The Optics of Progressive Lenses – Part 3

By Darryl Meister, ABOM

The Progressive Corridor

Recall that the umbilic represents the centerline of the progressive corridor, which is the channel of relatively clear vision connecting the distance and near viewing zones. It was also demonstrated mathematically that the width and intermediate utility of the progressive corridor depend upon the corridor length of the lens for a given Add power. The lens designer is primarily concerned with three features related to the geometry of the umbilic:

1. Total length of the umbilic
2. Horizontal placement of the umbilic
3. Management of power along the umbilic

Earlier, we defined corridor length as the vertical distance between a point along the umbilic producing the lowest mean power (in the distance zone) to a point producing the highest mean power (in the near zone, where the maximum Add power occurs). The point producing the lowest surface power at the start of the umbilic is generally located at the distance reference point (DRP) of the lens, while the point producing the highest surface power at the end of the umbilic is generally located at the near reference point (NRP).

In most cases, the umbilic is rotated so that it is aligned with the natural movement of the eyes as they converge to see nearby objects through the intermediate and near zones. This ensures that the eyepath, or the path the line of sight travels over the lens surface as it approaches the near zone, remains properly centered within the progressive corridor. As with lined multifocals, the lateral placement of the near zone is referred to as the inset. The total rotation of the umbilic will depend upon the inset. The eyepath of more advanced lens designs is precisely computed by calculating the theoretical reading distance that corresponds to the mean power at each point along the umbilic.

The change in mean power (or Add power) along the umbilic is referred to as the power profile of the lens design. The corridor length of the lens design primarily drives the power management geometry of the umbilic, since the length of the corridor will dictate how rapidly the mean power must change in order to reach the full Add power. The average rate of change in mean power along the umbilic is inversely proportional to the corridor length:

Rate of Change in Power = Add Power ÷ Corridor Length

However, it is actually possible to vary the mean power along the umbilic in a non-linear fashion in order to achieve a degree of flexibility in the optical performance of the lens design. For instance, the power profile often ramps up in mean power rather slowly at first to minimize the excess plus power in the distance zone, which could otherwise blur vision. The power profile may then ramp up more quickly in the progressive corridor to deliver a sufficient amount of mean (Add) power for reading vision well above the near reference point, so that the wearer may realize some degree of near utility higher in the lens. Careful control of the power profile is also necessary to ensure that the change in Add power reflects the natural inclination of reading materials.

In practice, corridor length is measured from the fitting point, not the distance reference point, since the fitting point is aligned with the optical system of the eye. (The location of the distance reference point is somewhat arbitrary; it is generally located to ensure error-free verification prescription.) Furthermore, it i common to specify the a percentage of the specified Add power, such as 85% or 95%. Consequently, the advertised corridor length is often shorter than the distance to the near reference point.

Although in theory a progressive lens could have a corridor length that approaches zero, there are certain optical and visual considerations that place practical limitations on the range of acceptable corridor lengths. Several factors must be taken into account when choosing the corridor length for a progressive lens design. For example, shorter corridor lengths offer the following advantages:

• More near vision utility in smaller frames
• Reduced eye declination during near vision

Every additional 1 mm of corridor length requires roughly 2° of additional ocular rotation to reach the near zone. If the corridor length is too long, the wearer may not be able to reach the full Add power of the near zone without awkward postural adjustments. However, shorter corridor lengths offer the following disadvantages:

• Less intermediate vision and mid-range utility
• More rapidly increasing power and astigmatism

The shorter the progressive corridor, the more the optics of the lens design must be effectively "squeezed" together. This results in higher levels of peripheral blur, reduced intermediate utility, or narrower viewing zones.

Consequently, the length of the corridor should be carefully chosen in order to offer the most utility with the least amount of compromise.

Viewing Zone Configuration

Unlike lined multifocals, the central viewing zones of a progressive lens are not well defined. The size and utility of the viewing zone will vary depending upon both the wearer's tolerance to blur and the nature of the visual task.

