progressive addition lenses- optics, designs and performances

pal

1. Progressive Addition lenses - design, optics & performances Sabina poudel b. optometry institute of medicine maharajgunj medical campus

2. Presentation layout Introduction to PAL Structural features and optical characteristics Optical description of progressive lenses PAL designs Special design PALs Performance characteristics of PAL Prism thinning in PAL Lens design selection Summary

3. WHAT ARE PROGRESSIVE ADDITION LENSES? A lens designed for presbyopes with power gradually increasing from the distance zone, through a progressive zone to the near zone Curvature of surface increases from its minimum value in distance zone to maximum value in near zone

4. There is no visible reading segment No any dividing line between distance and near portion No image jump Eye rotation is required to see from distance to near vision area and head movement is required to see across the lateral areas of astigmatism

5. PALs blend the transition between the distance and near zones to provide addition power without any segment lines or ledges Blending is achieved by incorporating plus-cylinder at an oblique axis to join sections of two surfaces with different curvatures in the lateral regions of the lens surface

6. Structural features & Optical characteristics Distance zone: A stabilized region in the upper portion of lens provides the specified distance prescription Near zone: A stabilized region in the lower portion of lens provides the specified Add power

7. Progressive corridor: A corridor of increasing power connects these two zones and provides intermediate or mid range vision The length of corridor is the distance from the center of the fitting cross to the position where 85% of the near add is achieved Blending region: The peripheral regions of the lens contain non prescribed cylinder power and provide only minimal visual acuity

8. Umbilicus A vertex line along which spherical add power increases towards the bottom of the lens Surrounding the vertex line are increasing amount of unwanted astigmatism Umbilical line Distance Zone Near Zone Intermediate Zone Lens Radius Changes Along Umbilical Line

9. In well designed PALs, unwanted cylinder power in the periphery is generally reduced to its mathematical limits However some level of unwanted cylinder power is ultimately necessary to blend any surface with Add power

10. M inkwitz’s Theorem The rate of change in unwanted cylinder power (∆ Cyl ) at a small distance away from the centerline of progressive corridor is nearly equal to twice the rate of change in Add power (∆ Add) over an equal distance along the centerline of the corridor

11. The average rate of change in Add power along the progressive corridor is equal to the total add power divided by the corridor length of lens ∆ Add = Add power Length of corridor

12. The rate of change in cylinder power away from the progressive corridor increases as the length of the progressive corridor decreases The rate of change in cylinder power away from the progressive corridor increases as the Add power of the lens increases

13. PAL References Distance reference circle Near reference circle Fitting cross Prism reference circle

14. Optical description of progressive lenses

15. Power profile The curve represents the power progression of the lens along its meridional line from distance to near vision

16. Contour plot Two dimensional map of the lens representing either the distribution of power or of astigmatism The map shows lines of equal dioptric values ( iso -power or iso -astigmatism) Between two consecutive lines, the power or astigmatism varies by a constant values

17. Grid plot The grid highlights the distribution of prismatic effects of the lens by showing how they alter a regular rectangular grid

18. Three dimensional plot A 3-D representation which plots vertically the value of a given optical characteristic at each point of lens in relation to a reference plane May be used to show the distribution of power, astigmatism, prismatic effects, gradients of power variations More demonstrative of lens characteristics than contour plot

19. Principal parameters of PAL

20. Pal designs Hard design Soft design Spherical and aspherical distance portions Symmetrical design Asymmetrical design Mono design Multi design Prescription based design

21. HARD DESIGN VS SOFT DESIGN Hard design Soft design Spherical distance zone Aspheric distance zone Wider distance and near vision zone Narrower distance and near vision zone Narrow and short intermediate corridor Wide and large intermediate corridor Rapid increase in unwanted astigmatism Gradual increase in unwanted astigmatism

22. HARD DESIGN Advantages Disadvantages Large distance & near area free from astigmatism High intensity aberration at periphery More accessible with downward rotation of eye Distortion for longer and more difficult period of adaptation Wider near zone even at high Rx Swim effect

23. SOFT DESIGN Advantages Disadvantages Decreases intensity aberration at periphery smaller field at sharp vision Easier, more rapid adaptation Need dropping of eye farther near to read Less distortion of peripheral viewing Reduce swim effect

24. Hard design Soft design Wide distance and reading zones Narrow intermediate zones Close spacing of contour lines Reduced distance and reading zones Wider intermediate zone Wide spacing of contour lines

25. Indication for selection of hard design and soft design Hard design : Previous successful hard lens wearers People who do a lot of reading Soft design : Young presbyopes Active outdoor profession Professional driver

