Essentials on Optical Dispensing

P.S.SelvakumarFaculty

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 displacementImage 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.

Commercially failure.

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Trifocal DesignsTrifocal Designs

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 segment: same advantages like Double-D.

Contd.,

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Occupational Lenses (Continued)Occupational Lenses (Continued)

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.

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Progressive Lens DesignHard Vs Soft Lens DesignProgressive Lens DesignHard Vs Soft Lens Design

Hard Design Wide distance and near

zones Narrow intermediate

corridor Rapid increase in

unwanted astigmatism Shorter corridor

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 = C x F 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