Demystifying 'High Light Utilization' claim by Multifocal IOLs
- 4 hours ago
- 8 min read
High light utilization has become one of the most frequently cited performance metrics in the marketing of modern trifocal intraocular lenses (IOLs). Most manufacturers report light-utilization values approaching 90%, while some platforms claim values exceeding 93%. There is little doubt that significant advances in diffractive optics and lens design have enabled contemporary trifocal IOLs to achieve substantially higher optical efficiencies than earlier generations of bifocal lenses, which typically reported efficiencies in the range of 80–82%.
At first glance, these figures appear straightforward: if more light reaches the retina, visual performance should improve.
However, the relationship between High light utilization and patient experience is not necessarily linear. High light utilization is desirable only when the transmitted light contributes meaningfully to image formation. Light that is dispersed into halos, straylight, higher diffraction orders, or other unwanted optical phenomena may still be counted as "utilized" light, yet contribute little to useful vision and may even degrade visual quality.
Another important consideration is that there is no universally standardized methodology for reporting light-utilization values. Measurements are often performed under highly controlled laboratory conditions, typically using monochromatic green light and a fixed pupil diameter of approximately 3 mm. Such conditions differ considerably from the polychromatic and dynamically changing optical environment encountered by patients in everyday life. Consequently, comparing light-utilization values between different IOL platforms may not always be as straightforward as the numbers suggest.
As clinicians, therefore, we should resist the temptation to accept high light-utilization figures at face value. When a company representative claims that an IOL achieves exceptionally high light utilization, several important questions should immediately arise. How was the measurement performed? Under what pupil size and wavelength conditions? Does the reported value account for light scattered into halos and photic phenomena? Most importantly, how much of that light is actually concentrated into the retinal image in a manner that improves visual quality?
This article explores the optical principles underlying light utilization, examines the assumptions and limitations behind commonly quoted efficiency values, and discusses why the concentration of light into useful image-forming information may be more clinically relevant than the absolute percentage of light reported as "utilized." By understanding these nuances, clinicians can better evaluate claims that high light utilization automatically translates into fewer halos, less glare, and superior patient satisfaction.
When the company claims 'High light utilization', is that light being referred to is a light 'throughput' through the IOL?
Light throughput is simply the measurement of light that passes through the IOL. It does not consider what amount of light reach the retina, or how is that light concentrated on the retina.
Thus, The simplest interpretation is:
In the above case, light utilization is simply a metric of light transmission through the IOL. This information is not helpful to understand the quality of image on the retina. Not all light that pass the IOL reach the retina, some are lost to the forward scatter of light at larger retinal visual angles, that work as straylight to the patient.
Is light utilization a measurement of diffraction efficiency?
What many manufacturers mean by light utilization is the percentage of light reaching the three focal points:
Far: 50%
Intermediate: 20%
Near: 23%
Thus the total light utilization that may be quoted in this example is 93%. This is what usually many manufacturing companies actually mean. The challenge with this approach is that, it does not talk about quality of the light being focussed in the focal points. While in this case, a high light percentage is being directed to the focal points, it does not talk about the quality of the light organized or concentrated in each of those focal points.
If by high light utilization what is being meant is light being directed to each of the focal points, then what arises as a question is the nature of the Point Spread Function (PSF) of each of the focal points.
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 an object 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.
So the question is, if by high light utilization, what is being referred to is the PSF, then which
part of the PSF is being referred to? Is the 93% light utilization (as an example) is being quoted as the light that reflects in the airy disc, or is it the sum total of airy disc and airy rings. Remember, the light reaching the airy rings, is of no use for the image quality. Thus in image 2, if total light utilization being claimed is a sum total of the Airy pattern, that is the light of the candle (airy disc) and the diffraction pattern (airy rings), then it may not be very meaningful for the clinician.
What matters to the patient is not
how much light reaches the retina, but rather, how much light is concentrated into the useful retinal image? Two lenses may both claim 93% light utilization. But Lens A concentrates most energy into the central lobe. Lens B spreads much of that energy into rings and halo structures.
Both may have identical utilization numbers.
Patients will not perceive them as equivalent.
Another question that clinicians may ask is that what is the total percentage of light that is directed in the clinically relevant retinal visual angle of less than 1-2 degrees. Several studies have pointed to the fact that, light directed at higher visual angles of more than 2 degrees creates a veil of luminance on the retina. This washes of the contrast of the image and may be a source of bothersome glare. This is often referred to as straylight.
So when a company claim very high light utilization, keep in mind the following questions?
Is the light utilization quoted is the percentage of light that cross the IOL? That is total transmitted light?
Is the light utilization quoted is the percentage of light that is directed into the three diffraction orders, or focal points?
