Progressive lenses Part 1
How progressive power is obtained
31 | May 20 | 2005 OT
Continuing Education and Training
progression zone is governed by the power
law for the design. The power law may be
linear, or may be more complex to provide a
greater or lesser increase in power at the start
of the progression.
Compared with bifocal and trifocal
designs, the progressive power lens offers:
• Vision at all distances – since the addition
increases over the progression zone
• More natural use of accommodation –
accommodation does not need to
fluctuate when vision is transferred from
one zone to another
• Absence of image jump – there is no
abrupt change in power
• The appearance of a single vision lens –
there are no dividing lines on the lens
The second of these benefits is immediately
apparent when the accommodative demands
and ranges of vision through a trifocal lens
and a progressive power lens are compared.
Consider a subject who has an amplitude of
accommodation of +1.00D and who is
prescribed a correction of +3.00, Add +2.25
for near. The accommodative demands and
ranges of vision, which are obtained when
this specification is dispensed in trifocal form
and progressive power form, are illustrated in
Figure 1.
Mo Jalie SMSA, FBDO (Hons), Hon FCGI, HonFCOptom, MCMI
Module 2 Part 5
Lens Dispensing Today
The drawbacks of trifocals are much the
same as with bifocal lenses. They have
zones of fixed power, which provide the
wearer with a limited focusing range
through each zone, there is jump at the
dividing lines unless visible, no-jump
designs are used, and the presence of the
segment dividing lines detracts from the
appearance of the lenses. These drawbacks
are eliminated by dispensing progressive
power lenses, the use of which has
expanded rapidly in the last few years.
Without doubt, progressive lenses have
become the first choice of multifocal design
for the correction of presbyopia.
Progressive power lens design
A progressive power lens is designed to
provide continuous vision at all distances
instead of the predetermined working
distances of bifocal and trifocal lenses. The
lens can be considered to have three distinct
zones just like a trifocal design – a distance
zone, a progression zone, or simply, the
progression, and a near zone. Unlike a
trifocal lens, the progression provides an
increase in reading addition from the
distance portion to the near portion. The
rate at which the power increases in the
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T
hirty years ago, if a subject required a correction for distance,
intermediate and near vision, it might have been provided in the
form of trifocal lenses.
About the author
Mo Jalie is Visiting Professor at
the University of Ulster.
Sponsored by Rodenstock
Progress through partnership
www.rodenstock.co.uk
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Figure 1 Comparison of accommodative demands and
ranges of vision through trifocals and progressive power lenses
AccommodationinDAccommodationinD
Accommodation in D
Working distance in cm
0 20 40 60 80 100 200
0 20 40 60 80 100 200
Clear
vision
Blurred
vision
Acc in D
+2.0
+1.5
+1.0
+0.5
+2.0
+1.5
+1.0
+0.5
RP range
IP range
DP range
Figure 1a
Figure 1b
Continuing Education and Training
32 | May 20 | 2005 OT
Mo Jalie SMSA, FBDO (Hons), Hon FCGI, HonFCOptom, MCMI
Sponsored by Rodenstock - Progress through partnership
Figure 1a assumes that a trifocal
correction with an intermediate addition of
+1.25 has been provided. This intermediate
addition would provide continuous vision
from 80cm, down to the near point at
31cm through the intermediate and near
portions of the lens. Figure 1b assumes
that the correction has been dispensed in
progressive power form and it is seen that
the accommodative demand is of the same
form as that enjoyed by the subject before
the onset of presbyopia. Naturally, the
actual position of the curve depends upon
which zone of the lens the subject happens
to be using.
In order to obtain an idea of the
geometry of a progressive surface consider,
first, an E-style bifocal whose bifocal
surface is made from two different spherical
surfaces placed together so that their poles
share a common tangent at point D (Figure
2). Needless to say, the two surfaces are
continuous only at point D. At all other
points, there is a step between the two
surfaces whose depth increases as the
distance from D increases. If we wished to
produce a truly invisible bifocal design,
that is one whose dividing line cannot be
detected, the two surfaces need to be made
continuous by blending the two surfaces
together making the DP surface and NP
surfaces continuous at all points.
In principle, a progressive lens may be
considered to have spherical DP and NP
surfaces connected by a surface whose
tangential and sagittal radii of curvature
decrease according to a specific power law
between the distance and near zones of the
lens.
In theory, to make a surface where the
curvature increases at the correct rate to
satisfy whatever power law it is required to
adopt, we need to be able to combine
small segments of spheres of everdecreasing
radii, all tangential to one
another in a continuous curve. It will be
understood that these sections will only be
continuous along a single, so-called,
meridian or umbilical line and that at all
other points on the surface the sections
must be blended to form a smooth surface.
