Science Behind Diffractive Multifocal IOL
- Subhabrata Bhattacharya
- Jun 14, 2021
- 12 min read
Updated: 2 days ago
The author visiting Lighthouse (optics) in Kerala, India in 2018.
There are very few people who would not stop by to see, if ever they pass by a Lighthouse. A Lighthouse is one of those few things that has always intrigued us and caught our imagination. Standing by the sea or the ocean, the Lighthouse has stood tall through the test of centuries to help men navigate through the many vagaries of sea life and reach destination safely.
Though the history of Lighthouse can be traced back to the Roman Empire, or beyond, the modern Lighthouse optics is credited to Augustin Jean Fresnel in the 18th Century. Prior to Fresnel, Lighthouse optics consisted of huge lenses that collected the light from an equally big lamp and threw it parallel and collimated many hundred miles over the sea for the ship to navigate. Fresnel understood that much of the refraction done by these huge lenses could be replaced by a thinner lens with a stepped design, each of these steps would bend the light by a certain degree to make the lights emerging from the lamp of the Lighthouse parallel. Thus Fresnel lenses allowed the construction of Lighthouse optics with short focal length and large aperture, without the mass and volume of material that went into prior medieval Lighthouse optics.
Of course, Fresnel lenses were constructed into many types, for the benefit of different types of Lighthouse and their objectives. Thus a Fresnel Lighthouse optics could be of Zero Order ( to throw light to infiniti for ships to see from hundreds of miles away in the ocean ) , First Order to Sixth Orders ( all focusing at a particular distance from the Lighthouse, depending on the need. A sixth order lens would throw light at a shorter distance, specifically placed near harbor.
The modern day diffractive multifocal lenses are a concept from Fresnel Lenses. As you are aware, these lenses are stepped or saw tooth designed to create multiple focal points. The Fresnel lenses applied in diffractive multifocal lenses is to collect parallel rays and focus it on the retina. In the Lighthouse, it is however the other way round , where the light is collected from the lamp, and thrown parallel to the sea. The basic optics however remains the same.
The Fresnel optics together with Thomas Young’s Diffractive Interference of Light have helped create the modern diffractive multifocal lenses. Young’s interpretation of the wave theory of light showed the world how light could be played with, to create constructive interference and thus “orders” or focal points. Ophthalmic scientists are heavily indebted to Fresnel and Young, for their contribution to optics and paving the way for modern diffractive multifocal lenses.
Young’s Double Slit Experiment :
Thomas Young took forward Hyugen’s principle on wave theory. Hyugen stated that every point on the wavefront is in itself a source of secondary wavelets, the sum of all wavelets together forming the waves.
So light travels in waves ? Not in straight lines ? To this debate then in the 18th century, Thomas Young brought to an end with his now famous Young’s Double Slit Experiment.
In Young’s double slit experiment, the science on which all diffractive multifocal IOL is based, light is passed through the first (single slit) on the left of the image on Fig 1. The light that passes through the slit spreads as it comes out of the opening. This spreading of light, as light passes through the slit is called diffraction. The spreading of light through the slit or opening is inversely proportional to the opening in the slit, that is, the larger the slit the less spread out the light is. Or less the opening of the slit, the more the spread out of the light as it passes through the opening. When you look at the optics of any diffractive IOL (fig 4), you can relate the steps or rings on its optics as slits or openings which help light to diffract and fall at a particular focal point. If you look at the optics more closely, the distance between the steps are not uniform. The distance between the steps decreases as you move from the centre to the periphery of the diffractive optics. This is because, as you move away from the centre, you will have to bend the light more, inorder to make it fall on the focal point. This is the simple rule of diffraction.
Coming back to Young’s double slit experiment in fig 1, the light that passes through the first slit now spreads out and then passes through two slits kept at a little distance away. As the light waves passes through these two parallel slits, they in turn form two separate wavefronts. These wavefronts would now interact with each other and ultimately fall on a screen that is placed at a few metres away from the second pair of slits. You will find alternate bands of bright and dark spots on the screen. The bright spot (maxima) that is exactly at the centre of the two slits is of maximum brightness and is called 0 order. As you move from the zero order, all subsequent bright spots will be of diminishing intensity (see fig 2). If you have to relate it to modern day diffractive IOL, the focal point that receives light from the central bull’s eye or the centre of the diffractive steps, is the dominant focal point and is often the distance focal point or zero order of the lens.
Keeping our focus on Young’s double slit experiment, if you look at fig 2 , you will see a number of bright and dark spots. The bright and dark spots are the result of crests (high) and trough (low) interacting with each other as they come out of the two slits. This is the law of interference of light. As the crest of one wave emerging from one slit, meets the crest of another wave emerging from the second slit, a bigger wave is created - the result of which is seen as several bright spots on the screen. On the corollary, as the trough of one wave meets the crest of the other wave, dark spots are seen on the screen. You can relate this to the waves of the sea, wherein the high/crest of one wave meeting the low/trough of the other results in a negation of the waves, while the high/crest of one wave meeting the high/crest of another wave results in a bigger wave formation.
Relating this to the science of diffractive IOLs, it is always desirable to utilize as much light as possible all directed at the two focal points of a bifocal IOL. This can be only achieved, when the crest meets the crest of the wavefronts of light passing through the diffractive IOL optics. That being said, some light loss is inevitable as the crest of one wave meets the trough of the other.
The question here is that, how can we space the steps/rings of the diffractive IOL in such a way that all light passing through the diffractive optics falls on the two focal points of a bifocal IOL ? At what distance should these steps be spaced to each other to utilize the maximum light waves for the focal points ? Herein comes the importance of understanding wavelengths, and its importance in diffractive IOLs.
