Simultaneous Vision: The Science Behind the Art
MULTIFOCAL CONTACT LENSES
Simultaneous Vision: The Science Behind the Art
Learn the real meaning of simultaneous vision and why it's important in practice.
By Pete Kollbaum, OD, PhD, FAAO
As I sat down to write this piece on "simultaneous vision," I did what I imagine any good "scholar" would do these days — I Googled it. In 0.2 seconds, I had access to 5,490,000 different entries. Catching my eye was the third entry on the list: "Simultaneous vision – Umberto Boccioni." Upon clicking this link, I learned that an artist named Umberto Boccioni (1882-1916) had painted a piece called "Simultaneous vision" in 1912 (Figure 1).
Figure 1. "Simultaneous vision" by Umberto Boccioni.
As I stared at this piece, which is classified as a form of cubism, and futilely tried to make sense of it, something clicked. If fitting simultaneous-vision contact lenses is described as an art, maybe the art of fitting these contact lenses is as easily misunderstood as my view of this painting. In this article, I'll aim to clarify potential areas of misunderstanding about the science behind simultaneous-vision contact lens designs. Then, I'll highlight how some of these design features have been optimized in Ciba Vision's new Air Optix Aqua Multifocal lens.
WHAT DOES "SIMULTANEOUS VISION" MEAN?
In the clinical literature, simultaneous-vision contact lenses are often said to form a "distance image" and "near image." As a result, some have argued that these lenses should in fact be called "simultaneous image" lenses. Regardless of what they're called, the terminology used to describe these lenses has the potential to be confusing.
This is because the target can be "distant" or "near," while the powers of the lens are also referred to as "distant" or "near." So, when the terms "distant" or "near" image are used, it�s sometimes unclear if this means the image of a distant (or near) target, or the image produced by the distance (or near) optics. A possible solution is to refer to the object distance as well as the portion of the lens creating this image when describing the optics of these lenses.
It's important to remember that regardless of the target viewing distance, the portions of the lens containing a distance refractive and near refractive power will both contribute to the image that is formed. For example, assuming a perfect correction of the eye's distance spherical power, when viewing a distant object, the light passing through the distance-powered portion of the lens will be focused on the retina, while the light passing through the near-powered portion of the lens will generate a defocused image surrounding the focused image. Conversely, when the target is at the correct near-viewing distance, the near-powered portion of the lens will focus the light on the retina, whereas the light through the distance-powered portion of the lens will generate a defocused image. In both of these cases, the resultant image is that of a sharp image superimposed over a defocused image. This defocused image is sometimes described as "ghosting" by a lens wearer. By manipulating either the lens optics or how this lens positions on the eye, the location of the defocused image relative to the focused image can be altered.
As the location of the focused and defocused images can be altered by the lens design, so can the intensity of the defocused image relative to the focused image. This can be done by altering the percentage of the pupil that's covered by the near- and distance-focusing portions of the lens. For example, Figure 2 shows three different cases where the same pupil size is used to view a distant target. In one case, 70% of the pupil area is covered by the near-focusing portion of the lens (Figure 2a). In another, an equal percentage of the pupil is covered by the near and distance-focusing portions of the lens (Figure 2b). In the last image, 70% of the pupil area is covered by the distance-focusing portion of the lens (Figure 2c). Notice that when viewing at distance, the instance where most of the light is passing through distancefocusing optics (Figure 2c) looks the best.
Figure 2. Simulated images of a wearer viewing at distance through a simultaneous vision lens in which (a) 70%, (b) 50% and (c) 30% of the pupil is covered by the near refractive lens power.
With today's newest lens designs, such as the Air Optix Aqua Multifocal, iterative design optimizations have occurred to determine the optimal proportion of the lens devoted to distance and near optics. This optimization process aims to provide the ideal percentage of light passing through the lens in front of the pupil for distance and near vision, minimizing the visibility of the defocused image, while providing the best possible all-around vision for the wearer.
HOW DO SIMULTANEOUS-VISION LENSES WORK?
Simultaneous-vision lenses can be a bifocal or multifocal design. In either design, the center of the lens can contain the distance (CD or center distance) or near (CN or center near) refractive power. Multifocal designs generally are composed of two or more distinct zones of two powers. In soft contact lenses, they're generally a concentric design consisting of two circular zones, or an annular design consisting of several concentric circular zones.
Multifocal designs, however, generally are aspheric and have a gradient power change from the lens center outward. This gradient power change from lens center to periphery is spherical aberration. Generally, multifocal lenses have slightly higher amounts of spherical aberration than those found in typical single-vision lenses in order to provide multifocality (for example, to expand the depth of focus) for the wearer. This is a slightly simplistic view, as multifocal lenses could also contain a combination of a concentric zone or zones, each with asphericity. This combination of features is actually the design of most higher-ADD multifocal lenses.