The simplest concept of this latter
surface is a section taken from an oblate
ellipsoid, as illustrated in Figure 3 where
the radii of curvature of the spherical
surfaces which represent the distance and
near portions are shown as rD and rN
respectively. It can be seen that the solid
ovoid, which is obtained by inserting the
ellipsoidal section between the two
hemispheres shown in the figure, will result
in a surface which has no discontinuities.
Along the meridian line, DD’, a cross
section through the surface would be
circular and the radii of the circles in a
plane parallel with either the distance or
near portion circles do indeed decrease
from rD to rN continuously.
The ability to mass-produce surfaces of
such complex nature was made possible by
the introduction of computer numerically
controlled (CNC) grinding machines. CNC
machining methods have enabled the
design and production of both progressive
and aspheric lens designs in the last 30
years. Basically, the CNC cutter, which is
often a single point diamond tool, cuts a
surface under computer control, the cutter
sweeping in an arc over the workpiece with
the program positioning the cutter in
exactly the right place as the cutter traverses
the workpiece.
The drawback to the direct machining
method is that no matter how accurately
the surface is generated, it must still be
smoothed and polished. These final stages
used to be accomplished by means of a
floating pad system and it was essential to
ensure that the pads did not remove any
more glass than intended thereby
maintaining the correct surface geometry.
In recent years, CNC polishing has been
developed where the blocked lens may be
transferred directly from the generator to
the polishing cycle without de-blocking to
ensure that the path of the polishing tool
follows that of the generator precisely
(Figure 4).
Slumping consists of using ceramic
slumping moulds which are themselves
produced by CNC cutting, upon which the
glass blanks with carefully polished convex
spherical surfaces are placed (Figure 5).
The assembly is then heated to a high
temperature at which the glass starts to
flow. The back surface of the blank then
conforms in shape with that of the ceramic
mould and the convex surface of the blank,
which is the progressive surface, slumps to
the required geometry. The initial shape of
the mould must be very carefully calculated
and highly sophisticated temperature
control is necessary to ensure that the glass
Figure 4
Schneider Opticmachines ALG 100-4 CNC
(by courtesy of Schneider Opticmachines)
Figure 2
E-style bifocal made by placing together two
spherical surfaces with a common tangent at D
Spherical DP surface
Spherical NP surface
D
Figure 3
Concept of a progressive surface.
Section of an oblate ellipsoid is inserted between two hemispheres
of radius of curvature rD for the distance portion and rN for the near position
D
D’
rN
rD
rD
rN
D
D’
Continuing Education and Training
34 | May 20 | 2005 OT
Mo Jalie SMSA, FBDO (Hons), Hon FCGI, HonFCOptom, MCMI
Sponsored by Rodenstock - Progress through partnership
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flows correctly.
Usually, the slumping is assisted by
vacuum forming, where the glass is heated
to just beyond its softening point at which
temperature it is incapable of flowing by
gravity alone, when a vacuum is applied to
the interface between the forming block
and the concave surface of the blank,
effectively, sucking the surface into shape.
The actual geometry of a given
progressive surface, naturally, is regarded as
proprietary information by lens
manufacturers but some insight into how
the design of a surface might proceed can
be obtained by developing the concept
illustrated in Figure 3.
The CNC cutter can be programmed to
cut a series of arcs, each one of which is
independently controlled, but which will
result in a continuous surface of any
geometrical configuration. For example, it
is possible to cut a conicoidal surface such
as the oblate ellipsoid which, in fact, looks
promising at first sight for a progressive
power surface since both tangential and
sagittal surface powers increase quite
rapidly as the distance from the pole of the
surface increases. The rate of increase in
curvature can be controlled by altering the
asphericity of this conicoid, and the change
can be determined as follows. If an oblique
pencil of rays is traced through a spherical
lens there is likely to be an error, δT, in the
tangential oblique vertex sphere power of
the lens. Of course, in the case of a
minimum tangential error lens form, the
error, δT, may be zero. The ray-trace also
yields the incidence height, y1, on the front
surface of the lens, the radius of curvature
of the front surface, F1, being denoted by
r1.
It is possible to show1
that for any given
tangential error, there must be a conicoid
which will eliminate the error. If the error is
positive, a conicoid whose p-value is less
than one must be employed, whereas, if the
error is negative, a conicoid whose p-value
is greater than one would be employed.
If the tangential error is known for a
given incidence height on the front surface,
then the asphericity can be found from the
equation:
p = 1 + (r1/y1)2
.(1 - (F1 / (F1 + δδT))2/3
)
This relationship provides some of the
preliminary information that is needed to
design a progressive power surface. In
progressive power surface design, we want a
tangential error to occur, the error being the
magnitude of the required reading addition.
