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

5. • 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

6. • 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

8. 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

9. 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

11. 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
ZoneIntermediate
Zone
Lens Radius Changes Along
Umbilical Line

12. • 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

13. Minkwitz’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

14. • 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

15. 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

16. PAL REFERENCES
Distance reference circle
Near reference circle
Fitting cross
Prism reference circle

17. OPTICAL DESCRIPTION OF PROGRESSIVE
LENSES
Power profile
Contour plot
Grid plot
Three dimensional plot

18. POWER PROFILE
• The curve represents the power progression of the
lens along its meridional line from distance to near
vision

19. 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

21. GRID PLOT
• The grid highlights the distribution of prismatic
effects of the lens by showing how they alter a regular
rectangular grid

22. 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

24. PRINCIPAL PARAMETERS OF PAL
Size of the distance and near areas
Types and intensity of aberrations
Depth and usable width of the
progressive corridor

25. PAL DESIGNS
• Hard design
• Soft design
• Spherical and aspherical distance portions
• Symmetrical design
• Asymmetrical design
• Mono design
• Multi design
• Prescription based design

26. 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

27. 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

28. 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

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

30. 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

31. 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

32. Circles
Link
Circles

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

34. 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

35. • 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
• The 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.

37. • 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

38. • 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

39. 1
2
RL
Greater temporal eye rotation from a central point

41. • 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

43. 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

44. 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

45. Progression profile of Multi design PAL

46. 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.

47. NEW PAL DESIGNS
• Atoric progressives
• Position of wear or as worn lens design
• Personalized progressives

48. 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

49. • 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

50. 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
The practitioner specify the sphere, cylinder and axis
measures along with vertex distance and pantoscopic
tilt

51. • 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

52. 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

54. SPECIAL PURPOSE PALS
• Short corridor progressive lenses
• Near variable progressive lenses
• Occupational progressives that include distance
powers

55. 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

57. NEW VARIABLE FOCUS LENSES
• Started out as a replacement for single vision reading
glasses

59. 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

60. • E.g. AO Technica, Hoya Tact

61. PERFORMANCE CHARACTERISTICS OF PAL
Control of accommodation
Foveal vision
Extrafoveal vision
Binocular vision

62. 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

64. 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

65. - 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

67. • 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

68. • 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

69. 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

70. • 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

71. • 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

72. • 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

73. 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

74. • . 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

76. 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 down prism

78. • 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

79. LENS DESIGN SELECTION
• Consider how the wearer uses their lenses
• For most wearers a good modern progressive lens
design is the best solution
• But not all designs provide wide fields of view at
distance, intermediate and near
• Consider 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

80. 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

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

82. SUMMARY
• Any lens for presbyope is a compromise and so is the
PALs
Advantages 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

83. • 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

84. • 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.

85. REFERENCES
• System for Ophthalmic Dispensing 3rd edition ,
W.brooks, M. Borish
• Clinical Optics 2nd 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