Essentials on Optical Dispensing
P.S.Selvakumar
Faculty
Aravind School of Optometry.
Multifocal lenses
and
Designs
History
 1270 - Convex lens used to read in China.
 1784 - Benjamin Franklin invented first bifocal.
 1884 - Cemented bifocal introduced.
 1890 - Fused bifocals
 1906 - Solid / one piece bifocal types launched.
 1959 - Progressive lens launched commercially.
Corrective lens
 Convex / Concave lenses in the form of
- Sphere
- Cylinder
- Sphere & cylinder combination.
 Prism
 Available in mono focal /bifocal /trifocal /
varifocal lens designs.
Multifocal lens
Two types based on processing method
 Solid / one- piece type
 Fused type
Solid /One-piece bifocals
 Benjamin Franklin bifocal.
 Same materials used throughout the lens.
 Curvature changes varies the lens power.
 Available in Glass / Plastic materials.
 Canada balsam is used in cemented segments.
 Can make in any power ranges.
Executive bifocal
 Executive bifocal is a modern version.
 Upper half for distance & lower half for near.
 Optic center and the reading segment are same.
 No image jump.
 Weight
 Less pleasing cosmetically.
 Recommend only on specific request.
Fused bifocal
 Two different materials used.
 Segment material has higher refractive index.
 Segment is fused into main lens.
 No change in lens curvature.
 Image jump is one of the inherent effect.
 Near OC is center of the segment.

In 22mm segment, OC is 11mm below the top of segment.
 Mild image displacement &strong image jump in high ‘+’
lens segment.
Fused - Flat-top bifocals
 Comes either in ‘D’ or ‘B’ shapes.
 Segment width ranges from 22,25,28 & 35mm.
 Segment top is placed 5mm below the distance OC.
 Near OC is 3mm below the segment top.
Image jump & Image displacement
Image jump
 An abrupt displacement of the target as the line of sight
crosses the bifocal edge.


Strong in round segments and
Very minimal with flat-top segments.
 Occurs between the segment line and near OC.
Image displacement
 Occurs even if not see through OC.
 Causes the target to be seen displaced from its true position
when viewing.

Upward displacement in ‘-’lens & downward in ‘+’lens.
 Total displacement is the sum of prismatic effect of dist and
near lens.
Trifocal lens
 Same process is followed like bifocal.
 Designated by size of segment.
 Available in similar designs.
 Three focal points are located.
 Can make either in plastic or glass.
 Intermediate power is half of the reading add.
Trifocal Designs
 Commercially failure.
12
Invisible bifocal lens
 Bifocal without the visible lines
 Look just like ordinary lenses
 Segments are totally invisible.
 Introduced for providing good cosmetic
appearance.
 Technical skill required for proper dispensing.
 Invisible trifocal lens is a first pattern of PAL.
Multifocal Occupational Lenses
To perform a particular job and are not meant for
everyday wear.
 Double-D segment :


A half-moon-shaped segment at bottom & an upside-down flat-top
at the top of the lens.
Suitable to one who need to see well at the near both looking down
to read as well as looking up above their head to work.
 Double-round
Occupational Lensessegment
(Continued): same advantages like Double-D.
Contd.,
Multifocal Occupational Lens –
continues…
E-D trifocal:

A dist.power in upper half of the lens &

Intermediate power in the bottom half in Executive bifocal style with

‘D’ half-moon segment contain the near power that resides within the
lower segment.

Suitable to one who must see at the intermediate distance in a wide
field of vision & who also must see clearly both close-up and in the
distance.


A electronic technician is a best suitable person for this lens.
Sometimes a regular multifocal becomes an occupational lens by
changing the way it is fitted into frame.
Varifocal lens - PAL
Progressive Addition Lens

Provides clear central vision at all distance( distance,
intermediate and near).

Look just like single vision lens without visible lines.

Increases the power by changing the lens curvature

Avoid abrupt image jump.

Adventitious astigmatic error on either side of progressive
corridor.

Habitual head movers adapt easily.

Over 150 designs introduced.

70 designs are currently available.
PAL – Markings and Measurements
Wavefront Lenses
 To correct refractive errors along with some obscure vision
errors known as ‘higher order aberrations’.
 A normal person (6/6) may have vision distortions that
create problems such as double vision or halos at night.
 To achieve crisper vision beyond what conventional lenses
provide.
 Helps to eliminate certain vision distortions.
 Still under research to enhance contrast sensitivity with low
vision / visual loss due to common eye diseases including
macular degeneration.
Criteria for prescribing…
 Visual needs.
 Previous power.
 Present problem with spectacle,if used.
 Current correction.
Common technical errors with
power glasses
 Incorrect PD measurements.
 Incorrect fitting heights.
 Inaccurate refraction.
 Inappropriate frame selection
 Improper fitting.
 Base curve changes.
Centring
 Optic centre a centre point on the lens through which
light goes undeviated.
 OC coincides with pupillary centre to avoid unwanted
prism called Centring.
 If it is displaced from the pupillary centre then it is called
as Decentration that induces prismatic effect in the visual
axis
 Prentice rule to calculate the induced prismatic effect
P=CxF
Where
P - prismatic effect, C - decentration in cm, F - power of the lens.
 If 1.0 D lens is decentred by 1cm,it causes prismatic
effect of 1 prism diopter at a distance of 1M.
Vertex Distance
 Is measured between the apex of the cornea and
back surface of lens.
 Lens optical effect may vary with vertex distance.
 Vertex distance responsible for decrease of vision.
 If moves away from eye,
+ lens becomes stronger.
-- lens becomes weaker.
Key points…
to success with Power spectacle
 An accurate refraction.
 Correct size of frame and adjustments.
 Proper monocular PD measurements.
 Perfect fitting.
 Proper instruction on use.
 Motivate the patient to adapt the new type of
vision.
Thank You