Relation between the mesopic pupil size and diffractive zone of multifocal IOL
- May 17, 2025
- 7 min read
What should be the maximum size of the diffractive zone of a multifocal IOL that provide patients with good distance vision in mesopic conditions and yet help patients be comfortable to read? How could I arrive at a cut off value of the pupil size, based on the particular diffractive IOL I use?
These questions have often been raised especially in the context of diffractive multifocal IOL. But there is no easy answer to these questions. In this article, I will try to provide insights that may help to arrive towards some logical conclusion. In my view, both the questions above are interrelated. The size of the diffractive zone is constant on the IOL, but the pupil size of patients differs from one to the other as well as for varying light conditions in an individual. Thus finding the cut off value for mesopic pupil size may be important for clinicians to ensure that patients have a good distance vision even in low light. The cut off value may however differ from practice to practice, based on the device used to measure the pupil size, the light conditions in the clinic, etc. For example, Gedik and co-authors saw a large difference in measured mesopic pupil size value between the Nidek OPD Scan II and the Lenstar LS 900 II. Nevertheless, a broad idea may help in patient selection process.
Entrance Pupil: The pupil that you measure with optical biometry, is not its actual size, as the measurement is affected by the corneal magnification, whose overall effect is like that of a convex lens. Thus, we do not measure the physical or real pupil size at the iris plane, but the image of the pupil on the object side, thus measuring larger than the real pupil due to the corneal magnification. This is the entrance pupil, which is the image of the pupil at a point that is anterior to the iris. The entrance pupil is located approximately 3.0 mm away from the corneal vertex (1). On an average eye, with a corneal power of 44 diopter, ACD of 4.0 mm, the entrance pupil measured is almost 14% bigger than the actual pupil size (2). Jean-Philippe Colliac in his chapter on Gaussian Optics(3) provides a mathematical relation between the entrance pupil and actual pupil size. The values of actual pupil measured should be multiplied by a factor of 1.13 to arrive at the entrance pupil size. For a real pupil with a 2 mm diameter during photopic vision, the entrance pupil diameter is therefore 2* 1.13 = 2.26 mm. However, since the pupil measured in a clinic is the entrance pupil, therefore a measured value of 2.26 mm must be divided by a factor of 1.13 to arrive at the actual pupil size.
Exit Pupil: Like the entrance pupil, the exit pupil is the size of the aperture (pupil) as seen from the image side. It is the beam of light that will pass through the actual pupil and fall on the image side and on the IOL plane. Therefore, the size of the aperture or pupil that is seen from the image plane is the exit pupil. In the Gullstrand eye the position of the exit pupil from the corneal vertex is 3.664 mm, while the position of the entrance pupil from the corneal vertex is 3.045 mm.
In the image above entrance pupil is drawn as a dashed line (blue) because it's not a physical structure — it's the image of the iris as magnified by the cornea, sitting anterior to the real pupil and about 1.13× its diameter. Real pupil is the only solid element (gray iris blocks with a true gap) because it's the one physical aperture stop in the whole chain — everything else is an optical image of it. Exit pupil is also dashed (purple) since it's likewise virtual — this time the image of the iris as seen through the crystalline lens from behind, at 0.92× the entrance pupil. The amber ray lines trace the actual light path: they narrow slightly at the real pupil (the true limiting aperture), then the boundary carries straight through to the IOL plane — showing why the exit pupil diameter, not the entrance or real pupil diameter, is what determines how much of the diffractive zone gets illuminated. The teal band on the IOL marks the diffractive zone; the surrounding gray is the outer refractive zone, matching the "3.2 mm diffractive zone" |
To arrive at the size of the exit pupil, the diameter of the entrance pupil is to be multiplied by a factor of 0.92. Thus, if the entrance pupil size is 3.4 mm, the exit pupil is 3.4* 0.92= 3.1 mm. For the same entrance pupil, the actual pupil size will be 3.40/1.13 = 3.0 mm (as said before).
The exit pupil is important because, ultimately, the size of the aperture here will determine what portion of light will pass through the diffractive multifocal IOL to form the image on the retina. Thus, with the above example, a patient with 3.40 mm of mesopic pupil size at the entrance pupil position, will have only 3.12 mm of the IOL exposed that will contribute to vision.
If this relationship is well understood, let us now focus on the average pupil diameter for cataract patients in mesopic condition. The average bilateral pupil dilation in Indian patients was 3.92 in mesopic conditions (4). The median mesopic pupil size in a large cohort of 18335 eyes in Germany was found to be 4.1 mm (5). Wang and co-authors in a study on 716 patients of mainly Caucasian patients in USA found the average mesopic pupil size as 5.68 mm between all age groups. However they reported a reduced mean mesopic pupil size of 4.67 mm for patients above 65 years of age (8). It is widely acknowledged that pupil size reduces after cataract surgery. Camelin and coauthors reported a 10-13% average miosis after cataract surgery (6).
Considering all the above, if we assume a preoperative entrance mesopic pupil size of 4.0 mm, with a 10 percent miosis of pupil size post cataract surgery, thus bringing the size of actual pupil to 3.60 mm, the post operative mesopic exit pupil will amount to a size of 3.31 mm (3.60*.92). This is the effective size of all light passing through the physical pupil after surgery and being refracted or diffracted by the IOL and contributing to form the image on the retina. Thus, any light that falls outside the 4.0 mm of preoperative mesopic entrance pupil will be blocked by the iris postoperatively, and will not reach to form an image on the retina. Therefore, for patients to read even in a mesopic and low light situation, the minimum diffractive zone of any IOL will have to be 3.31 mm considering an entrance pupil of 4.0 mm.
