double-Executive
bifocals.

Franklin style Executive
segments
 Franklin style lenses are the most
commonly known by the trade name
Executive.
 It is a one piece lens with the segment
extending the full width of the lens.
 The lens has the advantage of a very
wide near viewing area.
 Disadvantages of Executive-style
bifocals include greater weight and less
cosmetic appeal.

Franklin style Executive
segments
Executive bifocal (Franklin bifocal)
Larger flat-top segments, as high as 40 or
even 45 mm, offer a compromise for these
factors.

Blended bifocals
 Blended bifocals are a type of invisible
multifocal.
 This is a round-segment bifocal that has
been made less visible by blending the
edge of the bifocal segment over a
small zone.
 These blended bifocals can be
cosmetically more appealing to
patients.

Bifocal Style Benefits Disadvantages Availability
Executive widest near portion - cosmetically
unappealing
- heavier in weight
Glass
Round Segment
(Kryptok)
- cosmetically better than
any other bifocal
- cheapest of all bifocals
- most downward eye
movement required
by patient to read
- high image jump
while shifting focus
from distance to near
Glass & Plastic
D' or Flat Top - larger useable reading
area
- less downward eye
movement required by
patient to read
- less amount of image
jump experienced while
shifting focus from
distance to near
costliest of all bifocals Glass, Plastic &
Polycarbonate

Trifocals
 Trifocal Lenses contain three focal
points, usually for distance,
intermediate and near vision.
 A tri focal lens has an added segment
above the bifocal for viewing objects in
the intermediate zone, which is
approximately arm's length away.
 Trifocal lenses are available with the
reading segments in a variety of shapes
and widths.

Flat-top Trifocal
 The most popular is the flat-top (also
known as straight-top or D-segment),
28 millimeters wide.
 The segments vary in width from 22mm
to 35mm and in depth from 6mm to
8mm.
 A lens with depth of more than 8mm
considered as all time wear lens.

Flat-top Trifocal
Flat Top 7x28 Trifocals
Flat Top 8x35 Trifocals

Executive (Franklin)
Trifocals
 The Executive style lens is a full width
segment lens with a 7mm full width
intermediate.
Executive Trifocals

The E-D trifocal
 An E-D trifocal has a distance correction
along the top half of the lens,
 separated from the intermediate
correction in the bottom half by a line
that goes all the way across the width
of the lens in the Executive style,
 and a D half-moon segment containing
the near correction that resides within
the lower half of the lens.

The E-D trifocal
 The E-D trifocal is for someone who
must see at the intermediate distance
in a wide field of vision and who also
must see clearly both close up and in
the distance.

Multi-Focal Occupational
Lenses
 Multifocal are suited for performing a
particular job or hobby and are not
meant for everyday wear:
 Occupational bifocal is a Double-D,
which has a half-moon-shaped flat-top
bifocal at the bottom of the lens and an
upside-down flat-top at the top of the
lens. The rest of the lens area is for
distance correction.

Double-D segment
 . Car mechanics, who need to see well
at the near point both looking down to
read as well as looking up above their
head to work on the undercarriage of a
car on a lift, would benefit from a
Double-D

 Double-round segs are also
occupational lenses that can be used
for the same purposes as the Double-D.

Quadrafocals
 Quadrafocals are special-use lenses
that combine the concepts of trifocals
and double-segment bifocals .
 In addition to a trifocal lens, in these
lenses another near segment is placed
above eye level to fill a special need.

Rede Rite/Minus Add Up
Curve
 Also known as Rede-Rite bifocal or
upcurve bifocal.
 It has large round segment at the top.
 A bifocal with huge add area at the
bottom and small distance area at the
top.
 It is for people who need full near
working area and also wants to see
clearly at distance.

Ordering correct lens
power
 Ordering correct lens power for
reading glasses:
 (Distance power)+ (near addition)=near
power
 The intermediate segment provides
50 percent of the added magnifying
power of the near seg.

Fitting Guidelines for
Bifocals and Trifocals
 Bifocals typically are fitted so the top line
of the near segment rests at the same
height as the wearer's lower eyelid.
 Trifocals are fitted a bit higher, with the top
line of the intermediate seg aligned with
the lower margin of the wearer's pupil.
 Eyes typically will look through the correct
viewing zone of your bifocals or trifocals
without a need to tilt your head back or
make other posture adjustments.

While fitting the bifocal into a
spectacle frame three factors
must be considered:
1.Segment Inset
2.Segment Width
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.

Image Jump
 The visible lines in bifocals and trifocals
are points where there's an abrupt
change in lens power.
 When a bifocal or trifocal wearer's line
of sight moves across these lines,
images suddenly move, or "jump.“
 This sudden displacement of image as
the bifocal line is crossed is known as
image jump.

Image Jump
 The discomfort caused by this "image
jump" can range from being mildly
annoying to creating nausea.
 To be seen clearly, objects must be within a
specific range of distances.
 Objects that are outside the distances
covered by the bifocal or trifocal lens
powers will be blurred.
 Image jump can be calculated by Prentice’s
rule

 Prentice Rule:
Δ= cF
 when the patient is looking from their
distance to near portion, they will
experience an image jump
In Plus Lens : BD
In Minus Lens: BU

What is the main factor that
will affect how much image jump
they will experience?
 The distance to the optical center from
the top of the seg.

Horizontal prism at near
by Segment Decentration
 Horizontal prism at near can be
induced by segment decentration.
 The steps are:
1. Seg inset=distance PD-near PD/2
2. Additional seg inset by Prentice’s rule
Ca=near prism/Fa
where Ca additional seg inset, Fa the
add power

 Net seg inset= seg inset+ additional
seg inset
 Minimum segment size needed to give
wearer enough reading area with:
Seg size= 2(10+additional seg inset)

 How to move it for BI or BO?
⁻ For BI move the OC inward
⁻ For BO move the OC outward

Ophthalmic Dispensing: Multifocal Lenses.pptx

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