Friday, 24 January 2014

OPTICAL CHARACTERISTICS OF GENERAL PURPOSE PROGRESSIVES

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.





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.