For instance, the blur produced by unwanted astigmatism in the periphery of a progressive lens may make certain critical viewing tasks—such as reading—difficult, though these regions may be perfectly acceptable for dynamic vision, which often requires only localizing and recognizing objects. Nevertheless, the lens designer can define the size and shape of the central viewing zones by establishing a limit on the amount of acceptable power error, including the maximum levels of unwanted astigmatism and mean power error. (For our purposes, the mean power error represents the difference between the mean power produced by the lens design and the Add power actually required for a given reading distance.)

For a given power error threshold, the size of the central viewing zones can be increased by pushing the unwanted astigmatism farther into the periphery. However, this trade-off results in a harder lens design with higher and more rapidly increasing levels of unwanted astigmatism. Because of this trade-off, lens designers must be careful not to increase the size of any particular central viewing zone any more than necessary.

The size of each viewing zone should be judiciously managed in order to provide the best overall balance by maximizing the global utility of the lens design. Therefore, lens designers must choose the best balance between the size of the distance zone and the size of the near zone, so that both zones function comparably for the wearer. Of course, this will ultimately depend on the nature and frequency of the visual tasks performed by the wearer.

Some lens designers feel that maximized distance utility is preferred by wearers, and create progressive lenses with an emphasis on distance vision. These lens designs will have relatively large distance viewing zones, often at the expense of the near zone size. Other lens designers feel that maximized near utility is more useful to wearers, and create progressive lenses with an emphasis on near vision. Consequently, the idea of "balance" depends on how the lenses will be used by the actual wearer, not necessarily on the relative physical sizes of the viewing zones.

Design of Periphery

Unfortunately, the surface astigmatism inherent in progressive lenses produces a significant amount of cylinder power, which the wearer perceives as blur. Clear vision through the peripheral regions of the lens, where this surface astigmatism is greatest, is often not possible because of this blur— particularly in higher Add powers. In addition to this blur, the changing prism and magnification produced by the power and astigmatism of a progressive lens can cause objects to appear to shift, distort, or even sway unnaturally in some cases.

An annoying visual phenomenon may arise when the apparent movement of the visual environment through the lens differs from the physical movement detected by the neuro-physical system because of variations in prism and magnification. The vestibular apparatus, which is located within the inner ear, is linked to the visual system and helps to both maintain your sense of balance and stabilize vision while in motion. The appa "rocking" produced by this neurological conflict is known as image swim, and may result in a sensation of vertigo, or motion sickness. rent Objects—such as straight lines—may also appear curved or skewed when viewed through the lateral areas of the lens. This apparent shearing of images is called skew distortion, and occurs because the unwanted astigmatism in the periphery of progressive lenses causes unequal magnification in oblique directions. (Recall that the surface astigmatism of a progressive lens is generally obliquely oriented.) This unequal oblique magnification causes the vertical and horizontal lines of images to tilt and stretch (or skew). The more skewed a vertical line appears, the less orthoscopic—or straight—the image is through that particular zone.

It should now be obvious that the design of the periphery of a progressive lens is critical to dynamic vision performance and wearer acceptance. Often, the distribution of optics in the periphery of a progressive lens is carefully optimized during the lens design process in order to minimize astigmatism, image swim, skew distortion, and other unwanted features. A well-designed progressive lens will reduce unwanted astigmatism and its associated effects to its mathematical limits.

Generally, the maximum level of unwanted astigmatism in a modern progressive lens will be similar in magnitude to the Add Power, so that a +2.00 Add progressive lens will frequently have a maximum level of astigmatism of roughly 2.00 D. Image swim, which can result in the apparent acceleration of images, can be minimized by controlling the rates of change in prism and surface power, particularly along meridians that are orthogonal (perpendicular) to the line of sight. Skew distortion can be minimized by controlling the axis of the unwanted astigmatism in the periphery so that it is generally oriented at a less oblique axis.

Lens Design Symmetry

Early progressive lenses were symmetrical in design, so that the right and left lenses were identical. To achieve the desired inset for the near zone, the lens blanks were simply rotated 9 to 11°. The principal drawback to this approach was the disruption of binocular vision as the wearer gazed laterally across the lens, since the astigmatism and power differed between the nasal and temporal sides of the distance zone. The binocular field of view, or the combined region of clear vision seen simultaneously through both lenses, was also severely restricted.

