Article Date: 1/1/2011

Using Power Profiles to Evaluate Aspheric Lenses
LENS EVALUATION

Using Power Profiles to Evaluate Aspheric Lenses

Profiles can help practitioners gain insight into designs and performance for near-vision tasks.

By Alexis K. S. Vogt, PhD; Kirk Bateman, MS; Tim Green, MS; & Bill Reindel, OD, MS

As visual demands continue to increase with the growing number of personal electronic devices, multifocal lenses are playing a more prominent role and are being more widely prescribed. The increased need for near, intermediate, and distance vision has driven the multifocal market, and simultaneous vision multifocal contact lenses have proven to be the preferred method of soft contact lens fitting.

With the added optical design complexity of multifocal contact lenses, the fitting process may seem more complex to inexperienced practitioners compared to spherical contact lens fitting. A comprehensive understanding of the optical characteristics of multifocal contact lenses can assist practitioners with selecting the optimal multifocal design for each individual patient.

The Power Profile

One method of characterizing the optical design of a contact lens is through power profile measurements gathered from Hartmann-Shack wavefront sensing instruments. Hart-mann-Shack wavefront sensors have historically been used as optical metrology tools to create qualitative wavefront maps of second-order aberrations of a contact lens, but they lacked intuitive refractive power detail. Recent technological advancements now provide not only insight into the optical quality of a lens, but also information on the refractive power characteristics of a lens.

The power profile of a contact lens provides insight into the spherical or aspheric nature of the lens and the presence of any multifocal features in the contact lens. The method also offers the added benefit of directly measuring the contact lens multi-focality without the additional subjectivity associated with clinical data.

Hartmann-Shack wavefront sensing instruments can record more than 2,800 unique measurements over the central 6mm of a contact lens. The data can be displayed as a plot of the local power as a function of radial distance from the center of the lens. Figure 1 shows power profiles of a traditional spherical contact lens as well as an aspheric multifocal contact lens, in which each data point represents the measured power over a narrow annular zone. This graph shows the power of the lens from the center (on the left side) out to the periphery of the optical zone (on the right side). With the traditional spherical contact lens, the power profile remains relatively uniform across the optic zone, while the power profile of the aspheric contact lens changes across the optic zone, becoming more negative toward the periphery, as is characteristic of an aspheric optical design.

Figure 1. Power profiles of −3.00D spherical and aspheric lenses.

Utilizing high resolution Hartmann-Shack wave-front sensing to create contact lens power profiles is useful to compare multifocal contact lenses that integrate aspheric optical designs. Both the PureVision Multi-Focal (Bausch + Lomb [B+L]) and Air Optix Aqua Multifocal (Ciba Vision [Ciba]) lenses use aspheric optical designs to create center-near adds, with the PureVision design addressing the needs of emerging, functional, and absolute presbyopes with two different add powers and the Air Optix Aqua design with three.

Comparing Lens Designs

To compare similarities and differences between designs, wavefront measurements were gathered for PureVision Multi-Focal Low Add (PVMF LA), PureVision Multi-Focal High Add (PVMF HA), and for Air Optix Aqua Lo Add, Med Add, and Hi Add lenses. For a comprehensive analysis, single vision PureVision (B+L) and Air Optix Aqua (Ciba) lenses were also measured to establish a baseline.

For each multifocal or spherical lens design, three −3.00D lenses were measured and the median power of the three measurements was plotted. Figure 2 shows the resultant power profiles for the three Air Optix Aqua multifocal designs and the Air Optix single vision lens. These results highlight similar power profiles for the Air Optix Aqua Med Add and Hi Add lenses and similar central power profiles for the Air Optix Aqua Lo Add and single vision lenses. These similarities in power profiles could translate into similar on-eye performance.

Figure 2. Power profiles of Air Optix Aqua Multifocal Lo, Med, and Hi Add lenses and Air Optix Aqua single vision lenses.

