Since the launch of the RayOne Galaxy lens, there has been a significant amount of attention focused on spiral optics and how such design provides multiple focal points to provide depth of vision to patients. The adoption of spiral optics to provide such multifocality in IOLs or contact lenses is new, and not many literature is available that would help a layman like myself, to comprehend. So here I set out to reflect what I have understood whatever reading material is available, and by talking to some of the experts in the subject.
Have you ever seen water in the kitchen sink slowly draining out but also creating a vortices of water (Image 1). What exactly is a vortex? Vortex is a fluid particle rotating about a center creating a hollow section in the center. This rotation of fluid particle is always present even if the drain pipe is clogged and water flows out slowly. If you have tiny food particles in the sink, you will find them rotating as water slowly drains out of the sink. When the drain is unplugged and their is no major obstruction on the drain pipe, gravity causes the water to wash down and the rotation is intensified and we see the formation of vortex. The same vortex is also seen in tornado, which is created by warm ocean water. Such phenomenon can be seen in your coffee mug, as you stimulate with the spoon for the sugar to absorb.
Vortices are not just seen in the kitchen sink, coffee mug, or the hurricanes. It is seen in whirlpools in the ocean, wind direction created by the flying aircraft, etc. However, all of these have something in common, angular momentum. Momentum is a product of mass of the object and its velocity. By momentum, we generally mean its movement in a linear direction. By angular momentum we refer to its momentum when it moves in circles or rotation. The earth's movement on its axis is an example of angular momentum. Angular momentum can be derived by speed multiplied by the radius of the circle it is moving in and the mass of the object. If the ball in image 2 starts moving towards the center while still maintaining its rotation, the angular momentum will increase and it will start spinning faster. Thus the velocity of fluid in the kitchen sink is inversely proportional to the distance from the vortex . This is exactly what happens with the water in the kitchen sink or in a tornado. As the water spins and come closer to the kitchen outlet, it spins faster (generates greater angular momentum) creating a vortex of water. Thus water surrounding the vortex will be shaped in a tubular form.
Let us now come to light which is our subject of interest. In a previous article in this website, I had described how two wavefronts of light passing through two slits or openings, can be made to interfere so that it creates constructive (and destructive) interference of light. This property of light has been exploited by IOL scientists to help provide multifocality and depth of focus. Another exciting aspect that can be created of light property is vortices of light, like image of light forming as donuts. Thus light has both linear and angular momentum like a top spinning. A laser light moving on an angular momentum, will create a dark spot in the center (vortex), surrounded by light. This dark spot will be similar to the vortex of water created in the kitchen sink, or the eye of the tornado.
In a toric IOL, there are two meridians of power to correct the astigmatism of the cornea. The two meridians of power are 90 degree away, and helps converge the light to a single focal point. Thus a toric lens surface, consisting of two different refractive powers are designed to bring focus all rays of light to a image point. This may be called a stigmatic lens.
The spiral optics is designed to help elongate this focal point. To do this, a lens based on spiral optics would have different refractive powers in different meridians of the lens. Thus, unlike the toric IOL, it is designed to elongate the focal point as a zone of focus by the help of angular momentum (vortex in kitchen sink) such that a tubular zone (vortex) is created which stretches the focal points by exploiting the angular momentum of light.
For those of you who have stayed in the industry long enough, would remember the zonal refractive multifocal IOL (image 3), like the Array. Such zonal refractive multifocal lenses had alternating zones of two different refractive powers, one meant for distance and the other dedicated to the near. The Array, had its central zone, dedicated for the distance, and beyond this zone was alternating zones of near and distance. Altogether there were five refractive zones. Spiral optics may be designed similar to such concepts where alternate zones of spirals with definite powers are created (Image 4)
Different forms of spiral optics-
The design can be shaped in a Fermat spiral pattern (image5), that is in a parabolic shape pattern of alternating steep and flat meridian in a spiral shape. In a Fermat or parabolic spiral shape, the area between two consecutive curves remain the same. Thus as you move from the center of the spiral to the periphery, the spirals tend to be more closely spaced to maintain the same area at a given angle between them.
The design can also be shaped in an Archimedean spiral shape. In an Archimedean spiral shape, the distance between each spiral remains the same from the center to the periphery.
On the other hand, it could also be created in a logarithmic spiral shape wherein each of the spiral spreads out further and further when starting from the center.