Most modern lens designs, however, are now asymmetrical in design, with separate right and left lens designs. The amount of astigmatic error o either side of the progressive corridor can now be adjusted independently. This allows the near inset to be achieved without rotating the lens design; instead, the umbilic is designed from the start with a slight nasal ward inclination. This reduces the disruption in binocular vision, and provides better overlap between the distance, intermediate, the right and left lenses—affording view a larger binocular field of view.

While asymmetrical designs improve binocularity and ensure a better binocular field of view, levels of power and unwanted astigmatism may be higher to the nasal side of the progressive corridor as a result of achieving the near inset without rotating the design. Consequently, unless this unwanted power and astigmatism are carefully managed, differences in prism, magnification, and power may still exist between corresponding point o involve both eyes working together in unison, these binocular differences may result in less comfortable binocular fusion.

Horizontal symmetry extends the notion of an asymmetrical lens design by minimizin differences in power between points at equal lateral distance to the right and left of the eyepath, essentially "mirroring" the optics of the surface horizontally at each point along the umbilic. This improves binocular performance by reducing the differences in power between the righ minimizing power differences between corresponding points on the lenses, the two eyes perceive virtually no difference in prism, blur, and magnification, which ensures more comfortable binocular vision.

Multi-Design Lenses

Originally, progressive lenses employed a single unique lens design that was essentially scaled for each Base curve and Add power combination in its range of semi-finished lens blanks. The lens design used for the steepest Base curve was identical to the lens design used for the flattest Base curve; the progressive surface was simply "scaled" up. Similarly, the lens design used for the highest Add power was simply scaled up in a linear fashion from the design used for the lowest Add power. For instance, the lens design for the +2.00 Add power had twice as much surface astigmatism, as the +1.00 Add. A progressive lens product that utilizes the same basic lens design across every Base curve and Add power combination is now referred to as a mono-design lens series.

While a mono-design lens series represents the easiest approach to progressive lens design, lens designers soon realized that the overall performance of the lens series could be improved by tailoring the lens design slightly in order to account for optical effects that were specific to the wearer's distance prescription and/or stage of presbyopia. For instance, the lens design could be adjusted by Base curve to compensate for the differences in magnification or prism produced between different distance prescriptions. Similarly, the design could be adjusted by Add power to compensate for the differences in surface astigmatism produced between different Add powers.

The first stage in the evolution from traditional mono-design lenses occurred in the late 1980s, when lens designers started adjusting the basic progressive lens design for each Add power. A progressive lens product that utilizes a different basic lens design for each Add power is now referred to as a multi-design lens series. The first multi-design lenses employed softer designs in the low Add powers in order to provide emerging presbyopes with a more single vision-like visual experience, while employing harder designs in the higher Adds in order to offer greater viewing zone utility for advanced presbyopes.

The next stage in the evolution of progressive lenses occurred in the mid 1990s, when lens designers began adjusting the basic progressive lens design by both Base curve and Add power. A progressive lens product that utilizes a different basic lens design for each Base curve and Add power combination is often referred to as a Design by Prescription lens series. In addition to changes based on Add power, the earliest Design by Prescription lenses employed designs with slightly larger near zones in the steeper Base curves to compensate for the increased magnification and reduced field of view produced by plus prescriptions. This also allowed the flatter Base curves to have slightly larger distance zones, which reduced the loss in distance vision that results from the interaction of the progressive surface astigmatism with the oblique aberrations produced by minus prescriptions.

A mono-design lens series uses a single basic design that is simply scaled for each Base curve and Add power combination. For a range of Add powers from +0.75 to 3.50 D, a typical multi-design lens series could result in up to 12 unique lens designs. For 5 Base curves and a range of Add powers from +0.75 to 3.50 D, a typical Design by Prescription lens series could result in up to 60 unique lens designs.

Variable Near Inset

Conventional progressive lens designs employ a fixed, 2.5- near zone. This approach has several limitations, and does not take into consideration how the wearer's reading distance or prism from the distan prescription will affect ocular convergence:

• As the Add power increases, the working distance of the Add decreases, requiring additional ocular convergence.
• Plus power in the distance prescription will induce Base Out prism at near, requiring additional ocular convergence. Conversely, minus power will require less ocular convergence.

These factors will influence the interpupilla curve and Add power combination. Using this approach, the exact near inse required for each base curve an add power are carefully computed to ensure that the centers of the near zones are properly align with the lines of sight during near vision. This improves the wearer's binocular field of view through the near zone.

