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 difference in refractive power across the meridians (or zones) may be two or more such that the focus is no longer at one point, but across two or more points, and additionally 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 a dedicated zone could be created as spiral optics, but other zones would be refractive only dedicating light to either near or distance. We will describe the spiral shaped zone that contributes to depth of focus and intermediate vision by extending from the near focal point to the distance. In this context, we will focus on the Galaxy IOL later in this article.
Spiral optics on the IOL material is designed by molding the material to its desired topological charge. Topological charge here means how much one wavelength of light (distance between two successive crests or troughs) is made to twist and turn on its axis. In the world outside IOL, such change is induced by having different optical thickness across different meridians. This is called phase plates, wherein each plate has a different refractive index. It could be also made through curvature changes that is changes in asphericity of the lens;. Such curvature changes (asphericity changes) could be in a helix form (image 4) very much like a spiral shaped staircase along the optical axis of the lens . In either way the refractive power will shift as you shift across meridians of the lens.
Light travelling across a meridian, will twist and turn differently depending on how close it is to the optical axis. That is, its orbital angular momentum will be different in different places across that meridian. Thus the amount of twist and turn that light would make will determine the depth of field that the patient is going to experience with the lens. The spiral phase plates will twist and turn the photons like water in the kitchen sink and make it fall over a tubular region. This accounts for the depth of focus. The way of doing this is by creating regions of variable thickness of material (phase plates of variable refractive index), or curvature in a helicoidal pattern depicted in image 4.
As light passes through these phase plates in the material, the incident light will undergo a phase shift that will be different for light travelling through different areas of the IOL. This is constructed from the IOL material with gradually increasing, spiraling thickness or curvature that affects the speed of light while travelling through the medium thus creating phase shift. Such phase shift will create an angular momentum of light passing through particular areas with vortices and fall in different zones creating a tubular focus region.
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 sprial 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 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.
Subha Bhattacharya
India
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