If we substitute the addition for the
tangential error in the above formula, it will
give us the required asphericity to reach the
value of the addition in the tangential
meridian at a given distance below the
optical axis of the surface.
For example, in the case of a +6.00D
surface worked on a material of refractive
index 1.50, to obtain a near addition of
+1.00D at 14mm below the pole of the
surface would require an asphericity of
+4.5. For an addition of +2.00D, we would
need an asphericity of +7.2 and for an add
of +3.00D, an asphericity of +9.4. These
p-values describe oblate ellipsoidal surfaces.
Lenses which employ oblate ellipsoidal
surfaces are relatively easy to analyse using
ordinary trigonometric ray-tracing
techniques and in the case of a plano
distance lens which employs a convex
oblate ellipsoid whose p-value is +7.2
(which is designed to produce an Add of
+2.00D), we obtain the field diagram
depicted in Figure 6. It can be seen that for
about a 25˚ rotation of the eye, which
corresponds with a point about 14mm
from the pole of the surface, the tangential
oblique vertex sphere power is indeed
+2.00D, the value which it was set out to
achieve.
The ellipsoid alone, however, will not
produce a good progressive power surface.
The conicoidal surface is astigmatic, the
surface astigmatism is normally chosen to
eliminate the aberrational astigmatism of
Figure 5
Slumping a glass blank to the required geometry
b) Slumping a glass blank (forming)
a) Ceramic mould
Continuing Education and Training
35 | May 20 | 2005 OT
Sponsored by Rodenstock - Progress through partnership
oblique incidence.
The field diagram depicted in Figure 6
indicates the oblique astigmatism in the
refracted pencil and it is seen that the
astigmatism almost matches the increase in
mean oblique power of the surface. Since
the field diagram indicates the amount of
oblique astigmatic error in the refracted
pencil, it actually gives us the amount by
which the sagittal curvature must be
increased in order to eliminate the
astigmatism. For example, at 25˚ where the
astigmatism is -1.50D, if the sagittal power
of the surface is increased by +1.50D, it will
eliminate the astigmatism for this zone.
The field diagram gives a first indication of
the path that the CNC cutter must take, as
it traverses the progression zone to
eliminate the increasing surface
astigmatism of the simple ellipsoid.
Figure 7 illustrates the power variation
along the meridian line, AD, for progressive
lenses which are assumed to have spherical
distance and near portions. The separate
plots shown in Figure 7a indicate that the
tangential and sagittal powers differ, as is
shown in the field diagram in Figure 6.
Whereas the tangential power along the
meridian line has reached the near portion
power of +4.00D, the sagittal power only
reaches +2.62D at the bottom of the
progression zone. When the sagittal
curvatures are increased point by point
along the meridian line, however, the
sagittal powers can be increased to match
the tangential powers in the progression
zone resulting in the power variation
illustrated in Figure 7b.
Note that this description of the
correction for astigmatism only applies to
points, which lie along the meridian line
itself. As the cutter moves away from the
meridian line, the form of the surface
becomes more complicated depending
upon the performance, which the designer
wants to achieve in the periphery of the
progression zone.
It will be understood that the image
formation within the progression zone
does not depend only upon the
astigmatism at single points on the surface,
for the power of the progressive surface
changes across the refracted pencil, which
is admitted by the eye.
If the power law through the
progression zone is linear then the change
in power depends upon the near addition,
A, and the length of the progression zone,
h. For each 1mm of progression zone the
increase in power, δF, through the zone is
given by:
δδF = A/h
Thus, if the reading addition is +2.00D and
the length of the progression zone is
10mm, the surface power through the
progression zone must be changing at the
rate of 0.20D per millimetre.
When the eye uses the progression zone,
the refracted pencil which fills the pupil
must exhibit a skewed form of astigmatism,
even along the meridian line. The
astigmatic nature of the pencil is illustrated
in Figure 8, which depicts the tangential
and sagittal fans of a narrow circular pencil
of light, which is limited by the eye’s pupil,
emanating from a region of the progression
zone, which is bisected by the meridian
line.
