The Impact of Smoothing Diffractive Steps in IOL Science on Patient Visual Quality
- 21 hours ago
- 6 min read
Updated: 1 hour ago
Intraocular lenses (IOLs) have transformed vision correction for cataract patients, offering not just restored sight but enhanced visual experiences. Among these, diffractive IOLs stand out for their ability to provide multifocal vision, helping patients see clearly at different distances. A key area of innovation in diffractive IOL science is the smoothing of diffractive steps—the tiny, concentric rings on the lens surface that split light to create multiple focal points. This post explores why many companies focus on smoothing these steps and what this means for the visual quality patients experience.
Understanding Diffractive Steps in IOLs
Diffractive IOLs use a series of microscopic steps etched into the lens surface. These steps create constructive interference patterns that split incoming light into different focal points, allowing patients to focus on near, intermediate, and far objects without glasses. An explanation of how diffractive IOLs work to provide multifocality is described in the below video.
Traditionally, these steps have sharp edges, which effectively separate light but can cause unwanted visual effects.
Sharp diffractive steps can lead to:
Increased light scatter causing glare and halos around lights, especially at night.
Reduced contrast sensitivity, making it harder to see fine details in low-light conditions.
Visual disturbances that can affect patient satisfaction.
In the article 'Understanding the Point Spread Function and Strehl Ratio of a lens system' (https://www.quickguide.org/post/points-spread-function ) I have given a detailed understanding of the PSF. PSF is the ability of the lens to capture a point source of light, as a point source of light on the image plane. That is, in an ideal lens, the image of the point source of light (the object) will be captured exactly as a point source of light. But we do not live in an ideal world. Thus, in reality, the image of the point source of light is spread out. How much of the light is spread out, is measured by PSF. Thus, point spread function or PSF is a measurement of blur of a point source of light in the image plane and it defines what the point source of light will in reality look like.
The PSF contains the airy disc and the airy rings. The more the light is concentrated in the airy disc, better the resolution and contrast of the image. However, some amount of light will always leak and create an airy ring or diffraction halos. This part of the light that reach the airy rings bring down the contrast of the image, and could create the halo or glare effect.
Smoothing these steps means modifying the lens surface to reduce the abruptness of these edges, aiming to maintain multifocality while minimizing side effects.
Relating PSF with the retinal visual angle
Relating the Point Spread Function to Retinal Visual Angle
The point spread function (PSF) describes how an optical system images a single point of light on the retina. In an ideal diffraction-limited optical system, most of the incoming light is concentrated within the central Airy disc, while only a small fraction is distributed into the surrounding Airy rings. The greater the proportion of light confined to the central lobe, the higher the retinal image contrast and the finer the spatial detail that can be resolved. Conversely, when more light is redistributed into the peripheral rings because of diffraction, aberrations, or scatter, the central peak becomes broader and less intense, reducing image contrast and visual resolution.
The significance of the PSF becomes clearer when it is related to retinal visual angle. Human visual acuity is traditionally expressed using a Snellen chart, where a person with 20/20 (6/6) vision resolves a minimum angle of approximately 1 minute of arc, equivalent to 1/60° or approximately 0.017° (≈0.02°). This angular resolution corresponds to the smallest separation between two retinal image points that the visual system can distinguish. Therefore, an optical system whose PSF remains largely confined within this angular range is capable of supporting normal visual acuity.
Visual acuity, however, represents only one aspect of image quality. Contrast sensitivity depends on the eye's ability to resolve patterns across a range of spatial frequencies. Peak human contrast sensitivity occurs at approximately 3–18 cycles per degree, corresponding to retinal visual angles ranging from roughly 0.33° to 0.06° for one cycle. Consequently, the width and shape of the PSF influence not only high-contrast letter recognition but also the visibility of lower-contrast objects encountered in everyday vision.
In practical terms, the central lobe of the PSF determines the eye's ability to resolve fine detail (visual acuity), whereas the surrounding distribution of light into the Airy rings influences contrast sensitivity and, when excessive, contributes to straylight and dysphotopsias such as halos and glare. Thus, evaluating how light is distributed within the PSF provides a direct optical link between intraocular lens design and the retinal visual angles that underlie human visual performance.
Why Companies Are Moving Toward Smoothing?
The drive to smooth diffractive steps comes from a desire to improve patient comfort and visual quality. This is achieved through reducing the forward scattering of light, especially at larger visual angles, beyond 1-2 degrees. Thus any light that reach the green zone could be regarded as forward light scatter, creating a veil of luminance and washing off the contrast of the image.
Several factors motivate this trend:
Reducing Visual Disturbances: Patients often report halos and glare after receiving traditional diffractive IOLs. Smoothing the steps softens the light transition, reducing these effects.
Improving Contrast Sensitivity: Smoother steps scatter less light, which helps maintain better contrast, especially in dim environments.
Enhancing Intermediate Vision: Some smoothing designs optimize light distribution to improve vision at intermediate distances, which is crucial for daily activities like computer use.
Increasing Patient Satisfaction: By reducing side effects and improving overall visual quality, smoothing can lead to higher satisfaction rates and fewer postoperative complaints.
Different ways of smoothing diffractive steps:
Two important ways of smoothing of diffractive steps are captured in the below picture:
Sine-wave smoothing replaces the abrupt diffractive step with a smooth sinusoidal transition, much like the regular rise and fall of ocean swells. In contrast, Gaussian smoothing redistributes the phase according to a bell-shaped profile, concentrating the transition near the centre while allowing the edges to taper gradually, analogous to the smooth wake of a boat that gently emerges from and fades back into calm water. Because the Gaussian profile contains less high-spatial-frequency content, it generally produces lower halo intensity and a cleaner point spread function than sinusoidal smoothing.
What does this mean to your patient:
The goal is not to eliminate glare—that is impossible with a diffractive optic—but to reduce the likelihood that glare becomes bothersome.
The formation of multiple diffraction orders is an inherent consequence of diffractive optics. As a result, some degree of positive dysphotopsia—such as halos, starbursts, and glare—is an expected optical trade-off when creating multiple focal points. The objective of modern diffractive IOL design is therefore not to eliminate these phenomena, but to manage how light is distributed around the intended retinal image.
When abrupt diffractive step transitions are replaced by smoother phase transitions, the amount of light redistributed into unwanted diffraction orders, higher-angle scatter, and straylight may be reduced. More of the transmitted light remains concentrated within the useful central retinal image, while less contributes to veiling luminance at larger retinal visual angles.
From the patient's perspective, this distinction is important. Many patients can tolerate some glare or halos if these are mild and do not interfere with daily activities. What affects satisfaction is not simply the presence of glare, but whether that glare becomes bothersome—reducing contrast, impairing night driving, or creating disability glare.
Ultimately, the success of a premium diffractive IOL should not be judged solely by how much light reaches the retina, but by how efficiently that light is concentrated into the useful retinal image while minimizing the light that contributes to visually disturbing phenomena. The engineering challenge is therefore not to eliminate diffraction, but to use it in a way that maximizes image quality and minimizes patient-perceived visual disturbance.
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Reference:
(1) Hakan Kaymak 1,2, Richard Potvin 3, Kai Neller 1,2, Karsten Klabe 1, Robert Donald Anello 4,✉; On behalf of the NINO Study Group, Customizing Clinical Outcomes with Implantation of Two Diffractive Trifocal IOLsfof Identical Design but Differing Light Distributions to the Far, Intermediate and Near Foci, https://doi.org/10.2147/OPTH.S456007