Lenses designed to correct or manipulate optics in any way can be thought of in simple terms as lenses with relative thickness changes across them. This is most easily imagined if we consider light as a wavefront.
In Figure 3, a wavefront is merely the joining of the heads of the rays of light. For example, at the far left of Figure 3a, we show a myopic wavefront with its periphery advanced relative to the center, which is classified as retarded. This shape provides more material of higher refractive index at the lens periphery to "slow" the propagation of light in the periphery of the wavefront, which is relative to less material at the lens center to "speed up" the propagation of light in the center of the wavefront. This results in a flat, aberration-free wavefront (far right, Figure 3). Notice that this lens is shaped like a typical negative-powered lens that would be used to correct myopia.
Figure 3. Schematic showing the correction of a (a) myopic and (b) comatic wavefront by the introduction of a lens.
This same line of thinking may also be used when creating lenses to correct higher-order aberrations, such as coma or spherical aberration. Figure 3b shows an eye with coma and the corresponding lens required to correct the coma of this eye. Again, notice that the shape of the lens needed to correct the aberrated wavefront mirrors the shape of the aberration we aim to correct. It's important to realize that the thickness differences needed to induce these optical changes are only a few microns or less. In creating a multifocal contact lens, the situation is slightly different than both of these cases, in that we don't want to correct an aberration, but rather induce it. More specifically, we want to induce spherical aberration to provide multifocality for the lens wearer.
As mentioned previously, multifocal lenses can be a CN or CD design. With CD lenses, positive power gradually increases from the center to the periphery, which is positive spherical aberration. Figure 4 shows the two-dimensional (Figure 4a) and cross-sectional (Figure 4b) wavefront of the measured (ClearWave, AMO) SA within the Air Optix Aqua Multifocal LO and HI ADD lenses. In both cases, there is negative SA within the lenses, as expected from their CN design. Also notice that this measured level of spherical aberration increases only slightly between the increasing add designs. As described above, the HI ADD lens contains an additional central zone of higher near power, eliminating the need for inducing large amounts of spherical aberration, which may degrade vision.
Figure 4. Schematic showing the correction of a (a.) myopic and (b.) comatic wavefront by the introduction of a lens.
Most lenses contain levels of SA that vary with lens power. This may be problematic for multifocal lenses, however, as this may cause their on-eye performance to be variable. For example, the levels of SA needed to provide multifocality may be present in a −6.00D lens, but not in a +4.00D lens. Figure 4a shows the spherical aberration of a series of −3.00D Air Optix Aqua Multifocal lenses. However, measuring a range of lens powers from +4.00D to −6.00D yields virtually identical results. What this means is that these lenses should provide a fairly constant ADD effect regardless of the required distance prescription of the wearer. This constant add effect should result in improved vision, more predictable clinical results and decreased chair time when fitting these lenses.
Based on the above description of how the change in optics occurs in these lenses, we would expect these lenses to have a subtle relative thickness change across the lens in order to provide the necessary spherical aberration. Figure 4c shows thickness measurements of the LO and HI ADD Air Optix Aqua Multifocal lenses. Notice, there's a gradient change in thickness across the lens which mirrors that of the spherical aberration-induced. Also notice, as anticipated from the similar levels of spherical aberration in the LO and HI ADD lenses, that there's little measured difference in lens thickness between these two ADD designs. This equal thickness profile across lens designs should provide similar on-eye performance and comfort, regardless of ADD design.
THE ART OF FITTING FOR SIMULTANEOUS VISION
With improved knowledge of the science underlying the designs in the newest simultaneous-vision lenses, there's a little more understanding of the art of fitting the lenses. So, unlike me staring blankly at a piece of art, hopefully you won't be left staring at your patients, trying to make sense of what they're telling you. Instead, you can use the new design features in today's simultaneous-vision lenses to optimize your fitting success and your patients' satisfaction.
DR. KOLLBAUM IS AN ASSISTANT PROFESSOR AT THE INDIANA UNIVERSITY SCHOOL OF OPTOMETRY, WHERE HE TEACHES AND PERFORMS RESEARCH IN THE AREAS OF CONTACT LENSES AND OPTICS, AND HE IS DIRECTOR OF THE CLINICAL OPTICS RESEARCH LAB. AFTER RECEIVING HIS OD DEGREE FROM INDIANA UNIVERSITY, DR. KOLLBAUM WORKED IN PRIVATE PRACTICE IN IOWA PRIOR TO RETURNING TO INDIANA UNIVERSITY TO COMPLETE A MS IN CLINICAL RESEARCH (INDIANA UNIVERSITY SCHOOL OF MEDICINE, INDIANAPOLIS) AND PHD IN VISION SCIENCE (INDIANA UNIVERSITY, BLOOMINGTON). HE HAS BEEN AN ADVISOR/CONSULTANT TO ALCON, CIBA VISION, COOPERVISION, VISIONEERING TECHNOLOGIES AND VISTAKON.
Contact Lens Spectrum, Issue: March 2010