Assuming that the diameter of the beam
of light traversing the progression zone is
4mm, with a surface that provides a near
addition of +2.00D over a progression
length of 10mm, there will be a difference
in the tangential and sagittal surface powers
across the zone of 0.40D. If the
astigmatism across the pupil is defined as
the difference in vergence between the
tangential and sagittal fans across the pupil,
then the astigmatism is -2A/h D. This
approximate, but important, rule tells us
FFiigguurree 77
Power variation for progressive power lenses with ellipsoidal progression surface of depth 14mm Rx +2.00 Add +2.00, before and after adjustment to
sagittal curvatures in progression zone
FFiigguurree 66
Field diagram for plano lens made with a convex oblate ellipsoidal surface
Field diagrams for spectacle lenses
Tangential and sagittal oblique vertex sphere powers
Ocular rotation in degrees
Obliquevertexspherepowers
10.00
8.00
6.00
4.00
2.00
0.00
F’T
F’S
40 35 30 25 20 15 10 5 0
rD
rN
Sagittal meridians
Tangential meridian
A
D
A
D
a) Progression zone if pure
oblate ellipsoid
b) Progression zone with
modified sagittal radii
to reduce astigmatism
S
T ST T
+2.62 +4.00
+1 2 3 4 5 +1 2 3 4 5
30
15
0
-15
-30
Continuing Education and Training
36 | May 20 | 2005 OT
Mo Jalie SMSA, FBDO (Hons), Hon FCGI, HonFCOptom, MCMI
Sponsored by Rodenstock - Progress through partnership
that the astigmatism across the pupil is
proportional to the addition, but inversely
proportional to the length of the
progression zone. In other words, the
smaller the addition and the longer the
progression zone, the smaller the
astigmatism becomes.
As the eye moves away from the
meridian line, the total astigmatism
increases, approximately linearly but, of
course, dependent upon the exact nature of
the cross-section of the lens.
The corridor of clear vision through the
progression zone is often described as the
region in the zone where the astigmatism
does not exceed +1.00D. It is conventional
for manufacturers of progressive designs to
compare the width of the corridor of clear
vision, either as a stated number of
millimetres or by an isocylinder diagram
where the contours illustrate the change in
surface astigmatism in different zones of
the lens. Such a diagram is illustrated in
Figure 9, together with an iso-mean power
diagram, which shows how the power
varies across the lens. On the basis of a
1.00D limit on surface astigmatism, the
lens illustrated would be described as
having a corridor width of nearly 20mm at
10mm below the geometric centre of the
lens.
The iso-mean power diagram illustrates
that the surface increases in power slightly
as the eye rotates upwards from the
geometric centre of the lens, which is due to
the method chosen to blend the surface
between the distance and near portions.
Also the full reading addition of +2.00D is
seen to have been reached at a point 20mm
below the geometric centre of the lens.
A second consequence of the progressive
surface is depicted in Figure 10, which
compares the appearance of three vertical
lines viewed through a bifocal lens and a
progressive power lens. Since an increase in
power is inevitably accompanied by an
increase in magnification, and it is
inevitable that if there is one then there
must also be the other, vertical lines viewed
through the progression zone exhibit skew
distortion.
The directions of the lines can be
illustrated by means of a vector plot, which
shows how their orientation can be
expected to vary in a real image of the lines.
The skew effect can be minimised by
decreasing the surface curvature as the eye
moves away from the meridian line. The
lower down the surface, the greater the
reduction in curvature needs to become.
The development of progressive lenses
over the last 40 years can be discussed in
terms of the way in which the CNC cutter
has been employed to blend the DP surface
with the NP surface. In the first
commercially successful lens, the Varilux 1
design from Essel Optical Company (now
part of Essilor International), the DP and
NP surfaces were virtually spherical and the
CNC followed a path, which described
circles of ever-decreasing radius as it
traversed from the spherical distance
portion to the spherical near portion. This
necessitated very severe blending of the
distance and near portions with large
amounts of surface astigmatism at the
peripheries of the progression. However, the
distance portion was almost completely free
from surface astigmatism. The second
generation Varilux 2 design used a series of
conic sections of varying asphericity to
reduce the astigmatic nature of the earlier
design and, at the same time, used
aspherical DP and NP surfaces to further
reduce the severity of the blend. Without
doubt, the Varilux 2 design pointed the way
for later generations of progressive power
designs.
Third generation designs, such as the
Truvision OMNI design, combined
aspherical DP and NP surfaces, which
spread the blending further into the
distance portion, softening the definition in
the distance portion but considerably
reducing the astigmatism in the lateral
portions of the lens. The power profile of
lenses, such as the OMNI, can be compared
with that of an up and down curve trifocal
design, with the full distance prescription
FFiigguurree 88
Astigmatic nature of the progression zone
F
F+δF
F+2δF
Isocylinder lines Iso-mean power lines
FFiigguurree 99
Isocylinder and iso-mean power lines for progressive power lens, plano add +2.00D
0.25
0.50
0.75
1.00
0.25
0.25
0.50
0.75
in 0.25D intervals
FFiigguurree 1100
Skew distortion in a progressive power lens
Flat-top bifocal Progressive power lens
25 15 5 5 15 25 25 15 5 5 15 25
20
10
0
10
20
20
10
0
10
20
Continuing Education and Training
37 | May 20 | 2005 OT
Sponsored by Rodenstock - Progress through partnership
obtained near the top of the lens and the
near prescription at the bottom. Such a
long progression, of course, enabled the
lens to exhibit remarkably low levels of
surface astigmatism in the ‘intermediate’
peripheral zones.