Is the light utilization quoted reflecting the amount of energy in the PSF? Or is it in the Airy disc?
What is the percentage of light encircled in the clinically relevant retinal visual angle of less than 1 degree.
Why a High MTF Does Not Necessarily Mean Low Dysphotopsia
Another common misconception in multifocal IOL evaluation is that a high Modulation Transfer Function (MTF) automatically translates into superior visual quality and fewer photic phenomena. Manufacturers frequently quote impressive MTF values, particularly for the distance focus, and clinicians often assume that a higher MTF must imply a lower propensity for glare and halos. However, this is only part of the story.
MTF describes how effectively an optical system preserves contrast at different spatial frequencies. In simple terms, it tells us how much of the object's contrast survives after passing through the optical system. While this is undoubtedly an important measure of optical performance, MTF alone does not provide a complete picture of image quality.
To understand why, consider a real-world analogy.
Imagine two groups of fifty soldiers marching over a distance of one kilometre. Both groups cover the distance in exactly fifteen minutes. If one were to judge performance solely on speed, the two groups would appear identical.
However, there is an important difference. The first group marches in perfect synchrony. Every soldier moves in step with the others, creating a highly coordinated formation. In the second group, although the overall speed is identical, some soldiers are slightly out of step. Most soldiers may have their left foot forward while a few have their right foot forward. The group still reaches its destination at the same time, but the movement is less orderly and less coherent.
An MTF measurement is somewhat analogous to measuring only how quickly the groups completed the march. It tells us how much contrast is preserved, but not whether the optical wavefront remains perfectly organized while doing so, very much like if the soldiers where marching in a well synchronised rhythm.
The missing information is contained in the phase of the optical system. Two lenses may exhibit very similar MTF values and therefore preserve a comparable amount of contrast. Yet one lens may concentrate light more effectively into the central retinal image, while the other may distribute a greater proportion of light into diffraction rings, PSF wings, halos, or other unwanted structures. From the patient's perspective, these two lenses may produce very different visual experiences despite having similar MTF values.
This distinction becomes particularly relevant in diffractive multifocal IOLs. A lens may achieve an excellent MTF at a given focus while simultaneously generating significant light distribution outside the central image-forming region. The patient may therefore experience glare, halos, or reduced subjective image quality despite the apparently impressive MTF performance.
For this reason, MTF should be viewed as only one component of optical quality assessment. A complete understanding requires consideration of additional metrics such as the point spread function (PSF), encircled energy, Strehl ratio, light distribution among diffraction orders, and wavefront phase behaviour. These metrics help answer a question that MTF alone cannot:
"Not only how much contrast is preserved, but also how effectively the light is concentrated into the useful retinal image".
Conclusion
The purpose of this article is not to suggest that high light utilization or high MTF values are unimportant. On the contrary, both are valuable indicators of optical performance and reflect the remarkable advances made in modern diffractive IOL design. However, problems arise when these metrics are interpreted in isolation or assumed to directly predict the patient's visual experience.
As clinicians, we must remember that the retina does not measure percentages of light utilization, nor does it perceive MTF values. Patients experience vision through the final retinal image produced by the optical system. What ultimately matters is not merely how much light reaches the retina, but how effectively that light is concentrated into useful image-forming information while minimizing unwanted light distribution into halos, glare, and straylight.
High light utilization is only valuable when the light is utilized where it matters most — inside the retinal image, not outside it.
A lens may demonstrate excellent light utilization by directing a large proportion of incident light into its intended focal points. Likewise, a lens may exhibit impressive MTF values, indicating good preservation of contrast. Yet neither metric alone reveals how much of that light is concentrated within the central image-forming region of the point spread function, how much is distributed into surrounding diffraction rings, or how the phase structure of the wavefront influences the quality of the final retinal image.
Perhaps the most important lesson is that optical quality cannot be reduced to a single number. Light utilization, diffraction efficiency, MTF, phase behaviour, Strehl ratio, encircled energy, PSF morphology, chromatic performance, pupil dependency, and tolerance to real-world imperfections all contribute to what the patient ultimately perceives.
Therefore, the next time an IOL is promoted as having "94% light utilization" or an exceptionally high MTF, the clinician's response should not be immediate acceptance, but informed curiosity. How was the measurement performed? Under what conditions? Where does the remaining light go? How much of the transmitted light contributes to the central retinal image? And most importantly, how do these laboratory metrics translate into the visual experience of real patients?
In the end, the goal of cataract surgery is not to maximize a number on an optical bench. The goal is to maximize the quality of vision experienced by the patient. The closer our metrics come to describing that reality, the more meaningful they become.
