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Optics of Free-Form Lenses – Part 2, Single Vision

By Darryl Meister, ABOM

Faculty/Editorial Board:

darryl meisterDarryl Meister is a Certified Master Optician, technical marketing manager for Carl Zeiss Vision, technical representative to the VCA and ANSI, has been a key contributor to many important industry initiatives and writes and lectures frequently on ophthalmic optics, lenses and dispensing.

Learning Objectives:

Upon completion of this program the participant should be able to:

  1. Learn the limitations of contemporary best form single vision lenses and their designs.
  2. Understand the way that prescriptions can be customized using position of wear values.
  3. Know how lenses can be customized for conformal optics i.e., for an 8 base wrap frame regardless of almost but extreme Rx’s.

Credit Statement:

This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO).
Course #10-45

This course is supported by an educational grant from Carl Zeiss

Although free-form progressive lenses customized to the visual requirements of the wearer have become increasingly common, some lens manufacturers have recently begun introducing free-form single vision lenses that are also customized to the visual requirements of the wearer.

Limitations of Traditional Single Vision Lenses

For over a century, lens designers have understood that the field of clear vision through a spectacle lens is limited by various optical aberrations, particularly oblique astigmatism. These optical aberrations result in unwanted sphere and cylinder power changes from the desired prescription away from the center of the lens. This reduces the quality of peripheral vision for the wearer. For spherical prescription powers, it is possible to minimize these optical aberrations either through the proper choice of front (base) curve or through the use of an aspheric lens design.

In order to eliminate these optical aberrations completely, however, a unique base curve or aspheric lens design would have to be used for each spherical prescription power according to “best form” optical design principles. Unfortunately, this represents an impractical requirement for cost and inventory management. Consequently, traditional single vision lenses are produced from lens blanks that are factory-molded with a limited number of front (base) curves, upon which relatively broad prescription ranges must be grouped (Figure 1). Moreover, for prescriptions with cylinder power, no base curve or aspheric design can eliminate the optical aberrations produced by both the sphere power meridian and cylinder power meridian simultaneously.



Figure 1. Each prescription power ideally requires a unique front (base) curve or
aspheric design in order to eliminate optical aberrations, yet powers are generally
grouped onto a limited number of base curves to minimize costs.

Additionally, the position of wear of the lens can also influence optics and vision quality. The position of wear represents the position of the fitted spectacle lens on the wearer. This includes pantoscopic tilt, face-form wrap, and vertex distance. Tilting a lens also introduces oblique astigmatism, which results in unwanted cylinder power and changes to the sphere power (Figure 2).



Figure 2. Although vision may be clear through the center of a lens with no tilt, vision is often blurred when viewing
through the periphery of the lens or through the center of the lens when tilt has been added due to oblique astigmatism.

Fortunately, with the advent of free- form surfacing, single vision lenses can now be designed and manufactured on demand for each wearer. Some lens manufacturers have begun using this technology in conjunction with sophisticated optical design software to customize the lens design to the unique visual requirements of each wearer, immediately prior to fabrication. Using the wearer’s prescription and fitting parameters, a unique lens design is calculated in “real time.” The final lens calculations are then transmitted directly to precision free-form surfacing equipment.

Customization for the Prescription

When the wearer looks obliquely through the peripheral regions of a spectacle lens, unwanted sphere and cylinder power errors are introduced by oblique astigmatism and other aberrations. This results in errors from the desired focus of the lens. These unwanted power errors from the desired prescription produce blur that degrades image quality and narrows the field of clear vision for the wearer (Figure 3). Lens aberrations can also cause the field of clear vision through the lenses to become distorted in shape, particularly in the presence of cylinder power, which impacts the binocular vision and stereopsis of the wearer. With traditional single vision lenses, each base curve will typically deliver optimum optical performance only for sphere powers located near the center of the prescription range associated with each base curve (Figure 4). Other prescription powers will frequently suffer from residual aberrations in the periphery of the lens because of this optical compromise that increase as the power deviates from the ideal prescription power.


Figure 3. For many prescriptions, the field of clear vision may be significantly
reduced and distorted in shape by uncorrected lens aberrations.