The latest generation of designs has
combined the features of low-power
aspheric lenses with a progressive surface
design, and also adopted different power
laws for different near additions.
This feature of progressive lens design,
the ability to control in which areas of the
lens the blending between the DP surface
and the NP surface occurs, has enabled
manufacturers to decide which areas they
should prioritise for optimum vision. If the
designer requires a large distance portion
with the surface astigmatism confined to the
lower portion of the lens, rather like the
earliest progressive lens designs, the result is
a hard progressive design. The arrangement
of the surface astigmatism in a hard design
is shown in Figure 11a. It is seen that this
design enables a large DP area and a
relatively large NP area to be obtained, but
there are rapid discontinuities in the
astigmatism in the lower portion of the
lens.
If the designer wishes to reduce the
amount of astigmatism which occurs in the
lower portion of the lens, to speed the
patient’s adaptation to progressive lens
wear, it can be spread into the distance
portion as indicated in Figure 11b. This
arrangement results in a soft progressive
design, and there can be no doubt that
when the addition is low and, hence, the
surface astigmatism is low, the soft
progressive design has proved to be the
most successful in enabling rapid wearer
acceptance of progressive design.
Some manufacturers produce progressive
lens series that are deliberately soft in
design for the low addition lenses in the
series, the design tending to become harder
as the additions increase. These are known
as multi-design series. Figure 12 illustrates
how the power law differs with a multidesign
series for the additions, +1.00D,
+2.00D and +3.00D. It can be seen how the
length of the progression zone reduces as
the addition increases for these series.
Another important feature of progressive
power lens design relates to the symmetry
of the power distribution across the lens. In
Figure 13, the eyes are supposed to be
viewing an object at B, the visual axes
intersecting the lenses at P in the right eye
and P’ in the left eye. If the surface power
at point P were to differ much in
magnitude or orientation from the surface
power at P’, there will be different prismatic
effects at the two points. For ease of fusion
in all directions of gaze, the vertical
prismatic effects at corresponding points
must be approximately equal. This is more
likely to be the case if each lens is
individually designed for the right and left
eyes, rather than producing a single design
that is rotated inwards, in opposite
directions for the two eyes. Progressive lens
designs that exhibit approximately equal
vertical prismatic effects at corresponding
points, are said to possess horizontal
symmetry.
Although isocylinder and vector
diagrams are informative, it is foolhardy to
suppose that they can be used to predict
wearer adaptation and acceptance of the
lens.
Despite the consequences of the
progressive surface, the brain quickly adapts
to the effects of surface astigmatism and
skew distortion. The adaptation required of
the visual system is probably no greater
when patients wear their first pair of
progressive lenses than that required when
given their first pair of bifocals. Indeed, if
progressive power lenses are introduced at
the onset of presbyopia, when the reading
addition is low, the adaptation period is
probably less than it would be for the first
pair of bifocal lenses. In an attempt to
evaluate the performance of new
progressive designs, most manufacturers
submit the lenses to clinical trials, the
information that the wearers report being
fed back into the design loop.
Patients who, typically, find it difficult
to adapt to progressive lenses are those
with high reading additions who have
successfully worn bifocal lenses in the past
and have become accustomed to a wide
reading field of view through a large
segment. Problems with adaptation to
progressive lenses are almost always due to
either an incorrect prescription for distance,
too high or low a near addition or poorly
fitted lenses2,3
.
