Essilor’s Varilux Physio 360o Enhanced lenses correct HOAs using parameters like age, pupil size, working distance, and lighting conditions.
The optics of the human eye are much more complicated than you might guess. The fact is that the eye is a complex optical system made up of many components, surfaces, indices, and “materials,” spaced apart from each other. The traditional designations of spherical or astigmatic for the eye’s refractive conditions are somewhat inadequate too. That’s because an eye’s imperfect focus is more than spherically or astigmatically in error; it actually is in error in a more intricate way. The three images in Fig. 1 were made with a Placido disc (an eye chart consisting of a series of concentric alternating black and white circles). If the image of a spherical lens was assessed with a Placido Disc as the target, its image would appear like the “sphere” image above and remain circular, although larger for plus lenses and smaller for minus lenses. When assessing the image of a cylindrical or sphero-cylinder lens, the Placido Disc image would become a series of larger or smaller ovals depending on the lens’ powers. These round and oval image shapes indicate that the optics of spherical and cylindrical lenses is fairly simple.
The image on the left is a representation of an eye assessed with a Placido Disc. Notice how irregular it is. Because it’s not round or oval, it means that its power is not spherical or cylindrical, it’s irregular and is caused by Higher Order Aberrations (HOAs).
HIGHER ORDER ABERRATIONS Eyecare professionals (ECPs) have traditionally been taught about spherical and astigmatic errors of the eye because frankly, they were the only kinds that could be corrected. With the advent of LASIK surgery, ECPs became aware of another level of aberrations known as HOAs. These also affect vision, especially in low light, and low contrast situations. Correcting these aberrations can improve vision too. In fact, it is theorized that human vision could be corrected to 20/10 if the eye’s HOAs were corrected. And yes, even those whom ECPs consider emmetropic could benefit from HOA correction.
There are five levels of HOAs illustrated in Fig. 2. The first, zero, indicates no error. Levels one and two represent the spherical and cylindrical errors which ECPs are familiar with. The other three levels have interesting names like coma, trefoil, and pentafoil.
What’s important to know here is that HOAs reduce the quality of vision and correcting them can improve it. Until recently, that wasn’t the case.
VISION AND HOAs HOAs affect vision in several ways. As you might suspect, it can affect acuity, causing reduced performance on a Snellen vision test. HOAs also affect contrast and create distortions, which results in patient complaints like seeing halos, starbursts, or comet-shaped glare around lights at night and reduced color saturation in objects.
ADDRESSING HOAs
HOA-CORRECTING CHALLENGES Correcting HOA with a spectacle lens is not easy because the eye roves behind the lens. This means that any attempt at correcting HOA has to be a compromise for various angles of gaze through the lens. This is no different than Best Form or Corrected Curve lenses of old that attempted to reduce marginal astigmatism. The result is the same: a compromise. A less compromising situation would be a contact lens with HOA correction because that kind of lens, for the most part, stays centered over the pupil as the eye roves.
Three spectacle lens companies have lens products that address HOAs: Ophthonix, Inc., Carl Zeiss Vision Inc., and Essilor of America, Inc. Ophthonix was the first in 2005 with its iZon lenses that are designed to address the symptoms of HOAs of the eye. It does this by assessing the eye using the company’s Z-View® Aberrometer (a wavefront measuring instrument) and processing its iZon lenses using that data. Essilor’s Varilux® Physio 360◦ Enhanced lenses correct HOAs of the lens using a number of parameters like age, pupil size, working distance, and lighting conditions. Carl Zeiss Vision’s i.Scription® process uses their i.ProfilerPLUS® instrument (a combination aberrometer, autorefractor, and topographer) along with a subjective refraction and electronic fitting measurements to craft one of three of their lenses.
Ophthonix touts that its iZon process is, “… the world’s first fully integrated vision correction system.” It does this through detection, measurement, and correction. The detection is done using the Z-View Aberrometer. The Aberrometer measures more than 10,000 points over a 6mm pupil diameter to provide an accurate assessment of the patient’s lower order aberrations and HOA results. The Z-View uses this data to create what the company calls an iPrint®, which represents the unique optical fingerprint for each patient’s eye. The iPrint actually is a graphical way of illustrating the different HOAs and the degree of these aberrations of the patient’s eye. Using this data, the software uses a unique algorithm to determine a wavefront prescription that comprises the best surface optics for the patient. With this data, a customized iZon lens is manufactured using free-form technology.
A SIMILAR APPROACH Carl Zeiss Vision takes a similar approach. Using its i.ProfilerPLUS instrument, it measures the patient’s refraction and takes HOA measurements at small and large pupil diameter settings. It also measures the patient’s corneal topography. This data and the patient’s subjective refraction are sent to the company’s i.Scription software. Using the company’s i.Terminal™, the patient’s fitting measurements (PDs, fitting heights, pantoscopic and face-form tilt, and vertex distance) are taken and provided to the i.Scription software as well. All the data are used to determine the best optics for the patient, which is processed into the company’s Zeiss Individual™, GT2 3DV, and GT2 3D Short lenses using free-form technology.
Ophthonix’s iPrint actually is a graphical way of illustrating the different HOAs and the degree of these aberrations of the patient’s eye.
Carl Zeiss Vision’s i.Scription process uses their i.ProfilerPLUS instrument (a combination aberrometer, autorefractor, and topographer).
Essilor of America, Inc.’s Varilux Physio Enhanced lenses address HOAs using W.A.V.E. Technology 2™ by addressing the factors that affect pupil diameter: the patient’s age, working distance, lighting conditions, and their prescription. According to Essilor, normal pupil sizes vary from 2mm to 8mm and wavefront perception and visual sharpness depend on pupil size. As pupil size increases, so does the degree of HOAs that manifest themselves, which affect vision. Since pupil diameter is affected by a person’s age, working distance, ametropia (their Rx), and lighting conditions, W.A.V.E. Technology 2 considers all these factors and customizes the Physio Enhanced lens design using a “pupil mapping” approach.
HOA correction represents the frontier of innovation for lenses. As eye doctors begin to discover the benefits of aberrometry in their examination routines, we’ll undoubtedly see more lens advancements in this category.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.
HIGHER ORDER ABERRATIONS