Figure 4. Unlike traditional lenses, the optical performance of customized
single vision lenses is not necessarily limited by the availability of factory-molded
base curves, so every wearer enjoys the best optics possible with minimal lens aberration

Recall that a unique base curve or aspheric lens design is required in order to eliminate—or at least minimize—these aberrations in spectacle lenses with a spherical prescription power. Further, when the prescription contains cylinder power, no conventional base curve or aspheric design can eliminate the aberrations produced simultaneously by both the sphere and cylinder power of the lens. Moreover, because prism effectively tilts the optical axis of the lens, prescribed prism also introduces oblique astigmatism.

With sufficiently advanced software and a free-form delivery system, it becomes possible to customize the lens design based upon the unique prescription requirements of each wearer. By fine-tuning the optical design of the lens for the exact prescription using a complex aspheric optical design, residual lens aberrations are virtually eliminated. As a result, fully customized lenses will deliver wider, more symmetrical fields of clear vision compared to traditional lenses. Wearers will enjoy the widest fields of vision possible and improved binocular utility, regardless of prescription (Figure 5).


Figure 5. This free-form single vision lens from Carl Zeiss Vision
is precisely customized for the wearer’s exact prescription,
which ensures wide, symmetrical fields of clear vision.

Customization for the Position of Wear

The position of wear is the position of the fitted lens relative to the actual wearer, including the pantoscopic tilt, face-form wrap, and vertex distance of the lens. Spectacle prescriptions are typically determined using refractor-head or trial-frame lenses that are positioned perpendicular to the lines of sight. Unlike the lenses used during the ocular refraction procedure, eyeglass frames generally place the spectacle lenses at an angle with respect to the lines of sight once fitted to the wearer’s face.

Unfortunately, tilting a lens introduces oblique astigmatism, which results in unwanted sphere and cylinder power changes across the lens. These are proportional both to the power of the lens and to the magnitude of lens tilt. Moreover, changes in the fitted vertex distance will also influence optical performance, since the viewing angle to a given point over the lens design varies as a function of the distance between the center of rotation of the eye and the back of the lens. Consequently, position of wear can have a significant impact upon the optical performance of spectacle lenses, particularly upon the quality of straight-ahead vision (Figure 6).


Figure 6. Vision may be significantly degraded by the
position of the fitted lens on the eyeglass wearer.

With sufficiently advanced software, it is possible t customize the lens design based upon the unique fitting parameters of each wearer (Figure 7). If the wearer’s pantoscopic tilt, face-form wrap and vertex distance are supplied, the performance of the lens in its position of wear may be mathematically modeled in order to apply the necessary optical corrections across the lens surface during the optical optimization process. Wearers can therefore enjoy the best optical performance possible, regardless of their unique fitting requirements (Figure 8).


Figure 7. Plots of ray-traced optical astigmatism demonstrate that the optics of the lens design can also be
tailored to the exact fitting requirements of the wearer, ensuring that every lens performs exactly as intended,
with no unwanted prescription changes that could otherwise degrade vision quality through the central viewing zones.



Figure 8. This free-form single vision lens from Carl Zeiss Vision
is precisely customized for the wearer’s exact fitting parameters, which ensures clear vision straight-ahead vision.

Traditional spectacle lenses are often designed to exhibit the specified optical performance only when measured using a conventional focimeter, such as a lensometer. Because Zeiss Individual SV is designed to provide the wearer with the prescribed optical performance once the lens in the actual position of wear, small differences from the original prescription are required at the verification point of the lens. These power adjustments are supplied as a compensated prescription, which represents the correct lens powers to verify with focimeters in order to provide the actual wearer with the specified prescription.

Until recently, the real-time optical design benefits afforded by “free-form” or “digital surfacing” technology were limited to progressive lenses. While presbyopes could enjoy the clearest, most comfortable vision possible, pre- presbyopes wearing single vision lenses were left to tolerate the inherent optical compromises of traditional, semi-finished lenses. Consequently, single vision lens wearers often endured reduced fields of clear vision or a reduction in visual acuity through the center of the lens.

In fact, because of optical aberrations introduced by the tilt and peripheral optical performance of the lens, the visual acuity of the wearer may be reduced from 20/20 to 20/40 or even worse. In addition to losing up to three or more lines of visual acuity on a Snellen chart, the wearer’s appreciation of contrast is also greatly reduced. This causes colors to lose sharpness and definition.