26. Spherical distance portion PAL Originally , PALs were designed to maintain an upper half just like a regular single vision lens Upper half had a spherical front surface E.g. Varilux lens

27. Circles Link Circles

28. In aspherical design, both the upper and lower portions are aspherical instead of just in the lower section containing the progressive corridor E.g. V arilux 2 lenses

29. Symmetrical vs asymmetrical design Symmetrical designs Conventional PALs Right and left lenses were identical The lens blank were rotated 9 to 11° nasally to achieve the desired near inset

30. Raised the unwanted cylinder power in the nasal region of lens well into the distance zone, resulting in disruption of binocular fusion as the wearer gazed laterally and a reduction in binocular field of view Limited inset control for near vision since the inset path of the progressive corridor would have to fall along a straight line T he lens is required to be ‘swung’ upward nasally in order to try to align the reading zone with the eyes’ near visual point when converging to read.

31. Asymmetrical design Separate designs for the right and left lenses Amount of cylinder power on either side of progressive corridor is adjusted independently, which allows the near inset to be achieved without rotating the lens design The progressive corridor is initially designed at an angle with the necessary nasalward inclination Provides better binocular alignment between the right and left viewing zones with large binocular field of view

32. Levels of unwanted cylinder greater on nasal side of progressive corridor as a result of achieving the nasal inset without rotating the design Produces difference in prism, magnification and power between corresponding points on the two lenses as the eyes move across them in unison

33. 1 2 R L Greater temporal eye rotation from a central point

34. Horizontal symmetry - Lenses were asymmetrical but designed to give the wearer equal acuities and prismatic effects at all corresponding points of gaze in order to achieve excellent binocular vision

35. Mono design It classify hard and soft Maintain design principles throughout the range of addition It describe the characteristics of progressive zone with a range of power for a given design

36. Multi design In 1988, Essilor introduced PAL that used a different design for each reading addition Incorporates the best features of hard and soft lenses Low reading additions were combined with a soft design which become harder as the add power is increased The reading area remain almost constant throughout the range Ensure the visual comfort and ease of adaptation at each stage of presbyopia

37. Progression profile of Multi design PAL

38. Prescription based design Dedicated design for every base and add Design by base-different designs for hyperopes,myopes and emmetropes Design by add- effective near zone sizes change as the add increases Near inset position varies relative to level of presbyopia and reading distance Corridor length also varies relative to both base and add.

39. New PAL designs Atoric progressives Position of wear or as worn lens design Personalized progressives

40. Atoric progressives Oblique astigmatism can be corrected for spherical lenses by using an aspheric surface But if the lens had two different powers i.e when prescribed cylinder power is present, then oblique astigmatism could only be corrected for both meridians at once if an atoric lens design is used In PAL, oblique astigmatism caused by lens aberration combine with unwanted cylinder in lens periphery

41. Free form technology used to produce atoric surfaces Process begins by generating the lens surface using a three axis computer numerically controlled (CNC) generator With three possible axes of movement, single point cutting tools can produce any lens surface with a high degree of accuracy and smoothness E.g. Ziess Gradal Individual, Varilux Physio 360

42. Position-of-wear or as-worn lens designs Includes following factors in the design of lens on an individual basis Pantoscopic tilt Vertex distance An asheric or atoric surface T he practitioner specify the sphere, cylinder and axis measures along with vertex distance and pantoscopic tilt

43. When the prescription is received, an optimium base curve is chosen for the front surface of lens and prescription is modified to allow for tilt and vertex distance Then the amount of asphericity needed in each major meridian is calculated E.g. Rodenstock Multigressiv 2 lens

44. Personalized progressives Designed to match the unique head and eye movements of the wearers Uses an instrument called VisionPrint System to measure head and eye movement The lens is designed so that the near viewing area will match the personal viewing habits of the wearer E.g. Varilux Ipseo

45. Special purpose pals Short corridor progressive lenses Near variable progressive lenses Occupational progressives that include distance powers

46. Short corridor progressive lenses Allows a PAL to be worn in a frame with a small vertical dimension Faster transition from the distance and near portion of lens Wearer is quickly into the near portion when looking downward Minimum fitting height should be suitable for the frame

47. New variable focus lenses Started out as a replacement for single vision reading glasses

48. Occupational progressives with distance power Used for small office environments and computer viewing Include a small distance portion located at the top of lens Intermediate area of the lens positioned in front of eye Intermediate and near zones considerably wider than standard progressives but not as wide as near variable focus lenses

49. E.g. AO Technica , Hoya Tact

50. Performance characteristics of pal

51. Control of accommodation In single vision lens, eye’s accommodation supported for near vision only In a bifocal lens, the eye’s accommodation experiences abrupt changes when gaze shift from distance to near across the segment For each point of the progressive lens meridian, the power exactly correspond to the eye’s focusing distance