To compare these designs, add power was calculated from each power profile. For a center-near multifocal design, the add power is located in the central portion of the lens and can be calculated by subtracting the average power measurement over the central 2mm diameter of the lens from the labeled power. Using the clinical metric of 0.25D to indicate a clinically significant change, power differences within 0.25D were considered to represent the same spherical power and similarly, add differences within 0.25D were considered to represent the same add power.

The mean lens powers over the central 2mm diameter for the center-near Med Add and Hi Add Air Optix Aqua multifocals were −1.70D and −1.59D, respectively. The power difference of 0.11D (95 percent confidence interval [CI]: 0.02D, 0.20D) did not meet the criterion of being different by 0.25D, and hence the Med Add and Hi Add multifocals could be considered as having similar power. The mean central lens power for the Lo Add Air Optix multifocal was −2.75D and was considered distinct from the Med and Hi Add lenses. The spherical lens power profile was relatively flat, and the mean power in the central portion of the lens was −3.01D. The difference in means between the spherical lens and the Lo Add Air Optix lens was 0.26D (95 percent CI: 0.18D, 0.35D).

Similar Hartmann-Shack wavefront measurements were gathered for PureVision Multi-Focal and PureVision single vision aspheric lenses. Figure 3 illustrates the power profiles of the PureVision Multi-Focal Low and High Add center-near aspheric design lenses, as well as the PureVision single vision aspheric lens. Mean lens powers over the central 2mm diameter for the PureVision Multi-Focal Low Add and PureVision Multi-Focal High Add were −2.41D and −1.16D, respectively. With a difference in central powers of 1.25D (95 percent CI: 1.18D, 1.31D), the power profiles revealed two distinct design profiles. The mean power in the central portion of the single vision lens was −2.97D, with a difference in means between the single vision lens and the Low Add PureVision lens of 0.56D (95 percent CI: 0.50D, 0.64D).

Figure 3. Power profiles of PureVision Multi-Focal Low Add, High Add, and single vision lenses.

The add of each multifocal lens was calculated as the difference between the labeled power, in this case -3.00D, and the power of the central portion of the lens (Table 1). The results indicate that the Air Optix Aqua Lo Add offers +0.25D of add, while the PureVision Multi-Focal Low Add offers +0.59D, more than twice the add. The difference between add powers was statistically significant (0.34D [95 percent CI: 0.26D, 0.42D]). The Air Optix Aqua Med and Hi Add designs have very similar amounts of add, +1.30D and +1.41D, respectively, with a difference in add of only 0.11D (95 percent CI: 0.02D, 0.20D). In comparison, the PureVision Multi-Focal High Add offers +1.84D of add. The add difference between the PureVision High Add Multi-Focal lens was significantly greater compared to the Air Optix Hi Add (0.43D [95 percent CI: 0.35D, 0.51D]).

Summary

High resolution Hartmann-Shack wavefront sensing is a useful tool to provide information on the refractive characteristics of contact lenses and to evaluate relative similarities and differences in power profiles, particularly of aspheric multifocal contact lenses. The add powers calculated from the power profiles can provide insight into fitting lenses and on-eye performance for near vision tasks.

Our analysis of power profiles of the Air Optix Aqua multifocal lenses showed that there are effectively just two distinct designs. The power profile for the Lo Add estimated the add to be 0.25D. With an add of this magnitude, practitioners may find a need to adjust the distance power to increase the effective add. With the Med Add and Hi Add lenses having similar adds, practitioners may find limited use for the Med Add as the patient requires a greater add need.

Our measurements of the two PureVision Multi-Focal lenses confirmed two distinct designs that offered more add when compared to the Air Optix Aqua Multifocal lenses. Greater add power helps reduce the inconvenience of changing lens powers as reading needs increase. CLS

Dr. Vogt is an optical design engineer and part of the Lens Design group at Bausch + Lomb (B+L).
Kirk Bateman is the manager of Statistics and Data Management at B+L.
Tim Green is the manager of Lens Design at B+L.
Dr. Reindel is the director of Medical Affairs for the global vision care business of B+L.


Contact Lens Spectrum, Issue: January 2011