25 15 5 5 15 25
20
10
0
10
20
20
10
0
10
20
25 15 5 5 15 25
0.25
1.00
2.00
1.00
2.00
0.25
0.50
0.75
1.00
a) Hard progressive lens b) Soft progressive lens
FFiigguurree 1111
Comparison of isocylinder lines for hard and soft progressive lens designs
of power, plano add +2.00D
FFiigguurree 1122
Power laws for +1.00, +2.00 and +3.00 additions for a multi-design series of progressive power
lenses. Note the designs tend to become harder as the reading addition increases
DRP
NRP
Add +1.00D
Add +2.00D
Add +3.00D
FFiigguurree 1133
Horizontal symmetry at
corresponding point on the lens
B
P P’
DRP = distance reference point NRP = near reference point
0.00 +1.00 +2.00 +3.00
Continuing Education and Training
38 | May 20 | 2005 OT
Mo Jalie SMSA, FBDO (Hons), Hon FCGI, HonFCOptom, MCMI
Sponsored by Rodenstock - Progress through partnership
Thinning prism
When a progressive lens is worked to
individual prescription by surfacing the
concave surface of the blank, it is usual to
incorporate a thinning prism in the
specification of the back surface. The
purpose of the thinning prism is to equalise
the edge thickness of the finished lens,
usually at the top and bottom edges of the
lens, and its magnitude depends upon the
power of the lens, the cylinder axis
direction if the lens is astigmatic, and the
position of the distance reference point
with respect to the box centre of the lens
(Figure 14). Typically for plus spherical
lenses, the magnitude of the thinning prism
is about two-thirds of the reading addition
and its base usually lies at 270˚. As with all
ordered prisms, the magnitude and
direction of the prism incorporated in the
lens must be measured at the prism
reference point, which is normally the
geometric centre of the original blank.
It is sensible to apply an anti-reflection
coating to any lens that incorporates prism
in order to eliminate the possibility of the
ghost image, which is formed by total
internal reflection at the lens surfaces
annoying the wearer.
Fitting progressive
power lenses
A suggested fitting routine for progressive
power lenses is as follows:
1. Select the final frame that the patient is
to wear and adjust it to fit properly. As a
general guide, the frame should be
closely fitting with the smallest possible
vertex distance and the correct
pantoscopic angle, and should provide
adequate depth beneath the centre of
the pupil to accommodate the reading
zone of the lens. A minimum depth of
22mm is usually suggested for designs
with corridor lengths of 18mm, or a
minimum depth of 14mm for compact
designs. Manufacturers usually give a
recommendation upon both the
required minimum depth of lens below
the pupil centre, and the required height
of lens above the pupil centre in order
to ensure that the subject has an
adequate field with the design
(Figure 15).
2. If the frame is empty, attach vertical
strips of transparent adhesive tape to
each eye to enable reference points to be
marked. Otherwise, the fitting cross
positions can be marked on the existing
lenses.
3. With the correctly adjusted frame in
position, ask the patient to look straight
into your eyes. If necessary, adjust the
height of your stool to ensure that your
eyes are on exactly the same level as that
of the patient. Some manufacturers now
recommend that measurements for the
height of the fitting cross should be
taken with the patient standing when it
is easier to judge whether their normal
head position is more or less upright
than when they are seated.
4. Direct the patient to look straight into
your open left eye and using a fine-tip
marking pen and, preferably, a light
coloured ink, place a dot in front of the
centre of the patient’s right pupil.
5. Direct the patient, without moving their
head, to look straight into your right eye
and place a second dot in front of the
centre of their left pupil.
6. Remove and replace the frame on the
patient’s face and repeat the above
procedure, this time without making
any marks, to ensure that the dots which
you have marked do lie in front of the
centres of the pupils.
Record the fitting point positions and
check by means of an appropriate blank
sizing chart that the lenses can be obtained
from the available blank diameters. When
ordering the lenses, it is necessary to give
the fitting point positions (the vertical and
horizontal fitting distances and directions
of the fitting points from the boxed centres
of the lens shapes). The horizontal fitting
point positions are deduced from the
difference between the monocular CDs
measured from the centre of the bridge of
the frame, and half the horizontal box
centre distance of the frame. It is very
common for the monocular CDs to differ
between the right and left eyes. The
specification should be written, for
example, as 34/32, which indicates that the
right eye monocular CD is 34 and the left
eye monocular CD is 32. The heights of the
pupil centres may also differ for the right
and left eyes.
It is sensible, whenever possible, to
dispense a frame that allows some vertical
adjustment of the height of the distance
reference point at, or subsequent to, the
final fitting. This enables the lenses to be
raised or lowered, if this is found to be
necessary, to aid adaptation.
