The Science behind Diffractive IOLs
An understanding of the concept of the science behind diffractive multifocal IOLs should begin with the elementary concepts of the physics behind such lenses. Diffraction is the spreading of light when light passes through an opening. One fundamental condition that however needs to be met is that the wavelength of light that we are talking of must be smaller than the opening through which a mono chromatic light pass. As the human vision peaks at a specific wavelength in each of the three lighting conditions, research around this lenses are typically with the wavelength of light that accounts for peak photopic sensitivity. The reality however is a lot different. Patients seldom are exposed to a mono chromatic light source, and hence the acceptance of such IOLs depends on specific spectral sensitivity of individuals.
That being said, let us draw our attention to some of the fundamental concepts behind the creation of such multifocal lenses. At the core lies the wavelength of the monochromatic light and how it can be tweaked to create two or more focal points for the patients to see a range of vision. So, what is a wavelength?
A wavelength is the completion of a cycle of the wave. Thus it is the distance from the crest to the crest, or the trough from the trough. Light is measured by wavelength (nanometers). The number of times the waves of light passes through a given point is called the frequency. You can measure the light waves in units hertz . That is frequency of light. Shorter the wavelength of light, higher is the frequency. If you ask your friend to hold the other end of the rope while you hold its one end, and both of you swing it vigorously, you would see that as you shake it harder, its vibration increases, and the wavelength decreases. In physics, the more you move towards UV light rays, the more the frequency, and less the wavelength of light. The entire discussion of the harmful effect of shorter wavelength of Blue Light rests on this.
Let us draw our attention on the diffractive IOLs. All diffractive lenses are created through a combination of refractive and diffractive elements. You would have walked to eat a pizza at a popular food joint. The bread of the pizza is prepared first (to fill your appetite) and then the several toppings are spread on the bread (to suit your taste buds). The popular diffractive multifocal IOLs are accordingly made by preparing the monofocal optic first, and then the steps are imprinted to give the multifocality. Therefore, the first precondition of a surgeon success with a multifocal IOL is to have its monofocal lens tasted first.
All focal points are created by bringing the incoming light to focus on the multiple focal points. In physics, these focal points are referred to as “orders”. The distance focal point is called the zero order, as all light that pass through the base power of the lens is directed to the zero order. The base power of the lens is the refractive component, like the bread of the pizza. The central bull’s eye of any diffractive multifocal IOL is the refractive element and therefore, directs light to one focal point only. This is usually the distance or the zero order, but in case of tri focal IOLs, they are often directed to the intermediate, or the first order.
Let us now have a look into how the near focal point is created. In case of the bifocal IOLs, the near focal point is often called the first order, and in case of tri focal IOLs, they would be called as the second order. As light passes through the steps, the later slows down the speed of the light and increases the wavelength of light on the vitreous. This is called the phase delay, wherein the wavelength of the light changes, that is it slows down, and is directed to the first order. The step heights determine the amount of light that is to be directed to the first order. Usually the step height is a direct function of the number of focal points desired multiplied by the monochromatic wavelength of light and then divided by the refractive index change between aqueous and the lens material. Just for those, who are married to science.
It is necessary that all light that passes through the cornea and the lens, is utilized and directed to the zero and first order. Since we are now discussing on the first order only, the steps (diffraction gratings ) has to be arranged in such a way, that the light traversing through the optic meets at the first order. That is why you will see the distance between the steps are gradually reduced, so that light passing further away from the optical axis of the lens is bent more to direct it to the first order. In diffraction, the less the aperture, more the bending. Hence, the distances between the steps are reduced to help the light passing through the periphery bend more to reach the first order.
That being said, the step boundaries are generally put whenever the difference of wavelength of light passing through two successive steps exceeds by one wavelength. Why ? Because that is the only way you can ensure that all light to be directed to the first order has the same phase difference. In physics, this concept is Young’s Double Slit. In Young’s Double Slit, all crests that create the maxima ( here focal points) needs to be in phase. Therefore the wavelength difference between successive waves that are in phase cannot exceed one wavelength. The successive maxima that is created in such experiments are a direct summation of the difference between the wavelengths.
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