Higher-Order Aberrations

Pete Kollbaum

Introduction

We all do many things every day of our lives without giving them much thought. Have you ever arrived at work in the morning and could not really remember the trip there? How often do you have a cup of coffee without thinking about how that coffee came to be made? Too often, I would say, we do not ask enough questions about what is behind what we do, or we don’t put enough thought into why something tastes good or why something works (or does not work). When I first started in practice, I would prescribe a multifocal contact lens to one patient that optically performed great, but when prescribing that same lens to another patient it might fail miserably.

At the time, although I immediately moved on to prescribe something else, I remember feeling frustrated with this lack of predictability and with the lack of information for making educated decisions on lens selection to best treat my patients. Understanding lens optics may increase the likelihood of success. Fortunately, in the last 10 to 20 years, technology has changed remarkably so that practitioners now have the necessary details to take much of the guesswork out of successfully prescribing contact lenses for our patients. Knowledge about optics and aberrations is key, though, in this process.

Take the guesswork out of contact lens optics

Optics of eyes are well-described but vary across the population (Thibos 2002 JOSA) for many reasons. Age is one of the more important ones and is better known, but the optics of each eye also vary with accommodation, tear film, pupil size, etc. What ultimately matters for all patients and their vision, however, is how the optics of their lens combine with the optics of their eye.

Optical principles that we all learned in school taught us that the optics of the lens as measured in air equal the optical impact of that lens on the eye - a somewhat miraculous outcome. However, this is not always the case for high-powered lenses that may fail to conform completely to the eye (Kollbaum 2013 OVS). Further, the optics of the lenses themselves may change as a function of lens power.

"Understanding contact lens optics may increase the likelihood of success”​

In all of these instances, how the lens centers on the eye is also critically important, as decentration of any lens on the eye relative to the pupil center induces aberration. For example, decentering a lens that contains spherical aberration induces coma in direct proportion to the amount of spherical aberration within the lens and the amount of decentration.

Although manufacturing wavefront-guided contact lenses to optimally correct an eye is quite possible (see the GSLS highlights below in this newsletter on Jason Marsack’s great work in this area), even these remarkable corrections will be static and optimal only for one instant in time. Fortunately, it does not have to be this complicated, and visually significant benefits can be obtained by utilizing some key pieces of information.

On-Eye-Off-Eye

Off-eye (e.g., Kollbaum 2008 OVS) and on-eye (e.g., Singh 2019 OVS) optical metrology technology has changed over recent years so that manufacturers (and others) can accurately identify the optics manufactured in lenses (as opposed to assuming the intended optics were manufactured) and how they interact with the eye.

In addition, and possibly even more importantly, manufacturing technology has evolved over recent years so that manufacturers can now generate lenses in most cases exactly as they want, with optics added in very precise and specific ways or with precise optical junctions, etc. For instance, companies can essentially minimize lens-to-lens variation and aberrations, which were historically introduced during lens production. They can also now accurately control spherical aberration as a function of lens power (Kollbaum 2013 OVS).

In fact, several companies, such as Bausch + Lomb and CooperVision, aim to do this in many of their (aspheric) lens designs, whereas other companies have chosen to retain spherical optics and levels of aberration that vary with lens power. Further, companies that manufacture many of the lens designs that control spherical aberration as a function of lens power also have chosen to largely counteract for the levels of spherical aberration found in the population average. The thinking behind these mass-produced lenses is that they will on average decrease eye-plus-lens aberration and thus improve vision for much of the population. That may be an improvement, to some degree at least. One step up would be custom-made lenses for individual eyes.

Increasing Importance

The issues that impact contact lens optics are now of increasing importance as companies aim to develop and manufacture complex lens designs to better correct the presbyope (without inducing aberration) or to slow myopic eye growth in children. Regarding myopia control, the more traditional coaxial and more recent non-coaxial contact lens (and spectacle) designs are quite remarkable. Many aim to induce myopic defocus without significant degradation of image quality. Much discussion related to myopia control has happened lately regarding the impact of lenses on the eye’s peripheral optics. The issues of conformation and decentration remain critical here in predicting what peripheral optics are attained. Resolving issues related to the eye, lens fit, and lens optics may ultimately drive the successfulness of even the most sophisticated myopia control correction options in slowing myopia progression, so these issues should not be negated.

The Higher Importance of Contact Lens Optics

Let’s continue to urge manufacturers and researchers to help clinicians understand the detailed knowledge of the optics of every lens so that when practitioners prescribe a lens, they can make educated decisions on why it should work and what to do next if it does not work as intended. It is important to give contact lens optics and the issues described above some thought let’s give them the place they deserve within our industry and profession. This will help us each time that we prescribe a lens to maximize the likelihood of successfully helping every patient.

References

- Kollbaum P, Jansen M, Thibos L, Bradley A. Validation of an off-eye contact lens Shack-Hartmann wavefront aberrometer. Optom Vis Sci. 2008 Sep85(9):E817-28

- Kollbaum PS, Bradley A, Thibos LN. Comparing the optical properties of soft contact lenses on and off the eye. Optom Vis Sci. 2013 Sep90(9):924-36.

- Singh NK, Jaskulski M, Ramasubramanian V, Meyer D, Reed O, Rickert ME, Bradley A, Kollbaum PS. Validation of a Clinical Aberrometer Using Pyramidal Wavefront Sensing. Optom Vis Sci. 2019 Oct96(10):733-744.

- Thibos LN, Hong X, Bradley A, Cheng X. Statistical variation of aberration structure and image quality in a normal population of healthy eyes. J Opt Soc Am A Opt Image Sci Vis. 2002 Dec19(12):2329-48.