Essentials on Optical Dispensing

1. Essentials on Optical Dispensing P.S.Selvakumar Faculty Aravind School of Optometry.

2. Multifocal lenses and Designs

3. 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.

4. Corrective lens • Convex / Concave lenses in the form of - Sphere - Cylinder - Sphere & cylinder combination. • Prism • Available in mono focal /bifocal /trifocal / varifocal lens designs.

5. Multifocal lens Two types based on processing method • Solid / one- piece type • Fused type

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

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.

13. 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.,

14. 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.

15. Varifocal lens - PAL Progressive Addition Lens • Provides clear central vision at all distance( distance, intermediate and near). • Look just like single vision lenswithout 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.

16. PAL – Markings and Measurements

17. 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.

18. Criteria for prescribing… • Visual needs. • Previous power. • Present problem with spectacle,if used. • Current correction.

19. Common technical errors with power glasses • Incorrect PD measurements. • Incorrect fitting heights. • Inaccurate refraction. • Inappropriate frame selection • Improper fitting. • Base curve changes.

20. 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 FWhere 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.

21. 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.

22. 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.

23. Thank You