Typically, flaw near insets. Since the rea can be no longer than the reciprocal of the Add power, higher Add powers require shorter working distances. Consequently, higher Add powers are typically designed with greater near insets. A low-Add, low-Base lens may only require a near inset of 1½ mm, while a high-Add, high-Base lens ma require 3½ mm or more of near inset.

As-Worn Optimization

Many progressive lenses utilize flatter base (or front) curves to provide flatter, thinner, and lighter lenses. In order to maintain accurate peripheral optics by minimizing the aberrations introduced by using flatter lens forms, the distance zone of the lens may be aspherized. Aspheric surfaces use surface astigmatism to neutralize the oblique astigmatism introduced by viewing through the periphery of the lens. The application of asphericity to the distance-viewing zone of a progressive lens allows the use of flatter, thinner lenses, without unnecessarily compromising the peripheral optical performance of the lens design.

Another recent innovation in progressive lens design is the process of optically optimizing the lens design for the as-worn position or position of wear, which represents how the lenses are actually worn. This process uses optical ray tracing and lens-eye modeling to refine the optical powers as perceived by the wearer with the lenses in their intended positions. This takes into account the influence that oblique aberrations, lens tilt, vertex distance, and other variables affecting viewing conditions have on the optical powers perceived by the wearer. For instance, tilting a high-powered lens effectively increases the sphere power and introduces cylinder power.

Conventional progressive lenses, on the other hand, are designed to maintain distance and near zones that are optically clear when measured for vertex power by standard focimeters—including vertometers and lensometers—not as seen by the actual wearers. Consequently, as-worn optimization ensures that the lenses will deliver the prescribed powers du actual use. In som escription, which is the prescription you should verify in a focimeter. A lens that produces the correct compensated Rx in a focimeter will produce the correct prescribed Rx for the wearer once the lenses are in the worn" positions.

A single Base (front) curve can only deliver optimal peripheral optical performance for a single prescription power. Generally, this single po sphere power located toward the middle of the prescription range recommended for that p viates from this single "optimal" power, the optical performance of the len design deteriorates. When cylinder power has been prescribed, this deterioration in optical performance can also result in an effective shift central viewing zones.

Fortunately, free-form surfacing surfacing frees lens designers from the constraints of traditional Base curve limitations. Free-form surfacing equipment can generate both simple and complex surfaces, including progressive lens designs, directly onto a lens blank "on-the-fly." When used in combination with sufficiently advanced optical design software, free-f surfacing t w the wearer's specific parameters. This ensures that every pair of lenses performs exactly as intended.

Prism-Thinning

Earlier, it was demonstrated that the curvature of a progressive lens surface must become increasingly steeper towards the bottom of the lens in order to provide a gradual—or progressive—change in power. This surface geometry has several important consequences for a progressive lens blank:

• Thicker upper edge and thinner lower edge
• Increased lens thickness across the entire blank
• Increased lens weight as a result of the additional thickness

Fortunately, it is possible to minimize the excess thickness and weight of progressive lenses. A technique known as prism thinning can be employed to improve the cosmetics of a progressive lens. Prism-thinning (also called equi-thinning) is the process of grinding prism into a progressive lens blank in order to minimize the thickness difference between the top and bottom edges of the lens, as well as to reduce the overall thickness of the lens.

Most commonly, prism thinning typically involves grinding base down prism into a progressive lens in order to remove a wedge of base up prism from the lens blank. Prism thinning is particularly beneficial for plus lenses and lenses with high Add power, but may also be applied to minus lenses when necessary. Prism thinning should be treated like prescribed prism, and verified at the prism reference point of each lens.

Conclusion

The design of a progressive lens is complicated. Today, an optician can order and deliver progressive lenses that meet general-purpose requirements or lenses manufactured for very specific tasks (sports, computer, mostly reading, etc.). The key is an understanding of the basics and the design trade-offs made by suppliers.

Visit 2020mag.com/CE for discussions of the optics of customized, personalized and optimized lenses. Review lab and lens brand literature for a better view of availability of designs and their benefits. Most importantly – use the newest of design technologies, patients benefit the most.