Diffractive orders with multifocal IOLs, echelette design and the concept of chromatic aberration correction
- Subhabrata Bhattacharya
- Mar 8, 2025
- 8 min read
Updated: Mar 21
Earlier in the article Science Behind Diffractive IOLs ( https://www.quickguide.org/post/the-science-behind-diffractive-presbyopia-correcting-iols ) I had explained the concept of diffraction and how multifocality of vision is achieved with diffractive IOLs. In this article, I will explain some of the design variations that IOL companies bring in, to negate chromatic aberration and its effect on image quality.
The first lens to be introduced that negated the effects of chromatic aberration with an echelette design was the TECNIS Symfony extended depth of focus IOL(1). The Rayner trifocal uses a -1, 0, and +1 diffractive order, as opposed to the usual practice of 0, +1, and +2 diffractive orders. In this article I will focus to give a simple explanation of what such expressions (like orders and echelette design) mean, and what the IOL manufacturing companies try to achieve by such design.
In my earlier article 'Science Behind Diffractive IOLs', I had explained through Young's Double Slit Experiment (image 1), how light passing through two slits or openings create two waves and how they interact with each other that helps create constructive and destructive interference of light. As light travels in waves, there will be a high of the wave (also called crest) and a low of the wave (also called trough). As the crest of one wave meets the crest of another wave, constructive interference is created that results in bright light formation on a screen kept close by. These areas of light formation is called 'orders'. Out of all the orders, the '0' order is the place of maximum bright intensity as light from the two openings or slits, are travelling equal distance to reach the O order. For all other orders, the light starts to loose its intensity, such that the +2 order (or -2 order) will be less bright than the +1 order ( or -1 order). Or in other words, light becomes less bright, that is the intensity of light loss is more as you move from the 0 order to other orders.
The standard way of diffractive trifocal IOL manufacturing has been to use the 0 order for distance, and the +1 and +2 orders for intermediate and near (image 2-Finevision, Zeiss). The base curvature of the trifocal or bifocal IOL is designed in such a way, that the refractive base curvature throws light to the 0 order and creates the distance focal point. The diffractive steps or rings, are shaped in such a way that the 0 order is created at this focal point. This explains how such multifocal IOLs have the greatest light devoted to the distance, and since distance is the primary demand of the patients, more so in mesopic conditions, the distance focal point or 0 order gets the maximum light from the photon budget.
