The
first successful progressive addition lenses were designed to maintain some of
the characteristics of a bifocal. One criterion considered to be important was
maintaining traditional lens optics in the upper half of the lens. If this is
done, the power from the midline upward corresponds exactly to the prescribed
distance power. At the midpoint of the lens and downward following the expected
path of the eyes, plus power begins to increase. Once the full add power is
reached, lens power does not vary.
The
progressive zone connects distance and near lens areas. These types of lenses
are said to have spherical upper halves because the front surface of the
upper half of the lens is spherical, rather than aspheric.
The
first really successful progressive lens was the original 1959 Varilux lens. ”The
1959 Varilux lens used this design philosophy.”
The original Varilux lens was designed to maintain a spherical surface in the upper half of the lens. It had two large and spherical distance and near vision zones linked together.
Unwanted Cylinder
Unwanted cylinder is the greatest problem inherent in progressive
addition lenses. Although the progressive zone gives clear vision when properly
fitted and dispensed, the area to either side of this zone will have some
unwanted cylinder power. This cylinder varies in amount and orientation,
depending on design and adds power. It will be noticeable if the eye moves far
enough laterally from within the progressive zone.
Interrelating Progressive Design
Factors
Here
are some general design factors that may influence unwanted cylinder power and
other lens parameters.
1.
Add power—as add power
increases, so will the amount of unwanted peripheral cylinder.
2. Rate of progressive power change—progressive
power can change from distance to near zones in either a rapid or slow fashion,
making the progressive corridor either short or long. A rapid change means
that
the progressive zone surface curvature changes over a very short distance
resulting in a short corridor lens.
When
the power changes rapidly:
i) The intermediate zone width will
generally be smaller.
ii) The near zone is generally wider and
larger. If the progressive zone is longer, the plus power changes more slowly.
A longer progressive zone means less unwanted cylinder; a shorter progressive
zone means more unwanted cylinder.
3.
Intermediate zone width—a
larger minimum zone width is associated with lower amounts of unwanted cylinder.
The smaller the intermediate zone width and area, the greater the unwanted
cylinder will be.
However,
there is not as direct a relationship between the amount of unwanted
astigmatism and near-viewing zone size.
4.
Zone widths—distance and
intermediate and near zone widths influence each other. When one zone is made
larger or wider, the other two zones will become narrower and smaller.
Hard Versus Soft Designs
When an individual (wearing a progressive addition lens) is using
the near-viewing area of the lens and slowly looks to one side, the eyes begin
to leave the region of the near zone. Outside of this near zone, the power begins
to change, and unwanted cylinder power increases.
Hard
Designs
With
a bifocal lens, there is a distinct, lined border between the near-viewing area
and the rest of the lens. There is no question as to where the near portion
ends. With some types of progressive addition lenses, the change in power and
increase in astigmatism is more demarcated than in others.
For
example, the unwanted cylinder may rapidly increase from nothing up to 0.50 D,
then move quickly to 1.00 D, and on up to 1.50 D in the space of only a few
millimeters. Because of the rapid change along the border between viewing
areas, this type of design is known as a hard design.
Hard
designs generally offer larger and more delineated areas of unvarying optical
power for distance and near viewing.
Disadvantages
The
disadvantages of hard designs are linked with the rapid increase in cylinder
power and the areas in which that unwanted cylinder is concentrated.
1.
Distortions caused by more rapid power
change may mean a slightly longer period of adaptation.
2.
Straight lines may appear more curved
when viewed through the lower half of the lens than they do with other designs.
3.
The intermediate viewing area of the
lens may be more limited both vertically and horizontally, requiring the wearer
to zero in more consciously to view intermediate objects with clarity.
Soft
Designs
A
soft design is one in which the change from the near zone to the peripheral
area is gradual when compared with a hard design. As the wearer’s eye begins to
leave the near zone laterally, the amount of unwanted
cylinder increases, but more gradually.