Of equal importance to the success of
dispensing progressive power lenses to new
wearers is the confidence shown by the
practitioner when prescribing this design
for the correction of presbyopia, and
especially what advice is given in the
wearing and use of the lenses at final
fitting. Needless to say, the advantages of
FFiigguurree 1144
Thinning prism
FFiigguurree 1155
Minimum requirements for frame depth
a) Semi-finished blank
without thinning
prism
b) Base-up prism to be
removed to equalise
thickness at the top
and bottom of blank
c) Base-up prism removed
provides equal
thickness at top and
bottom of blank
Mean power plot add +2.00D
for a progressive design with a
corridor length of 18mm does not
provide an adequate near portion
in a shallow frame
Mean power plot add +2.00D for a progressive
design with a corridor length of 14mm shows
that the lens may be mounted in a shallow
frame. Note the recommended frame depth
below the pupil is 17mm, with a minimum overall
frame depth of 29mm
12
17
Continuing Education and Training
39 | May 20 | 2005 OT
Sponsored by Rodenstock - Progress through partnership
progressive designs over other forms of
multifocal correction should be spelt out in
simple terms at the time of dispensing:
• “These lenses will enable you to focus at
all distances”
• To first time young presbyopes:
“They will be easier to get used to than
bifocal lenses”
• “In wear, the lenses will restore the
vision of youth”
• “There are no tell-tale dividing lines on
the lenses”
At final collection, it is important to
re-emphasise these advantages and
demonstrate the use of the lenses. A
suggested routine for the final delivery of
progressive lenses to patients when they
come to collect their new spectacles is as
follows.
1. Firstly, make sure when checking the
lenses that not only the powers at their
recommended measuring points and the
prism at the prism reference point are all
as ordered, but also that the horizontal
painted lines are parallel to the line
joining the centres of the engraved
circles. Also ensure that the position of
the fitting cross on each lens has been
restored so that it is clearly visible.
2. The frame into which the lenses are
mounted should have been set up to fit
the patient properly at the time of
dispensing. However, the lens mounting
process may have disturbed the initial
set-up of the frame, so it is important to
ensure that the frame is returned to the
adjustment that was made when the lens
measurements were taken. Any small
final adjustments that may be needed at
the time of final fitting should be
undertaken first.
3. Verify that the positions of the fitting
crosses do coincide with the centres of
the patient’s pupils. The same procedure
which was used for dotting the desired
fitting point positions should be
adopted.
4. Clean off the painted marks and ensure
that the lenses and frame are completely
clean and look like new.
5. Ask the patient to look at a distant
object directly in front and verify that
the anticipated visual acuity is obtained
for distance vision.
6. Ask them to look at a laterally placed
object at the same height as the straightahead
distance target, and point out if
necessary that it might be necessary to
turn the head to obtain optimum acuity
for this object.
7. Ask them to look at a test chart at near,
which you should guide into their hands
so that it is held directly in front of the
subject in the usual reading position,
and verify that the anticipated acuity for
near vision is obtained.
8. Ask them to move the near chart to one
side and, again, ensure that the patient
understands that they may need to turn
their head and point their nose in the
direction of view in order to see the near
target clearly.
9. Move the near test target to a position
where it is about 40cm to 50cm directly
in front of the patient, and just below
the original object chosen to verify the
distance acuity. Point out that they can
also see clearly at the intermediate
distance, if necessary by a slight
adjustment of the visual axes in the
vertical meridian. In particular, allow the
patient to confirm that the gaze can be
transferred from the distant target to the
intermediate target without moving the
head, simply by adjusting the direction
of gaze of the eyes.
10.Point out the visual tasks which are not
easy with progressive power lenses, such
as reading a notice with small print
which is above head height, reading
when lying down or trying to watch
television when lying down in bed, and
looking over their shoulder as they
might when reversing a car.
Most important of all is to reassure the
patient that these visual tasks are well
understood, as is the optical performance
of their progressive lenses, and that most
people need no more than a short period
to get used to this state-of-the-art correction
for presbyopia.
Insetting progressive
power lenses
When single vision lenses are dispensed for
near vision, they are decentred inwards so
that the visual axes pass through the optical
centres of the lenses. This enables the
centres of the near fields of view to
coincide and avoids the introduction of
unwanted horizontal prismatic effect for
near. In the case of multifocal lenses whose
distance portions have been properly
centred for distance vision, the main lens
exerts prismatic effect at the near visual
points, which alters the convergence
requirements for objects viewed through
the near portion. The purpose of insetting
the near portion is to bring the near fields
into coincidence4
and in order to achieve
the desired effect, it is necessary to take the
prismatic effect of the distance portion into
account (Figure 16).
It can be seen from Figure16 that, in
order to place the centre of the near
aperture in the correct position (i.e. the
centre of the segment in the case of bifocal
lenses, or the centre of the near zone in the
case of progressive power lenses), the
insetting must be greater in the case of plus,
and less in the case of minus distance
portions when compared with the inward
decentration of single vision lenses
designed for near vision. Thus, for each
combination of base curve and reading
addition, the designer must calculate a
different inset for the progression corridor
on the lens to ensure that the wearer enjoys
the widest possible widths for the
progressive corridor and the near vision
zone.