Earlier designs on spiral optics were proposed on Fermat spiral shape (1).
Why was a Fermat spiral shape chosen over an Archimedean shaped spiral?
In a Fermat spiral, since the area between successive spirals remains the same, the focal points created by such spirals can be controlled or limited to a given defined space, thus creating a depth of focus. Thus light passing through the Fermat spiral shape will cause it to focus more closely, that is the light passing through the meridians of alternate powers will be more closely spaced than light passing through the Archimedean spiral. This will allow the light that passes through the spiral region of the Galaxy IOL to be focusing between the focal points created between the inner refractive zone and the outer refractive zone.
However, in image 4, the contact lens designed on spiral optics shape is more on a logarithmic pattern.
The Galaxy IOL from Rayner optics:
The Rayner Galaxy IOL is the first IOL to be designed on spiral optics. Not many information is there on the design of this lens. However, from what I understand the entire optic is not based on spiral optics concept. A specific zone of the lens is designed so. In my understanding the lens would have three zones, the innermost zone is a non spiral refractive zone that focus power to a particular focal point. This could be the near or distance. The spiral zone is included between the inner zone and outer zone, as a middle zone. This twists and turn the incoming light passing through the zone as the vortex in the kitchen sink. The outer zone may be dedicated totally for the distance. To the best of my knowledge the diameter of these zones are - first zone 1.1 mm, second zone of spiral optics pattern is between 1.1 to 3.2 mm, and the outer periphery beyond this dedicates light completely to the distance focal point. Thus the IOL is not based on diffractive optics but a combination of refractive and spiral optics.
The picture on the right may help in understanding how a spiral optics lens may
work. In the surface we can see two sets of Archimedean spirals (alternate white and gray spiral colors of two sets each having a constant width). Each spiral will have its own additional power that is over and above the base power of the lens. The near focal power may arise from the 24a and the intermediate from 24b. 56a and 56b denote the lens and pupil center, respectively (4).
You may question that regions of alternating spiral powers may give rise to dysphotopsia. Thus areas of transition between such zones may be source of light scattering. To avoid such issues, such lenses may have transition slopes or areas between two spiral tracks in adjacent zones.
For example, h1a and h1d are the transition height of spiral zones as described in the patent application. They can all be of equal height or heights of varying size to help create a smooth transition between spirals of different powers to keep the dysphotopsia profile low.
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Challenges with spiral designing lenses that create optical angular momentum for increased depth of focus:
Such shapes are complex and can introduce significantly more aberrations unless controlled by careful optical design compensation.
Currently under US FDA norms, a 21 diopter IOL will have a tolerance limit of .3 diopters. Thus the actual power of a 21 diopter IOL may in reality be anywhere between 20.70 to 21.30 diopter. Thus IOL designing has its own limitations, and such complex design could itself pose to be a daunting task to maintain its design accuracy.
Though currently available spiral design multifocal IOL is not based entirely on such design, that is it the majority of the IOL optic is based on refractive design, yet alignment of such IOL may be crucial.
Despite creation of transition zones between spirals, there would be regions of rapid (but not abrupt) power transition, that is changes in the radius of curvature between two spirals. How far patients may adapt to these transition zones is yet to be seen.
The earliest note on spiral optics can be seen in literature dating back to mid-nineties. However, interest in the subject for IOL and contact lens manufacturers have started only recently. It remains to be seen, if spiral optics helps eliminate the drawbacks of diffractive lenses, or it is just a flash in the pan. After all, there is no free lunch in optics, and we may not find the perfect lens, in our life time.
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Reference:
Spiral diopter: freeform lenses with enhanced multifocal behavior Laurent Galinier,1 Philippe Renaud-Goud,2 Jean Brusau,1 Lucien Kergadallan,3 Jean Augereau,3 AND Bertrand Simon3,*
Patent (pending) disclosure (https://patents.google.com/patent/EP3990979A1/en)
http://patents.google.com/patent/US20030117577A1/en20030117577A1 (image 7).
https://patents.google.com/patent/US20230338137A1/ko
(" INTRAOCULAR LENS WITH FOCAL PERFORMANCE TAILORED TO PUPIL SIZE DEPLOYING REFRACTIVE POWER MODIFICATION ALONG SPIRAL TRACKS")