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.
What is meant by echelette design or blazed grating and why does companies like Rayner utilize a diffractive order that is different than that of standard trifocal IOL diffractive orders?
It is evident that companies like Rayner, do not use the 0 order for the distance. This is shown in image 2, wherein the diffractive orders are -1, zero, and +1, for distance, intermediate and near respectively. To understand why and how this is done, we have to understand the concept of chromatic aberration.
Chromatic aberration
In my earlier article 'Why we should stop asking about Abbe Number with Intra Ocular Lenses' (https://www.quickguide.org/post/abbe-number-and-iols ) I have explained what is chromatic dispersion of light.
Chromatic aberration or chromatic distortion is the failure of light to come to focus at a single point. When white light passes a glass prism, it breaks into its several components. The short wavelength blue light falls before the green light, and the green light falls before the red light. This process of white light splitting into several of its constituent colours is called dispersion of light. Light travelling through a prism from air bends towards the base of the prism. This bending towards the base happen differently for all constituents of white light according to their wavelength.
Chromatic aberration with a diffractive grating or steps is opposite to a refractive monofocal lens. While, white light passing through a refractive lens will have the blue light refracted the most and the red light refracted the least (2)(image 3), however, when white light pass through a diffractive grating, blue light is diffracted the least and red light is diffracted the most.
However, what is to be noted here is that, the zero order in a diffractive grating do not suffer from chromatic aberration. This is because light travels equal distance to reach the zero order, that is the Optical Path Difference (OPD) between different wavelengths of light is zero. Other than the zero diffraction order, all other orders, negative or positive suffers from chromatic aberration in air. It is important to state how do we identify a negative and a positive diffraction order? If you draw a normal, that is a straight line through the diffractive grating and its image point, the angle that are positive between the normal and the order are positive orders, while the negative angle created are negative orders.
Thus diffractive IOLs that are designed on refractive base curvature of the lens, compensate for longitudinal chromatic aberration for the +1 and +2 orders ( as the chromatic aberration generated in the positive orders is in opposite to the chromatic aberration generated by the base refractive lens). But this is not true for the 0 order, because at 0 order, that is distance for a standard diffractive IOL, there is no chromatic aberration generated by the light (explained before). Thus there is no compensation for refractive lens base curvature generated longitudinal chromatic aberration for the zero order, by a diffractive multifocal IOL that utilizes 0, +1, and +2 orders for distance, intermediate and near, respectively.
To overcome this, lenses with echelette design (Tecnis Symfony)(3) or Rayner trifocal use a different design. The Rayner trifocal uses a blazed grating or an echelette grating, that is the steps of the trifocal IOL is specifically oriented in a way so that the incoming light maximum intensity is not at the zero order, but at the -1 order (for Rayner trifocal IOL). Thus the shape of the grating or steps/rings are significantly different to trifocal IOLs that utilize 0 order for the distance (image 5).
Thus for Rayner, this echelette or blazed grating creates -1 order as distance, and the base curvature of the lens is designed in such a way that the monofocal base curvature power is matched to provide the distance focal point at the -1. In other words, the light that is focused by the base curvature of the lens are matched with the blazed grating shape of the diffractive steps so that the distance focal point and the -1 order create a singular distance vision for the patient.
Since -1 order is created for the distance with the echelette or blazed grating shape, the base power of the lens is adjusted. For example, a Zeiss At Lisa trifocal IOL that uses 0 order for the distance has a base power of 20.0 diopter. To create the same lens for the patient, the Rayner trifocal IOL that utilizes the -1 order for the distance will have a higher power in its base curvature, say 21.50 to compensate for the difference in power between the -1 and 0 orders. If the same lens, utilized the +1 order for distance, then the base curvature power would have been reduced (image 6).
What does the companies gain when they utilize the -1 order for distance?
As earlier pointed out, the chromatic aberration generated by a diffractive structure is in opposite to the chromatic aberration generated by a refractive lens.. Since there is no chromatic aberration compensation at the zero order ( a diffractive grating does not have any chromatic aberration at the zero order) there is a large chromatic aberration at the distance focal point due to the monofocal refractive base lens. By shifting the distance to -1 order through a blazed or echelette grating shape, the distance is relatively freed from chromatic aberration, as the sign of the chromatic aberration generated by the diffractive steps will be in opposite to the sign of the chromatic aberration generated by the refractive base curvature.
What to consider for such design?
The effect of chromatic aberration on the human eye may be limited. This is already explained in the article 'Why we should stop asking about Abbe Number with Intra Ocular Lenses' (https://www.quickguide.org/post/abbe-number-and-iols ). Thus the real and ultimate benefit of such lens design to the patient, is yet to be understood. The risks of chromatic aberration is mitigated by the presence of macula pigment and the fact the peak cone sensitivity is towards longer wavelength of light. There is also a risk of overcompensation of chromatic aberration by shifting the distance to higher or lower diffractive orders. The benefit of such design is still to be explored and more evidence on real world outcomes awaited.
References:
Design concepts for advanced-technology intraocular lenses; Michael Simpson, Susana Marcos & co-authors https://doi.org/10.1364/BOE.544647
Extended depth of focus intraocular lens:chromatic performance; MARIA S. MILLÁN* AND FIDEL VEGA; https://doi.org/10.1364/BOE.8.004294