52. Foveal vision Used for critical direct viewing of an object Distance zone Distance zone should be designed such that central vision is excellent through 14 degree of eye excursion in any direction Relatively few compensatory head movements occur until the lines of sight are directed at least 14 degrees away from primary gaze position

53. At a vertex distance of 14 mm, 14 degrees amounts to only 3.5 mm from distance MRP For large ocular excursions, patient should learn to make head movements Distance objects in the inferior field must be viewed by tilting the head down in order to use the superior distance zone Shorter vertex distance increases the field of view through the viewing zones of lens

54. Near zone Stable near zones on PAL are also larger than required to maintain 3.5 mm of excursion in any direction from near MRP Difficulty comes from inadequate gaze depression to get the lines of sight below the top edge of stable near add Pantoscopic tilt brings the near zone closer to the eye and increases the field of view through the near zone

55. Progressive corridor Usually not wide enough to permit 3.5 mm excursion from umbilical line Patient must make unnatural compensatory lateral head movements in order to align objects near the midline when viewing at intermediate distances

56. Extrafoveal vision Refers to the visual perception provided by the periphery of retina When an object is imaged in the periphery of the retina, the physiological factors that become important are: - Locating an object in space - Perceiving the object’s form - Detecting the object’s movement The optical clarity becomes less of an issue due to the fall-off in retinal acuity

57. However, progressively changing add power and unwanted cylinder power in lens periphery produce rapid variation in prism and magnification These variation produce image swim in which objects appear to shift , distort or even sway unnaturally

58. When the apparent movement of the visual environment through the lens differs from the physical movement or orientation detected by wearer because of image swim, produce an unpleasant rocking sensation similar to vertigo or motion sickness Result of vestibulo ocular conflict

59. Unwanted cylinder power in lens periphery oriented at oblique axis, unequal magnification in oblique direction Referred as skew distortion- causes vertical and horizontal edges of image to stretch and tilt

60. Binocular vision For optimal fusion, the images produced by the right and left lenses must be formed on corresponding retinal points and display similar optical properties For ease of motor fusion , both right and left lenses must offer approx. equal prismatic effect on each side of progressive zone To ensure sensorial fusion, the power and astigmatism encountered on corresponding points of two lenses must be approx. equal

61. . When the patient's gaze is lowered for reading at near, the eyes naturally converge to maintain a fused, single binocular image. The PAL design should ensure that the power progression follows this path of convergence downwards. Thus , the progressive meridian is oblique to follow the downgaze path of the visual axes; the two meridians follow a V-shape

62. Prism thinning in pal Increase thickness of PAL when the distance powers are either plus or low minus Result of steepening front curve in the lower half of lens To reduce the thickness, base down prism can be added to whole lens k/a yoked base d own prism

63. The amount of prism needed to thin the lens varies according to the strength of addition, size and shape of lens after edging, and design of lens Varilux suggests adding prism power amounting to approx. two thirds of the power of the add

64. Lens Design Selection Consider how the wearer uses their lenses F or most wearers a good modern progressive lens design is the best solution B ut not all designs provide wide fields of view at distance, intermediate and near C onsider the design that will suit the wearer - general purpose : balanced fields of view - mainly for reading : wide near visual fields - mainly for computer : wide intermediate visual fields

65. Patient selection for PAL Who are good candidate ? Those who require add power for certain task but prefer edge not visible Presbyope complaining image jump Emerging presbyopes Person needing trifocal

66. Who are Poor candidate ? Having motion sickness Satisfied with bifocal High add requirement(3.00D) Significant vertical muscle imbalance Anisometropia (>3Ds)

67. Summary Any lens for presbyope is a compromise and so is the PALs A dvantages of PALs Uninterrupted vision from distance to near More natural use of accommodation Absence of image jump No dividing lines on the lens, cosmetically better

68. Disadvantages of PALs Unwanted astigmatism at the periphery of lens which causes image swim and distortions while looking through that part of lens Increase in eye and head movements Longer adaptation time Expensive

69. Many designs are available with their pros and cons A successful optical correction depends on an accurate assessment of the patient’s visual requirements, the physiology of the eye’s performance for various viewing distances, and a knowledge of the advantages and limitations of the ophthalmic lens design.

70. References System for Ophthalmic Dispensing 3 rd edition , W.brooks , M. Borish Clinical Optics 2 nd edition, Theodore Grosvenor Borish’s Clinical Refraction, William J. Benjamin Fundamentals of Progressive Lens Design, Darryl Miester Progressive Addition Lenses, Essilor Academy CET articles Internet