With most modern progressive designs,
the inset is individually calculated taking
into account both the working distance and
the distance and near prescriptions, in
addition to the monocular centration
distances.
Part 2 of Progressive power
lenses on June 17 will look at
the latest generation of
progressive lens designs.
References
1. Davis JK and Fernald HG (1976) US
Patent 3960442. Ophthalmic Lens Series
2. Essilor International (1997) Personal
communication. In-house analysis of
wearer rejection of progressive lenses.
3. Sullivan CM and Fowler CW (1990)
Investigation of progressive addition
lens patient tolerance to dispensing
anomalies. Ophthal. Physiol. Opt. 10: 16.
4. Jalie M (2003) Ophthalmic Lenses &
Dispensing. Second edition. Butterworth
Heinemann, Oxford.
FFiigguurree 1166
Insetting progressive power lenses
a) Plus lenses b) Minus lenses
OC OC
Continuing Education and Training
40 | May 20 | 2005 OT
Mo Jalie SMSA, FBDO (Hons), Hon FCGI, HonFCOptom, MCMI
Sponsored by Rodenstock - Progress through partnership
1. What jump is exerted by a
progressive lens made to the
prescription, +1.00D add +2.50D?
a. 1.00D
b. 2.50D
c. 3.50D
d. None
2. A progressive power lens with an
addition of +3.00D has a
progression zone 15mm in length.
Assuming a linear power law
through the progression, what
addition would you expect the
wearer to obtain 10mm below the
start of the progression?
a. +1.00D
b. +1.50D
c. +2.00D
d. +2.50D
3. Why does a section taken from a
simple oblate ellipsoid not form a
useful progressive surface?
a. The sagittal curvature is insufficient to
provide no astigmatism along the
meridian line
b. It does not join the distance and near
areas without a discontinuity in the
surface
c. It does not provide sufficient near
addition in the tangential meridian of
the meridian line
d. It provides too great an addition along
the sagittal meridians of the meridian
line
4. Why does skew distortion occur in a
progressive power lens?
a. Because all lenses exhibit distortion
b. Because the power becomes more plus
as the eye moves down through the
progression zone
c. Because the power becomes less plus
as the eye moves down through the
progression zone
d. Because the magnification becomes
less as the eye moves down through
the progression zone
MCQs
Progressive lenses Part 1 – How progressive power is obtained
Please note there is only ONE correct answer
Module 2 Part 5 of Lens Dispensing Today
5. What are the characteristics of a soft
progressive design?
a. No astigmatism in the distance area of
the lens
b. No astigmatism in the lower half of the
lens
c. Astigmatism is allowed to spread into
the distance portion of the lens
d. Astigmatism is confined to the
peripheral portions of the lens
6. What is the main characteristic of a
multi-design series of progressive
lenses?
a. The power law varies with the reading
addition
b. The design becomes softer as the
addition increases
c. The design varies with the distance
prescription
d. The length of the progression zone
increases with the near addition
7. What is meant by the term ‘horizontal
symmetry’ when applied to a
progressive power lens?
a. The horizontal power of the lens is the
same at corresponding points in a pair
of lenses
b. The horizontal prismatic effect is the
same at corresponding points in a pair
of lenses
c. The vertical prismatic effect is the same
at corresponding points in a pair of
lenses
d. The thinning prism which has
been incorporated is the same for each
eye
8. Which of the following does not
usually hinder adaptation to a pair of
progressive lenses?
a. Too high a reading addition
b. A low reading addition
c. An incorrect distance prescription
d. Poorly fitted lenses
9. If a standard prism thinning is
applied to a progressive power lens
whose power is plano / +1.00 x 90
add +1.50, how much prism would
you expect to find at the distance
reference point of the lens assuming
this to lie 5mm above the prism
reference point?
a. 1.00D base down
b. 1.50D base up
c. 1.50D base down
d. None
10. If no prism thinning is applied to a
progressive power lens whose power
is -5.00 add +2.50D, how much prism
would you expect to find at the
distance reference point of the lens
assuming this to lie 4mm above the
prism reference point?
a. None
b. 1.00D base up
c. 2.00D base down
d. 2.00D base up
11. On verification of a progressive power
uncut lens the power at the distance
reference point, 4mm above the
geometrical centre of the uncut, is
found to be +4.00D and it is noted
that the prismatic effect at this point
is 2.6D base down. What thinning
prism has been incorporated in the
lens?
a. None
b. 1.00D base down
c. 1.60D base down
d. 1.60D base up
12. Minus power progressive lenses
should be inset in which one of the
following ways?
a. The same as plus lenses
b. More than plus lenses
c. Less than plus lenses
d. The same for all minus powers
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