Ophthalmic Lens

1. Ophthalmic Lens R.D.Gopinath Supervisor – Optical Sales

2. Introduction • Many patients come to determine the refractive status and correction. • Spectacle lens commonly used therapeutic tool. • Around 70% of the patients require spectacle correction. • Optical dispensing is a significant part in eye care.

3. Ophthalmic lens prescribed to … • Correct the refractive errors. • Enhance the near vision performances. • Have a therapeutic effect on the eyes. • Relieve the symptoms of muscular imbalance. • Prevent eye suppression & stimulate the visual performances. • Protect from cosmetic and environmental situations. • To protect the eye from light radiation and dusty atmosphere.

4. 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.

5. No one knows for certain when eyeglasses were invented, although documents from the 13th century prove the existence of eyeglasses at that time. Several sources quote a manuscript written in Rome in 1289 by a member of the Popozo family that says, "I am so debilitated by age that without the glasses known as spectacles, I would no longer be able to read or write." A painting done by Tommaso da Modena in 1352 includes the first known artistic representation of eyeglasses. • Historians credit the Chinese with carving the first frames more than 2,000 years ago, but apparently those frames did not contain lenses and were used to protect their eyes from "evil forces." The frames were carved from tortoiseshell, a sacred material. • The use of a magnifying glass was first recorded in about 1000 A.D. It was called a reading stone and was placed on top of reading material to magnify letters. Monks used it to copy manuscripts. Later, Venetian glassblowers constructed lenses that could be held in a frame in front of the eyes. Glasses for distance vision first appeared around the middle of the 15th century, and there are various references in literature of that time to spectacles for "distant vision." • In the 15th century, the printing press was invented, making reading materials more available to the public and increasing the need for glasses. Early eyeglasses were held by hand in front of the eyes or designed to "perch" on the nose. It wasn't until the 17th century that a London optician perfected the use of side pieces that rested on the ears. • In 1784, Benjamin Franklin invented a bifocal lens with the top half for viewing at distance and the bottom half for reading. • In the 19th century, a method was found for examining eyes and prescribing eyeglasses. Prior to that time, those who needed glasses simply tried on various pairs until they found one that worked. Dr. F. C. Donders of Holland wrote the first textbook on examining and prescribing eyeglasses. This book stimulated an interest by eye physicians in prescribing glasses, while opticians set up their shops to fill prescriptions. Some of these opticians elected to also examine eyes and dispense their own prescriptions. This group evolved into the modern profession of optometry.

6. The first high-quality optical glass was developed in Germany toward the end of the 19th century. Prior to that time, eyeglass lenses were made from Brazilian quartz crystal and were poor in quality. Until 19l4, most glass used in optical lenses was imported from Germany. At that time, John Bausch and Henry Lomb developed a high-quality glass in their Rochester, N.Y., factory. • Glass remained the primary material for eyewear lenses until the 1940s when the Pittsburgh Plate Glass Company developed a plastic lens material named CR-39. This material remains the choice for most lenses today because it weighs about half as much as glass and is more impact-resistant. Like glass, it can also be ground, polished, and tinted. • The other plastic used in lenses is polycarbonate (poly), a lightweight, almost unbreakable material developed by NASA as part of the space program. This is the material commonly used in children's, sports, and safety glasses. A major advantage of poly is that it can be ground much thinner than either glass or CR-39. It is, however, more expensive and usually needs an anti-scratch coating because it is softer.

7. Lens materials • Natural media - quartz / rock crystal, topaz, ruby, etc. • Glass materials - crown & flint / barium crown • Plastic materials

8. Glass material- Crown glass • A soda-lime silica material. • Contains 70% silica, 12% calcium oxide, 15% sodium oxide and potassium, borax, arsenic in few % • Refractive index - 1.523. • Widely used material.

9. Glass material- Flint glass • Is used in making bifocal /achromatic lens. • Contains 60% lead oxide, 30% silica, 8% soda and potash & arsenic in small %. • Lead oxide increases the Refractive index. • High chromatic aberration.

10. Glass material- Barium crown • Barium crown is now replaced. • Contains 35% barium oxide,30 % silica and lime, zinc, zincorium,aluminium,boran in small %. • Increases the refractive index. • No chromatic aberration.

11. High index glass • Material unknown. • Used in making a lens in thin form. • Refractive index is 1.806. • Suitable for anti-reflection coating. • Gives good appearance to crown glass. • Not suitable for fused type bifocals.

12. Plastic materials • Original plastic lens from PMMA. • Modern hard resin lens from Allyl Diglycol Carbonate.(1.498). • Thermoplastic lens from Polycarbonate.(1.586)

13. Plastic lens • Half of the weight of glass. • High impact-resistant. • To be tinted for proper UV protection. • Is hardened by chemical /heat process. • Polycarbonate comes in a thin form • Thickness :center -1.5mm ; peripheral -2.2mm.

14. High index lens • Mostly comes in plastic lens materials. • Material :Allyl resin and poly urethane polymers. • Available in various refractive index of 1.60,1.70 and 1.80. • Make into remarkable thin & light weight. • Chromatic aberration occurs. Polycarbonate lens is advisable to all.

16. Plastic lens materials Materials Refractive Index Abbe value CR 39 1.498 58.0 Spectralite 1.537 47.0 Hilux II 1.556 40.0 Polycarbonate 1.586 30.0 High index 1.600 36.0 Hyperindex 1.660 32.0

17. Base curve • Used to designate the lens form. • Single vision spherical lenses : weaker of the two curves. Base curve will be the back / concave side of ‘+’ lens and the front / convex side of ‘—’ lens. • Single vision cylindrical lenses: the lesser (weaker/flatter) of the two curves on the side in which the cylinder is ground. In ‘+’ cyl lens, the cylinder is ground on the front surface of the lens & ‘—’ cyl lens, the cylinder is ground on the back surface of the lens. Almost all lenses are designed in ‘—’cylinder form. • Standard base curves +4.25D,+6.25D,+8.25D. • Multifocal lenses: the base curve is on the spherical side containing the reading segment.