Cosmetic and Conformal Optics

Optical customization frees lens designs from the constraints of traditional “best form” lenses, which must adhere to specific base (front) curve guidelines in order to maintain quality vision. Some customized single vision lenses rely on an extremely sophisticated optical optimization process to refine points over the entire back surface of the lens, regardless of whether a flatter or steeper front curve is utilized. The result of this optimization process is a complex “aspherization” of the initial lens design. A flatter profile is often chosen in order to reduce lens thickness, weight, and magnification. Wearers can therefore experience the clearest optics possible in a lens that is flatter, thinner, and more attractive-looking than conventional lens designs that rely on steeper, spherical base curves (Figure 9).


Figure 9. This free-form single vision lens from Carl Zeiss Vision
employs point-by-point optical aspherization to deliver exceptional optical performance in a flat, slim lens.

It is also possible to manipulate the optics and form of the lens based on the overall “shape” of the frame and other opto-mechanical requirements. With the increasing popularity of steeply curved and highly wrapped eyewear, which often necessitate complex atoric lens designs on relatively steep (8.00) base curves, this application of free-form technology is becoming increasingly relevant to achieve maximum optimal performance. When the form of the lens has been manipulated to “conform” to certain mechanical requirements, the lens design must be suitably tailored to the shape of the lens in order to prevent a reduction in optical performance.

Free-Form Lens Surfacing

Unlike traditional lenses, which rely either on a set of mold or hard lap tools in fixed increments of surface power in either eighth-diopter (0.125 D) or tenth- diopter (0.10 D) steps, free-form lens surfacing utilizes “soft” lap tools made, from a compliant foam or similar material, which are not restricted to fixed increments of surface power. Hence, a computer-controlled soft lap polisher can polish surfaces cut by a free-form generator in hundredth-diopter (0.01 D) steps. This results in more precise optical powers.

Manufacturing complex lens designs using free-form surfacing requires meticulous process engineering to assure quality and ongoing quality control. Although free-form lens surfacing can theoretically produce extremely accurate and precise lens surfaces, in practice the kinematics of the single-point diamond turning and soft lap polishing processes used in free-form lens surfacing can result in unwanted variations in surface power and waviness. Dozens of different cutting and polishing parameters must be carefully adjusted and tested for each combination of lens design, prescription, and material in order to ensure quality results (Figure 10).


Figure 10. Without extensive process engineering and ongoing quality control,
a free-form surfacing process can actually produce lenses with unwanted variations in power and waviness over the surface.

Failure to verify production quality of a free-form surfacing process on a regular basis can also lead to inferior optical quality compared to traditional lens production methods. Fortunately, lenses from the more experienced free-form lens suppliers must typically meet stringent quality guidelines and design specifications. These free-form lens suppliers may use sophisticated optical metrology instruments to “map” the optical characteristics of lenses periodically in order to compare them against the intended lens design (Figure 11). This quality control ensures that every lens delivers the precise optical powers that the wearer requires.


Figure 11. Engineers at Carl Zeiss Vision
rely on sophisticated optical metrology tools to ensure that
free-form lenses deliver consistently superior quality and optics.

Some free-form lens suppliers emphasize the value of free-form lens surfacing over traditional lens surfacing because it eliminates the rounding errors of hard lap tools. The actual optical error due to these rounding errors is typically small, with a maximum power error on the order of either ±0.0625 or ±0.05 diopters, depending upon the tooling increments. The optical error produced by either changes in the position of wear or the use of a limited number of base curve designs, on the other hand, can be considerably greater.

Conclusion

As with free-form progressive lenses, the use of free-form surfacing to deliver truly customized single vision lenses is the most meaningful visual benefit of this technology to wearers. The full potential of free-form technology will only be realized when utilized in conjunction with powerful software tools capable of “real-time” optical design using input specific to the individual wearer.

With sufficiently advanced optical design and free-form delivery system, lens designers can deliver lenses that provide wearers with the best optical performance possible, regardless of their prescription requirements or fitting characteristics. With the most advanced customized free-form lenses, patients can enjoy up to 50% wider fields of clear vision as well as sharper, more natural straight-ahead vision.


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