A
soft design has a slower vertical change in power as the wearer looks from
distance to near.
Advantages
The
advantages of a soft design are easier, more rapid adaptation times; less
distortion of peripherally viewed objects; and less “swim” of objects with head
movement.
Soft
designs typically start with a smaller near zone and allow aberrations to
spread over a larger area, including parts of the upper half of the lens. This
means that the dioptric power of the unwanted cylinder will not be as large.
A and B on left side( hard design), and A and B on right side (soft design)
Disadvantages
The
disadvantages of soft designs include the possibility of a slight reduction in
visual clarity in the upper peripheral areas of the distance lens, the
necessity of dropping the eyes farther to reach the full add power and a
“smaller” near zone. It should be noted, however, that wearers do not always find
the near zone to be functionally as small as it may appear on an astigmatic contour
plot. Because the amount of unwanted cylinder increases so gradually as the
eyes leaves the near zone laterally, the wearer may be able to use the outer
limits of the near area anyway, even though these areas contain a certain
amount of unwanted cylinder power.
Free Form Progressives
It
is now possible to individually shape a lens surface to a unique form with a
varying surface curvature and then polish that surface to optical quality. This
type of manufacturing has commonly been referred to as free-form generating.
Here
are some examples of what these changes in manufacturing mean in terms of
possibilities for progressive lenses. Some possibilities may be used by one
design, some by another. Not all will be used for the same lens.
1.
The back surface of the progressive can be made as an aspheric or an atoric
surface. Atoric curves can reduce the peripheral aberration called oblique
astigmatism. This is especially important for progressive addition lens wearers
with cylinder. When uncorrected oblique astigmatism is present, it combines
with the peripheral distortion inherent in progressive addition lenses and can
further degrade peripheral vision. An atoric design can improve peripheral
vision.
2.
Progressive lenses are normally made as semi finished lenses with certain fixed
base curves. These Semi finished lenses are then surfaced in the laboratory.
With free-form generating, the front surface can be custom surfaced to any base
curve and the progressive optics included during surfacing. Then the back
surface is generated at the completion of the front surface. This way the base
curve can be more closely matched to the power of the lens.
3.
If a frame is fit with a specific vertex distance, the prescribed power of the
lens can be altered for the vertex distance of the frame. These power changes
are not limited to quarter diopter increments. The smoothing and polishing
process no longer uses power-specific tools to bring the surface to optical
quality.
4.
When a lens is tilted, there is a change in the sphere power, and a cylinder is
induced whose axis is in the meridian of rotation. This power change can be
compensated for on an individual basis, whether the tilt is pantoscopic tilt or
face form. Again the compensation may be done more exactly because it is not
limited to quarter diopter increments.
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5.
With this type of generating, it is possible to make a progressive lens to
order with the progressive power on the front of the lens, the back of the
lens, or on both the front and the back of the lens. (The Definite lens is made
this way with the progressive add split between the front and back surfaces.)
6.
This type of generating allows for the progressive portion of the lens to be
made at different widths, depending upon the needs of the wearer.
7.
The progressive zone of a lens can be shortened or lengthened to custom fit the
vertical depth of the frame and the vertical height of the wearer’s eyes.
POSITION-OF-WEAR OR
AS-WORN LENS DESIGNS
A major change in progressive lenses that took place because of
free-form generating resulted in lenses sometimes referred to as position-of-wear
or as-worn designs. A primary example of this is the Rodenstock
Multi gressiv2 lens. This lens includes all the following factors in the design
of the lens on an individual basis:
# Pantoscopic tilt
# Vertex distance
# An aspheric or atoric surface to optimize correction of lens
aberrations the practitioner specifies the sphere, cylinder and axis measures,
along with vertex distance and pantoscopic tilt. When the prescription is
received, an optimum base curve is chosen for the front surface of the lenses,
and the prescription is modified to allow for tilt and vertex distance.
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