18. +9 4 +9 5 2 +9 +9 0 Base Curves Positive Spheres +4D +5D +7D +9D Negative Spheres 9 8 3 +2 +2 7 +2 +2 –1D –6D –7D –5D

19. Why base curve important? • To have a same radius that coincides with eye ball rotations. • To maintain the equal vertex distance. • To avoid /minimize the peripheral distortion. • To avoid the eye lash touching with lens.

20. Base Curve and Ocular Curve • The base curve, ocular curve, and lens power are related as follows: • Base Curve (BC) = Lens Power (P) - Ocular Curve (OC) • Ocular curve (OC) = Lens Power (P) - Base Curve (BC) • Example:  If the lens power is -6.00 and the base curve is +2, what is the power of the ocular curve? • OC = P - BC • OC = -6.00 - 2 = -8 • Example:  A patient is wearing a +3.00 Sphere lens with a base curve of 6 D.  If the prescription changes to +1.00, what will the new base curve have to be to keep the ocular curve the same. • First we calculate the ocular curve of the current glasses: • OC = P - BC • OC = 3 -6 = -3 • Now we plug this into the BC version of the formula: • BC = 1 - (-3) = 4 • We will need to specify a base curve of +4 in the new glasses to keep the ocular curve the same with a lens power of +1.00.

21. Lens Effectivity • 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. Monofocal lens • Single focus lens. • consists of a single sphere of appropriate radius provides only one correction. • Prescribed only for single vision. - Distance vision glass - Near vision glass - Vocational glass

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

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

25. 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.

26. 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.

27. Fused bifocals • 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.

28. Fused -Round / Kryptok bifocal • Near OC is center of the segment. • In 22mm segment, OC is 11mm below the top of segment. • Mild image displacement and strong image jump in high ‘+’ lens segment.

29. 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.

30. 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 distance and near lens.

31. 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.

32. 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.

33. 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.,

34. 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.

35. 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.

36. High power - Lenticular lens • Strong distortions occur inherently in edges of lens. • Circular lens in small diameter. • Thin Plano-carrier support to fit a lens into a frame easily. • Bull’s eye effect causes poor cosmetic appearance • More conspicuous than other lens.

37. Aspheric lens • To eliminate the ‘pin-cushion’ distortions. • Center is more thicker than periphery. • Minimal prismatic effect in edges. • Suitable for Aphakic patients. • Induces more reflections off the flatter back surface of the lenses which can be prevented by A-R coating to improve the vision through the lens. 

38. Lens aberrations • Distortions • Barrel & Pin-cushion. • Curvature of image • Coma • Magnification and minification. • Aberrations • Spherical • Chromatic

39. Wavefront Lenses • To correct refractive errors along with ‘higher order aberrations’. • 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 associated with or not by conventional lenses. • Still under research to enhance contrast sensitivity with low vision / visual loss due to common eye diseases including macular degeneration.

40. Wave front method • Traditional methods may fail to genuinely reflect what's wrong with our vision. • Vision errors are detected automatically and almost instantaneously in this technique. • A method involves • beaming light into the eye, and then "mapping" how light waves travel after they are reflected off the retina. • If light rays are not refracted properly by cornea and crystalline lens, then focusing problems can occur.

41. Tinted lens • A dying material is added with white lens. • Gives UV, Glare and cosmetic protection. • Allows better visual function in many ways by • improving the contrast sensitivity. • improving the dark adaptation & • reducing the glare and light sensitivity • Special filters help certain eye conditions. • Not suitable for presbyopes. • Available in either constant or variable tints (Photochromatic lens) . • Lens dyingprocess : Dipping / high vacuum process.

42. Fresnel lens • Polyvinyl chloride sheets in 1mm thickness consist of a series of parallel ridges. • Each ridge has a designated prism angle. • Refractive Index 1.525. • Used as a ‘light-condensing lens’ in ships. • Ideal for aphakic patients. • Poor cosmetic appearances due to concentric circles. • Availability: +20.0D Sph / -14.0D Sph / up to 30 prism diopters. • Useful • For higher prism correction. • As temporary trial prisms for heterophorias. • In recovering muscle paresis / unstable muscle condition. • To expand the field vision in low vision.

43. Polaroid lens • Thin layer of Dichronic crystals is sandwiched between two layers. • Reflected light is polarized and absorbed by middle layer. • Light transmission reduce by 50% improves vision and comfort. • Useful for water activities and highway driving. • Available in combination with tints. • Unlike ordinary sunglasses, polarizing lens eliminates 99.9% of glare from horizontal surfaces (roads, water and snow). • Gives 100% UV protection.

44. Anti reflection coated lens • Reduce reflections from the surfaces for better light transmission. • Allows more light (99%) into the eyes to improve vision. • Best in low-light conditions like computer & night driving. • Reduces glare that causes "ghost images". • Eliminate the reflections that can contribute to eye strain. • Material: Magnesium fluoride.

45. Safety lens • Plastic hard-resin lenses. • Does not have the sharp splinters typical of a broken glass. • Polycarbonate lenses • Introduced initially for industrial use. • Now all power lenses comes in polycarbonate material. • Heat –treated impact resistant lenses / ‘Toughened lenses’ • Laminated lens. • A sheet of plastic is sandwiched between two glasses. • Best for all children, sportsmen and industrial workers.

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

47. Thanks for your attention!